Home How-to Articles Cummins 4BT and 4 ISB Diesel Engine Conversions

Cummins 4BT and 4 ISB Diesel Engine Conversions

by Moses Ludel

The Cummins R2.8L turbo-diesel crate engine conversion has a strong following. For many, however, this swap is too costly. Some find the Cummins 4BT and 4 ISB 3.9L conversion more affordable.

The Cummins 2.8L crate turbo-diesel engine has enjoyed a following. At the 2019 SEMA Show, Las Vegas, this painstakingly restored Scout II boasted an R2.8L turbo-diesel transplant engine. Installations like this are inspirational.

Cummins built the 4BT engine for industrial, agricultural and commercial automotive use. Introduced in 1984, the Cummins B-Series engines included the 6BT 5.9L inline six and a 3.9L four-cylinder 4BT. The 4BT architecture is essentially the 6BT minus two cylinders, making 4BT swaps easier.

For engine conversions, a Cummins/Ram 5.9L truck can be a donor for the factory engine to transmission adapter, flywheel or flexplate and mating to a transmission like the Getrag 360, NV4500, NV5600 or G56 manual. Automatic transmission options would be the 727 Torqueflite (3-speed, fully mechanical and easiest to adapt) or an A518, 47/48RE or RFE overdrive automatic. The Ram offerings beyond the A727 would be more difficult to adapt due to electronic sensors and the need for a control module. On a budget, builders can benefit from recycled Ram truck parts.

The 4BT turbo-diesel became a popular step van choice, often described as the “bread truck” engine. Mechanical injectors and other basic features make this engine easier to service and less costly to maintain. Early engines are 8-valve, while the later model 4 ISB turbo-diesel has 16 valves. Like the BT6, the BT4 evolved into an ISB engine with ECM triggered common rail diesel (CRD) injectors.

Engine swaps should all look this “factory”. Cummins R2.8L turbo-diesel comes with a list of peripherals that ease the installation. This Scout II swap includes power brakes and air conditioning, power steering and a large output alternator. Neat hose routing, wiring and incidentals make this a show stopping vehicle…A 4BT or 4 ISB would also fit well in this engine bay. The Scout II is a good candidate with its stout ladder frame and Dana 44 front and rear axles.

Evolved Cummins 4 ISB Design

In late 2000, Cummins launched the high tech 16-valve ISB engine to replace the B-Series 4BT. To better meet emissions standards, the 6BT had already been updated to a 24-valve ISB design for “1998.5” and newer Ram 5.9L and 6.7L Cummins trucks. In a press release, Cummins detailed the new 4-cylinder ISB engine. Here are key differences between B-Series engines and the newer ISB:

“Today (10/29/00), in San Diego, California at the American Trucking
Association, Cummins introduced the new electronic four-cylinder ISB 4
engine. A blend of the experience from the past 20 years and progressive
feature improvements, the 4 cylinder ISB product continues the B Series
tradition of quality while improving driver satisfaction. This new
4-cylinder engine will replace the current 4-cylinder B3.9 engine that is
used in automotive walk-in van applications.

The new 4-cylinder ISB 4 engine will be available in 145hp and 170hp
ratings, with 420 lb-ft of torque for both horsepower ratings. Weighing in
at only 773 lbs., this compact 3.9 liter package is praised for its
improved performance. It joins the Interact Systems (IS) family with many
of the same features customers enjoy today with the current six cylinder
ISB product, as well as a number of new features.

Like the 5.9 liter ISB, the four-cylinder ISB 4 product has:

*Full Authority Electronics – compatible with Cummins electronic software
for diagnostic and maintenance functions
*Vertically Centered Injection – improved performance, oil control and
*4 Valve/Cylinder – 16 valve cylinder head for improved combustion

New features include:

*High Pressure Common Rail fuel system – improves performance and fuel
economy at lower emissions
*Rear Gear Train – improves performance and reduces noise
*Sculpted Block – integrated oil and coolant passages for fewer leaks

The benefits for customers are numerous! The advantages of the 4 cylinder
ISB include:

*20% improvement in fuel economy. Results from field test units indicate
customers will have significant improvements in fuel economy, compared
to the B3.9. Cummins’ own test truck had fuel economy improvements of
20 – 30% over the B3.9 with an on-highway route. This improvement in
fuel economy is primarily due to the engine electronics and vertically
centered injection. With the high price of diesel fuel today, these
savings can really add up for a customer!

*15,000 mile oil change intervals. The 4 cylinder ISB product has the
same maintenance interval as the 6 cylinder ISB. This is more than
double the maintenance interval for the B3.9. Thanks to the vertically
centered injection, customers will have less downtime and spend less

*4-5 dBA quieter than its predecessor. This means it sounds about half
as quiet to the ear. The difference is significant in the seat of a
walk-in van — and one the drivers will really appreciate.

At launch, the 4 cylinder ISB engine will only be available in walk-in van
and street sweeper applications. The 4 cylinder ISB is available at
Freightliner Custom Chassis and Workhorse Custom Chassis. Beginning in
January 2001, the first 50 engines are expected to be delivered to the
OEM’s. These initial engines are expected to be in the field later Q1
2001. The OEM’s have committed to keeping the first 50 engines in 8
distributor territories, in order to help Cummins fulfill parts and channel
training requirements while taking care of customers. The 8 distributor
territories have been notified and Cummins Service has prepared the
network. Full production of the engine is expected to begin in Q2 2001,
with many distributors receiving the ISB engine in their territory no later
than Q3 of 2001.

The ISB 4 cylinder engine illustrates Cummins’ continued investment in new
technology. This technology allows Cummins to provide customers with
engines that are cleaner, more efficient, and better performing. The
advancements with the ISB 4 engine make it an exciting story to share with
Cummins customers!

Cummins, Inc.

Why a 4BT or 4 ISB Diesel Engine?

The main cost saving with the 4BT or a 4 ISB swap is a used engine option. If a complete bread truck engine with transmission can be found in good condition, this can be the foundation for an affordable package. Some 4BT step van applications were equipped with a common 2WD version of the Borg-Warner T18 four-speed (non-overdrive) transmission. This makes the swap practical for taller (numerically lower) axle gearing on the highway or lower (numerically higher) axle gearing for off-road use on a budget.

Advance Adapters makes an adapter kit to mate a 2WD or 4WD Borg-Warner T18 to common 4×4 transfer cases. Click here to access Advance Adapters’ official website and kits like the T18 to Dana 300 transfer case adapter, which would be common for 1980-86 Jeep CJ applications. There are scores of other adapter approaches. The Cummins SAE adapters or Ram truck OEM pieces provide another route.

While the 4BT engine’s horsepower ratings seem less than impressive, this is deceiving. Like a “big truck” or industrial diesel, these engines produce exceptional torque at a low rpm. The quick torque rise from idle to peak torque rpm is a hallmark of truck, marine and industrial turbo-diesel engine performance. Unmistakably, the 4BT delivers.

For those bent on horsepower figures or better fuel efficiency, the 4BT can be modified for greater horsepower output and higher torque. Be aware that a heavy throttle foot will reduce diesel fuel efficiency. Higher rpm in each gear and shift point, or rpm beyond the engine’s torque peak at cruise speed, will also increase fuel consumption.

Cummins 4BT engine can be adapted to Ram/Cummins OEM transmissions. The 6BT and ISB six-cylinder factory adapters will mate the 4BT to an NV4500 and other Chrysler/Ram transmissions.

The NV4500 is a popular transmission choice with good gear ratios plus overdrive with an iron case. 4WD Ram versions will mate with a Dana 300 or newer NP/NV transfer case, using Advance Adapters clocking ring and the correct spline output/input shaft. Advance Adapters also mates the NV4500 to earlier Model 20 and Spicer/Dana 20 transfer cases. There are many adapter options available for the NV4500 and other suitable transmissions. Transmission choice should be a heavy-duty light truck type.

Advance Adapters’ Dana/Spicer 20 (Jeep®, Scout and Ford applications) to NV4500 adapter kit. There are a variety of transmission to transfer case adapter kits available. See Advance Adapters website.
There is an Advance Adapters kit to mate a Dodge truck NV4500 to the Ford Bronco Dana 20 transfer case. There is a GM version of the NV4500 as well. This kit is busier than the Jeep®/Scout approach. Advance Adapters specializes in a wide range of popular transmission and transfer case combinations and choices.

4BT and 4 ISB Fuel Mileage Gains

For fuel efficiency, this engine is fuel miserly in stock or near stock form. Claims of 30 mpg or higher appear achievable when looking at the 4BT or 4 ISB torque curves. However, these claims are dependent upon the vehicle application, gearing and driving style. Maximum fuel economy would be more likely with a lighter weight Jeep CJ, Wrangler, XJ Cherokee, MJ Comanche or similar size pickup, a Toyota FJ40, classic Land Rover, Defender 90 or an SUV like the ZJ/WJ Grand Cherokee or a KJ Liberty.

Big Bear Engine Company has considerable experience with the Cummins 4BT engine. They explain that the 4BT must be modified to achieve 28 or more miles per gallon. In Big Bear Engine Company’s experience, 4BT miles per gallon in a Jeep® 4×4 conversion follows this formula:

“The standard 4BT will [yield] 18-20 mpg in the city and 22-25 mpg on the highway. A normal upgraded engine will yield 28-30 mpg…with some high level performance mods to the aftercooler, turbo and variable (lift and duration) timing of the camshaft being able to get 35-40 mpg.”—Big Bear Engine Company

If a 4×4 Jeep® conversion can attain this fuel mileage, amortizing the cost of a diesel conversion would take far less driving miles. For examples of 4BT fuel mileage results in Jeep® 4x4s and other conversions, or to determine necessary engine modifications and the cost of these parts and services, contact Big Bear Engine Company. The company offers advice and quotes.

Doing the math, a heavier SUV or full-size half ton rated 4×4 pickup intended for hauling or towing will get less fuel mileage. With a stock 4BT engine, expect the 20-23 mpg range unloaded and 12-18 mpg (depending on trailer weight and wind resistance) when towing a trailer. Again, gearing is critical, engine speed as well. Typically, running a diesel engine beyond its torque peak rpm will result in reduced fuel efficiency.

Also in play, trucks and SUVs or Jeep-type vehicles are notorious for poor drag co-efficiency and unfavorable aerodynamics. Add a chassis lift and oversized tires, and the fuel mileage gets even worse. 4BT or 4 ISB mileage expectations of 30 mpg or higher might be achievable on paper. In real world driving conditions, this can change.

The best fuel mileage for our lifted Ram 3500 5.9L CRD Cummins is at 1600-1900 rpm in overdrive. Over 1900 rpm, each 100 rpm will see a distinct decrease in fuel economy. Some of this is load, much of it is drag and wind resistance. With Hypertech Max Energy tuning, the peak torque is at 2,100 rpm. (Stock tune torque peak would be 1600 rpm.) By 2100 rpm, fuel efficiency is already down 2 mpg. Road speed, drag and wind resistance are up. Each factor contributes to this mileage decrease.

Note: To drop engine speed below 2,100 rpm at interstate road speeds, a 7-speed medium duty overdrive transmission conversion and an axle gearing change was on our drawing board. That thought ended with a reality check conversation. A friend and former platform engineer at Chrysler shared flatly that our DH Ram Cummins truck, like many other trucks, had a poor drag co-efficient. He noted that vehicle speed and resistance played a much heavier role in fuel consumption than engine rpm. Additionally, when considering the truck’s chassis lift, big tires, massive winch bumper and hefty winch, the removal of the air dam and other frontal area modifications, gearing alone will not fix the fuel mileage issue.

Swapping a four-cylinder Cummins turbo-diesel into a square nosed, lifted 4×4 with oversized tires, a winch, auxiliary fuel tank and a full complement of body armor is a recipe for lowering fuel mileage expectations. At interstate cruise speeds or in a headwind, frontal resistance is like pushing a billboard down the road. Add a trailer, watch the mileage drop further.

Amortizing the Cummins Swap Option

Amortizing or recovering the cost of this conversion takes several variables into account: 1) the cost of the 4BT or 4 ISB engine and peripherals; 2) installation parts and any sublet labor for the installation; 3) cost of engine modifications to improve fuel efficiency or performance; 4) the fuel mileage difference between the OEM gasoline engine in good operating condition and a 4BT or 4 ISB turbo-diesel and 5) the cost difference between unleaded regular gas and pump diesel fuel…Mileage gains are relative.

Weigh the cost of a new or rebuilt turbo-diesel engine conversion, including all peripheral components and sublet labor. Consider fuel savings per mile over the OEM gasoline engine. Given the fuel mileage of a stock (unmodified) 4BT engine, the amortization point for this conversion could be 300,000 or more miles. There is a way to lower this amortization mileage. According to Big Bear Engine Company’s projections, substantial fuel mileage gains are possible with 4BT engine modifications.

For comparison purposes, a new R2.8L Cummins crate engine conversion with adapters and the peripherals is currently $16,995 (US) plus the labor. (Contact Axis Industries for details.) The Axis Industries Jeep TJ Wrangler package is not unreasonably priced when considering the scope of components—the new crate engine and a thorough installation package is costly. Do the math and determine whether a diesel conversion makes financial sense for your needs.

Of course, driven properly, these industrial turbo-diesels exceed gasoline mileage by a wide margin. Keep in mind, however, that if the goal is higher rpm and constant interstate driving, a contemporary gasoline engine like the high tech GM 5.3L LS V-8 iron block light-duty light truck (LDLT) engine, or a 5.7L Mopar Hemi V-8, will deliver good performance and reasonable fuel mileage in the right vehicle with correct gearing. GM LS engine swaps are popular for that reason. Additionally, as a recycled package, the LS engine swap may be less expensive than a diesel engine conversion.

Steve Roberts at Advance Adapters walks through a 50-State emissions legal Jeep TJ Wrangler with a GM LS Gen 3 V8 engine conversion. For some, a high-tech gasoline V8 in a lightweight Jeep® 4×4 is sufficient. Compare cost and consider your goals. Advance Adapters worked with Cummins around the contemporary R2.8L turbodiesel swap using the Aisin AX15 transmission.

Add to all this the current cost of diesel fuel. Once priced consistently below leaded or unleaded regular, diesel fuel is ranging forty to sixty cents a gallon above 87 octane gasoline. (This is our local finding, your area may differ.) When considering the cost amortization for a diesel conversion, keep this factor in mind. Unless the cost of fossil fuel diesel or bio-diesel come into line at some point, diesel fuel pricing will impact the overall cost per mile.

Note: We bought our 2005 Ram 3500 new in October 2004 and have seen a steady increase in the cost of diesel fuel over unleaded regular gasoline ever since. Meanwhile, high-tech gasoline engines have become more fuel efficient and economical to operate. Fuel mileage, a strong reason for buying a diesel powered truck, has become less of a bargaining point. We don’t pull a hot shot load or a 9-horse, live-in trailer. Most of our driving is under light load with occasion travel trailer or car hauler towing. Perhaps this is good reason to consider a contemporary gasoline V-8.

4BT and 4 ISB Weight and Vibration

Diesel engine tune or fuel calibration determines the power output and fuel efficiency. For more power output, the engine design must have the stamina for added turbocharger boost and increased fuel flow. Here, the 4BT and 4 ISB have the same architecture as the robust 6BT and 6 ISB Cummins engines. This is an entirely different paradigm than the lighter weight R2.8L engine design. The B-series and ISB engines can work hard all day. Visualize a loaded bread truck, street sweeper or large agricultural equipment.

To meet these work loads, the 4BT or 4 ISB is not a light weight package. The engine with an SAE adapter, flywheel, clutch and engine-driven accessories is well over 800 pounds. Compared to an iron Jeep/AMC inline six, these Cummins engines are 250-300 pounds heavier. Add an A/C compressor and other power accessories, and weight bumps up further. Provisions must be made for handling the high engine torque, engine weight and the four-cylinder diesel vibration.

Yes, the 4BT and 4 ISB come with a reputation for vibration. When the engine is not installed and mounted correctly, this can be an issue. Farm Strong, Inc. knows the 4BT well. They have engineered custom motor mounts that reduce vibration. Farm Strong takes the angle of the mounts, position of the crankshaft centerline and engine support position into account. A 4BT or 4 ISB engine conversion should follow the Farm Strong, Inc. guidelines. These engine mounts are a wise investment.

Engine Sourcing and Resources

Big Bear Engine Company at Denver, Colorado is an excellent resource for insight into 4BT engine options and conversion needs. A supplier of new, used and re-manufactured Cummins engines, the company has a lengthy history with the 4BT. To learn more about 4BT diesel engine performance, tuning specifications and conversion engine applications, visit the Big Bear Engine Company’s 4BT website at https://www.4BTengines.com. The company’s Facebook page is https://www.facebook.com/bigbearengineco/.

Big Bear Engine Company serves the diesel industry with a machine shop, new and rebuilt replacement parts and expertise at engine tuning for specific equipment or vehicle applications. The company has provided many 4BT engines specifically for conversions into 4×4 Jeep®, Land Rover, Land Cruiser and light-duty truck applications.

Big Bear Engine Company has extensive experience and guidelines for 4BT and 4 ISB engine conversions. Shops or serious DIY enthusiasts considering a 4BT or 4 ISB engine swap can contact Big Bear Engine Company for details and guidelines.

For a Cummins 4BT engine swap, Big Bear Engine Company provided this photo overview of a 4BT engine conversion into a Jeep 4×4 chassis. Jennifer at Auburn, Maine took these photos of her engine swap in progress. Thanks, Jennifer, for sharing:

Back end of Cummins 4BT 3.9L inline four engine is similar to the 5.9L 6BT inline six. These engines use an SAE adapter or can mate to Ram/Cummins transmissions. Used step van or “bread truck” applications are sometimes packaged with a BW T18 or other common gearbox attached.
Four-cylinder length makes this a good choice for a Jeep CJ, YJ, TJ or JK driveable chassis. The engine can also work with a Jeep XJ Cherokee uni-body; however, the BT4 weight, torque and vibration require proper frame trussing and Farm Strong, Inc. universal type motor mounts as a place to start. This engine would also work well for an FJ Land Cruiser, Land Rover or a lighter 1/2-ton rated 4×4 pickup truck. 4BT engines find their way into vintage Power Wagons and 4×4 pickups.
Good fit for this Jeep 4×4 chassis! Proper motor mounts are crucial. Farm Strong Inc. mounts or a similar mount layout are a must for the 4BT or 4 ISB swap. Note mount position above the crankshaft centerline to reduce vibration.
Any swap involves careful adjustments for peripheral parts. Consider the brake master cylinder, steering column, steering shaft and adequate body clearance. Unlike the Cummins R2.8L crate engine*, there is no “bolt-in” kit for a 4BT/4 ISB conversion at this point. A qualified shop or experienced DIY enthusiast can perform this swap. Metal fabrication tools and powertrain experience are a must! Diesel experience can be helpful.
*See Axis Industries for a complete R2.8L crate engine installation package.
This starter motor has excellent clearance. A variety of hoses and engine peripheral parts require attention during fit-up. Expect the conversion to take considerable time—all engine conversions do! You are mating a hefty turbocharged diesel engine with a gasoline engine chassis. Focus on the goals.
A turbocharger with inter-cooling or after-cooling means big tubing and lots of it! The fitment challenge is getting this plumbing safely routed and under the hood. Aesthetics come into play, too. Most want an impressive under hood look. Why not? This is an impressive, industrial strength engine package!
Fortunately, this Jeep has a relatively tall hood. Height of the engine, measured from the oil pan base to the top of any plumbing, should be considered before attempting this swap. Clearance above the axle is also a concern. The oil pan sump must be at the rear for typical beam 4×4 front axle clearance. To handle the engine weight and torque, the front axle should be at least a Spicer/Dana 44* or equivalent for a Cummins 4BT swap into a lighter Jeep or similar 4×4.
*The Jeep TJ or JK Rubicon Dana 44 front axle has a smaller axle tube size than a full-size beam axle 4×4 truck with a 44. The 4BT requires a truck version of the Spicer/Dana 44 front axle. Truck axles have a higher axle weight capacity. The aim is not just a bigger ring gear design but also more front axle weight capacity. Each front beam live axle comes with a load rating. So does a rear beam axle.
Details and quality parts matter. Oil lines provide crucial lubricant and must be reliable. Off-pavement, remote use of the vehicle demands attention to details like these braided oil lines and fittings.

Caution: If your 4×4 fords streams, the air filter should be mounted high—away from water—and shielded. A windshield-high snorkel is advisable for deep water crossing…Drawing water into the air induction system and the engine’s cylinders will hydro-lock the engine. This results in severe connecting rod, piston, cylinder head, block and crankshaft damage.

Attention to detail and clean, tidy tube routing are obvious on this swap. The finished result is a reliable, aesthetically pleasing Cummins 4BT in this Jeep…Great work and worth the effort!
This 4BT fits like it should! Routine service work is accessible. The electric cooling fan and radiator are nicely done for this engine. Horsepower equates to BTUs, and the moderate 4BT horsepower output does not require a massive radiator.

Note: This 4BT turbo-diesel engine should run cool. An engine-driven mechanical fan, fan clutch and safety fan shroud are an option if space permits. Years ago, an engine driven fan provided far more CFM air flow than an electric fan. Aftermarket electric fans have come a long way; many swaps now rely solely on an electric fan.

The Cummins 4BT engine looks at home in Jennifer’s Jeep! The alternator and other serviceable parts are easier to reach, consistent with a bread truck’s doghouse service requirements. The serpentine belt is a Cummins mainstay, an easier way to drive accessories.

Vehicle Registration and Emissions Issues

While emissions legality was once an eye wink for many, states like California now assure motor vehicle emissions compliance. Regulations and the means of enforcement continue to tighten, and a six-pack of beer may no longer get you a bogus emission inspection from a friend or acquaintance willing to risk his or her smog license.

Aside from annual registration renewal, you may not be able to resell your 4×4 if a donor engine came from the wrong type vehicle. Many states require an emissions inspection during the transfer of ownership. The buyer may be on your doorstep after a failed attempt to smog, title and register the vehicle.

If you do a legal engine swap, be certain to keep all paperwork and receipts. Stow papers from a referee station approval, including pollution tailpipe tests and the test facility’s confirmation that the engine conversion meets state emissions and “engine change” (conversion) standards. States like California issue an official sticker label for the engine bay once the vehicle has successful passed a Referee Station visual inspection and tailpipe test. This enables the vehicle to get future biennial inspections at a regular smog station.

Upon resale of the vehicle, you will have proof that the process met state motor vehicle department requirements. If the conversion is legal, the buyer can title and register the vehicle at your state; however, this may not mean the vehicle can be registered at all other states. Each state has its own guidelines for engine conversions or “changes”. State engine conversion regulations range all the way from no emissions enforcement and no inspection requirements to a policy that mimics California’s engine change guidelines.

Traditionally, California set the most stringent emissions standards in the U.S. These guidelines have been adopted by many other states. Years ago, vehicles were built to Federal 49-State standards (EPA) and additional guidelines for the California (50-State) market. Since 2016, U.S. vehicle manufacturers have been building new engines and chassis emissions equipment to a collaborative EPA/California emissions standard.

In the Ram 6BT and 4BT era of mechanical fuel injection, there were engines built with different emissions hardware for two emissions markets: 49-State (Federal/EPA certification) or California (EPA plus California certification). Today, it’s impractical to manufacture some vehicles with 49-State emissions devices and others with devices for the California market. Mechanical equipment is identical for all 50 States. Prior to model year 2017, some models with electronic engine controllers (ECM) were software flashed to meet a CARB emissions tune requirement if sold in the California or “Border States” market.

For diesels, there were slightly different powertrain software calibrations for California-bound new trucks through model year 2016. 2017 and newer model vehicles meet an identical California and Federal emissions standard. CARB and EPA now collaborate and set the same emissions requirements for all motor vehicle testing.

Example: We purchased our new 2005 5.9L Cummins Ram 3500 truck at Nevada. The sales zone was near California, and DaimlerChrysler delivered the truck with a “Border States” emissions calibration. The ECM software included a white smoke tune adjustment to meet California Air Resources Board (CARB) requirements. The Nevada dealership had “Federal” (49-State) and “California/Border States” software available. As a Nevada based and registered vehicle, our dealership was able to re-flash the ECM under warranty with the 49-State Federal software, which gave the engine a slight performance edge…In every other way, the engine and chassis emissions devices were the same for all 50 States.

California’s Clean Air Resources Board (CARB)

There is currently a fervor in some political circles to “deregulate”. California’s right to set emissions standards for the Golden State has come under attack. Ironically, the Mulford-Carrell Act that established the California Clean Air Resources Board (CARB) was signed by none other than Governor Ronald Reagan in 1967. The measure was nationally recognized as necessary.

Air pollution was extreme at Los Angeles, Orange County and other California cities. As late as the 1980s, Disneyland’s Matterhorn was not visible from the adjacent Santa Ana freeway (I-5). The Federal 1970 Clean Air Act, signed into law by President Richard Nixon, granted California a waiver to set higher pollution control standards. Nixon’s home away from Washington, D.C. was San Clemente, California. In the 1990 Clean Air Act Amendments, Congress further expanded the California waiver to include off-road (off-highway equipment and agricultural engines) and non-automotive engines (lawn mowers, generators and so forth). President George H.W. Bush signed that legislation into law.

Deregulating or eliminating the CARB influence on engine conversions would place all states under EPA 50-State regulation. Since 2016, the EPA’s emissions rules have been a collaborative effort with CARB. The CARB engine change (conversion) guidelines remain in place for California as a reasonable path for installers to perform engine changes while not compromising air quality. Without CARB influence, EPA could set different engine change policies or even make it illegal to swap engines. Eliminating California’s CARB influence under the pretext of “deregulation” could have the opposite effect: Any Federal/EPA decision about engine changes (conversion swaps) would arbitrarily apply to all 50 states. Individual states could lose the right to determine engine change guidelines.

Additionally, California’s CARB has become a vital means for aftermarket engine performance parts manufacturers to get emissions approval (CARB E.O. exemption numbers, i.e. 50-State legal status) on tested and qualified products. Nullifying CARB would leave the EPA as the sole agency controlling aftermarket parts approval—if the EPA were willing and equipped to do so. If the EPA ruled to not approve aftermarket products, that would end the 50-State legal automotive aftermarket. California’s CARB is currently the only 50-State recognized path for approving aftermarket engine-related parts or crate engines.

As an active SEMA Member, 4WD Mechanix Magazine supports the specialty equipment industry and also appreciates California’s willingness to emission lab test and approve non-polluting aftermarket products for 50-State legal use. The Summit Racing and JEG’s catalogs are packed with 50-State legal CARB E.O. exempted products—from intake manifolds, ignitions and headers to select GM LS V-8 and Ford Coyote V-8 crate engines.

California’s CARB is recognized by legitimate manufacturers and the EPA as the benchmark for emissions legal aftermarket products—plus engine change or conversion guidelines. The Federal EPA interprets the 1970 Clean Air Act and 1990 Clean Air Act Amendments. California’s CARB functions under a Clean Air Act waiver.

California’s CARB works closely with the automotive aftermarket to lab test and approve products and crate engines that do not compromise emissions standards. California has been home to aftermarket automotive parts manufacturers like Banks Power, Edelbrock and Derale. If CARB were “deregulated”, how would the EPA respond to engine changes and aftermarket parts approval? Regardless of your political persuasion, deregulation of California’s CARB would serve no useful purpose.

A practical engine installer should be aware that more states could adopt California standards and guidelines for engine conversions. Some states will likely rubber-stamp the established California “engine change” (conversion) rules. If so, emissions guidelines and methods of smog inspection could be similar, and these standards would impact vehicle registration and titling. At our home state of Nevada, densely populated zip codes at Washoe and Clark Counties currently use California-type formulas for vehicle emissions testing, titling and registration.

A Diesel “Engine Change” at California

As recently as the 2000s at California, a diesel engine swap into a gasoline powered vehicle was fully acceptable and straightforward. The vehicle registration process required only a fuel type change. Logic surrounded the goal here: a properly running automotive diesel engine could produce less CO, CO2 and HC than most gasoline engines. A diesel engine swap can actually improve on some elements of air quality. However, diesels also produce NOx, sulfur dioxide (lower now with ULSD), other pollutants and visible particulates. Some of this has been addressed with later model diesel truck engines.

When diesel vehicles began using catalytic converters to combat these pollutants, that all changed. In 2010, California started biennial tailpipe emission checks for 1998-up diesel powered trucks up to 14,000 pounds GVWR. (Nevada Emissions Control Program, covering Clark and Washoe County zip codes, requires smog inspection of 1968 and newer diesel powered cars and trucks up to 14,000 pounds GVWR. (Here is a snapshot of state regulations for OEM diesel vehicles: https://drivinglaws.aaa.com/tag/emissions-inspection/.)

As diesel trucks fell under biennial inspection guidelines and closer emissions scrutiny, so did diesel engine swaps. California changed registration requirements from a fuel type change to full-on engine change guidelines similar to gasoline engine conversions. California diesel engine swap standards are fairly straightforward for light-duty light trucks (LDLT), a 4×4 Jeep-type vehicle or an SUV:

1) The engine donor vehicle must be the same emissions tier and GVWR class as your 4×4 chassis. The donor vehicle and swap engine must be the same year or newer than your chassis. For emissions purposes, California will not allow swapping a heavy-duty light truck (HDLT) or medium duty truck engine into a light-duty light truck (LDLT), a passenger car or an SUV (like a Jeep 4×4) that fits a lower class GVWR. Know your vehicle’s GVWR, it’s emissions category, and the conversion engine’s emission category. Verify your state’s registration guidelines for the diesel swap engine and your GVWR chassis. California example: A medium duty Isuzu or Cummins emissions certified diesel engine cannot be installed in a lighter weight (under 6,000 pounds GVWR) vehicle.

Here are two links for official California rulings and engine change guidelines. If you live at California and must register, smog and title your vehicle within that state, the 2019 Smog Check Reference Guide (link below) is especially useful. “Appendix D” spells out the aims and specific rulings for an engine change (conversion). Without raining on anyone’s parade, it’s always better to know the facts before buying a conversion engine or making the actual engine change:



Note: In California BAR or CARB language, an “engine replacement” is the same type gasoline engine and emissions package as your vehicle’s original engine. This term applies to exact replacement engines, a new or rebuilt long block and so forth. By contrast, an “engine change” is installing a different type engine (i.e. a diesel or a GM LS V-8) in place of an OEM Jeep® gasoline engine. Know the terms and follow the “engine change” guidelines. Make an informed choice.

2) Notably, a GM Duramax V-8, Ford Powerstroke V-8 or Cummins 5.9L and 6.7L heavy hauler is always in a 3/4-ton or heavier chassis (HDLT) or a medium duty, higher gross weight truck chassis. From an emissions legality standpoint, these engines cannot be swapped into a lighter truck like a 1500 series, a Jeep-type 4×4 or an SUV under 3/4-ton capacity, which is anything but a 3/4-ton Suburban or Ford Excursion. As for the Suburban or Excursion, a diesel conversion would likely be a newer Duramax V-8 or Powerstroke V-8, respectively. A 1991 or older Suburban might be a candidate for a 5.0L Cummins V-8 (a recycled Nissan Titan engine), a Duramax V-8 or a 5.9L/6.7L Cummins ISB.

2) Installed, the engine must have all components considered part of the donor vehicle’s original emissions package. Pay close attention on later diesel engine packages and your 4×4’s chassis year. Depending upon the swap engine year (which must be the same model year or newer than your chassis), a swap can mean the catalytic converter(s), engine sensors upstream and downstream of the exhaust manifolds/headers, all oxygen sensors and other parts that were originally certified with the conversion engine. Many newer engine swaps include the donor transmission and its control module. Read through the engine change guide’s details on the exhaust system, OBD-II and the diagnostic link.

3) Lastly, and really what matters in terms of pollution, your vehicle must pass a chassis, diagnostic link (if an OBD-II vehicle) and tailpipe emissions inspection. Tailpipe readings must meet or improve upon the standards set for the vehicle’s original engine in good operating condition. Any newer, correct tier and class diesel or gasoline engine in good condition (with all emission devices in place) will meet this benchmark if installed in an older Jeep, Land Cruiser, Land Rover, Scout, Scout II or light-duty light truck. California recognizes that engine changes in many cases will reduce or at least not impact the vehicle’s pollution output. This is why California allows engine changes. The end game is healthier air quality, less visible smog and lower greenhouse gas emissions.

Cummins R2.8L Crate Engine and California Emissions

By design and from testing at Cummins and elsewhere, the R2.8L will easily meet emissions standards for the emissions tiers targeted. Cummins has pursued a California E.O. (Executive Order exemption number) for an engine change into 1999 and earlier Jeep® 4x4s and light-duty light trucks (LDLT). This standard also applies to passenger cars from 1993-back.

That E.O. number is not in place at this date, and despite a very clean emissions score, the Cummins R2.8L crate engine does not currently qualify for an engine change in California. We have been strong supporters of this engine since its launch and want Cummins to succeed at gaining 50-State legal emissions status. The fuel savings and low emissions from this engine would make a significant contribution to better air quality.

R2.8L California CARB Emissions Status

As of March 2020, below is the official statement from Cummins regarding emissions and status. Check the https://cummins/engines/repower link for updates on R2.8L emissions status. 4WD Mechanix Magazine has been a staunch supporter of this engine’s development and suitability for many vehicle applications and “engine changes” or conversions. The Cummins R2.8L turbo-diesel crate engine package can meet tailpipe emission standards for its tier and deserves a CARB E.O. exemption number.

Cummins Repower Website Statement:

The Cummins R2.8 Turbo Diesel (PN 5467046) crate package is not CARB-approved at this time.

“Due to state/provincial law restrictions, these engines are not currently available for purchase in the state of Texas, state of West Virginia or the province of Quebec. Warranty service with respect to the engines and loose components will not be available in the states of Texas and West Virginia.

“The Cummins R2.8 Turbo Diesel has demonstrated (in a representative vehicle with a manual transmission) through the testing procedures prescribed in 40 CFR Part 86 to meet EPA’s Tier 1 LDT2, LDT3, and LDT4 end of useful life emissions standards. The same vehicle also demonstrates Tier 0 LDT1/LDV end of useful life emissions standards. This means that the R2.8 generally is suitable for Model Year 1999 or earlier light duty trucks and Model Year 1993 or earlier passenger cars. However, the suitability of the R2.8 engine to any vehicle is the responsibility of the installer and may depend on state or area laws. Installation of this part in a vehicle or application for which it is not intended may violate U.S. and Canadian laws and regulations related to motor vehicle emissions and will void the applicable engine warranty. Check your state and local emissions requirements before purchasing.”

A possible reason for the CARB stalemate is that no emissions certified production vehicle in the U.S. uses the Cummins R2.8L engine. There is no prototype vehicle with an R2.8L turbo-diesel that has been certified for highway use through the EPA and California. A Nissan Frontier concept vehicle with the Cummins R2.8L drew immediate attention and would have been a huge market success. That has not developed yet.

Unfortunately, to date that concept model has not evolved into an emissions certified production vehicle, sold and distributed in the U.S. If such a “mini-Titan” truck came to market, the R2.8L engine would eventually be available through recycling yards from donor Frontier pickups. California would recognize the engine as certified and emissions legal in a Nissan Frontier. A given year and model Frontier R2.8L engine could serve as a crate engine prototype.

The Cummins R2.8L engine can easily pass California emissions lab standards for its emissions tier. This crate engine with fixed tune calibration would qualify as a complete engine “package”. If an R2.8L turbo-diesel came from a production vehicle prototype with EPA/California emissions certification, there would be no problem. Installers could do engine changes and pass smog requirements with a trip to the nearest California BAR Referee Station. Referee Stations are also a helpful resource for determining necessary crate or donor engine equipment that must be included in the conversion.

The crate engine concept has worked for other engine manufacturers, notably GM’s E-ROD crate engines. In these cases, however, the E-ROD crate engine also has an EPA/California emission legal counterpart, an approved and certified motor vehicle application. This could be a given model and year production car like a Camaro or whatever.

Caution: If your vehicle is registered in California or a state with similar emissions standards and registration/smog inspection guidelines, follow the latest Cummins Repower developments for the R2.8L turbo-diesel crate engine. Be sure you can register and title your vehicle before making this swap.

GVWR and Diesel Emissions Categories

When installed in an EPA or California certified motor vehicle like a step van (bread truck), the 4BT or 4 ISB turbo-diesel engine originally met guidelines for that GVWR emissions class. If the donor truck has a medium duty chassis, confirm whether the engine will meet emissions requirements for a swap into your vehicle’s chassis. At California, if the donor vehicle is a heavy-duty light truck (HDLT) or a medium duty chassis, the engine cannot be swapped into a light-duty light truck (LDLT), passenger car, Jeep-type 4×4 or light SUV emissions vehicle.

Confirm the chassis GVWR for the 4BT or 4 ISB donor vehicle. When using the California emissions guideline, if you own a Class 1 emissions Jeep CJ, Wrangler, Cherokee, Land Cruiser, Scout II, light-duty light pickup (LDLT) or similar vehicles, a heavy-duty light truck or medium-duty truck donor engine will not qualify for emission certification in any of these vehicles. If the donor engine does not qualify, a Referee Station will send you home without a tailpipe test or any paperwork to register the vehicle. Better to inquire before buying the swap engine.

By contrast, the Volkswagen’s TDI engine, Jeep KJ Liberty 2.5L or 2.8L VM Motori diesel, or the 3.0L V-6 Grand Cherokee diesel are in the same emissions GVWR class as light duty emissions 4x4s like a Jeep CJ, Wrangler or XJ Cherokee. These diesel engines would be legal for an engine change/conversion. The 3.0L V-6 might be a consideration, and the VM Motori 2.8L drew attention with its Export status in applications like the military JK Wrangler platform. A Referee Station will confirm the emission tier or class of the donor engine and its acceptable GVWR emissions class use.

None of these light-duty diesel engines are a match for the 4BT and 4 ISB. The 3.9L Cummins engines are built for industrial, agricultural and commercial automotive (truck) use. For longevity, high stamina and low-speed torque, these Cummins engines are superior. Arguably, the 4BT and 4 ISB architecture is superior to the lighter Cummins R2.8L turbo-diesel crate engine.

The appeal of the R2.8L is light weight, newer technology and an easier to install, well conceived crate package. Whether the R2.8L proves itself for longevity is an unknown at this point. Vehicle weight, loads and use will be determinants. The R2.8L has a Brazilian Ford F350 prototype. This Ford truck application should be monitored over time. A late seventies Ford High Boy 4×4 with an R2.8L crate engine in its bay is difficult to imagine. Such a truck deserves a Cummins 5.9L or 6.7L ISB inline six, a Navistar 7.3L V-8 or a late Cummins 5.0L V-8.

Due to installation ease, the Cummins R2.8L has become popular for a wide range of applications. This engine design fits the light-duty vehicle (passenger car, light-duty light truck and light SUV) emissions category if Cummins is able to get CARB approval for the crate engine package. A light diesel design should work well for passenger cars or light-duty trucks and lighter weight SUVs. The Liberty KJ 4×4 was not exactly light with a curb weight up to 4,312 pounds, yet the VM Motori 2.8L turbo-diesel survived. So did its 2.5L predecessor, though power was not its strong card. Add vehicle weight, heavy loads or significant towing to the equation, and the stouter 4BT or 4 ISB makes far better sense.

Licensing a Diesel Conversion

If you live in a state like California or one that requires periodic emissions inspection, check your state regulations before considering a Cummins R2.8L, 4BT, 4 ISB turbo-diesel or any other engine conversion. Know your state and local emissions guidelines before investing in a diesel engine swap.

Note: For a quick overview and details on initial registration or a biennial smog inspection at California visit: https://www.dmv.ca.gov/portal/dmv/?1dmy&urile=wcm:path:/dmv_content_en/dmv/vr/smogfaq. The statements there reference original equipment diesel vehicles and not a gasoline vehicle with a diesel engine swap. Worth noting, 1975 and older vehicles are exempt from smog inspections.

There is one path to titling and registering a light 4×4 vehicle with a Cummins 4BT or 4 ISB engine conversion. States have a vehicle age point that no longer requires emissions inspection for registration or titling. For California, 1975 and older vehicles are exempt from inspection. For Nevada emissions program zip codes, 1967 or older vehicles are exempt. Montana does not require emissions inspections; however, before considering a diesel engine conversion at Montana or any other state, review your state’s vehicle registration laws relating to diesel engine and other conversions. Be certain that the vehicle can be titled as well as registered without a mandatory visual and/or tailpipe emissions inspection.

If we were to undertake a Cummins 4BT or 4 ISB conversion at Nevada, a vintage 1967 or older vehicle and chassis would be a safe bet. Our ’99 XJ Cherokee registers outside the Washoe and Clark County emissions zones and does not require emissions inspection. However, we still would not convert the XJ Cherokee unless the diesel engine could meet statewide emissions requirements.

Were we to sell the vehicle to a party at neighboring California or a Nevada emissions testing locale (as close as 30 miles from where we reside), the 4BT or 4 ISB engine may not qualify for an emissions inspection. If the engine were deemed a heavy-duty light truck or medium-duty GVWR emissions class, the XJ Cherokee would not pass a Referee Station inspection. Additionally, our immediate area is slated for 20,000 acres of industrial development and accompanying residential growth. This rural county could at some point become an emissions testing zone for vehicle registration.

Some 4×4 vehicles suited for the 4BT or 4 ISB swap at Nevada would be a pre-1968 model year Jeep CJ, 4WD Willys Pickup or Willys Station Wagon, a Land Rover, Nissan Patrol or Toyota FJ40 and FJ45. For California’s 1975 or older exemption, the 4×4 chassis could be a Jeep CJ, a Scout II, a Land Rover or a Toyota FJ40 or FJ60 Wagon. With some creativity, any of these vehicles could be built to a heavier standard, using appropriate axles, drivelines, Saginaw power steering and upgrade brakes. Some of these vehicles already have these features or equivalent components. Our build-up goal would be contemporary handling, braking and steering with a turbo-diesel powertrain. Vintage, exempted vehicles can be an interesting place to start.

Diesel Swap Overview

While exploring the 4BT and 4 ISB conversion, 4WD Mechanix Magazine gathered information from a variety of sources. Here are considerations related to this swap:

1) There are different horsepower ratings available for the 8-valve 4BT. Bread truck (step van) engines are typically 105-130 horsepower versions, depending upon fuel tune and calibration. Torque is 325 lb-ft for the 130 horsepower engine; the 105 horsepower version rates 265 lb-ft. These engines use high pressure pumps similar to the 6BT. The 4 ISB engines have 16 valves, and in stock form they produce up to 170 horsepower and 420 lb-ft torque. Each of these engines can be boosted and fueled to reach much higher horsepower output. According to Big Bear Engine Company, 4BT fuel efficiency can be improved substantially with engine modifications.

2) The 4 ISB engine requires an ECU/ECM controller, sensors and a wiring harness. This should be supplied from the donor vehicle. If not, Cummins and Big Bear Engine Company do sell the 4 ISB as a standalone engine package for industrial and other applications. This would include the necessary controller, sensors and harness. It would save considerable cost to recycle these components from the donor vehicle.

3) Installers are on their own around emissions legality. Some states might see a bread truck 4BT or 4 ISB turbo-diesel engine donor as an EPA certified engine application and acceptable. However, the concern could be which emissions tier or class did the donor bread truck fit? None of the engine or kit suppliers are able to assure emissions legality. This is the responsibility of owners and based upon your state motor vehicle emissions regulations. Do your homework before undertaking a diesel engine swap.

4) In terms of fuel efficiency, 30 mpg is attainable in a lighter Jeep® 4×4 with a modified 4BT engine. What conditions apply? Taller gearing and the right engine calibration. If fuel efficiency is your primary concern, the 105 or better yet the 130 horsepower (with a respectable 325 lb-ft torque) version is a place to start. Though not a world beater for 0-to-60 mph performance, the stock 4BT’s impressive torque and quick torque rise are reason enough for consideration. The stock 4BT 130 horsepower engine has more torque than the R2.8L. A 4 ISB beats the R2.8L in both horsepower and torque. Torque rates 310 lb-ft (R2.8L) versus 420 lb-ft torque (4 ISB). Whether gasoline or diesel power, there is no substitute for cubic inches: 171 (R2.8L) versus 239.3 (3.9L 4BT and 4 ISB).

5) If you buy a bread truck engine used, rebuilding a 4BT or finding parts will be easier at the new olddutchdiesel.com website. A spin-off from the Big Bear Engine Company website, enthusiasts can follow 4BT parts developments here. For precision machining and exchange parts, contact Big Bear Engine Company.

6) Big Bear Engine Company recommends a Cummins damper to smooth out the vibration on the 4BT. Farm Strong Inc. engine mounts drop the crankshaft centerline below the frame mounting points and angle the mounts properly. Visit the Farm Strong, Inc. website for details. Do not overlook this measure, or vibration will be an issue. Drivers of step vans with the 4BT engine describe their teeth rattling. The 4 ISB is smoother.

7) Expect a 250-300 pound engine weight difference over the front axle, depending upon the powertrain placement. Farm Strong, Inc. illustrates the correct powertrain and engine mount locations, which place the mounts in correct orientation to the crankshaft and frame. Reducing vibration is possible with these mounting kits and guidelines!

8) Boosting chassis spring rates is wise for the additional engine weight. The front axle load capacity must be rated for the 4BT/4 ISB engine weight. A Dana 44 or equivalent axle type (Toyota Land Cruiser, beam 1/2-ton 4×4 Ram truck, Scout II 44, beam axle GM 4×4, Land Rover, etc.) would be the minimum axle advisable. The AAM 9.25″, Dana 60 and the stronger Toyota FJ axles would work. What likely won’t work is a Spicer 25 or Spicer 27 closed knuckle, a Dana 30 or the smaller tube size of a TJ or JK Wrangler Rubicon Edition’s Dana 44 front axle. Larger axles with increased drum or disc brake size and a matching master cylinder would be valuable.

9) The rear axle also needs to handle the torque output of your 4BT or 4 ISB when in 2WD mode. A Dana 35 is not the candidate. A 44 or larger rear axle would be minimal, even larger if the vehicle has a heavy complement of accessories or the engine torque gets boosted. Upgrade the front and rear axles and the driveshafts (U-joint size, tube size, etc.) to match your vehicle’s intended use and the engine’s torque output.

10) Manual transmission options are broad, although the bare minimum should be a transmission designed to handle this kind of turbo-diesel torque. The T18/T19 (okay), SM420, SM465 or NP435 (each much better), NV4500 (stout with a fifth overdrive), NV5600 (six speed overdrive, some known synchronizer troubles and other issues) or Mercedes G56 (the six-speed Ram Cummins version) would each make good manual transmission choices for a vehicle up to 8,500-pounds gross vehicle weight (GVW). Modifying the engine for more power will require a heavier duty transmission choice like the NV4500 or G56.

11) Any of the described transmissions should come from 4WD donor applications if swapped into a 4×4 with an attached transfer case. A 2WD/RWD transmission would need a 4WD output shaft and an adapter for direct mate-up to a 4×4 transfer case. See Advance Adapters for details on transmission to transfer case adapter kits.

Note: There are older 4×4 trucks, vintage NAPCO conversions and Power Wagons that use a divorced transfer case and a 2WD style transmission output. Here, the transfer case mounts separately from the transmission with a short driveshaft between the two units. The arrangement suits long wheelbase pickup and stake beds or medium duty trucks, although we built a 1971 W100 R-Series Dodge Power Wagon SWB pickup years ago that used a divorced version of the rugged New Process 205 gear drive transfer case.

12) Engine to transmission adapters could also be a standard bread truck variety for the BW T18/19. Recycled Ram truck/Cummins factory adapters are another cost-effective option. The 4BT and 4 ISB use common SAE adapters as well; a medium duty truck transmission could be used for a much heavier duty application. This is unlikely, though, as a vehicle beyond 8,500 pounds GVWR would do better with the inline six-cylinder Cummins 6BT, 5.9L ISB or 6.7L ISB turbo-diesels.

Have a one-off adapter need? Phoenix Casting and Machine makes SAE adapters for medium duty truck engines like the Cummins 4BT/6BT or ISB. The company’s phone number at Nebraska is (402) 751-2135.

13) There are B-series Cummins automatic transmission options like the Chrysler/Ram approaches. Avoid transmissions that require a controller or modules and electrical work. These would require the electronic modules and wiring harnesses from a Ram donor vehicle. (Some signals come from a 6BT or 6 ISB engine’s ECU/ECM. Unless there is a step van/bread truck application of the transmission, electronic shifting could be difficult to adapt.) The 727 three-speed from an early Gen 1 Ram Cummins truck would be easiest to install but lacks an overdrive—if that’s an issue. This is a fully mechanical, standalone transmission with easier to adapt kick-down controls. In good shape, a 727 could handle the torque. (This was Chrysler’s choice for hemi-powered muscle cars and Class A motorhomes with 413 and 440 V-8s.) Early 6BT powered Ram/Cummins 4x4s use a 727 with an output adapter to the bulletproof NP205 gear drive transfer case.

14) There were also freight vans (UPS in particular) that used Cummins 4BT/6BT SAE adapter patterns with an Allison automatic. Some work could make an Allison swap possible. Again, the transmission output must mate with a 4WD transfer case, and this means matching the Allison output spline count and shaft stick-out length to a 4WD Allison transfer case adapter. (4WD Allisons with a transfer case adapter are late model electronic shift designs.) A hybrid Allison automatic would need to be built around an early, mechanically controlled Allison transmission from a 6BT-powered step van. An option would be a divorced transfer case if your 4×4’s wheelbase is long enough.

Worth exploring: Can the earlier 6BT freight van version of the Allison (step van donor) be modified with a later 4WD output shaft and 4WD adapter to the transfer case? If possible, this might enable use of the earlier, non-electronic Allison automatic. Consult an Allison parts specialist about the feasibility of modifying an early, mechanically controlled Allison transmission to mate up with a common transfer case.

15) The oil pan sump must be at the rear for 4×4 beam front axle applications. This should not be a problem with the Cummins 4BT or 4 ISB. Confirm your chassis, axle and oil sump needs. Verify the oil pan sump location before buying an engine. Know the peripherals parts you need from the donor vehicle. It is always best to get everything as a package, including the transmission (if desirable), any control modules and related wiring harnesses. Hunting down these parts later is costly and time consuming.

16) Among the virtues of a Cummins diesel engine is longevity. A new or properly rebuilt (contact Big Bear Engine Company) engine should last at least 500,000 miles in a lighter weight vehicle. Some expect a million miles from a B-Series or ISB engine. In any case, the 4BT or 4 ISB is capable of an exceptional lifespan under the right driving conditions and with proper maintenance, correct lubricant and quality air filtration. Long life, strong torque and fuel efficiency justify these 4BT and 4 ISB swaps. However, unless you plan to keep the vehicle for a very long time, you may not amortize the cost of this conversion. A new or rebuilt 4BT or 4 ISB engine with all peripherals plus installation parts and upgrades can be an expensive proposition.

Note: The pulse cycling of a 6BT or ISB inline six-cylinder engine is different than a 4BT or 4 ISB four. Inline sixes have better inherent/harmonic balance than four-cylinder engines. This helps extend inline six-cylinder engine life. Inline sixes generally last longer under heavier loads. The Cummins crankshaft damper is a sensible upgrade for the 4BT engine.

17) Additional expenses related to swaps are the engine mounts, cooling system, exhaust system, intercooler or aftercooler; piping, routing of hoses and power driven accessories like the power steering pump, power brakes (vacuum pump needed for a vacuum booster or a hydraulic booster powered from the power steering pump); the air intake system and a starter motor plus the engine, charge system and starter wiring. Where Cummins pieces are involved, there is easier access to new and used parts. Other parts will come from the aftermarket and often take considerable time to fit up or fabricate. This swap involves a lot of labor and is not for the faint of heart. If subletting the labor, cost will be substantial to do the job correctly. Fabrication work requires a quality welder, cutting equipment and shaping tools.

18) The Cummins R2.8L engine swap into the common Jeep models has spawned a “kit” source. The Axis Industries kit, without considering any labor costs, includes a new Cummins crate engine assembly and necessary components to complete the swap. The package for a TJ/LJ Jeep Wrangler, using the stock (original) manual transmission, is currently $16,995—parts only, add labor to this. The R2.8L crate engine comes with many peripheral pieces. A recycled or rebuilt 4BT or 4 ISB engine may or may not come with all of its accessories. To see the components that make up the R2.8L crate engine package, review the two video/article links below. This will provide insight into what the Cummins R2.8L turbo-diesel crate engine “package” looks like:



19) Optimal vehicles for a 4BT or 4 ISB swap have a stout frame and axles. Frames capable of handling these four-cylinder turbo-diesel engines would be the 4×4 Jeep CJ, YJ/TJ/JK Wrangler, FJ Land Cruiser, a Land Rover, the Scout and Scout II. Some install these engines in 1/2-ton pickups although buying a 3/4-ton 4×4 (new or used) with a factory diesel engine might make better sense. The Jeep XJ Cherokee is also a prospect; however, the unibody/frame at the engine mounting and mid-chassis areas should be reinforced. RuffStuff has some well-conceived frame reinforcement pieces that would be useful when swapping a 4BT or 4 ISB into a Jeep XJ Cherokee:



Summing Up the 4BT or 4 ISB Turbo-Diesel Swap

The Cummins 4BT or 4 ISB conversion requires motivation and a price-is-right access to the engine and peripheral components. Though mileage close to 30 mpg is attainable with some vehicle applications and the engine modifications described by Big Bear Engine Company, fuel efficiency is governed by aerodynamics and driving habits. Our 2005 Ram 3500 4WD Quad-Cab with 5.9L ISB 24-valve CRD diesel once achieved 25 mpg on an unloaded run from the Reno Area to Portland, Oregon. Curb weight was 7,800 pounds.

The engine stayed between 1600 and 1900 rpm for the entire trip, most often in the 1600-1750 rpm range. At the time, none of the current aftermarket accessories were on the truck. The 18K pound capacity front winch and winch bumper, aftermarket rear bumper, an ARE bed cap, beefy 37″ diameter tires and spare, a 4-inch chassis lift, White Knuckle rock sliders and a 75-gallon Transfer Flow auxiliary fuel tank came later. So did the horsepower and torque boost from Hypertech MaxEnergy tune software. Today, the best mileage at posted interstate speed limits without a trailer in tow is 19-21 mpg…Fuel mileage is relative.

This is our 2005 Ram/Cummins 24-valve 5.9L ISB powered truck at 180,000 miles. Bought new, the Ram provides all the power, comfort and towing ability we need. 900 pounds heavier than stock when fueled to capacity at 110 gallons, the fuel efficiency did drop with the accessory load, chassis lift, front winch bumper, drag coefficient change and loss of aerodynamics. 4.56 axle gearing helps offset the 37″ tires…Some trade-offs are worthwhile. For better fuel mileage, we slow down.

If you contemplate this swap, and if the conversion is legal in your state, plan to keep the vehicle long enough to amortize the cost of the Cummins 4BT or 4 ISB engine conversion. Consider the initial engine cost, the cost of engine modifications, the conversion parts and any sublet labor. Consider diesel fuel pricing as well. If you do the work yourself, that saves substantial cost, but your time is worth something.

You will be assured better fuel efficiency than a gasoline engine if you drive the vehicle like a diesel. This means using gears and keeping a lid on rpm. With the vehicle unloaded, upshift points should be 1200-1500 rpm. Cruise rpm should be 1600-2100 rpm for maximum mpg. Choose axle ratios according to your tire diameter and anticipated road speeds in overdrive.

Redline can be 3,000 rpm or higher although such speed is pointless with enough gears. A practical redline for a Cummins 4BT or 4 ISB engine would be 2500-2700 rpm, beyond this only under severe load or in a white knuckle passing situation. If you prefer any other driving method, forget about diesel fuel efficiency or stick with a gasoline engine.

Cummins 4BT or 4 ISB durability is uncontested, these are industrial strength engines. The rugged iron block and cylinder head have been engineered for the long haul. Making the 4BT, 4 ISB or the R2.8L decision should take into consideration the overall price of admission for converting to diesel power. If this conversion in your state and local venue is emissions legal and without vehicle titling or registration obstacles, and if your commitment and finances are ample enough, this is a viable option.

A recycled 4BT swap should center on the 130 horsepower version. If you choose the higher tech 16-valve Cummins 4 ISB four-cylinder engine, be certain the controller, all sensors and wiring harnesses are intact and included. Make sure you get the engine peripherals with the package, including any emissions related components. Otherwise, nickels will turn to dollars while you fabricate mounts and add costly accessories.

You will need a heavy-duty transmission behind the Cummins 4BT or 4 ISB. The Aisin AX15, the A150 Toyota version and the Wrangler NV3550 work well behind Jeep inline gas sixes, a Toyota V-6 or even the crate calibrated Cummins R2.8L turbo-diesel four. However, these and other light vehicle transmissions are out of their league with the high torque of an industrial strength 3.9L Cummins turbo-diesel, especially a 4BT or 4 ISB with modifications.

Once you confirm that the conversion will not create a vehicle registration or titling challenge, determine what the project entails. Get the engine and installation parts lined up before pulling out the gasoline engine. There are many 4BT and 4 ISB turbo-diesel conversions with happy owners. If you undertake a Cummins B-series four-cylinder swap, make sure you’re one of them!

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Vicente Gonzalez September 29, 2020 - 12:23 pm


I have a 3.9 ISBe engine on my Vw 8-150 Work Truck.

Does the ISB engine have a Killer Dowel Pin problem in the rear geartrain, as the 4BT and 6BTs ??

Are there any other mechanical problems on the ISB i should be on the lookout for ?

My truck is 160,000+ miles and has been well maintained.

Thank You very much in advance

Moses Ludel September 30, 2020 - 11:18 pm

Hi, Vicente…The Killer Dowel Pin issue affects 5.9L/6BT 12-valve and early 5.9L 24-valve ISB engines. The notoriety comes from Dodge Ram Cummins model years 1989-93 (12-valve BT) and 1998.5-2002 (24-valve ISB engines). There are other truck and off-highway applications of these engines built during these periods that would also be vulnerable to the dowel problem. For those not familiar, the issue is a common timing cover dowel pin that comes loose from the block and falls inside the timing cover. This can raise havoc with a variety of expensive parts, sometimes to the extreme. Fortunately, I’m not plagued by this prospect, as my 2005 Ram 5.9L H.O. 24-valve ISB engine is not in the group that requires a KDP Kit. For more information on the repair kit for the “KDP”, here is a link to Geno’s Garage:

  • https://www.genosgarage.com/product/tst-kdp04/engine-upgrade
  • The kit is simply a guard that attaches to an adjacent bolt and covers the dowel head, preventing the dowel pin from loosening and falling into the timing gear mechanism.

    Regarding your 3.9L ISB engine, by design the 4BT and 4 ISB 3.9L four-cylinder engines do have timing gear and cover arrangements similar to the inline sixes. 3.9L ISBs built around the same time as the early 24-valve ISB sixes could have a similar cover and dowel arrangement. If so, the issue can be remedied, and kit parts are relatively inexpensive; however, there is considerable labor to access the dowel. Before considering the KDP Kit, I suggest that you contact a Cummins engine dealership parts department or the Big Bear Engine Company mentioned in this article with contact information. They can identify whether your 3.9L ISB engine, by serial or I.D. number, is in need of a KDP Kit or not. I trust this helps. Let us know what you discover.


    Jennifer Gary May 28, 2023 - 5:36 am

    The black cj7 pictured in this article is my Jeep I used a ford zf5 speed transmission in it ….

    Moses Ludel June 23, 2023 - 10:24 am

    Very nice Jeep CJ7 and thanks for sharing the transmission choice. The ZF 5-speed was a Ford/PowerStroke diesel mainstay, a smart pick, Jennifer!


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