Cummins 4BT 101

4BT Basics And Performance Options

The popularity of diesel-powered vehicles here in the U.S. has really jumped in a positive direction in recent years. More and more manufacturers are starting to offer all-new diesel powerplants in half-ton trucks, cars and midsize SUVs—the introduction of a 3.0L diesel in the new Ram 1500 trucks, an EcoDiesel in the latest Chevrolet Cruze passenger car and there’s even been talk of a 4.5L Duramax coming down the pipeline. With the big car builders starting to offer diesel upgrades from the standard gas powerplants, there must be legitimate reasoning.


Maybe it’s better highway fuel efficiency and the giant increase in torque offered by a diesel engine. In reality, it’s probably an attempt at meeting ever-increasing CAFÉ (corporate average fuel economy) numbers. Either way, these smaller V-6 and four-cylinder engines will be sticking around. But back in the day, it was Cummins that developed their own small diesel engine market with the 3.9L 4BT.

2 The 4BT engine has been used in commercial utility trucks and industrial construction equipment since the late 1980s and because of its extremely basic functionality works well as a transplant engine when space constraints restrict the use of the larger six-cylinder 5.9L Cummins. With the cylinder head off the 4BT engine, someone with 12V Cummins experience should feel right at home; the 4BT is virtually the same, just two pistons shy, of course.

4BT History

The 4BT—which stands for four-cylinder, B-series, Turbocharged—was used mostly in midsize box trucks, agricultural equipment and small industrial vehicles, and is basically a smaller version of the popular 5.9L 12V Cummins found in the 1989-98 Dodge trucks. The 4BT shares virtually all its parts with its big brother the 6BT; i.e., pistons, connecting rods, injectors and valve train design.

Obviously, the major internal difference comes from the lack of two cylinders, which means two less journals on the crankshaft, a shorter camshaft, two less pistons in the injection pump, etc. But even these pieces are the same as the 6BT, just in an inline four-cylinder version. Because of these similarities, the 4BT Cummins has an extremely strong aftermarket, since many 5.9L 12V performance parts can be used in the smaller 3.9L engine. This, paired with the somewhat compact size of the 4BT, makes it a great candidate as a transplant engine for just about any vehicle. We’ve seen 4BTs in everything from Jeeps to old school rat rods. The simplicity of the mechanical injection system also helps make it an easy swap, as there’s little to no wiring or electronics required.

Since the engine was originally developed for industrial applications, big horsepower wasn’t a major concern and the 239-cid motor most commonly came from the factory at around 105 hp @ 2,300 rpm and 265 lb/ft of torque at 1,600 rpm. But these numbers can vary up to as much as 150 hp depending on the application it was installed in.

The 4BT engine weighs in at around 750-780 lbs, which is heavier than most of the gas engines it would be replacing in a conversion project, but much lighter than the six-cylinder 6BTversion. The inline-four also offers a fairly compact size at just 30.6 inches long and 37.7 inches tall, helping with space constraints. The engines came with a few different injection pump systems, but the P7100 is, by far, the most popular, since it’s capable of producing larger amounts of fuel and more easily modified by the aftermarket to help increase power outputs.

Performance Mods

As previously mentioned, the 4BT can easily be modified to produce higher than stock power levels when outfitted with the P7100 (P-pump). Fueling mods can be taken to the same extremes as with the 5.9L 12V Cummins; 12mm and 13mm pumps with laser-cut delivery valves, hi-rev governor springs, full-travel rack plugs along with modified injectors can all be used in the 4BT platform. Obviously, to go along with major fuel upgrades, common cylinder head and turbocharger upgrades would be required. But the aftermarket already has most of that covered as well with head studs, better valve springs, larger valves, and even performance exhaust manifolds and camshafts that are being built for the four-cylinder diesel. While these upgrades can take the 4BT to all-new levels, one thing that needs to be mentioned is that because of its lack of cylinders and nearly identical operating rpm range, each piston is going to undergo a combustion cycle much sooner than that of a 6BT Cummins, so things like camshaft profiles and pump timing become critical for maximum efficiency at higher horsepower levels.

1 The 3.9L 4BT Cummins diesel engine has become an extremely popular platform for conversion projects over the past couple years and for good reason. Sharing virtually every internal and external piece with its big brother, the 5.9L 12V Cummins found in the 1989-98 Dodge Ram pickups, and coming as regular equipment in hundreds of commercial and industrial applications, parts availability, power potential, fuel mileage and the lack of electronics make it an easy go-to engine when it’s time to squeeze a diesel platform into the frame rails of just about anything.

ACD Stroker Kit

Like any engine platform, it’s a known fact that more cubic inches generally means more power and there’s only so much a 239-cid motor will be capable of, at least safely and efficiently. With this in mind, ACD Engines of Salt Lake City, Utah, has developed what they refer to as a “Stroker Kit” that will take the 4BT to all-new performance heights.

Also known locally as “All Cummins Engines,” ACD has been a full-line Cummins dealer for more than 20 years, specializing in midrange and industrial engines. They offer virtually everything Cummins—new or remanufactured engines, new genuine Cummins parts, used engines and even salvage parts. Since they deal in nothing but Cummins, anyone looking for that hard-to-find part can most likely call ACD and find they’ll have it in stock or at least know where to get one. Through their years of experience, they’ve become extremely well versed on the 4BT platform and have the conversion process down to a fine science, inserting the small Cummins engine into Jeeps, small SUVs, pickups and even a mid-’50s ambulance.

In the search for more power, owner Robby Pederson began development of a Stroker Kit that would increase the length of stroke and add some additional cubic inches to the inline-four diesel. In the gas world, Stroker engines are nothing new, as the GM small-block 383 Stroker engines have been around for nearly 35 years. The 383 is built by using a standard GM 350-cid engine block with a modified 400-cid crankshaft, which changes how far the piston travels.

New Rods and Pistons

Not looking to do so much crankshaft work, as the factory 4BT crankshaft is a robust piece, Pederson opted for an all-new piston and connecting rod design to gain that additional stroke he was after. A factory 4BT engine runs a 4.02-inch bore with a 4.72-inch stroke; this is how we come to a 239-cubic-inch motor. The new parts from ACD will allow the overall piston bore to expand to a 4.402-inch and a much longer 5.430-inch stroke is used, which will effectively take engine output to 333 cubic inches, or 5.46-liters, almost that of the 5.9L 12V Cummins.

2 The 4BT engine has been used in commercial utility trucks and industrial construction equipment since the late 1980s and because of its extremely basic functionality works well as a transplant engine when space constraints restrict the use of the larger six-cylinder 5.9L Cummins. With the cylinder head off the 4BT engine, someone with 12V Cummins experience should feel right at home; the 4BT is virtually the same, just two pistons shy, of course.

Knowing that the clientele for such a kit would be after extreme power levels, only the best materials were used to be sure the ACD Stroker Kit would stand up to high-boost and high-cylinder pressures. Rather than use standard-cast pistons, like the stock units, ACD went with a much stronger forged piston design that offers a different bowl design to improve the air/fuel swirl effect, helping create a more efficient burn in the combustion chamber. The piston design also has a much shorter overall height, along with the wrist pin location being moved closer to the deck; this is where most of the additional stroke comes from.

To match the new piston, connecting rods had to be developed, which were made from billet 4340, like those being used in all the high-horsepower diesel engines. The new H-beam rod design is not only stronger than a factory connecting rod, but it also has a much smaller wrist pin journal, which will only work with the ACD piston. While the pistons and rods are the true heart to the Stroker Kit, ACD has also developed a host of 4BT parts to go along with them to ensure the true peak performance can be met. Specific camshaft profiles have been designed, along with custom cylinder head work, and even 4BT-specific adjustable injection pump timing gears. The short time between injection events on the four-cylinder engine require a little different engineering of these parts than that of a six-cylinder Cummins, and ACD thinks they’ve developed the perfect pieces to help turn your run-of-the-mill 105-hp 4BT into a tire-shredding 800-hp monster.

3 The 4BT engine uses a simple two-valve per cylinder system, obviously one for intake and one for exhaust. While this design is somewhat “outdated” with the introduction of the four-valve-per-cylinder design found in the 1998.5+ 6BT engines, it’s still a sufficient and robust design, especially on the smaller cubic inch low 130-180-hp applications that most of the 4BT engines were designed for.

4&5 ACD Engines of Salt Lake City, Utah, is a strong Cummins engine and parts dealer with years of experience in both the 6BT and 4BT platforms. Through their business practices, they’ve become well known for their 4BT conversion projects and have just developed all-new engine internals to take the average run-of-the-mill industrial 130-hp four-cylinder diesel to all-new performance heights. Starting with a one-of-a-kind connecting rod and piston design, ACD will soon be offering complete Stroker Kits for anyone looking to pump up the power in their 4BT platform. Shown in the first picture is their new H-beam 4340 billet connecting rod engineered specifically for the 4BT engine. These rods not only help increase the stroke of the engine, but will provide unmatched durability to hold up to the extreme cylinder pressures from running massive power and torque through these engines. The second picture shows the new H-beam rod (top) next to a factory 4BT rod (bottom). An untrained eye may not notice much, but the design and material of the ACD connecting rods is far superior.

6 A close-up of the two connecting rods piston ends show where some of the additional piston stroke comes from. Notice how much smaller of a diameter is used to encase the piston’s wrist pin? This new connecting rod design was developed to be run exclusively with ACDs custom-forged pistons that use a tool-steel wrist pin. The combination of rod and piston raise the 4BT’s piston stroke from a factory 4.72 inches to a massive 5.430 inches. This alone will increase the engine’s cubic inches from an OEM 239 to just over 275, turning your average 3.9L 4BT into a 4.5L engine.

7&8 As mentioned before, ACD had to develop an all-new piston design for their Stroker Kits to help increase the engine’s overall stroke and work with their newly designed billet connecting rods. Knowing that customers interested in a 4BT Stroker Kit would be looking to make as much horsepower as possible, ACD decided robust forged pistons would be the best option to ensure maximum strength and durability. The combination of the forged piston and tool steel wrist pin, with special coating to reduce wear and friction, should make for a near indestructible end product. The piston bowls have also been worked over compared to a stock piston to promote a better swirl effect for a cleaner more efficient burn in the combustion chamber.

9 Sitting on the bench beside each other, the stock piston on the left looks quite a bit different than that developed by ACD Engines. The shorter skirt and much higher wrist pin location were used to once again aid in the overall piston stroke, which will allow for more displacement and performance potential. More cubic inches means more air and fuel can be drawn into the cylinders to hopefully create more power.

10 For the new forged piston design, ACD opted for complete gapless piston ring sets to limit cylinder pressure blow-by and help keep all the combustion power inside the cylinder where it belongs. This ring design is also durable, but block machining is critical, especially with the use of forged pistons. Where a stock cast piston would only need, say, .005-inch clearance between it and the cylinder wall, a forged piston will swell more under heat and stress, so piston to wall clearance needs to be increased to keep the rings from sticking and potentially ruining the piston and engine block.

11 ACD Engines has plans of installing this first monster Stroker 4BT engine into their shop’s H1 Hummer that rests on custom-built axles and 40-inch tires. The shop’s looking to prove the power potential of the little 4BT and its extreme versatility as both the perfect diesel conversion engine for an older vehicle with just mild power upgrades (like ACD’s camshaft and injection pump) to increase a stock 4BT from its underwhelming 130 hp to 200 or even 250 hp to a full-on competition 700+ hp engine like this Stroker should become. Like any engine build, it all starts with a clean, bare engine block that needs to be machined to precise cylinder and crank measurements.

12 While the 5.9L 6BT Cummins can run into block flex and main cap issues at high power levels, the shorter 4BT won’t be as susceptible to those issues. However, ACD still prefers to use a bottom-end Gorilla girdle to help tie everything together in the lower part of the engine. ARP main studs are also being used to be sure the proper clamping force is torqued onto the girdle and crankshafts main caps.

13 On the cylinder head side of things, upgrades to the ACD Engines’ 4BT are much the same as those found in performance 12V Cummins builds: hi-rev valve springs, titanium keepers, chromoly pushrods, etc… For this particular application, ACD will also be running fully ported and polished intake and exhaust ports along with larger valves to increase air volume being fed and expelled from the engine. To help those larger valves perform, ACD has spent countless hours developing different camshaft profiles that maximize lift and duration in the 4BT engines. Since the injection events are so close together with the inline-four design, the right camshaft profile is critical to how these engines perform. ACD can recommend the correct cam for any build; whether it be for power or fuel efficiency, their cams can help just about any 4BT application.

14 In high-revving and high-horsepower engine applications, the factory press-in freeze plug design is a common failure point. To prevent that, ACD Engines has the cylinder head water ports tapped to accept threaded plugs.

15 Because this engine will be running higher boost pressures, the cylinder head will be machined to use a custom fire-ringed head gasket kit and ARP head studs. Due to the swap from the factory head bolts to the stronger ARPs, like any 6BT Cummins engine, the factory rocker pedestals must be machined for clearance.

16 The Bosch P7100 injection pump has come a long way from what it was originally designed and built to do on the 1994-98 Ram trucks. Originally developed to support the mid 200-hp range, the 4BT platform with just four cylinders moves even less fuel, as it was only needed to support 130-150 hp in most applications. Using knowledge from the massive 12V performance market, those same techniques and modifications were used in the ACD injection pump. This 13mm pump uses laser-cut delivery valves, max-rack travel plug, stronger hold-downs, and a custom cam design specific to the 4BT pump to maximize fuel injection under the quicker injection events the four-cylinder engine is subject to. This particular pump is capable of moving up to 800 ccs of fuel, which should be more than enough to help move the big H1 Hummer down the street (doing long third-gear burnouts the whole way).

17&18 Again, turning to their knowledge of the 12V Cummins aftermarket, the fuel injectors for the 4BT engine were also reworked to maximize the amount of fuel that could be efficiently injected and burned in the cylinders. The dual-feed injectors and custom-honed nozzles should be more than enough to support ACD’s 800-hp goal. The custom high-flow injection lines will also be used to eliminate any restrictions between the injection pump delivery valves and the injectors.

19 Turning to their background working on the industrial side of the Cummins engine platforms, ACD will be using these OEM Cummins valve covers on this high-horsepower build, which offer dual port ventilation. While the gapless piston rings should cut down on most of the engines blow-by, the high-flow breathers in these valve covers will be much more efficient than running the factory individual cylinder covers found on most 4BT and 6BT engines.

20 Since the cylinder head intake and exhaust sides were ported and polished to maximize airflow through the head, it’s only natural that an exhaust manifold upgrade be used as well. While there are different OEM-style manifolds for the 4BT that can be used to fit different installation needs, they don’t necessarily flow what a high-horsepower build like this one would need. So ACD Engines turned to Steed Speed, which offers these custom-fabricated manifolds that can be bought with both T3 and T4 turbo flanges along with center- and top-mount exit locations. These different turbo mount locations should be more than enough to help fit a 4BT into the chassis of just about anything.

ACD will soon be installing this completely built 4BT engine into their shop vehicle, a H1 Hummer Army body sitting on the chassis from a Dodge Ram 2500. The Stroker 4BT will help push fully built axles, custom suspension and drivetrain sitting on 40-inch tires down the road. We’ll be sure to do a complete write-up on the project when it’s up and running in another issue of Diesel World.

ACD Engines understands that the complete Stroker Kit may be more than most 4BT project vehicles will need, so they have an array of 4BT-specific products that can do everything from help increase power to just improving fuel mileage and engine efficiency. They also carry a full line of custom brackets and conversion pieces that may help your conversion or transplant project go a little easier. DW

4BT Land Cruiser Conversion

The team at ACD Engines has made 4BT conversions their specialty and had this 1992 Toyota Land Cruiser up on the rack while we were shooting photos of their new 4BT Stroker Kit. The owner of the Land Cruiser wanted more pep than the factory gas engine could offer, but didn’t want to sacrifice fuel mileage, making it the perfect candidate for a mildly upgraded 3.9L Cummins.

This 1992 Toyota Land Cruiser was on the rack while we were at ACD shooting photos of their 4BT Stoker kits; it’s already been converted over to a 4BT engine platform that uses one of ACD’s custom camshafts and reworked rotary VE injection pumps, which help pump power output from 130 hp to nearly 200 hp. The power and torque increase should be enough to move the Land Cruiser up and down the freeway with ease, while offering a good, usable power band while off-road and excellent fuel mileage while on-road.

ACD Engines are experts at doing these 4BT conversions and have shoehorned the small Cummins engine into just about everything from Jeeps, mini-trucks and even a mid-’50s ambulance. Under the hood of this Land Cruiser you’ll notice that the four-cylinder diesel fits near perfect and will retain full use of the factory heat and A/C with some custom bracketry built by ACD. The addition of an intercooler in front of the radiator will also help with the turbo and engine’s efficiency.

While you may think all the magic of this conversion is found under the hood of the Land Cruiser, ACD was right in the middle of installing the new transmission and transfer case. The mildly upgraded 4BT will be backed by the NV4500 five-speed manual transmission found in the 1994-2000 Dodge Ram trucks. The transfer case is also a one-off that with use of an adaptor ring allowed for a passenger-side tail shaft exit to allow proper driveline angles be kept with the Land Cruiser’s original front and rear axles.

Knowing that major horsepower and torque would be needed and looking to keep things budget friendly, they opted for the less common and slightly cheaper VE injection pump series motor, versus the more popular and easily modified P7100 injection pump model. The VE pump working with custom ACD injectors and camshaft should bring power output up from the original 105 hp to a more user-friendly 200 hp. While this may not sound like much, the low-end torque of the 4BT backed by the NV4500 five-speed manual transmission (from the 1994-2001 Dodge Ram truck) will make for a great daily driver and off-road combination. On this particular build, ACD will also be able to retain the factory A/C and heating system, and were even able to add an intercooler system behind the grille. ACD and the vehicle’s owner are hoping this package nets a consistent 25-30 mpg.

ACD Engines