Reload Image

Reload Image


THE POPULARITY OF diesel-powered vehicles here in the U.S. has jumped dramatically in recent years. More and more manufacturers are offering new diesel power plants in ½-ton trucks, cars and midsize SUVs. Maybe the reasons for the boost are better highway fuel efficiency and the giant increase in torque offered by a diesel engine, but it’s probably an attempt to meet everincreasing CAFE (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 its own small diesel engine market with the 3.9L 4BT.

4BT History

The 4BT—which stands for “fourcylinder 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 1989-98 Dodge trucks. The 4BT shares virtually all of its parts with its big brother, the 6BT (e.g. pistons, connecting rods, injectors and valve-train design).

The 3.9L 4BT Cummins diesel engine has become an extremely popular platform for conversion projects throughout the past couple of 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.

The major internal difference comes from the lack of two cylinders, which means two fewer journals on the crankshaft, a shorter camshaft, two fewer 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 smaller 3.9L engines. 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 because there’s little to no wiring or electronics required.

The 4BT engine has been used in commercial utility trucks and industrial construction equipment since the late-’80s, and because of its extremely basic functionality, it works well as a transplant engine when space is tight. 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.

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 at 2,300 rpm and 265 lb-ft of torque at 1,600 rpm. But these numbers could vary as much as 150 hp depending on the application.

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 While this design became somewhat outdated with the introduction of the fourvalve- per-cylinder design found in the 1998.5- plus 6BT engines, it’s still a sufficient and robust design, especially on the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

The 4BT engine weighs 750- 780 pounds, which is heavier than most of the gas engines it would replace in a conversion project, but much lighter than the sixcylinder 6BT version. The inlinefour also offers a fairly compact size at just 30.6 inches long and 37.7 inches tall, which is useful 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 increase power outputs.

ACD Engines of Salt Lake City is a strong Cummins engine and parts dealer with years of experience in both the 6BT and 4BT platforms. It’s gained a reputation for its 4BT conversion projects and has developed new engine internals to take the average industrial 130-hp four-cylinder diesel to impressive performance heights. Starting with a one-of-a-kind connecting rod and piston design, ACD will soon offer complete Stroker Kits to pump up the power of 4BT platforms. The new H-beam 4340 billet connecting rod (left) was engineered specifically for the 4BT engine. These rods not only help increase engine stroke, they also provide unmatched durability to withstand extreme cylinder pressures from running massive power and torque through these engines. The new H-beam rod (top right) is pictured with a factory 4BT rod (bottom right). An untrained eye may not notice much, but the design and material of the ACD connecting rods is far superior to the standard.

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, high-rev governor springs, full-travel rack plugs and modified injectors can all be used in the 4BT platform. To go along with major fuel upgrades, common cylinder head and turbocharger upgrades are 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 built for four-cylinder diesels. While these upgrades can take the 4BT to all-new levels, we have to mention 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 in a 6BT Cummins, so things like camshaft profiles and pump timing become critical for maximum efficiency at higher horsepower levels.

A close-up of the two connecting rods’ piston ends show where some of the additional piston stroke comes from. Notice the much smaller diameter used to encase the piston’s wrist pin. This new connecting rod design runs exclusively with ACD’s custom-forged pistons using a tool-steel wrist pin. The combination of rod and piston raises 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 the average 3.9L 4BT into a 4.5L engine.

Knowing that customers interested in a 4BT Stroker Kit would want 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 create a near indestructible 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.

ACD Stroker Kit

Like any engine platform, it’s a well-known fact that more cubic inches generally means more power, and there’s only so much a 239-cid motor can produce, at least safely and efficiently. With this in mind, ACD Engines of Salt Lake City has developed its Stroker Kit to take the 4BT to heretofore unheard of 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. It offers virtually everything Cummins: new or remanufactured engines, new genuine Cummins parts, used engines and even salvage parts. Since it deals in nothing but Cummins, ACD stocks many hard-to-find parts, and if it’s not in stock, the staff knows where to get what you need. Through their years of experience, the staff has become extremely wellversed in the 4BT platform and has the conversion process down to a science, inserting the small Cummins engine into Jeeps, small SUVs, pickups and even a mid- ’50s ambulance.

Sitting side by side on the bench, the stock piston on the left looks quite a bit different from the ACD version. The shorter skirt and much higher wrist pin location were used to assist piston stroke, allowing more displacement and performance potential. More cubic inches means more air and fuel can be drawn into the cylinders to create more power.

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; the GM small-block 383 Stroker engines have been around for nearly 35 years. The 383 is built using a standard GM 350- cid engine block with a modified 400-cid crankshaft, which changes how far the piston travels.

For the new forged piston design, ACD opted for complete gapless piston ring sets to limit cylinder pressure blow-by and keep 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 .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.

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

New Rods and Pistons

Because the factory 4BT crankshaft is a robust piece, Pederson didn’t want to do much crankshaft work, so he opted for an all-new piston and connecting rod design to gain the 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-ci motor. The new parts from ACD will allow the overall piston bore to expand to 4.402 inches with a much longer 5.430-inch stroke, effectively taking engine output to 333 ci, or 5.46L, almost that of the 5.9L 12V Cummins.

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 bottomend Gorilla girdle to tie everything together in the lower part of the engine. ARP main studs are used to ensure proper clamping force is torqued onto the girdle and crankshaft’s main caps.

Knowing that the clientele for such a kit would be after extreme power levels, only the best materials were used to ensure 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, and the wrist pin location was moved closer to the deck. These modifications account for most of the additional stroke.

On the cylinder head, upgrades to the ACD 4BT are much the same as those found in performance 12V Cummins builds: high-rev valve springs, titanium keepers, chrome-moly pushrods, etc. For this particular application, ACD will also run fully ported and polished intake and exhaust ports along with larger valves to increase the air volume fed to and expelled from the engine. To help those larger valves perform, ACD spent countless hours developing different camshaft profiles that maximize lift and duration in 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 for power or fuel efficiency.

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

To match the new piston, connecting rods were developed and made from billet 4340, like those being used in all of 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 of the Stroker Kit, ACD has also developed a host of 4BT parts to complement them and ensure true peak performance is achieved. Specific camshaft profiles were designed, custom cylinder head work was done and 4BT-specific adjustable injection pump timing gears were installed. The short time between injection events on the four-cylinder engine required a slight engineering change for these parts, and ACD thinks its developed the perfect pieces to turn your run-of-the-mill 105-hp 4BT into a tire-shredding 800-hp monster.

Because this engine will run higher boost pressures, the cylinder head will be machined to use a custom fi re-ringed head gasket kit and ARP head studs. The factory rocker pedestals must be machined for clearance to accommodate the stronger ARP hardware.

The Bosch P7100 injection pump has come a long way since its start in 1994-98 Ram trucks. Originally developed to support the mid 200-hp range, the 4BT platform with just four cylinders moved even less fuel because it 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 to which the four-cylinder engine is subject. This particular pump is capable of moving up to 800ccs of fuel, more than enough to move the big H1 Hummer down the street—doing long third-gear burnouts the whole way.

The fuel injectors for the 4BT engine were also reworked to maximize the amount of fuel that could be effi ciently injected and burned in the cylinders. The dual-feed injectors and customhoned nozzles should be more than enough to support ACD’s 800-hp goal. The custom highfl ow injection lines will also be used to eliminate any restrictions between the injection pump delivery valves and the injectors.

ACD Engines understands that the complete Stroker Kit might be more than most 4BT project vehicles need, so it has an array of 4BT-specific products that can do everything from increasing power to improving fuel mileage and engine efficiency. It also carries a full line of custom brackets and conversion pieces to make your conversion or transplant project go a little easier.

ACD will use the OEM Cummins valve covers on this high-horsepower build, which offers dual port ventilation. While the gapless piston rings should cut down on most of the engine’s blow-by, the high-flow breathers in these valve covers will be much more efficient than the individual factory cylinder covers found on most 4BT and 6BT engines.

Since the cylinder head intake and exhaust sides were ported and polished to maximize airflow through the head, it’s only natural to upgrade the exhaust manifold. While there are different OEM-style manifolds for the 4BT that suit different installation needs, they don’t necessarily fl ow what a high-horsepower build like this one would need. ACD turned to Steed Speed for help. It offers these customfabricated manifolds 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 fit a 4BT into the chassis of just about anything.


• ACD Engines