The legendary 5.9L Cummins little brother, the 4BT Cummins engine has long been maligned in the performance industry. After all, the engine is only 3.9L, how stout can it be? Well, we’re here to tell you the answer is: very stout. Richard Brown, of Brown’s Diesel in Riverdale, California often marches to the beat of a different drummer, so when it came time to replace the alcohol-burning 408ci Chevy in his 3-window coupe, he thought why not a diesel?

A 3.9L four-cylinder Cummins known as a 4BT powers Richard Brown’s 3-window ‘32 coupe. It’s not just any 4BT however, as the mill has churned out 549 rear-wheel horsepower on 75psi of boost, and 740rwhp when nitrous and water-methanol are injected.

The problem was a 6BT (5.9L) Cummins was just all wrong for the application. It was too heavy and too big dimensionally, and ‘32 Fords aren’t exactly known for extra space. So, a 4BT Cummins got the call. Like many projects, Richard’s Ford took on a life of its own. Originally starting with a 263-hp turned up stocker, the 4BT moved the 2,900-pound coupe along pretty well. After a while though Richard kept modifying and turning up the coupe more and more, and eventually windowed the stock block. The next engine would be a full build.

Built Up 4BT

The team at Brown’s started out with a stock block, that was equipped with a Gorilla girdle, ARP 625 head studs, and a fire-ringed block and head. The crank was factory, but the rods were from Carillo, and the engine received forged pistons from D&J Precision Machine. To help get the airflow moving, a camshaft from Hamilton Cams was installed, along with pushrods and valvesprings that bump the valves in the factory cylinder head. The long block is rounded out with trick tappet and front covers from Keating Machine.

A big part of the 4BT engine’s power comes because of plenty of fuel. A Scheid Diesel 13mm pump capable of a whopping 800cc is fed by a 200gph Airdog 4G high-pressure lift pump, and then sends the fuel to mammoth 5 by 0.025-inch injectors also from Scheid.


Richard knew the small-displacement engine would need help making power, so he went with every trick in the book–and then some. He started with a 12mm injection pump and some 5 by 0.018-inch injectors, but has since moved up to an 800cc 13mm pump from Scheid Diesel on an adjustable gear and massive 5 by 0.025-inch injectors. Since this is a heck of a lot of fuel, Richard built an equally serious turbo system for the car.

The Cummins runs the type of compound setup that is used on traditional high-horsepower diesels, but the turbos had to be downsized. The smaller of the two turbos is a 57mm S200, while the larger is a 69mm S300, both from BorgWarner. Brown’s made the trick custom piping in-house.

Air and N2O

The 4BT is a smaller engine, so the turbos had to be sized accordingly. Keeping this in mind, Richard went with a 57mm S200 and 69mm SX-E S300, both from BorgWarner. Together, the turbos combine to make a whopping 75 psi of boost. In addition to the boost, Richard also runs both water-methanol injection and nitrous on the 4BT, and quite a lot of both. The nitrous comes in four stages–a 0.035-inch spool jet, followed by three 0.080-inch jets that are ramped in via Hobbs switches between stages as boost increases. The water-meth is also ramped in, with two nozzles feeding the intake horn.

Part of the reason a 4BT was chosen was because of how easily it swapped into the frame. The factory-style engine mounts could be used, it cleared all the steering and linkages, and even the throttle linkage and radiator from the small-block could be re-used.

Power Transmission

It may be “just” a 4BT Cummins, but with a full-manual race valvebody with a transbrake from Goerend Transmissions, the automatic behind the 3.9L needed to be plenty stout. Luckily, Richard is a transmission builder, so he knew exactly the direction he wanted to go. In addition to the valvebody, the 47RE-based transmission was fed stronger 48RE internals, as well as aftermarket shafts from Sonnax and TCS. The converter was a slight mystery, since the smaller Cummins engine made a lot less torque. In the end, they went with an experimental triple-disc converter from Goerend that allows for ultra hard transbrake launches.

An intercooler was added to the compound setup in this cool front-mount scooped location. It keeps intake temperatures and boost pressures from going stratospheric.
We were surprised to learn that Steed Speed made a steel 4BT manifold, but they do. The manifold helps with both flow and durability on the little Cummins and is also equipped with a 45mm wastegate.
Since the tires hang out in the breeze, there’s no room to skip on rubber. The rear street slicks that give the hook are 33×16-inch Hoosier Quicktime Pros. Front skinnies are mounted on Weld Racing wheels. They hold up to the weight of the diesel just fine.
Richard’s ‘32 isn’t a polished show car, and it’s not a rat rod either, but it does have some rat-style touches, like this dipstick holder wrench.
Cummins 4BT engines weigh around 750 to 800 pounds, which actually isn’t that much heavier than a small block with a blower. This meant the entire front suspension could remain unchanged.
Front skinnies are mounted on Weld Racing wheels. They hold up to the weight of the diesel just fine.
The rear-end is a very strong fabricated 9-inch with 3.40 gears, 40 spline axles, and a strange spool. It’s hung with Varishocks from Alston’s Chassisworks, and features a ladder-bar rear suspension.
The Dodge-based four-speed 47RE was used because of its simplistic nature, and because it has an Overdrive gear and a lock-up converter. This keeps the turbos spooled and diesel in its powerband at all times. Overdrive and lock-up are on simple switches that Richard hits while going down the strip or street.
The intake horn on the ‘32 is quite a busy spot. With four nitrous lines and two water-methanol lines, we’d love to see what’s happening inside that intake manifold!

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