Diesels are setting all kinds of records from land speed to elapsed time. So if you’re not already dabbling in diesel performance, you may be soon.
By Larry Carley
Diesel performance is nothing new. What is new is the level of interest in diesels for all kinds of racing; everything from traditional truck and tractor pulling to drag racing, even sports car and endurance racing.
Diesel engines are a different type of animal when compared to gasoline engines. While both use the same four-stroke cycle of combustion, diesels use the heat of compression rather than a spark to ignite the air/fuel mixture. A diesel engine requires a much higher static compression ratio than a gasoline engine, and on most engines there is no intake vacuum because the engine is unthrottled. Speed and power outputs are determined by the fuel injection system.
Most diesel engines are also turbocharged, yet make most of their torque and power at relatively low rpm (typically 2,500 to 4,000 rpm, though in some forms of racing diesels are being revved to 5,000 to 7,000 rpm).
So what does this mean to our readers? It means if a customer asks you to build a performance diesel engine, it’s going to take a different approach than building a performance gasoline engine. The bottom end in most diesel engines is fairly stout already, with forged steel crankshafts and beefy connecting rods, so the amount of modifications that may be needed here are usually minimal. And because most street performance and pulling applications don’t require a lot of rpms, engine balance is not as critical as in a high revving gasoline engine.
Pistons may have to be cut, modified or replaced, however, depending on how much turbo boost the engine will be running, what kind of fuel it will be burning (diesel only, or diesel plus nitrous oxide or alcohol), and what kind of modifications are being made to the camshaft and valvetrain, which is the focus of this article.
Because diesels require a lot of compression, camshaft duration tends to be short with minimal overlap. Valve lift may also be limited by the tight piston-to-valve clearances in most diesel engines. So unlike gasoline engines, you can’t go hog wild with lift and duration to make more power. A bigger camshaft can provide more power, but only if the lift and duration are right for the turbochager, cylinder heads, pistons, valvetrain and application.
The most significant power gains in a diesel engine come from increasing the amount of boost delivered by the turbocharger(s). But there’s more to the secret than just bolting on a bigger turbo. The camshaft must have the right exhaust characteristics so it will spool up the turbo faster and keep it spinning at peak efficiency in the engine’s power band. For some types of pulling, the power band can be quite narrow, say only 2,000 to 3,000 rpm for a big agricultural diesel engine, or maybe 3,000 to 5,000...