TECHNICAL Q&A

ANSWERS TO YOUR BURNING DIESEL QUESTIONS

I was recently talking with my dad (who is a retired GM engineer), and he asked what limits the maximum rpm for a diesel engine. Now I’m curious.

Our pickup trucks usually redline around 3,500 or so, although according to those who have aftermarket power-adding devices that change rev limits this can be increased to around 4,000 with substantial power gains. In motorcycles, lighter reciprocating parts (special lightened bolts/hardware, knife-edge rods, etc.) are used to increase max rpm and power (a stock 600cc Yamaha R6, for example, redlines at 15,500 rpm).

Is it just the additional weight of the diesels that limits engine speed or that compression ignition mandates heavier parts? With a 6.5L diesel or similar 2-valve-per-cylinder engine, obviously the breathing is poor, but with a 32-valve Duramax I would think a higher rpm would be both beneficial and desirable.

What is the highest factory and non-factory rpm for a diesel anyone is aware of? Walter Via Email

These engines are really just big air pumps, and are designed to be most efficient within a predetermined rpm range. The Duramax 6600, for example, (from the factory) is most efficient at 3,100 rpm—less above and below that, according to an SAE paper we have for this engine.

The heavier reciprocating components in a diesel engine certainly tend to limit the max safe rpm due to the destructible nature of centrifugal force, but airflow and diesel combustion efficiency also enter into the equation. Diesel fuel is much slower to burn than gasoline. The propagation of the flame-front in a diesel combustion chamber needs time to fully develop and then impart the full benefit of pressure onto its piston. Increasing piston speed beyond certain limits reduces the amount of energy that can be imparted onto the crankshaft. This, too, is a limiting factor in maximum engine speed.

The newer Ford PSD has a higher governed rpm limit than the Duramax. We contend that this higher rev limit partly contributes to its horsepower advantage over other contemporary designs. Higher rpm tends to coincide with higher horsepower potential and an implied broader rpm range, meaning the drivetrain can stay in a lower gear for a longer period of time while accelerating or pulling hills (seeing a torque advantage over another truck due to lower gearing).

We once read that the 5.9L Cummins has a reciprocating failure rpm at approximately 4,250 or thereabouts. That’s why Banks limited its Bonneville truck and other race applications to 3,800 rpm.

We know the Duramax is factory governed at a max of 4,800 while in grade-braking, and Banks and others have run the Duramax to well above 4,000 rpm in performance applications. In a race truck we were involved with, the governed engine speed was raised to 4,000, which would have produced a top speed of 160 mph.

Editor’s note: To add to the initial response, in race applications where the engine is HEAVILY modified we’ve see around 7,000 rpm quite often. However, these are extreme, highly modified engines that are designed to push the limits of diesel performance to the ragged edge. They aren’t engines you’d likely see 100,000 miles of use from.