Why do big diesel engines and race car engines have such different horsepower ratings?

Let's start by answering the question and then look at why the world works that way.

The answer to your question has to do with the way the two engines are designed. Your 11 liter diesel engine has a long stroke. That means that the piston is traveling a relatively long distance up and down in its cylinder on each cycle. A racing engine, on the other hand, has a short stroke. The piston in a racing engine has a large diameter for the engine size, and it goes up and down a relatively short distance on each cycle. This means that a race car engine can run much faster -- up to 15,000 RPM in a Champ Car engine -- but has relatively little torque. A large diesel engine usually cannot get above 2,000 RPM, but has huge torque because of the long stroke. The torque is what lets your engine pull a huge load up a hill.


So why does an engine with huge torque and low maximum RPM get a low horsepower rating? If you have read the article entitled How Horsepower Works, then you know that one horsepower is equal to 33,000 foot-pounds of work per minute. By this measure, one horse can raise 33 pounds 1,000 vertical feet in a minute, or 330 pounds 100 feet in a minute, or 3,300 pounds 10 feet in a minute, and so on.

What an engine naturally produces, however, is torque. Think about one piston in a gasoline engine. When the gasoline ignites, it pushes on the piston, and the piston exerts pressure on the crankshaft, causing it to turn. The crankshaft feels some number of foot-pounds of torque in the process. There are three variables that affect torque:

  • The size of the piston face
  • The amount of pressure that the ignited fuel applies to the face of the piston
  • The distance the piston travels on each stroke (therefore the diameter of the crankshaft). The bigger the diameter of the crankshaft, the bigger the lever arm and therefore the more torque.

There is a direct relationship between horsepower and torque. You can convert torque to horsepower with the following equation:

HP = Torque * RPM / 5,252

That 5,252 number, by the way, comes from dividing 33,000 by (2 * pi). Imagine taking 33,000 foot-pounds and walking it around in a circle rather than a straight line. For example, if you took a 10 foot pole and attached it to a vertical axle, the circumference of its circle is:

circumference = 10 * 2 * pi = 62.8 feet

You can see from the horsepower equation that high RPM values favor horsepower. If you take an engine with a certain torque and run it at very high revs, it can generate lots of horsepower even though its torque hasn't changed at all. A racing engine can produce relatively low torque, but because it can rev so high it gets a great horsepower rating. A big diesel has huge torque, but "gets no respect" in terms of horsepower because it cannot ever get above 2,000 RPM. This "makes sense" -- if two engines produce the same torque, the one that can do it more times per minute does more work and therefore has more power.

The difference in maximum RPM ratings also tells you why trucks need so many gears. A race car engine might idle at 1,000 RPM and can accelerate to 15,000 RPM -- a multiplier of 15. A big diesel might have a multiplier of only 2 or 3. Because the RPM range between minimum and maximum is so small on a diesel, there needs to be lots of different gears to keep the engine in its productive RPM range at any speed.