Engine Power in Relation to Size

This came up today on the www.bimmerforums.com "7 series" forum I am a member of. The question asked: "why can some 6-cylinder engines make the same or more power as 8-cylinder engines or 12-cylinder engines?" I was getting fed up with half-asses answers or answers that just confused the question more so I wrote my own and felt this should be shares abroad because in today's world of high output 4-cylinders and comparatively low output 8-cylinders others might be curious as well.

It looks as if we are straying a bit from the original question. The bottom line is that the number of cylinders has NO universal effect on horsepower or torque. What DOES have an effect is the size of the pistons and bore of the combustion chamber along with the length of the stroke of the engine, all other things being equal.

An example is the old Jaguar XJC's (1985 or so) with the 12-cylinder 5.3L engines. To get 12 cylinders into their 5.3L package the pistons had to be very small with little individual displacement. Make those bigger and you have more torque, make the stroke shorter and you gain more horsepower in proportion to torque.

Same idea for a 4-cylinder engine. If you have massive pistons and a short stroke you will have torque and horsepower and depending on size and stroke can easily equal that of a 6, 8, 12-cylinder engine and vise versa.

Again all things being equal, the more pistons you have the smoother and quieter the engine will be. Hence why supreme luxury cars use mainly 8-cylinder or 12-cylinder V configurations for their cars because they are smooth and quiet. BMW could certainly have opted for a short stroke big bore 4-cylinder but you would clank down the road and barely be able to keep your espresso in your hand.

Another 'for instance' is my 1982 SeaRay 192 shore-runner boat with the old MerCruiser 470 sterndrive. The engine is basically just half of a Ford 360 big block and produces 180 horsepower and 260 ft lbs of torque from a 1982 4-cylinder engine! The pistons are enormous with a relatively moderate stroke but the thing is loud and rattles your bones.

Here is my last example of the role of size and layout to power: In regards to the high horsepower small distribution INDY cars they are like an oversized superbike motorcycle engine. They have a short stroke allowing high RPMs thus allowing for high horsepower and small pistons that allow the car to get to those high RPMs but in turn give it little torque. That is ok because torque isn't as important in those cars because they rarely drop to low speeds and weigh nearly nothing when racing so high amounts of torque is not necessary.

Basically just remember cylinder count has little to nothing to do with horsepower or torque it just affects how smooth that power is delivered.

- Mitch G