Engine Size, Cylinders and Torque

[ManyScalps]

Had a look at torque numbers for a SL65 AMG, supercharged 6 liter V-12; 604 hp and 768 lbs-ft.

I realize it is supercharged thus the high torque. This got me to wondering. Would a 6L 4 cylinder, 6L 8 cylinder and 6 liter 12 cylinder engine, all internals the same, produce the same torque, the only difference being the number and size of cylinders?

BTW, thats the full 768 lbs-ft at 2,000 to 4,000 rpm!

The car weighs 4,400 lbs or twice as much as a Cobra. Would a Cobra with half the torque and half the weigh be equally as fast?

[HighPlainsDrifter]

Each engine is totally different,you can't compare a low reving 4 cylinder with huge heavy pistons and long stroke to a high winding 12 cylinder with small lightweight pistons and short stroke.
As to which is faster, there are other variables such as traction,weight transfer electronic traction control,etc. My money would be on the AMG by a noticable margin ,in the real world a cobra doesn't hook up or stop anywhere near the #'s the AMG can.
Perry.

[ManyScalps]

So does the number of cylinders per cubic inch affect torque?

Would a 6.0 liter 16 cylinder motor produce the same torque as a 6.0 liter 4 cylinder all internals being equal (cam, heads, etc)?

[MidOHasp]

stroke affects torque, all else being equal.

i think what i am trying to say that if both 6.0L engines had 500HP, the 4 cyl would have massively more torque than the 16 cyl.

[Anthony]

There are so many variables, hundredes if not thousands, that I think it is a difficult comparison.

The basics are that an engine converts chemical/thermal energy into mechanical energy. Assuming 100% volumetric efficiency, a 6L 4 cyl engine should consume the same amount of air/fuel, and liberate as much heat as a 6L 12 cyl engine at the same rpms. I tend to think of torque as kind of a measure of the volume of air/fuel the engine can consume per revolution, i.e, the torque is related to the displacement of an engine, and how efficient that engine can convert thermal energy into mechanical energy.

Of the thermal energy liberated, a general rule of thumb is 1/3 goes out the exhaust, 1/3 goes out the radiator, and 1/3 goes out the driveshaft. A higher compression captures a bit more of the energy that wouold be lost through the exhaust.

The engine is not 100% efficient itself, and some of the the initial mechanical energy is lost to heat/friction, such as ther piston/rings rubbing against the cylinder wall, heat production in the bearings, parasitic loss of valve train, oil pump, alternator, etc. The more you can reduce internal friction of the engine, the more energy makes it out of the driveshaft.

The mechanical design/relationship of various parts of the engine impact how this mechanical energy is transferred out to the driveshaft, such as rod length/stroke ratio, bore/stroke ratio. Generally, a longer stroke engine generates more torque, hp, at low rpms which is what you want in a truck engine. A larger bore than stroke engine is better suited for higher rpms.

A smaller combustion chamber allows for better, more efficient and quicker combustion, allowing for I think better energy conversion, and also allows for higher rpm efficiency.

HP is torque X RPM's, so the hgiher an engine can run at maximum efficiency allows you to make alot more power, do more work. Therefore HP is related to size of the engine ( max torque), and the rpms of near maximum efficiency ( determined by capability of heads, intake, exhaust)

This is very simplified, but kind of how I understand how an engine operates.

Assuming everythihg else equal (adequate head flow, compression, ignition), I would guess the engine with more pistons, of overall same displacement, would have better combustion efficiency and produce ( convert more of the thermal energy) into more power, optimizing timing of each engine. Having too many pistons will increase friction and reduce output.


some other things I've heard that a straight 6 engine and a V-12 engine design offer near perfect or perfect balancing. A V-10 design optimizes engine output, slightly better than a V-12 or a V-8, with regard to efficiency, internal friction, etc.

Ferrari made engines in the 1960's, some of 12 cylinder design, and some of 6 cylinder design, and maybe 4 cylinder as well, of the same displacement, in the same race. The 12 cylinder versions were generally faster, but in some races, the 12 cylinder cars broke down, and the larger piston versions ended up winning the race. I can't remember the specifics.

[ManyScalps]

Thanks Anthony for the indepth reply. It was a loaded question and I did get my answer.

MidOHasp,

I didn't realize stroke affects torque so much.