Quote:
Originally Posted by DGthe3
All other things being equal (displacement, cam profile, rpm limit, bore to stroke ratio, etc), the one with the fewest cylinders would be slightly more powerful and a bit more efficient while doing so. The reason is friction, fewer moving parts means less power is wasted inside the engine & more is sent to the crankshaft. The difference would be fairly slight though. The shape of the torque curve would be roughly the same between all of them, just with slightly different values.
However, if the only limits are displacement & number of cylinders ... a large number of small cylinders could be made to rev much higher than a small number of large cylinders. Each piston would have a shorter distance to travel, and carry less mass making it much easier to move quickly. Now, if they both have the same displacement, then they'll both have similar torques. Give the many-cylinder engine a cam profile that is optimized for higher rpm's and it will end up making quite a bit more power, since power is the product of torque multiplied by the engines speed.
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Pretty much that.
Or I could go dig out my text book from GMI on engine design ......................... not
A 3.0 L 4 cylinder would be tough to rev. You don't see 4 cylinders much above 2.5L in production today. Pistons and rods would need to be upgraded for lower mass so the cost goes up and you might as well get a 6.
Cost is a big part of the equation. 2 pistons, rods and a longer crankshaft and a longer cam and 4 more valves, springs etc. aren't cheap.
Also have to keep in mind that a 4 cylinder will need a balance shaft as will any V6 that isn't 60 degrees or so. Inline 6s are also balanced, but take up a lot of room so unless you are RWD only, like BMW, these don't fit well in FWD cars. Although Daewoo had one when GM bought them out years ago. So the old V6 engines made from V8s all have balance shafts.
As pointed out, being able to rev the engine to higher RPM can create more power and a bigger piston makes that harder to do.
The other part of the equation is the number of cylinder firings per revolution. This is part of the sound difference between a 4, 6 and 8.
So think of some GM examples.
The 2.5L makes around 190 HP. Equal to 6.2/2.5 x 190 = 471 HP in a 6.2L. GMs all new 6.2 makes 450ish.
The 3.0L GM had for a while (crappy engine) made 260 HP. Equal to 6.2/3 x 190 = 390 HP in a Camaro sized V8.
Then you can go to the very unloved 2.9L 4 cylinder and 3.7L 5 cylinder GM put in the Colorado/Canyon. The 2.9L made 185 HP and got 18/25 MPG. Now keep in mind GMs new Silverado gets 16/23 MPG. And the 3.7L was 242 HP and GM added the 5.3 L at 300 HP.
So you start with how much HP you need and then look at the most effective means to achieve that.
ANNNNNNNNNNNNND you have to consider marketing and customer expectations.
Start a similar thread and ask if you could give the same HP and torque of the 6.2L V8 in a Camaro with a 4 cylinder and see how fast you get jumped on. You will get a lot of (chest puffed out), "that wouldn't be 'merican muscle. Who would ever buy that".
Good technical discussion on this thread though and a great question.