Displacement vs Number of Cylinders Question

[Cymaro]

I was just doing my usual thinking and thought I was ask you gear heads a question. Say you have a 3.0L inline 4 cylinder, a 3.0L v6 , and a 3.0L v8 engine. All DOHC to be consistent. Would they all make the same power and use the same amount of fuel? I was thinking they would make the same hp and tq but just at different points in the power band. What do you all think?

[banter]

If they were all made of the same exact materials, and somehow making the camshafts equal, same transmissions, etc... Then maybe, on paper, that would be true. Too many variables.

[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.

[Number 3]

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.

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.

[LOWDOWN]

Originally Posted by Cymaro

Say you have a 3.0L inline 4 cylinder, a 3.0L v6 , and a 3.0L v8 engine. All DOHC to be consistent. Would they all... use the same amount of fuel?

For the same reason DG3 mentions "friction" (aka "pumping losses"), those pumping losses GENERALLY contribute to slightly lower mpg ratings, per displacement, as you ADD cylinders. Fewer cylinders to fire requires, typically, less fuel, as well.

The OTHER side of this discussion: How do you make a 4 cylinder deliver more power (like a "bigger"/ more cylindered engine) while enjoying better fuel economy than a V6? V6 vs. V8?

"Boost", aka turbo- or super-chargers, which have become the EPA-challenged method of choice, but have been around literally for decades. Modern electronics, combined with usage of exotic materials and/or application of same (coatings) to updated internal "boost" components and technologies, are allowing these methods to be more widely applied.

[mikeyg36]

Keep in mind drivability and balance as well. You won't see too many 4 cyls above 2.5L because of vibration and it would just be easier for the manufacturer to step up to 6 cyl. I4s have perfect primary balance but imperfect secondary balance, V6s are not balanced, V8s are the same as I4s, and I6s and V12s are perfectly balanced.

[MikeSVX]

You question on power can't really be answered. There is much more to making power then just displacement and cylinders. You can have 2 different 3.0l V6 engines one making 180hp and another making 300hp.