RPM question

[AcampoDave]

Just wondering; does the amount of cylinders in an engine have any bearing on the rpms it turns? For instance: if the drive train was otherwise the same, would a 6 cylinder engine run at a different rpm than an 8 at the same speed? My initial thought is no because the final drive on a non o.d tranny (for instance) is 1 to1, and that should remain constant regardless of how may cylinders a motor has right? Funny that in 52 years of living I have never pondered this question before.

[geezer#99]

No!
A 4,6 or 8 would all be the same rpm.

[mattfranklin]

Main thing limiting RPM is stroke length.
Piston speed is approximately constant.
Tiny engines go 20k rpm and big ship engines go 50 rpm.
But the piston speed for both is about the same.
There is a chart for that somewhere in the engine text books.

[DieselSJ]

Originally Posted by mattfranklin

Main thing limiting RPM is stroke length.
Piston speed is approximately constant.
Tiny engines go 20k rpm and big ship engines go 50 rpm.
But the piston speed for both is about the same.
There is a chart for that somewhere in the engine text books.

Piston speed can vary. On any engine, piston speed at idle is different than piston speed at redline. You just want to keep it below the speed of sound.

[garyd1961]

Gears determine rpm not pistons.

[mattfranklin]

Originally Posted by garyd1961

Gears determine rpm not pistons.

Yes, for what actually happens with the engine. We were commenting more on the redline or normally designed operating range.

Do doubt you can take an engine designed for an operating rpm range and keep it in first gear longer and really get the rpms way up. But not for long. ;-)

[garyd1961]

The OPs question was whether the number of pistons make a difference in rpms with the same drivetrain, it doesn't.

[mattfranklin]

Originally Posted by garyd1961

The OPs question was whether the number of pistons make a difference in rpms with the same drivetrain, it doesn't.

Exactly.