% power, prop pitch, and rpm relationship

[alpinelakespilot2000]

Hypothetical question regarding fixed pitch props:

Assuming two identical engines are both producing 75% power but one has an underpitched prop and the other has an overpitched prop, would engine temperatures (CHTs primarily) be the same on both engines? Stated differently, suppose it it takes 2700 rpm for one engine to produce 75% power but only 2500 rpm for another engine to produce 75% power (because of differences in prop pitch), assuming all else equal, would you expect the higher rpm engine to generate higher CHTs due to faster moving parts or should temps be the same on both engines since both are producing the same power?

Thanks.

[skylor]

Originally Posted by alpinelakespilot2000

Hypothetical question regarding fixed pitch props:

Assuming two identical engines are both producing 75% power but one has an underpitched prop and the other has an overpitched prop, would engine temperatures (CHTs primarily) be the same on both engines? Stated differently, suppose it it takes 2700 rpm for one engine to produce 75% power but only 2500 rpm for another engine to produce 75% power (because of differences in prop pitch), assuming all else equal, would you expect the higher rpm engine to generate higher CHTs due to faster moving parts or should temps be the same on both engines since both are producing the same power?

Thanks.

I would expect the lower RPM engine to produce higher cylinder head temperatures than the same engine producing the same power at a higher RPM. The reason being is that the slower turning engine will have a smaller theta PP (crank rotation angle when peak cylinder pressure occures) and a higher maximum cylinder pressure. However, the slower turning engine will be more efficient due to less pumping losses (from higher manifold pressure/more throttle).

Skylor

[RV10inOz]

I would expect the lower RPM engine to produce higher cylinder head temperatures than the same engine producing the same power at a higher RPM. The reason being is that the slower turning engine will have a smaller theta PP (crank rotation angle when peak cylinder pressure occures) and a higher maximum cylinder pressure. However, the slower turning engine will be more efficient due to less pumping losses (from higher manifold pressure/more throttle) or leaner mixture.

The first part in bold is 100%, well done.

[skylor]

Originally Posted by RV10inOz

The first part in bold is 100%, well done.

And I corrected the mistake in my post by removing the last 3 words...