Setting max RPM

I am going to have my prop dynamically balanced soon. I want to also double check my rpm accuracy at that time and reset the maximum rpm on the CS prop. The engine is a IO540-A1D5 with the crankshaft weights moved to the C4B5 configuration. Lycoming lists 2575 as the rated RPM but others tell me that?s for the HP rating in the specific application for the original engine and that 2700 is the correct max RPM. Various configurations of the IO540 use 2400 to 2800 as the rated max. Where should this engine be set? I am also curious about the rated HP. The A1D5 appears to be rated at 290Hp and the C4B5 at 250. Both at 2575 RPM.
George

Some engine models rated at lower HP were rated that way because they were built to also be slightly lighter in net weight. This was done by using lighter weight internal components. I don't know much about the specifics but I think there is sometimes a difference in crankshaft and connecting rods.
These lighter engines have been operated at the higher HP RPMs in many aerobatic airplanes (they wanted as light as possible with as much HP as possible).

Our C4B5 was out of an Aztec and rated

at 250hp at 2600 and 260hp at 2700 --

Ron

It's experimental you can set what you want, with a big caveat. 2700 or 2775 rpm is not a big deal. If you want to be conservative 2700. You did your research well.

You make more horsepower and higher RPM. +75 rpm is a few HP, not a big difference. Prop efficiency? Swinging a big two blade prop you might actually lose efficiency at higher RPM. Smaller diameter 3 bladed prop might actually benefit from the higher RPM?

On the general topic of revving your engine to redline or over a lot of experimental airplane builders and operators over rev all the time without a lot of thought. Reno air race paticipants over Rev their engines by several 100's Rpm over red line. However in Lycoming overhaul manual it has hard limits with mandatory inspections for over-revving, up to and including overhaul. I forgot what that RPM limits are. I recall 5% over is the point they recommend teardown? Someone know. 5% would be 2970 rpm. I know folks that do that all the time. I'm sure Lyc is conservative as they should be.

Will you hurt your engine at 2775 or reduce its longevity or reliability? No

On the smaller 4 bangers with the Hartzell constant speed props there's an issue of RPMs and harmonic's at given RPMs. I don't think that's an issue on the 6 cylinder with the counterweight crank.

On my O-360 I set slightly above 2700 rpm and often manually dial it back to 2700 for take off or climb. If I want a little extra I can push the RPM up a bit. It actually is smoother at the higher RPM.

You mention instrument accuracy. With more modern electronic engine monitoring I'm sure it's a lot better than the old days with the mechanical tachometers. However that's a good thing to check.

rvbuilder2002 said:

Some engine models rated at lower HP were rated that way because they were built to also be slightly lighter in net weight. This was done by using lighter weight internal components. I don't know much about the specifics but I think there is sometimes a difference in crankshaft and connecting rods.
These lighter engines have been operated at the higher HP RPMs in many aerobatic airplanes (they wanted as light as possible with as much HP as possible).

Click to expand...

Just went through this converting a C1B5 to C4B5. Lyv parts manual shows all key parts to be identical beteween the C4B5 (250 hp) and the D4A5 (260 hp)

Whoever set the spec just used the same engne, but derated the max hp by limiting the max rpm. It was clear that this was not due to parts limitations

Larry

lr172 said:

Just went through this converting a C1B5 to C4B5. Lyv parts manual shows all key parts to be identical beteween the C4B5 (250 hp) and the D4A5 (260 hp)

Whoever set the spec just used the same engne, but derated the max hp by limiting the max rpm. It was clear that this was not due to parts limitations

Larry

Click to expand...

Absolutely correct, both from my research and from the folks at BPE.

lr172 said:

Just went through this converting a C1B5 to C4B5. Lyv parts manual shows all key parts to be identical beteween the C4B5 (250 hp) and the D4A5 (260 hp)

Whoever set the spec just used the same engne, but derated the max hp by limiting the max rpm. It was clear that this was not due to parts limitations

Larry

Click to expand...

Yea, I should have read the original post a bit more carefully.
There are lighter versions of parallel valve 540 but I think they are all the carburetored 235 HP versions.
It is these parts that I was thinking about and know have been used to build custom lightweight engines in the past.
I agree that if it is known that it is internally a totally standard IO engine version then the only difference is the RPM adjustment.

rvbuilder2002 said:

Yea, I should have read the original post a bit more carefully.
There are lighter versions of parallel valve 540 but I think they are all the carburetored 235 HP versions.
It is these parts that I was thinking about and know have been used to build custom lightweight engines in the past.
I agree that if it is known that it is internally a totally standard IO engine version then the only difference is the RPM adjustment.

Click to expand...

I thought that was what you ment and almost mentioned that. While I haven't researched them, I would suspect that there are differences, including lower compression.

Larry