High compression at altitude


Active Member
Supporting Member
Mar 9, 2015
Carmel CA

Ahoy, Homebuilt Aircraft & Kit Plane Forum, below you will find a dialogue between myself, and the smartest engineer who I have ever known. (Name redacted). He is the gifted designer of airplane engines, PSRU’s, and a lot of other projects. Please read my bottom discussion first. He ‘anticipated’ my idea.

Hello Bob.

RE the engine compression, I have done exactly what you described here, and well before electronic engine

controls, when a pilot could be relied upon to manage the MAP correctly.

And it did provide an increase in both power and efficiency at altitude.

One can even apply a bit more than the 21" limit depending on the OAT.

And I actually COULD achieve a 14:1 static CR in my O-200-R

Hope you stay well.


Ahoy, ****

A few thoughts on naturally aspirated airplane engines at altitude:

An airplane ‘likes’ to cruise at a greater altitude than sea level, say 10,000 feet altitude without supplemental pilot oxygen , which Is 20.6” mercury atmospheric pressure.

What if one increases the compression ratio such that (with modern control of mixture, timing), you have the same detonation margins at WOT/10,000” as the original engine has at sea level. A (defeatable) sea level throttle stop at 20.6 “ map ‘should’ see no detonation at sea level, but with the increased compression ratio, a not serious loss of power. (????)

WOT at 10,000 feet should see less loss of power with the high compression engine than the standard engine. I looked at some ‘standard’ hp loss with altitude formulas, but they don’t seem at all applicable in this case. The amount of O2 in the air this altitude is less, so there will be a power loss, but, how-much???

I suggest that such an engine mod, if successful and of benefit, would be simple, and quite easy to live with.

Enjoy /s/ Bob Belter -- [email protected]