% Horsepower Loss with Altitude?

[farmrjohn]

I have an O200A engine with fixed pitch prop but no manifold pressure gauge. Therefore using the altitude performance charts won't work since they require manifold pressure. Is there a rough range for power loss per thousand feet of altitude? The operator's manual has several rpm/hp figures for sea level.

On a somewhat related topic/question, is there a source for hp and torque rpm curves? John

 

[EdFred]

You're going to lose ~1 inch of manifold pressure per thousand feet. When I'm wide open throttle on the ground I'm around 29 in of manifold pressure (800MSL) when I at 5000 ft I am around 25 in of manifold pressure at 10000 ft I'm around 20 in of manifold pressure.

 

[GRG55]

Very rough rule of thumb is 3.5% loss of rated sea level hp per 1000 ft of density altitude.

That's VERY rough, as a lot of factors come into play including whether you have a fixed pitch or constant speed prop, what RPM and so forth.

 

[farmrjohn]

Thanks. The owner's manual has a sea level performance curve that shows roughly 29 inch manifold pressure so that look like a reasonable figure to use for the charts, and the 3.5% loss/1000' math works on that example as well.

edit: Well, it looks like that will only work for the O-200 at sea level and full throttle. Is there a way of estimating what manifold pressure would be with a reduced throttle and fixed pitch propeller using just the tach?