Percent Power Calculation

[NoahF]

I must still be missing something regarding the chart data. You say “notice the difference between 10,000 ft and 2,000 ft is about 8HP for a given setting”. If I start at 2000 feet on the right side of the chart and look at the intersection with the 2700 rpm line, I read 187HP in the middle of the chart. If I go to the 10,000 ft vertical line and again move up to the 2700 rpm line, I now read 140HP. This is a reduction of (187-140) 47HP, not the 8HP that you quote, if I am interpreting the chart correctly.

Further, the AFS manual, on page 58, states that for an O-360, the delta HP with altitude at 8000 ft is 9.1 HP. We all know the rule of thumb that normally aspirated engines cannot exceed 75% power above 8,000 ft. For an O-360 rated at 180HP, this would be a 25% reduction, or 45HP, not the 9.1 HP in the manual example. The chart I posted above for an IO-360 A model supports this as well; maximum power at 8000 ft. is indicated as 150 hp, reduced 25% from a sea level maximum of 200HP.

So I think I must be missing something, can you please explain your numbers? How did you get them?

[Mounz]

I can only agree with Noah. The delta horsepower on page 58 is way off. The MAP/RPM values corresponding to 55% and 75% power seem to be accurate. The presentation is however unfortunate and confusing as the values on the right hand side of the table have nothing to do with those on the left. For example it is obvious that you will never see 23.9" MAP at 10 000ft with a normally aspirated engine hence you will never be able to get 75% power from your engine. This obviously makes the Delta HP of 11.4 look ridiculous.
Gary, I am somewhat confused about "add 1.7" MP for each degree above ISA". Surely you are going to see less manifold pressure as the temperature goes up, density altitude increases and the amount of power your engine can produce decreases.

[Mounz]

OK Gary, I think I see what you mean. You have to add 0.17" MAP per degree ISA deviation to mantain any given % power setting. That ties up with my knowledge although I now think in centigrade so the addition is almost double.