Andrew:
My brother and I spent several hours searching for a formula that would convert
sea level compression ratio, to that at a different altitude. We couldn't really
find anything. We found a few formulas that would give pressure for a given CR
but it would take several formulas, and calculations to get any results.
However the following might provide some insight.
Air pressure at sea level is 14.7 lbs/sq. in.
Air pressure at 10,000 ft is 10.2 lbs/sq. in.
Air density in % at 10,000 ft is (14.7-10.2)/14.7 or 31% less. If an engine is
using a 10:1 CR at sea level, then at 10,000 feet the compessed air's density
would be 31% less. I would think a compression ratio of 6.9:1 would compress
the air 31% less than the 10:1 ratio.
So my guess is that a 10:1 CR at sea level equates to 6.9:1 at 10,000 feet.
The decrease in air pressure as the altitude increases is not a linear
function. The correction at 5000 ft wouldn't be half of the above. As a rough
guess, I would say that decrease in air pressure from sea level to 5,000 ft
would be approximately 2 lbs./sq. in. So the equivalent CR of a 10:1 might be
about 8.7:1.
This is just a guess.
Blake
|