Finley Atherton said:
I have an 0-320 from Aerosport with the higher compression 9:1 pistons. I got the higher compression pistons to hopefully give me a little extra ?oomph? on takeoff from my short farm airstrip at 4,000 ft AMSL. I am starting to wonder if I will in fact get any extra performance. The LSE Plasma III is retarded by 5 degrees from standard as per the LSE instructions for engines with compression over 8.7:1 and the engine data plate shows the timing at 20 degrees rather than the standard 25 degrees (I assume this is the setting for the single mag). So, am I gaining extra power with the higher compression pistons , but then losing it with the more retarded timing???
I've been on the road, so please excuse the late post.
The compression ratio is a major factor in determining the thermal efficiency of the engine - i.e thermal efficiency is the ratio of the mechanical power produced by the engine to the energy that is available in the fuel. For an Otto cycle engine like our Lycomings, the theoretical maximum possible thermal efficiency is equal to:
1 - 1/(CR^(K -1))
where CR = compression ratio
K = ratio of working fluid's heat capacity at constant pressure to its heat capacity at constant volume. This is almost exactly 1.4 for air. So, this formula becomes:
1 - 1/(CR^0.4)
The actual thermal efficiency that is achieved is much lower than the theoretical maximum, due to friction, heat loss during the power stroke, etc. But, if we change the compression ratio, we can use the above formula to get a pretty good approximation of the change in power, assuming no other changes are made.
P2 = P1 * (1 - 1/(CR2^0.4)) / (1 - 1/(CR1^0.4))
where CR1 = compression ratio of engine 1
CR2 = compression ratio of engine 2
P1 = power of engine 1
P2 = power of engine 2
For example, 150 hp O-320s have a CR of 7. 160 O-320s have a CR of 8.5. If we plug those values in the above formula, it predicts a power of 159.5 hp for O-320s with a CR of 8.5.
If we assume that an O-320 with a CR of 8.5 has a power of 160 hp, one with a CR of 9 should have a power of about 162.7 hp, if no other changes are made.
How much of that extra 2.7 hp would be lost by retarding the timing 5 degrees? Hard to say, but I'm betting the total change in performance (i.e. increase CR + retard timing) would be very small.