"I like the idea of using large prop for small scale flying, why do you need to lower the comp? "
Unless I am wrong, you need to lower the comp ratio because the larger prop is causing higher dynamic compression due to flywheel effect, the pressure is building higher faster and you want to eliminate preignition.
Or conversly the propeller being harder to turn over is causing faster pressure buildup at ignition causing detonation(not the same as preignition).
Can we do some thrust tests with different props at full and half throttle? Anything repeatable or is this not practical? I do not see my plane being operated at full throttle for the entire flight so full power numbers may or may not mean it is making good thrust at half throttle depending on a multitude of factors ie: Bore and Stroke , intake limitations, compression ratio , exhaust limitations just to name a few.
I just obtained a nice .61 and have been told that underpropping it for speed is not recomended and to place a slightly larger prop than specified in order to keep the revs low and use the torque instead.
I like the idea of a lazy prop dragging my plane around like a mule than a tiny prop buzzing like a bumble bee. Dunno just my opinion.[
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My experience comes from building racing automotive engines so I may be missing some hidden 2 stroke principals.[
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