AV8TOR,

Using the "Corrected Compression" determination given in the links RingWinger posted I belive is the same as the "effective compression" you were refering to since it takes the exhaust timing out of the picture by using the swept volume above the exhaust port.

Using this method, I determined the CCR for the Stihl, Shindawa, Echo 38 and McCulloch 25. Both the Echo and the Mac run well on the glow/87octane gas mix. The CCRs are as follows:
Stihl -- 10.61
Shindawa --9.85
Echo -- 7.89
Mac -- 7.14

The most significant differences between the similar engines is the shape and volume of the combustion chamber at TDC, with the Mac and Echo having about 30% more volume than the Shindawa and Stihl.
I ran the Stihl on EI with fixed timing at 20, 25, 28, 32, and 35 deg BTDC. The Stihl began to show some vibration(shaking) at the 3 to 5 krpm range at 28 deg. At 32 deg it increased, but was still tolerable. At 35 deg the vibration was unacceptable. I never heard any sounds like detonation, but the could have been masked by the prop and exhaust noise.

From this information I believe that my conclusion of needing to increase the octane value of the mix for the Stihl and Shindawa is valid.

Andy,

Regarding the head volume. I believe that I would shoot for a Corrected compression ratio of between 7 and 8. For my 23 cc Shindawa this would translate to a head volume of about 2.7 to 2.8 cc.

Next step, try some octane addatives and see what happens.