True. However, my main concern is when the port opens. It's relative duration may be the same but if the open/close events are much later on the 30cc.

Trying to describe the cylinder difference without a picture: While the muffler flange is in the same relative position (to the 25cc) the 30cc port descends to a much lower position at the clyinder bore wall. This would make the opening and closing events occur much later. As this is the only significant difference between the two engines it is the most likely cause of the performance differences.

I will degree the engines once I sort out a degree wheel and get an idea of what some of the more 'optimal' timings should be for aviation use.

Can anyone provide their port timing and performance figures, it would be greatly appreciated?