This has been on my mind for quite some time now. I am considering slowly switching all my glow powered planes to gasoline. Glow fuel prices always seem to skyrocket dummer the summer months and then slowly come back down towards the end of the fall. Gasoline is just so much cheaper.

How does one determine what size gas powered engine to use in any given sized model?
For example: Does a plane that calls for a 1.20 4stroke engine need a 40cc gasser?
Is there some kind of mathematical formula for converting glow engine sizes to their equivalents in gas engines?

Thanks for any info.