Ok, how about an offset cylinder? So it sits above the con rod / crank connection with the crank at 90*? That way it is possible to have less height in the whole crankshaft rotation except close to 90 ATDC because of the rod being angled, PLUS you get the power directly onto the crank instead of angled. I know there'd probably be problems with excessive rod angles causing bore wear on the up stroke, but I think it'd be a step in the optimum direction. EDIT: You could have a fairly substantial offset pin in the piston, then you wouldn't have to offset it as much, and the upstroke wouldn't try and rock the piston as much either.