To make the extra 5 hp from the same Displacement, OMC had to spin the engine several hundred Rpms faster. To do that, they had to change the Port Timing. To do that without moving the Ports in the Block, they relocated the Wrist Pin in the piston in relation to the Crown of the Piston, which effectively changes the Port Timing in relation to the Crankshaft. Doing so resulted in the piston not travelling as far up in the Bore, and that lowered compression, so a different Cylinder Head was needed.(a few Geniuses, have tried putting the 25hp head on the 18/20 engines, without changing the pistons. That results in serious detonation, from too much compression). To get the extra revs, the engine needs to be able to inhale better, so a bigger Carb and some mods to the reeds are needed..
