[Zeke]

AFA blueprinting I don't think there's much more bang for the buck than an H-4 air cooled motor. The case halves are machined and bored whereas V8's are simply line bored for the crank and cam, and the block surfaced square to the bore, but the block is a single lump. The H-4 has the additional setting of the cylinders that have to be at the same height to fit the heads right. This would be true with many motorcycle engines.

When getting to rod length and wrist pin height all multi-cylinders are pretty much the same. And then there's balancing. Believe it or not, a single cylinder engine can be balanced to a certain degree.

When I was racing karts a huge amount of time, effort and money was spent to get more HP out of a stock out-of-the-box engine. The ubiquitous Yamaha KT100 was good for another 2 HP with modifications strictly adhering to the rule book which did not allow timing changes to the ignition nor the 2-cycle ports. The differences between motors shipped out of the factory were subtle but they did differ. And the tolerances could be improved on. Mainly squaring the cylinder to the crank, establishing the ideal piston clearance and making sure the ports were right on the money. Plus CC'ing the head, as well as balancing. By doing all the work with ultimate precision, 15% more HP could be found. A really good motor would turn another 1000 RPM over a bone stock box motor.

However, 4-cycle engines have more things to play with, mainly the cam and valve train that a 2- cycle motor doesn't have. And don't forget flowing the heads.

I've seen some nasty B and S motors on karts that were way beyond mowing the lawn.