The ABC engine has the potential for more power and requires very little break-in. It is, however, less tolerant of dirt or dust that may be ingested by the engine. Some engines are advertised as being schnuerle ported. This may result in some of the fuel passing over the piston and right on out the exhaust port. With schnuerle porting the fuel arrives from different directions and converges at the center of the cylinder which allows more fuel to be burned, hence more power.
Most newer engines use schnuerle porting.