I'm thinking of making a thumper, but want to make sure a 500 cc engine will be enough. CCS Supersingles is out (MuZ 660/720's being so prevalent). Are there classes among AHRMA, WERA, etc that limit singles (4 stroke) to 500? Had good luck last year at the AMA's "Amateur Nationals", but would hardly consider building something based on 2 races a year (mid-week at that!). How about FSS limited to 250?
AHRMA has a sound of singles 2 class that limits 4 valve thumpers to 510 cc & 2 valve thumpers to around 605cc. They also have SOS 1 which is open thumpers.
