according to the info in HOW TO MAKE HP, second edition, a 2 1/2 inch system will cost some power at 500 hp, a three inch will not, provided the mufflers flow enough cfm and there is a good x pipe. If you intend on getting the most blast per buck with good noise reduction that book is a great resource. Author is David Vizzard. On mufflers, it takes about 2.5 cfm flow for each hp you have to get to zero restriction/hp loss. For 500 hp that would be two 625 cfm mufflers or better.