The Rex Inline Silencers (http://www.rexmar.com/page237.html) work pretty well on stock type motors with minimal power loss. 3", 3.5", and 4" are available. We don't recommend these for anything that's built much beyond stock because you will see some power loss as the HP rating goes up. They generally knock off 8-10 dBA which on stock motors should get you well under the CA limit in most cases.