Ed, it would be best if you posted some specifics like what prop you are running, what type of airplane and what rpm figures are you getting at full throttle? The Slimline is a fairly good muffler, better then the stock muffler but a canister setup would be the quietest option if you have room for it. That being said, you can use prop selection to load the engine enough to keep it below 6,500 rpm. This would reduce both prop and exhaust noise.