What experience does anyone have running CMI 4 inch switchable "aft fire" clamp on mufflers closed while running ? I'm just curious what others have experienced doing this. I need the quiet and cruise @3500 RPM.

Running HP500 w/long, through transom risers.

Anyone have any experience here ?

I've got Gaffrig switchables which are similar to the CMI's. I run them at 3500 with no problem. I've gone 4000 but only to try it. No problems, but I didn't do it for more than a few seconds.

I have tried many different positions on the muffler butterfly. One way to tell the possible H.P. loss at W.O.T. is to turn the mufflers on and look for any RPM loss . In one of our single engine boats with a 600 HP Innovation the mufflers could be partailly closed at 5200 resulting in about a 8Dba reduction. To hold the mufflers at this position a short piece of 1/4" id hose can be tied to the cylinder shaft.

Thanks for the replies guys. I'm primarily concerned about water backing up and causing a problem. I'm not really concerned at all about a little HP loss. I just need a little quiet for cruising sometimes and not just around the dock.

I do have long, through transom Stainless Marine risers that dump right at the very end of the pipes and just at the mufflers.

JohnJan, which Gaffrigs are you running ?

Again, thanks for the replies.