We have a 2000 38' Fountain 502 efis with cmi headers on it with silent choice. I've been told that you can only use silent choice while idling, is this the case? Or how fast can you run with the silent choice on?

I think it depends on the motor... think about it, lots of non-Mag 454's out there only have through prop exhaust and they dont' have a problem.

Thanks for the reply. I was told with it on it was hard on exhaust valves above low rpms.

I recall seeing some systems that say not to be used above 3000 rpms.

Idle

I think I remember reading about the HP500 had to be open above X rpm, but for the life of me I can't remember the details.

I asked the same question to a mechanic at a Baja dealership about My 496 mag ho with switchable exhaust. He said it's ok to run it either way, just don't switch it while at the higher RPM range, He recomended to switch at idle or low RPMs...

I've ran mine at 75 with no problems... I've actually heard in some cases that it's better out of the hole since you have more backpressure which gives you more low end torque.... but that depends on the engine.
I got a few exhaust tip mufflers made so I don't need to run on silent choice on the busy weekends at Lake Havasu... I'm sure the exhaust running on silent choice doesn't help out the drive temps when it's 115 degrees out.

Same here, I had Corsa on my donzi, only switch at low RPM, you could run WO either way, just no switching above 3000...

There was a "Teague on Tech" article in Powerboat about six months ago that said motors with horsepower above 300 should have the exhaust open above 3000 RPM's.