Why dont they run the pressure ones the other way and use manifold vac to pull them closed? Hit gas, lose vac and springs pull open. Pull off throttle and vac pulls them shut.

Good question. Same as the cowl induction hoods on Chevys back in the 70's.

I love this idea!!!!
As for noise level, I pass Wisconsin properly measured testing! Meaning waiting for the reader to stabilize! MN just tags you on the first reading and does not wait!
That being said, its 84 with t502's
It is noticeable open and closed, I will be on the hose again this week I will try and remember taking a video and see how it turns out.

That would be Awesome MnFastBoat. I look forward to seeing it.

TC G

But can u carry on a conversation idling or running around at 3000 rpm and not have to yell to the person beside u or the nav. seat.?

Opps never did a sound or video :(
Sorry

It is noticeable at idle and running from open to closed. But from the photo you can tell mine can be heard from all angles :)[ATTACH=CONFIG]548345[/ATTACH]
Due to MN patrols I typically run mine closed 80% of the time anymore :(
Even getting on plane! I have pushed it to about 4-4200 rpm but after that I open them.
Usually if I am running that speed I know the patrols are not around. :)
I will say that there is a huge power gain from open to close getting on plane!
If my backseat is full or I want to get on plane in a hurry I open them, and it is noticeably quicker getting up!
However I still love them!

Just do GGB Inserts and be done with it.

http://marineexhaust.ggb.ca/

The switchable ones make me nervous with an aggressive cam and I swear the swichable tips made me blow my exhaust manifold gasket.

^^^^^^^^THIS!^^^^^^

I am on a very strict lake when it comes to noise. My GGB inserts did a fantastic job of quieting the bark on stock 496 HO's and I didn't notice a speed loss. I was never once bothered by Marine Patrol and I saw plenty of other boats that were last season. I am curious how well they will do with the new exhaust and added compression next year.

I have GIL air operated. They'll blow open in the mid - high 3's, but can manually hold them closed up to about 4300 if there's a patrol in the area

I have been really impressed with the Gibson Full time mufflers that came on the Spectre. Haven't been checked by the marine patrol yet, (They don't really hassle us down here about noise, just MANATEE Zones :( ) but they are really well made and they really knock down the noise...

http://gibsonperformance.com/i-22957...n-110001.html#!