Hi , I changed my Nova with a 7.4L straight pipe/flappers for a Formula 242 ss with Silent thunder ( same engine). The silent thunder is garbage. There is no heart to my Formula without that sound I loved. If I cut out 2 holes in my swim platform directly behind the 2 open pipes from the engine how would you pipe it? Consider these 3 things.
1. I want that muscle car sound I miss
2. I would like it to be sorta quiet at idle so I dont get complaints again from the marina
3.silent choice is not an option,it needs to be DiY.
I was thinking of having the pipe tips just underwater at idle so once I get going they come out and get louder. Any ideas ? Thanks

Under water at idle is bad, when you come off plane you will get water in your motor even with flappers. If your stock with the stock exhaust, you don't make much noise. It's probably less than mine with mufflers. I hear a lot of them going down the river with through hull exhaust and they aren't bad. When you start your boat and the echo is deafening then you have a problem. When I start mine in the harbor without mufflers, I check to make sure no one is standing close to the edge of the dock.

They make 90° tips that turn down into the water so it’s quiet at idle and open when on plane. The bend also slow a wave from washing up into the pipes but I’d still run an internal flapper. I would also not run these with any sort of serious cam.

There’s boat out there with serious power running those turn down and other similar systems.

Thats cool. Question, if the quiet type exhaust runs through the outdrive and out the prop. What is the issue with tips under water or no flaps ? Since the standard boat exhaust is always under water.

The quiet type of exhaust typically have internal flaps, and there’s more than one story of them failing and destroying the engine when it gobbles water. Some folks have run open exhaust with no flaps for years with no issues while others have run flaps and still ingest water. Each setup and boat is different. My Baja 272 has a molded in swim step that traps an incoming wave and shoves the water up the pipes. Others have tips far enough above the waterline that it’s not even an issue.

I had Edie Marine S pipes the went under the swim platform just below the water line. Twin 509's that were not stock and never had any problems. Flappers on the tips. It was nice to be able to hear at idle.