While a blower engine may not benefit from a tuned exhaust, it's processing a lot more air and fuel and still needs a good exhaust system and will benefit just like any other engine. The manifold, while not tuned, may not be as much of a restriction on the smaller engine as it would on the larger blower engine and that restriction will kill power on the blower engine.

I've done many header comparisons on the dyno. Lot of our customers order custom headers with their engine combinations. We do initial testing with the dyno headers but will always do the final pull and tune with customers headers. One thing I can say about headers, especially with NA engines; bigger is not always better.

The IMCO's I've worked with seem to be good up to about 850-900hp. After that, they're going to start hurting power over something tubular and more tuned. I did a blown pump gas 540 that made 1020 hp with dyno headers, but when in the boat with the IMCO's the engines performed to right about 900 hp, given rpm, prop and pitch. This kind of goes to the above post where there's no way the IMCO's would kill 100hp on a NA 540 even over a set of tuned headers, but they kicked the blower motor in the nuts.

I can believe that entirely!

Good example is stainless marine gen 3 exhaust. Not to be confused with their standard manifold. The gen 3 is huge. Makes the imco,gil, emi stuff look tiny.. Giant elbow, 5 inch tailpipe, etc. That exhaust worked very well on big power blower engines, as thats what it was designed for, and has outperformed even some big headers on those engines. Now, i would not chooose that for a N/A exhaust .

My stellings "green" headers were designed for the merc 575. Which was a 540ci making 575hp in late 80s. 1 7/8 primary. In a recent conversation with a well versed offshore builder who is familiar with those headers, said they certainly can be hindering power a bit, even on my 800hp blower engines.

For support I have another set of trim rings on the inside of the transom with a thick non stretching rubber/ canvas type material. It is flexible to allow movement but not stretchable so it supports the pipes at the transom.

Another thing I was wondering about.....
My "dry" tails have six 3/8" water outlets right at the tip. I was considering welding those shut and welding in bungs near the ends of the tails and dumping the water that way.

Is there any advantage to either way of expelling the water? Can the water migrate back up the tailpipes even when it's introduced at the very tips of the pipes?

Furthermore:
When exhaust hit's the atmosphere, an energy wave is sent backwards towards the exhaust valves. This is why collector length is very important, yet no one seems to give a schit about it.

If you are going to weld the tailpipe shut 100 percent what you need to do is cut the outer sleeve off of the tailpipe first, then cut 1" out of the outer tailpipe and then slide a 6" piece of 4.5" wet exhaust hose up over the existing outer tailpipe then slide the outer piece you cut off back on and weld it up. This allows expansion of the inner pipe because it's much hotter then the outer and if it's butt welded at the end it'll always be prone to crack. Atleast I have experienced this. All this is inside the bilge obviously. Use the wet exhaust hose as a "coupler" if you will except the inner pipe will be continuous you're simply holding water from spraying out. Then add your dumps in the tailpipes. You should also consider drilling a 1/4" hole in the bottom of the outer pipe outside the boat so that way the bottom of the tailpipe can drain when you shut the engine down. I've had great success with this tailpipe setup for a long while.

And as far as your question about can your engine pull water back up from the very end of the tailpipe? Absolutely. I know of a SVL team that had issues with water reversion on their sealed 525 with 100 percent dry tailpipes. They caught on their camera systems at idle the dumps were too close to the tailpipe and water was getting sucked over and back up the dry tail at idle. Wouldn't have believed it if they didn't show me the video. They moved the water dump and the problem went away.

JMO . . . Loud pipes save lives and engines

Anyone ever test say a stainless marine, emi, imco, style manifold against a header at the 650-800hp N/A range ?