Has anyone ran a dyno test to find the the hp difference on say a 475-500 hp Chevy bb using stock manifolds and risers and then stainless marine manifolds and risers. I talked with them yesterday and didn't get a answer. I know engine combo's have alot to do with it but I'm talking basic hp package. Nothing crazy...

Not sure anyone has successfully made that much power with stock manifolds and risers. Even 420's came with Gil's.

Im gonna have two sets for sale in a week or so, buy them put em on and let us know

tough one to call, I'm not so sure exhaust adds a certain HP but rather just allows you to take advantage of engine mods so it is a direct function of the engine rather then the exhaust itself. I would suppose most performance systems would be very similar if compared side by side with cast or tube being about the biggest difference. Not sure there really could be an answer to the question other then your always better off getting that air pump in the bilge to breathe better

Thats kind of what Im trying to figure out. How much will the factory parts hold back? Im going to have to upgrade, thats a given, I was just curious how much I'm loosing. Boat went 66 the other day with a Bravo 24 3 blade at 5200 and just about the same mph @ 4900 with a Bravo 25 3 blade. Im hoping to get a bit more mph with the better exhaust. Stainless says some independent mag did a test years ago and got from 3.5 to 6.5 mph gain?

On my stock gen v 502, the SM manifolds got me just shy of 3 mph. estimate 30-40 horsepower on my boat.

One mag did a test on emi thunder exhaust in a baja islander, stock mpi motor. it picked up enough they had to reprop "supposedly"

Unless you have a fairly big cam or other engine mods where the exhaust is really holding you back you will be real lucky to see 2-3 mph IF even that. I was running a blown 502 at one point with merlin heads and a stock cam at 7-8 psi of boost, after great pains and work to install regular tubular headers I saw NO rpm or speed gain what so ever, when I added more boost or more timing the boat picked up speed so it wasn't like it was "stuck" at a certain speed, I just gained no top end with the headers over bone stock manifolds, Smitty

Any 475-500hp NA engine is going to need some type of aftermarket exhaust so it does not revert water.
A supercharged engine can compensate for the lack of good exhaust. I did not see any gains on my 525SC going from Gils to CMI's.

I will speak here. Theres no perfict exaust for a boat.You hull an weight will meen alot. Your tourq ban an horse power. Open exaust works on blower motors or turbos. How Your hull performs will depend on how much back pressure you need to get the best perfomance for you eng an hull. If You put a nice open flowing exaust on your boat. I have played with mufflers go gain power. It's the hole package You have to pick your exhaust an dile it in. Artie