I went from stock merc manifolds with aftermarket risers to gils with the low risers for silent choice on a blown 502 (not sure on hp). Picked up a solid 200 rpm and 2 mph on gps with same load and similar air temps.Good luck.

Stock 330's in a 272 Formula stock to Imco Power Flows picked up 1 to 2 mph

I have a procharged 502 MPI 6-7 psi boost. I was running stock manifolds and risers with EGT gauge. I switched to Gil manifolds and risers. Gained almost 300 RPM's and dropped my EGT temps almost 75º!!! Stock mercs are good below 450hp, they start robing HP above that.
Therse alot of good quality used manifolds on the forum, I bought mine here, and saved quite a bit of cash

You must be pushing 650+ hp to get that speed increase. That's impressive. I would think exhaust would definitely be in line next.

Tim
Buying used is always an option. but keep in mind there is no guarantee on what you have acquired.
Also keep in mind the the manifold manufactures are always competing and improving their product this means changes for the better in most cases and I'm sure you want the latest tecknology.
Another issue is appearance, any one of the replacement manifolds looks a lot better than the stock product.
Another consideration is fit, I went over to Impco's with the nice looking stainless steel exhaust tips. I had to do some work on the transome to get them to look right the bracketry that holds the shift linkage is real sensative to clearances between the stud and the manifold you can just barely get the nut on.
As we all know in the boating hobby nothing really fits like they say it does, plan on playing around with it.

If anyone is interested in Lightning headers I have a very good connecion for them. E-MAIL me if you want his name and number.

26ZX Donzi,

I agree. With a stock engine you will not see much of an improvement with an aftermarket exhaust manifold. On a stock engine, a 1mph gain and an increase of about 300rpm sounds about right with an aftermarket manifold which translates to probably about a 15-20hp increase. But, at least you have the better breathing exhaust manifolds.......so now any future mods that you do to the induction side of the engine(s) i.e. cams, intake manifolds, carbs, heads, etc, the engine will be more sensetive and responsive to these changes/mods. The mods you do to the iduction side of the engines, the better the aftermarket exhausts will perform which translates into more hp from the exhausts. It's a combination of things all working together. BUT, until you FIRST add the exhaust it really isn't going to happen much for you.

If you can't afford to do everything at once at least add the exhaust first. True, you won't see much of a gain---maybe like 1mph as "BigWavz" has experienced, but they'll look nicer, it will lighten up your boat a little, they'll sound better and who knows...they might give you a little better fuel econ. THEN as time goes on add an intake manifold or cam or what ever you can afford at the time. And as you do these things/mods you'll see more of a difference which each added mod and that is again because you already have the aftermarket exhausts working for you and taking up the slack so to speak.

Sorry if my explanation sounds so elementary----but I just want to make sure you understand. Best Wishes

Mark/KAAMA