Was wondering what kind of gains I can expect from my SBC by switching to stainless marine from stock merc exhaust. Its 350cid, lt4 hot cam, vortec heads, 9.8:1 or so compression, 650 demon carb. It's turning about 5200 rpm and getting me to about 63 mph with a 21p laser II prop. The boat is 1987 concord magnum II 20 ft.

You will see some minor gains because they do flow better than stock exhaust. But the main benefit is longevity, some SM setups out there are 20 years old and still working fine. Even in fresh water stock manifolds are pretty much toast at 7 years.

i bought a pair of ones from revolution marine for my vortec small block, great price everthing is included, check them out 1st

You may want to check out GLM which offers aluminum cast manifolds and risers for a much lower price.

Any gains would be be around 1 to 1.5 mph which is not bad for a bolt on and this would be partly from the weight savings from cast iron to alum. The GLMs are semi tuned (individual runners running close but not completely to the riser. As you can see in the stock Merc manifold, it is just a large cavern no runners at all.

Good product better&lighter than stock exaust!

I put them on a stock 6.2 a few years back saw about 100-80 rpm increase and dropped exactly 83lbs from the stock iron gained maybe 1 1/2-2 mph but I owe this to the weight savings in addition to the slight increase in RPM's. By dropping weight in the stern my boat handled better, always good to take the fatman out of the back seat lol. As info mine is a 18 Donzi, still have them after 5 seasons and they look as new! Great product IMO Steve

From what I understand, the stock Merc 496 manifolds are aluminum with individual internal runners similar to the aftermarket units. They also use the dry joint with the jumper hose, and the riser makes a more gradual bend than the older risers. If you look around, these come up for sale used occasionally in the $500 range. They share the same flange pattern as the older Mk IV/ Gen V/VI big blocks. It might be a good upgrade for the money.

Earlier ones were aluminum. Later cast iron. All had stainless risers.

Does anybody have any experience with imco or edddie marine on a small block? All of these companies claim big gains but most of them come with a big price and the emi thunder and imco power flow seem to be a little cheaper than stainless marine

I replace my stock (502) manafolds and risers with SM last year. Don't know about HP increase but I can tell it breaths better and it sounds beachen and the weight loss helped too! Do it!