Stainless marine makes 2 styles of manifolds. Their standard ones, and then the big boys. The GEN III manifolds, have huge elbows, with either 4.5" or 5' diameter tailpipes. These manifolds are normally used on big power marine engines, like say a blown 1000HP engine. Do they work as good as a header on that engine, probably yes, and maybe better than most headers. The catch is though, most 1000HP marine engines, have a blower sitting on top of them. Probably not the exhaust I would choose if I was running a big power N/A engine.

The smaller manifolds, are good products, but not really a big power exhaust. More for the average 500-600HP engine.

Also, not all CMI headers are created equal. Elbow tops or split tops might get smoked by a GEN III manifold on a 1000HP blown engine. CMI Big Tubes, maybe not...

I'd forgotten about the Gen III lineup.

Stainless marine make more horsepower on a dyno on big motors .I have tested both.And why would you build a good motor and put a lousy product like CMI .their warranty is 25 feet or 25 hours..lol

I think the gen III is a nice pice on a supercharged and most NA engines. I do not think a gen III could compare to a good header on a 600"NA 1100HP Fuel injected IR engine. If it could produce the same torque and HP if not more would put my headers in the dumpster.

The CMI's you are talking about are not a header, they are simply a tubular manifold. I've run Stainless Marines, Gils, Eickerts and CMI's. My CMI's leaked so I got rid of them, no performance difference between the Stainless Marines and the CMI's. The Eickerts I gained midrange response and about 1 mph on the top end. These were all on a 440 CI BBC that I spun to 6100 rpms. Eickerts were also the loudest of the three by a good margin. On my current boat I'm running Gils with pipes that are dry to the tips. The Gils work fine but if I was buying new I would buy Stainless Marine and not think twice about it. If money was no object, I would buy the Eickerts. I'm still kicking myself for selling the set that I had.

Remember to be a header, the runners also need to be at least 24", anything less than that and all you have is a manifold.

Looks like stainless marine could design a new elbow with longer individual runners pretty easily.

What is the primary length?

Love to see your dyno sheet- and which mills you tested this on.

I think the Stainless marine products are very nice, but Imco power flow plus product appears to have longer runners, and I don't think either truly compete with large ID and OD tube headers.

Companies like Ilmor and mercury can use anything they want - and choose CMI.

UD

runner length is important for making power, Also 4 into 2 then into 1 makes the best on an overall average of power outputs but really not applicable in most marine apps. I have heard on the average that equal runner lengths of 32 inches mandrel bent makes the best power for a 4 into 1 true header design.

Here's my real world comparison results.

30 Superboat, 525efi, Bravo XR w/ITS, 28P Bravo prop.

CMI's = 78 MPH. Seen 79-80 a few times, on the rev limiter.
Stainless marine = 76.5 MPH. Best speed so far, RPMs down slightly.

Not worrying about the CMIs anymore = Priceless!