View Full Version : To CMI or not to CMI

12-26-2012, 09:57 PM
I have a set of CMI's that came with the 502 I bought. They appear to be good with no signs of previous leaking.......but......there are so many horror stories out there about CMI's leaking, that I'm concerned about using them.
My other option is to use the 496HO manifolds from my other motor.
Not as much bling with the 496 exhaust, but I would have peace of mind that they wouldn't leak.


12-27-2012, 01:58 AM
Stainless Marine..proven tested and awesome

Merry Christmas

12-27-2012, 02:25 AM
Ditto that.


Just Say N20
12-27-2012, 07:51 AM
Pressure test them?

Carl C
12-27-2012, 08:53 AM
Are you supercharging this engine? If so then you won't need the scavenging effect of long tube headers and the aluminum version of the 496 manifold flows very well and is light. IDK if they will bolt onto a 502 head though. If not supercharging then headers will be an important part of your engine. Do the headers look in good shape? Do they have a short exposed section of the inner tube at the flange before the outer tube weld? I believe that means they are the newer style. I'd pressure test them and use them. They don't all leak. Many of the problem headers came off larger multiple engine boats with long runs of unsupported exhaust pipe. Also, a lot of people believe that only the Mercury Racing headers had problems because they were supposedly built to lower quality per Mercury to save money. Not sure I believe that. Now you might run into the same problem as others who have ditched the 496 for a 502 based engine. The exhaust tips may need to be relocated.

12-27-2012, 08:54 AM
If they are merc spec'd cmi I would not use them. If they are true custom cmi I would pressure test

12-27-2012, 09:06 AM
What Carl said about supercharging. As long as an exhaust system isn't restrictive, it's OK on a supercharged engine.

roadtrip se
12-27-2012, 11:21 AM
Not to repeat anything that has been said here, but pressure test, and go use them. I have been utilizing the same set of CMIs since 2001 with zero issues, including on the 540 now in place. When I asked Potter about going to big tubers, he just told me to use what I have.

12-27-2012, 12:42 PM
Thanks for the feedback.

Carl, I am supercharging and the CMI's are the old style with the outer tube welded to the flange.
Thriller, SM's are nice, but I'm not looking to buy another set of exhaust. The stock 496 is a good flowing exhaust and I have the early aluminum ones, so they aren't overly heavy.

The general consensus seems to be pressure test the CMI's and use them if they pass. I did notice a loose inner flange in the tails (silent choice), but it is easily accessible and can be rewelded. Those of you that have CMI's, do you test them annually?

12-27-2012, 12:59 PM
The other question is what version of CMI are they? The E Tops seem to be very reliable Vs. the sport tubes or the big tubes. I'm guessing that's why they finally started making a big E top as they are a lower cost and higher reliability option for race boats taking a beating.

12-28-2012, 07:44 AM
Glen, I think it's been hinted at a couple of times here, but since you're supercharging, there's nothing to be gained w/the headers.
They will not work as designed (scavenging, equal length tubes, etc. because the dynamics will have changed), and there's the potential for problems at some point, even if they're in great condition now.
My vote would be for something simpler.

12-29-2012, 12:31 AM
Vette, the CMI's are E-tops.

George, I know I don't need the CMI's. The stock 496 exhaust will work just fine. It's just that I have the CMI's and.....you know......it's a bling thing. :pimp:

12-31-2012, 09:24 AM
E tops are reliable. Pressure test them and use them. I'll put my money that the pressure test goes well.