tALL RISER OR SHORT RISER ON CMI HEADERS

[bigsbetter]

I've got this used set of CMI headers and I have a couple sets of different risers. My original stock manifolds had the 3 inch spacer between the manifold and riser, Which makes the short CMI riser about the excact same position. I was told that you should run as tall of a riser that you can fit for more performance. The taller risers are about 6 inches taller than the shorter risers, which makes them 1 inch taller than my flame arrestor/air cleaner. They will just barely fit under my hatch, but would look kind of silly, but is it worth the extra horsepower? How much more power will I get from taller risers, if any? Either way ,I'm going to have to do some rigging to make the exhaust pipes work, just will have to do a little more with the tall risers. Is it worth the extra work and the silly 'super tall riser look' ?

 

[happy]

ok.. since your talking about risers, i'm going to assume you have manifolds, and not headers, risers do not bolt up to headers, they bolt on to manifolds, and tail pipes clamp on to headers..

i don't think the riser height is going to effect your horsepower whatsoever.. what the riser height does is keep water from flowing from astern into your engine.. its a dimension from the water line of your boat to the top of the the manifold.. you should be at 13 inches.. if not you need riser spacers, which is what someone put on.. this adds 3 inches of height so the water in the y pipe cannot slosh into your engine..

if they are true headers, more than likely your transom cut out holes are not positioned right.. and you will have to do alot of work to get them right.. or call cmi, and send them the dimensions.. and have them build the right tailpipe.. if that is possible..

if your running manifolds.. you'll need to have tailpipes made too.. cmi has pulled the short riser from the market, they revert, they only offer long tail pipes, and if you have a y pipe, and silent choice installed, this no longer accomodates that.. you should check with your local law enforcement prior to eliminating your silent choice.

other than that, to do the job right you'l need to plug your transom and redrill your exhaust holes

hoped that helped

 

[vinny p.]

Daryl,

He has older style CMI's where the header and risers bolt together, then the tails slide onto the riser. He doesnt have the more common Uni-Tops.



Charlie,
With the 2 piece header and risers you have, the only reason to use those super tall risers is for clearance issues. You will find that there is no real performance gain to be found there. I would run the standard length risers, that would facilitate installation. The only benefit would be that you can move the point of which the water and exhaust gases mix, a bit further back. I dont think you need to , especially since you are running a supercharger. Us naturally aspirated guys need to squeek out every inch. Another reason why I went to full dry pipes.

 

[ERUDEELUDER]

Its a cmi two piece header. The tailpipes go thru the transom and are correct length for a bravo set-up. Since you are running them thru the transom with the same tailpipes, the slope of the tailpipes will be the same with both risors, so the only way using the higher risors will help reversion is because of the tailpipes exiting the transom farther up from the water. If it will fit, I would recommend using the black headers with standard risors. The black headers have larger ports (I think 2 1/8 and the polished is 1 7/8). Hope it helps man.

 

[bigsbetter]

Thanks for the input guys. I go with the short ones. It'll be easier and look better also.
Jery, I didn't realize the port diameter was different, but you're right, the polished ones have a smaller port. Also, about the header that is sopposed to have the crack. There is water stains in the header tubes from water laying in the bottom of the tubes. The stains are in the 2 cylinders next to each other. The stains had the excact amount of water height in them. I pressure tested the header with water in it and cannot get it to leak. I thought maybe it would only leak with heat, so I also heated them up some. Still no leak. Do you think the water could have come from a bad head gasket which would allow the water to be eaqual depth in each header tube.? It just seems funny how the stains are in 2 adjacent cylinders and are the excact same depth, which would have been about 1/4 inch deep inside the header tube as it was installed in the upright position on the engine. And your dad did say that the engines blew up right?