Originally Posted by MILD THUNDER (Post 4389086)

I vaguely recall recommending my buddy joe to check his port match of his imco manifolds which had a square port flange, to the afr's large round exhaust port . If i recall, the manifolds opening was slightly smaller than the heads port, in the corners. Hows the exhaust port flow when theres that going on ? Lol

Anyone ever check their exhaust port match ?

Originally Posted by MILD THUNDER (Post 4389089)

I think alot of header failures are due to lack of cooling, as well as lack of tailpipe support. I cant tell you how many guys ive seen running -12 an lines to feed their headers. I personally would never run anything less than a -16 line to feed headers, and always make sure the sea pumps are in good shape. Cmi had a youtube video on this topic.

Nothings harder on double walled stainless headers with lots of welds than drastic temp changes . Now, salt corrosion, thats another story.

I have seen some exhaust comparisons at 450hp levels for marine engines, and there wasnt a huge difference in power. Id like to see some of those same manifolds at the 700+hp NA level. Forced induction is a different animal.

I know cam guys want to know what exhaust will be ran on a boat engine for a reversion standpoint, but i dont know of any that have the tuning parameters of a Gil, imco, stainless marine , Eddie marine, etc, when it comes to primary ID, runner lengths, and so on.

Maybe if full force tim dynos his afr headed 540s, he can try it with his stainless marine manifolds, and then a set of CMIs.

Another thing most dont mention. Your typical manifold has a 3.5 inch gas pipe, with water being injected early on. At high rpm , being fed by a sea pump (not garden hose on dyno), there is an enormous amount of water being introduced into the exhaust stream. Which, consumes much of the gas pipes volume. The water itself, can be like putting a muffler, or smaller ID pipe in the exhaust stream, hindering flow.

Originally Posted by MILD THUNDER (Post 4389089)

I think alot of header failures are due to lack of cooling, as well as lack of tailpipe support. I cant tell you how many guys ive seen running -12 an lines to feed their headers. I personally would never run anything less than a -16 line to feed headers, and always make sure the sea pumps are in good shape. Cmi had a youtube video on this topic.

Nothings harder on double walled stainless headers with lots of welds than drastic temp changes . Now, salt corrosion, thats another story

Originally Posted by vintage chromoly (Post 4389122)

I would think that, along with water reversion, the cam grinder would want to/need to know if you were running a short primary ( 10 to 12 inche's like in a manifold system) or a longer primary (20" long as in a long tube header style exhaust) to be able to get his best shot at the cam. Not so much which manufacturer of the manifolds or headers, but wether or not the primary's are long or short.

Funny you bring up tailpipe support. I was brainstorming the other day about how to rig a support clamp on the inner transom to help hold the dry tails as they pass freely through the transom and I plan on hanging a pair of clamp on mufflers to exacerbate the issue. We need to modify my tailpipes to provide a downward kick to clear the rubrail and I'm going to talk to the tig welder to work up some kind of support as well.

Maybe we just hear about every failure with headers but those who have no issues have no reason to post as they have experienced good results.

I agree that spending a ton of time porting a pair of cylinder heads and paying no attention to the exhaust match is shortsighted.

Originally Posted by MILD THUNDER (Post 4389065)

Another question I have.

Back in the day, alot of the old crane, comp, and other BBC cams, were designed for the GM iron castings. It was no secret, those heads left alot on the table as far as flow, esp on the exhaust side. Big split duration cams worked with them, to help out the poor flowing exhaust ports.

Now, lets say you have some good flowing modern heads. Your ordering a custom cam. You cam guy wants the flow numbers. You provide him with the flow numbers , which say were flowed with a 2 1/8 pipe on the port. BUT, you don't have a 2 1/8 pipe/header, you have a 1 7/8 header, or even a manifold with a small ID, and very short exhaust tube before it dumps into a common pipe. Now, that head that flowed 300CFM with the 2 1/8 pipe, maybe only flows 280CFM with a 1 7/8 pipe. Does you cam guy take that into consideration when designing your cam, maybe adding some exhaust duration, or lift? Or is this irrelevant?

And if you're running a 1 7/8 ID header, should your head be bench flowed, with a 1 7/8 pipe?