MILD THUNDER

Kinda dead around here lately. Thought I would bring up the "exhaust" topic.

As we know, today, many guys are building more and more powerful engines. The days of a 500HP engine being a hot item, have transformed into a 600HP engine, being a "mild" engine.

Sooo.....the guy who's building the N/A 502, 540, 572+ with "custom" cam, high flowing heads, and rarely is the exhaust system discussed. Exhaust header primary diameter, length, collector diameter, collector length, all play a big part of the build, esp on N/A engines. With cars, its easy to get deeply involved with various header designs, and one surely would want to if he was building a 600,700+ hp engine for his Chevelle. But then on the boat stuff, a guy will argue over the proper CFM carb, the ideal textbook intake runner volume, which aftermarket head has the best flow number, order a custom cam with the ideal duration, lift, degree it in, port match things, and then slap on some general marine aftermarket manifold like a Gil, Stainless marine, imco, emi, etc.

At what point does a custom big block pumping out 1.3+ HP per ci, start seeing the effects of a poorly designed for the engine exhaust?

MILD THUNDER

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?

rexcramer1

Good question, what is the tube diameter of your standard Gil, S. Marine or IMCO manifold?

vintage chromoly

Your cam grinder will ask what exhaust you're using before he specs the cam so as to account for the limitations or lack thereof of the exhaust. In the planning stages of an engine build the exhaust is determined before the cam is specified.


In a car, there is much more liberty and availability of header primary diameter/ length, collector diameter and length and there is no consideration of the issues of water intrusion. Even a true custom race car header will cost a fraction of what a "shelf" marine header will. in a boat, we see "manifold" style exhaust systems because of the fear of leaking headers, fitment and budget. Ever see a manifold type exhaust on anything but a nearly stock automotive application or because of class rules as seen in "pure stock" or FAST racing?

I'm sure if one had an unlimited budget, access to a wet dyno for back to back testing and the tooling to produce jacketed headers in multiple sizes and configurations the optimal header for a specific engine could be built. That said, in the 650 HP BBC marine world, it would be much more cost effective to find that 40HP gain in another way.

I would think an exhaust system that is not matched to the rest of the build would effect any engine wether it's a 350HP smallblock, a 700HP big block or a maximum effort build. In my mind it comes down to reliability (leaky headers) fitment and cost when talking marine exhaust.

These are just some random thoughts off the top of my melon.

bck

For what it's worth, the shop rebuilding my engines is pretty confident he'll be able to run my big tubes on the dyno. Hopefully it works out and shows the difference between them and the automotive dyno headers.

MILD THUNDER

I do hear you on the cost, and choices being an issue. I do think the leaky header thing is a bit over rated. My stellings are now 20 years old, and still working just fine. I had a small leak at the collector, but nothing horrible. Another friend of mine, had lightning headers for 10 plus years or so with no issues. I certainly wouldnt be spending thousands on cnc ported heads, only to run an exhaust system that is a poor choice for the power level I am shooting for . I would like to think the power loss from a Gil manifold vs a teue header, is not nearly as great at 400hp, as it is at 700hp. How often do you see a "gil" manifold on a skater, mti, apache, nortech, etc. Heck, even mercury went from manifolds to headers on 500efi, 525efi, and about everything above that power wise.

Plus , there is the option of a cast header manifold, like the PF marine /eickert/ lightning manifold.

vintage chromoly

I'm going to run lighting headers on my engine with dry tails. Freshwater use.

I decided that early on and bought the system and went from there.
I hope you're right about the leaking header woes being exaggerated.

It seemed counter intuitive to build up a cool mill and then cap it with manifolds (although lots of guys submit that there is little to no loss with a good manifold system, IE. Stainless marine)

Time will tell, I suppose.

bck

Gotcha beat by a few years in the long lasting header dept. I called CMI to decipher the numbers/ letters stamped on the flange and found out they were made in 92.

MILD THUNDER

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 ?

MILD THUNDER

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.