This is great news since Im getting rid of my headers and going to manifolds looks like I`ll probably be doing 100mph next year with all the extra power I`ll pick up with more backpressure.
I gotta call all those catback companies and tell em they`ve been doing it all wrong.. more back pressure = more power, not the other way around!!!
Now Im thinking forget the big 5" manifolds, I can get the bravo 4" version for about 1/2 the cost and have more power to boot.
Maybe I`ll have my guy weld up some 2" restrictors.

Cheap way.
Smush in some Idaho Potatoes. maine potatoes work for the SBC's.

Poke a 3/4" straight hole thru the Potatoe. 1/2" for the Small Blocks.

If you can, stick a kid's handheld multicolored pin wheel in it. You'll get a slight Vortex effect with it.

It Works !

Edit in: Friday PM, doing paperwork at work, so I get real weird. No one take offense. Maybe worry for me a lil. LOL.

:bong::bong::bong::signs069:

I can't recall every witnessing a scenerio, where on a big block chevy making in the area of 1HP per cubic inch or more, where a smaller diameter header primary made more power than a larger diameter, over 5000RPM.

I really doubt the CMI split tops are larger than a 1 7/8'' ID primary tube.

We can get into all kinds of header runner lengths and diameters, that would be ideal. But whats the runner length in a GIL manifold? Stainless marine manifold? EMI manifold? Like almost nothing. Heck merc manifolds are just a common log.

My point, I wouldnt sweat it. Of course if you are installing this engine in your pickup truck to tow the boat, you may want to look into some TRI-Y headers for some low end GRUNT. :cartman:

Good read about marine exhaust

http://www.hardin-marine.com/instruc...comparison.pdf

I just measured 1 7/8 is the primary ID

Maybe you can find some info here, http://www.custommarine.com/pdf_docs/CMI-catalog-lo.pdf

Then you are all set. 2" would be better, but that's spliiting hairs.

My point is, if they where say 2 1/8", especially 2 1/4" or bigger, than it would not be splitting hairs.

Mild Thunder - you are mentioning mostly manifold system which have no tuning capabilities. They do offer low or no back pressure.

The more overlap your cam has (NA) (.050", .) the more tuned lengths and diameters help. On the flip side, the more 'wrong' lengths and dimaters can hurt.

I was witness to a 28hp gain on a SBC 450hp motor just over collector length.Why so big ? The lower hp length ended up being a 'wrong' length.

Yup, no schit.

Anyway - go run a 600hp 502 NA marine engine in a boat with Gill's or another free flowing manifold system. Then slap on the KE's. You'll see ! Serious.

God. damn. it. it's like I have esp.

It may affect your bottom end a tad but I don't believe you will see a difference in the top end. Unless you are looking to do hole shots all day I wouldn't bother changing your exhaust, you won't notice the difference while running. Bigger is better in my book not to mention a much nicer sound.