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650HP NA exhaust question.

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650HP NA exhaust question.

Old 10-25-2019, 01:19 PM
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Default 650HP NA exhaust question.

Iíve found quite a bit of conflicting information on this topic so Iím hoping to get some more information.I have an HP 500 Iím wanting to turn into a 509 or 540 NA 650ish horse setup. It currently has the GIL manifolds with 1 piece downturn risers and that come out well below the waterline. Boat has never been in salt water and never will be. Iím not wanting to deal with reversion so Iím planning on ditching the GILís. My question is more in regards to headers vs manifolds what is the best bang for your buck vs power gains you will actually notice. Iíve read tons of stuff on here about how CMIís will outflow any manifold via exhaust scavenging and keeping all the exhaust pulses headed the right way.. Iíve also read quite a bit on how they leak and by the time I have tailpipes made they will be quiet a bit more expensive then manifolds.On the manifold side Iíve read plenty of threads that say a good(IMCO Powerflow, Dana, Kieth Eichert, etc) will easily handle 650hp and are much less prone to leaking.In summary I have no desire to build a nice engine and choke it down. However if I can get a more reliable product that will handle the load and be cheaper, thatís a no brainer to me. So how much more power does CMI flow vs good manifolds? If itís a 654 hp vs 657 hp then Iím going the manifold route, if Itís more 650 vs 620 then Iíll be in line for a set of CMIís and pipes.
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Old 10-25-2019, 03:09 PM
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No personal experience but the Keith Ekhart headers seem to be the hot ticket followed by Stainless Marine Gen3 manifolds.
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Old 10-25-2019, 08:30 PM
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I ran Lightning headers on my 650 HP 509's
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Old 10-25-2019, 08:41 PM
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Its really not a matter of how much HP a particular exhaust setup can "support". You can make 800hp thru a set of gils with enough boost in a forced induction build.

Theres a lot of power in exhauatexhaust tuning. Back in the early days, the best power modification on a muscle car, was some longtube headers and a free breathing exhaust. Uncorking 75hp wasnt uncommon switching from a factory exhaust manifold/muffler setup.

When dealing with the intake side of things, we love talking about single plane vs dual plane, intake runner sizes , and so on. We often forget , that a well designed exhaust, actually helps draw air in the cylinder, not just get it out. Your typical big block in the muscle car world, making 650hp, would have nothing less than a set of 2" id full length headers, ideally larger, with 3.5-4" collectors, and 34+ inch long runners. A gil, imco, emi, stainless marine, cmi elbow top, etc is nowhere near those runner lengths. That leaves you with the CMI sport tubes, or diamond performance (eickert) style manifolds.

Also, the exhaust requirement for a forced induction engine, isnt as critical compared to a NA engine. Forced induction engines have plenty of help filling the cylinders, NA not so much.
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Old 10-26-2019, 05:33 AM
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Mild Thunder and I went through researching when I did my 540 builds, seemed aluminum manifolds robbed 30-38 HP over headers or a header type manifold, if I wanted top HP, use headers or a Gen3 stainless marine..
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Old 10-26-2019, 09:58 AM
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Originally Posted by Full Force View Post
Mild Thunder and I went through researching when I did my 540 builds, seemed aluminum manifolds robbed 30-38 HP over headers or a header type manifold, if I wanted top HP, use headers or a Gen3 stainless marine..
I doubt the SM is better than the cast aluminum TnL manifolds on an NA app.
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Old 10-26-2019, 10:23 AM
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Originally Posted by hogie roll View Post


I doubt the SM is better than the cast aluminum TnL manifolds on an NA app.
I agree. Ssm gen 3 are great on forced induction due to their sheer size. NA, they just dont have the runner length. Regular stainless marine manifolds, probably give up 40hp or better on a 650hp NA build.

Last edited by MILD THUNDER; 10-26-2019 at 10:27 AM.
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Old 10-26-2019, 06:00 PM
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Boat came with a Innovation Marine engine with the KE manifolds 668HP 540 got 5 seasons before replaced engine. Not sure how much prior time was on them but was all in salt water. Replaced them with another pair from Diamond Performance when engine was replaced with another 540. No complaints.
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Old 10-27-2019, 07:01 AM
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Originally Posted by hogie roll View Post


I doubt the SM is better than the cast aluminum TnL manifolds on an NA app.
just mentioned what we had discussed with my builds that’s all, there are other options always, I am not sure the hassle with huge manifolds are worth the gains, a buddy has them and with the sudden loss of impeller 2 burned up in seconds of no water, at collector..and says they are hard to get around when wrenching..

Last edited by Full Force; 10-27-2019 at 07:03 AM.
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Old 10-29-2019, 06:51 AM
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You can make all kinds of hp NA but the minute you bolt ANY mediocre exhaust manifold on your giving up SERIOUS hp. Im only part way thru my dyno research on exhaust but I will give you a couple examples.
Strong 509, made
695 ish hp on my dyno, dry, open race car headers, no mufflers
with mufflers: 680 ish
with EMIs WITH dry to tip tails 655 (and vs imcos, the EMIS make 10-15 more hp in test Ive done)
509-modded mpi intake, ching chong heads-
610 hp, dry , open headers
595 w mufflers
555 to 560 With EMIS (customer ran 81 at Loto shootout with his 280 PQ with this motor )
Stock 500 EFI-made 510 ish with imcos w short tips, made 518 (and 10 more ft lbs tq/hp peaked 100-200 rpms higher) with EMIS
Point of this is you can build a 650/700 hp NA deal and when you cork it up with ANYTHING less than 1 7/8 ID headers or Thunder and lightning manifolds, you can kiss 30/40 hp good bye and peak will happen earlier. I havent been able to build a NA 502 that made the insane torque claimed by some east coast and Florida shops (seen claims of 660/680+ ft lbs of tq from various 502s) BUT if what they are claiming is true, focusing on a motor that will make crazy torque in the sub 5500 range NA might be the best way to go . Not many marine exhaust that will let you peak in the 5800/6200 range Soooo will camming a motor to peak there do much?

Last edited by articfriends; 10-30-2019 at 06:35 AM.
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