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OEM exhaust manifold porting
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Old 08-06-09, 02:32 PM   #1
stinkfinger
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Default OEM exhaust manifold porting

I've noticed that the commercially available PnP manifolds (as well as the DIY threads) all seem to gasket match the cylinder head exhaust port inlet. However, when I place the manifold gasket against my cylinder head, it's obvious that the exhaust ports of the head itself are much smaller than the gasket holes to which the manifold is ported.

It would seem that the resulting large step transition from smaller exhaust port to larger manifold inlet would create turbulence thereby negatively affecting flow.

Are the PnP manifolds being sold designed to be used with cylinder heads that have had their exhaust ports gasket match ported?

Assuming an unmolested stock cylinder head, wouldn't it be better to not port the inlet from the cylinder head, or do only whatever subtle porting might be required to match the exhaust port size (rather than the gasket size)?
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2006 WRB, RallyArmors, RPF1s, AEM CAI, Perrin inlet, Dom 1.5, ported: [IC Y-pipe, throttle body, exh manifolds], GR TMIC, MAW TGVs, Aeromotive FPR & rails, ID1000s, xpipe, uppipe, 3-port, Tial MVS EWG, APS 3" catless, Walbro, 1-step colder NGKs, EBCS. RCE blacks, WL 24mm RSB, COM-Cs, ALK, free caster mod, cowl stays, CL X-Brace, Group-N susp bushes, WL rear subframe lock bolts, R diff & outrigger bushes, TiC shift & trans x-member bushes, SS brake lines. 19psi 352whp@6500/330wtq@3700-5500

Last edited by stinkfinger; 08-06-09 at 05:56 PM.
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Old 08-09-09, 12:14 PM   #2
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Default Re: OEM exhaust manifold porting

It seems nobody cares about this, but for what it's worth, I made a new gasket exactly the same size as the cylinder head exhaust ports. This I intended to use as a template for whatever match porting of the exhaust manifolds might be necessary. As it turned out, the exhaust manifold already matches the cylinder head exactly.

So, it's still a mystery as to why people hog the manifolds out to match an (arbitrary) gasket diameter...
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2006 WRB, RallyArmors, RPF1s, AEM CAI, Perrin inlet, Dom 1.5, ported: [IC Y-pipe, throttle body, exh manifolds], GR TMIC, MAW TGVs, Aeromotive FPR & rails, ID1000s, xpipe, uppipe, 3-port, Tial MVS EWG, APS 3" catless, Walbro, 1-step colder NGKs, EBCS. RCE blacks, WL 24mm RSB, COM-Cs, ALK, free caster mod, cowl stays, CL X-Brace, Group-N susp bushes, WL rear subframe lock bolts, R diff & outrigger bushes, TiC shift & trans x-member bushes, SS brake lines. 19psi 352whp@6500/330wtq@3700-5500
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Old 08-09-09, 01:52 PM   #3
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Default Re: OEM exhaust manifold porting

Well I know when I took my headers off there was at least a 1/8inch section where there was carbon build up(aka step blocking exhaust gasses). I proceeded to match the gasket diameter so that there wouldn't be such a big step for the gases to escape around creating turbulence. Not such if it did anything but once the headers were off I figured what the heck might as well. I belive I have pics in my journal. Very matching job but it for sure helped smooth things out if anything.
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Old 08-10-09, 03:29 PM   #4
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Default Re: OEM exhaust manifold porting

I noticed the ring of carbon build-up around my manifold inlets as well. In my case anyway, I'm certain that this is not the result of a step into a smaller diameter manifold hole, but rather an exposed surface due to a gap formed by the over-diameter gasket sandwiched between the head and manifold. In fact, I have the very same carbon ring around my cylinder head exhaust ports.
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Old 10-20-10, 10:01 AM   #5
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Default Re: OEM exhaust manifold porting

From PM:

Quote:
Originally Posted by Frank_Zuccarini
Hi.

Can you tell me, please, do you still feel that the head outlet and manifold inlet openings are the same dimension, negating any benefit from porting?

Are you happy that you bought the Grimmspeed crossover, and do you think that it helped spool-up?

Did you reinstall heat shields, heat wrap or ceramic coat the manifold? Result?

How has the crossover held up? Corrosion? Erosion?

Do you have enough data to discern if your efforts resulted in any more HP/Torque, or faster spool up?

Thank-you for your time....................Frank
Hi Frank,

Sticking with my conclusion, I ended up lightly porting the manifold inlets, without appreciably increasing the inlet diameter. Rather I focused on smoothing and blending the inner radii. As for the manifold outlets, I enlarged these quite a bit, matching the diameter of the Grimmspeed x-pipe.

I'm mixed on the Grimmspeed x-pipe itself. It has held up well so far, though my 18K mile 2006 doesn't get driven much, and I live in a dry, unsalted climate so I wouldn't expect rust to be an issue. The issue that I have with it, is the sewing machine noise that it added (search to read other complaints). The noise is almost certainly caused by the flex section, but I wouldn't want an x-pipe without a flex (due to thermal expansion). Since I didn't install the pipe in isolation, I can't conclude what difference in power it made.

I reinstalled the heat shields on the manifolds, and wrapped the x-pipe and Grimmspeed EWG up-pipe (which were also coated, btw).

Since, as I mentioned, I made several changes so I can't endorse any one part. FWIW, here's a before/after dyno plot and the changes below (+21whp, +16wtq)...

- APS header
+ Ported OEM manifolds
+ Grimmspeed x-pipe
+ Grimmspeed up-pipe
+ Tial MVS 38mm EWG
+ Grimmspeed 3 port solenoid
+ AEM CAI


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2006 WRB, RallyArmors, RPF1s, AEM CAI, Perrin inlet, Dom 1.5, ported: [IC Y-pipe, throttle body, exh manifolds], GR TMIC, MAW TGVs, Aeromotive FPR & rails, ID1000s, xpipe, uppipe, 3-port, Tial MVS EWG, APS 3" catless, Walbro, 1-step colder NGKs, EBCS. RCE blacks, WL 24mm RSB, COM-Cs, ALK, free caster mod, cowl stays, CL X-Brace, Group-N susp bushes, WL rear subframe lock bolts, R diff & outrigger bushes, TiC shift & trans x-member bushes, SS brake lines. 19psi 352whp@6500/330wtq@3700-5500
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Old 10-21-10, 05:55 PM   #6
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Default Re: OEM exhaust manifold porting

Thank-you so much for taking the time to follow up with these details.

Am I correct in understanding that, after you made these changes, the car was re-tuned to take advantage of them? You did not see this bump in HP and Torque without a new tune, did you?

Of high interest to me is that after your changes, it appears that you achieved the pre-changes level of torque at about 300 rpm sooner. That has to be noticable, I'd imagine. And pre-change HP level 5-600 rpm sooner.

Again, thank-you for sharing.

Frank
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Old 10-21-10, 08:56 PM   #7
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Default Re: OEM exhaust manifold porting

Yes, the plots that I posted were tuned for the respective mods. I drove untuned for a few days after replacing the APS header with the PnP manifolds/x-pipe/uppipe, and seat of the pants said that spool was improved. I'd hazard a guess that the APS header tubing diameter was a bit oversized for my stage 2 car, negatively affecting spool to begin with. So, I wouldn't interpret my results to mean that a PnP manifold setup would necessarily improve spool over unmolested stock parts.

Comparing the dyno plots, there are a couple of other variables worth mentioning. One is that while the tunes were done on the same dyno, the individual performing the tunes was different (Jarrad from PDXTuning, then Tim Bailey from Surgeline). The other thing is that the new tune keeps building power up to 6500 rpm, which is (according to Tim) largely due to the EWG. That the car gained on the top end over equal-length headers is really what surprised me the most.
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2006 WRB, RallyArmors, RPF1s, AEM CAI, Perrin inlet, Dom 1.5, ported: [IC Y-pipe, throttle body, exh manifolds], GR TMIC, MAW TGVs, Aeromotive FPR & rails, ID1000s, xpipe, uppipe, 3-port, Tial MVS EWG, APS 3" catless, Walbro, 1-step colder NGKs, EBCS. RCE blacks, WL 24mm RSB, COM-Cs, ALK, free caster mod, cowl stays, CL X-Brace, Group-N susp bushes, WL rear subframe lock bolts, R diff & outrigger bushes, TiC shift & trans x-member bushes, SS brake lines. 19psi 352whp@6500/330wtq@3700-5500
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Old 10-21-10, 09:48 PM   #8
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Default Re: OEM exhaust manifold porting

Quote:
Originally Posted by stinkfinger View Post
It seems nobody cares about this, but for what it's worth, I made a new gasket exactly the same size as the cylinder head exhaust ports. This I intended to use as a template for whatever match porting of the exhaust manifolds might be necessary. As it turned out, the exhaust manifold already matches the cylinder head exactly.

So, it's still a mystery as to why people hog the manifolds out to match an (arbitrary) gasket diameter...
I haven't made the modifications to the car myself so it's all theory but what you're saying makes a lot of sense. Either you make the head ports, and the manifold holes larger or you just leave them both alone. I think it would be pretty much useless to match the manifold to the gasket if the heads don't match also. I say either match both or leave them both alone. Making the gasket match the holes isn't bad either but I doubt you gain anything from it. The flex section has some nice wrinkles in it that probably affect the flow more than the thickness of a gasket where it's slightly larger.
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Old 10-22-10, 03:32 AM   #9
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Default Re: OEM exhaust manifold porting

Thanks for the additional detail, Mr. Stinkfinger, sir.

I bet that it's a really fun car to drive.

Frank
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Old 01-06-11, 01:25 PM   #10
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Default Re: OEM exhaust manifold porting

Quote:
Originally Posted by stinkfinger View Post
As for the manifold outlets, I enlarged these quite a bit, matching the diameter of the Grimmspeed x-pipe.
How did the x-pipe "outlet" diameter compare to the inlet diameter of the manifold section (the one with 3 inlets and connected to the up-pipe). Was there an appreciable step-down? If so, this would presumably cause more back pressure than the step-up from the other manifold into the x-pipe.

i hope that made sense...
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