Front exhaust manifold replacement

[Jkotsi]

I haven't seen anything posted so I'll ask here, is there a better GM produced exhaust manifold that could replace the stock 2004 GT unit?

PLog and headers can't be used. Modifying the restriction out of the stock manifold looks to be a challenge.
Was was hoping a newer model dealt with this issue.


Cheers Jim

[Explicit_Spade]

Why can't a plog or headers be used?

[Fivefingerdeathpunch]

Nope, all front manifolds are pretty much exactly the same.

Port the stocker is about all you can do if you can't mod the engine.

[Jkotsi]

The reason for keeping stock manifold is due to racing rules. Surprised there weren't improved versions of this part. Looks like I'll need to pick up a manifold to play with.

How thin do you think cast iron can be shaped and not crack?

[Jkotsi]

Not entirely related but would there be much advantage to and HV3 with no exhaust improvement?

[Explicit_Spade]

Not entirely related but would there be much advantage to and HV3 with no exhaust improvement?

I felt throttle improvement when I had an HV3 and stock exhaust, even before a tune.

Porting stock exhaust won't get you much. The cat and resonator are your big exhaust restrictions.


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[Jkotsi]

Resonator is gone as are mufflers, Cat is next. It is a 2004 so I didn't think the O2 sensor location (U bend) was an issue. I will get a tune from Overkill once all the mods are complete.

E-Vap and EGR will also be removed. Exhaust is 2-1/2" from back of Cat, a section that replaces he Cat will be added. Same diameter.

[coolone]

Resonator is gone as are mufflers, Cat is next. It is a 2004 so I didn't think the O2 sensor location (U bend) was an issue. I will get a tune from Overkill once all the mods are complete.

E-Vap and EGR will also be removed. Exhaust is 2-1/2" from back of Cat, a section that replaces he Cat will be added. Same diameter.

Wait...? You can do all that but not change exhaust?

[WarStryker13]

It can't be "aftermarket" parts that are installed, it has to be something actually made by GM.

[FordMan77]

Did GMPP or Holden ever make/have a better front exhaust manifold? Just throwing that out there. Never really looked.

[Jkotsi]

It can't be "aftermarket" parts that are installed, it has to be something actually made by GM.

Exactly, the GMPP equipped cars set the bench mark, so modding a GP to that point is a fair build.

[Jkotsi]

Wait...? You can do all that but not change exhaust?

Removing non essential equipment is OK. There is no inspection but to keep everything in line the rules have a claiming clause. At the end of a race evening anyone can claim your car for $600. This is intended to keep the builds at an even level.

Still the intent is to keep the cars close to stock.

[WarStryker13]

If you do go with porting the stock manifold, directly underneath the primaries is where it will usually crack, a few extra passes with a welder to beef up the area would be your best bet.

Once you pull it off, you'll see how bad a factory front manifold truly is.... But it's nothing a die grinder and a welder can't take care of. Lol

[Jkotsi]

How do you handle the restriction in the collector?

[WarStryker13]

I don't remember where it was that I saw it (different forum, I think), but there was one person who actually cut the manifold right after #5 cylinder primary and brought the ID to 1.78" in the collector and the small section to the crossover, then welded them back together...

IIRC he was from CA, so he cleaned up the outside with a grinder so it was all smooth and looked like a factory part.

Not perfect, but an improvement nonetheless.

[rickisrad]

It just takes a few hours. I used a Black and Decker corded drill.

[Jkotsi]

Thanks for posting the info and posting the pics. I'll have a look at some other w-body sites to see if there is anyone that posted removing the manifold restriction.

Time to hit the wreckers for some manifolds. It may take a couple before I get something that works. Increasing the ID from 1-5/8 to 2" means adding material approximately 1" wide.

[WarStryker13]

If you can find some pregens, you might be able to modify them to fit... I don't remember if they changed the flange pattern between the LN3s and the series 1/2/3 3800s... James might know better on that topic.

Where's the psychotic gearhead when you need him? Lol

[Explicit_Spade]

You can't run headers, but you can add material to stock manifolds? That doesn't make any sense.

What kind of racing is this?

[l67ss]

If you can find some pregens, you might be able to modify them to fit... I don't remember if they changed the flange pattern between the LN3s and the series 1/2/3 3800s... James might know better on that topic.

Where's the psychotic gearhead when you need him? Lol

pre series 2 flanges were quite a bit dif