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L81 Exhaust Manifolds Are Restrictive.

Tom Bryant

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1959 black 270hp (9/2/69) 1981 Beige L81(10/20/80)
I am modifying a pair of original L81 exhaust manifolds to use on my '69 stepside project. I removed the heat shields and the AIR tubes then started cutting away the access welding material in the ports to beguin the port matching process. I discovered that the openings on the manifold are quite a bit smaller than the stock ports and a lot smaller than my ported heads. They are also very misshaped due to a sloppy welding process. Also in the collector end of one manifold (outlet end) there was a huge glob of weld joining the 4 tubes together that affectively closed off at least 25% of one tube. All of this can be ground out and still leave a lot of weld for strength.

The picture shows the roughed out port on the manifold I have started and the unmolested stock port of the other one. The untouched one was by far the better of the two before I started. I still have to transfer the port shape from the head to the manifold and do the finish shaping but you can see that if you spend some time port matching your exhaust manifolds you can help your L81 to breathe a little better and pick up some free horsepower .
 

KANE

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Dark Blue 1982 Trans Am(s): Polo Green 1995 MN6
... They are also very misshaped due to a sloppy welding process. Also in the collector end of one manifold (outlet end) there was a huge glob of weld joining the 4 tubes together that affectively closed off at least 25% of one tube...


And people wonder why the big three do so poorly now- the reputation for those examples of "fine fit and finish" still haunt them.

The pics look great. It is amazing how just a little attention to detail can bring about more HP and better performance.
 

big2bird

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Aug 26, 2007
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178
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Anaheim,Ca.
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1981 Black Coupe
I am modifying a pair of original L81 exhaust manifolds to use on my '69 stepside project. I removed the heat shields and the AIR tubes then started cutting away the access welding material in the ports to beguin the port matching process. I discovered that the openings on the manifold are quite a bit smaller than the stock ports and a lot smaller than my ported heads. They are also very misshaped due to a sloppy welding process. Also in the collector end of one manifold (outlet end) there was a huge glob of weld joining the 4 tubes together that affectively closed off at least 25% of one tube. All of this can be ground out and still leave a lot of weld for strength.

The picture shows the roughed out port on the manifold I have started and the unmolested stock port of the other one. The untouched one was by far the better of the two before I started. I still have to transfer the port shape from the head to the manifold and do the finish shaping but you can see that if you spend some time port matching your exhaust manifolds you can help your L81 to breathe a little better and pick up some free horsepower .

I would throw them in the trash. They are restrictive, and crack easily. Ask me how I know. Put some headers on your truck, and sell them manifolds to a NCRS guy.;)
 
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Tom Bryant

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7,333
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Edgerton, Ohio, United States
Corvette
1959 black 270hp (9/2/69) 1981 Beige L81(10/20/80)
Doug Thorley has introduced a new header for these trucks that cures the clearance problems on lowered trucks and clears the front crossmember better than anything before but I need a heat riser to operate the factory type choke I'm using on my vintage '67 Chevy 300 horse Holley. There were a couple of cracks in the welds that had to be fixed but nothing in the stainless. I've never had a problem with our '81 cracking and we have had it since new.

Heres the finished product.

e7f3f5c9.jpg
 

big2bird

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178
Location
Anaheim,Ca.
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1981 Black Coupe
Doug Thorley has introduced a new header for these trucks that cures the clearance problems on lowered trucks and clears the front crossmember better than anything before but I need a heat riser to operate the factory type choke I'm using on my vintage '67 Chevy 300 horse Holley. There were a couple of cracks in the welds that had to be fixed but nothing in the stainless. I've never had a problem with our '81 cracking and we have had it since new.

Heres the finished product.

e7f3f5c9.jpg
Looks good. I would have ditched the heat riser too, but that's just me.:)
 
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Tom Bryant

Well-known member
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Nov 9, 2000
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7,333
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Edgerton, Ohio, United States
Corvette
1959 black 270hp (9/2/69) 1981 Beige L81(10/20/80)
That would have been the smart thing to do but I had the carb already (restored by Bob Kunz) and it matched the Offy manifold that I got off of Ebay for cheap a few years ago. I had to either use those parts and install a heat riser to make the Chevy choke work or spend a lot more for the headers and a new carb.

This truck is an out of pocket build with a goal of having a nice parts chaser that will run respectably for not much money. I have to keep the budget in mind.
 

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