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Discussion Starter · #1 ·
I’m thinking of getting rid of my 2” exhaust manifolds for some corvette replica 2.5” manifolds. I’m wondering though if the exhaust pipes where they connect to the manifolds would need to be changed or will they work with the bigger exhaust manifolds?

I’m also curious as to if these have the same tab on that the original 2” have? I’m taking the alternator off tomorrow to add a chrome alt case so I’ll have a look at that hopefully.

here’s the ones I’m thinking of getting-

 

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With the bigger Manifolds, you will need bigger Exhaust Pipes as well as bigger inlets on your Mufflers.
 

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GM OEM 2 1/2" exhaust (not only the 'vettes) had the 2 1/2" manifolds, 2 1/2" exhaust pipes (to the muffler), 2" tail pipes, mufflers with 2 1/2" inlets/ 2" outlets, and a 2 1/2" heat riser on the passenger side (replaceable w/a spacer).

Here's an old topic you may find helpful:


The Dorman 674-503 is a what I used. Cut off the bracket on one and used it on the passenger side. Mine looked almost as good as DZAUTOs and it was my first (only too) time.

"Corvette replica" is a sales pitch.

Pete
 

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When I did mine, I replaced the manifolds with some Covette reproduction manifolds and did a new exhaust system at the same time. The stock exhaust would be a 2-1/5 pipe from the manifold to the muffler, where it necks down to 2 inches and the pipes after the mufflers are 2 inch as well. It is pretty remarkable how much bigger the 2-1/5 manifolds are compared to the 2 inch. You might be able to replace the pipe between the manifold and muffler with a 2-1/5 pipe and use an adapter to neck it down to your 2 inch mufflers, which is pretty much what the stock system does, just inside the muffler. But I don't know if you can buy just that pipe. You would probably need to have a muffler shop bend one up for you.
30025
 

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Discussion Starter · #6 ·
Nice dcairns, I bet you got a few more horses out of the upgrade, which is my reason for wanting them.
 

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When I swapped the manifolds and exhaust on my old 350 based 383, it really made a big difference. It is also possible the rusty old mufflers were no longer flowing as they should. But I think the larger exhaust really helped, especially given the 383 cubic inches. If GM thought the 2-1/5 exhaust was good for a 327, it would be even more important for a 383.

Check with some exhaust shops and see what it would cost to get the 2-1/5 pipes between the manifold and mufflers made up for you. It might be affordable.
 

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Nice dcairns, I bet you got a few more horses out of the upgrade, which is my reason for wanting them.

Here's a system that fit my '63 perfectly:


'Show Cars' has misc parts and individual pieces (the heat riser and/or spacer, etc).

Even the big daddy 425 hp 409 had the 2 1/2" exhaust pipes and 2" tail pipes (as did the BB Chevelle, etc).

In your quest, try some ignition timing advance. Here's some info you may find helpful:


Pete
 

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Nice dcairns, I bet you got a few more horses out of the upgrade, which is my reason for wanting them.
I watched a show on TV where they dyno tested chevrolet stock cast iron manifolds and headers on a fairly stock 350 SB. The stock 2.5 cast iron rams horns came very close to the horsepower and torque numbers of 1/3/4 inch shorty headers. None of the other stock cast iron exhaust manifolds even came close. Very enlighting!
 

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This is probably more information than you were wanting... but:

I did a similar test on my 327 based 383 that I have in my 64 now. I had the engine dyno tested on headers and stock manifolds with 2-1/5 pipes. Of course, there were no stock mufflers with the 2 inch reduction on the inlet. The headers generally gave a plus 0 to 5 hp increase over the 2-1/5 inch rams horn. At at the very top end (5400rpm up to the 5600rpm cut off of the test) there was a noticeable jump of 10-20 hp, The dyno operator said the last data point (highest rpm) is usually thrown out. But they are pretty close, which is remarkable.

Youtube video of my dyno test:

1964 327 Dyno Test

Yes, you can get a lot more power out of a 383, but my goal was to have it look like a perfectly stock 300hp 327, have a idle that would allow for the power brakes and air conditioning and use the stock carb, intake manifold, heads, and exhaust. And I wanted to have near 409 performance. So a lot of compromises in that.






I repeated the stock manifold test with a 2-1/5 to 2 inch reducer at the end of the 2-1/5 pipe and that did impact the power in a noticeable way






I also repeated it with the stock air cleaner on, to see how that affected things. Not much difference but I did not try the combination of air cleaner and no reducer on the exhaust pipe, so hard to say which is limiting things.
 
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