2 1/2in exhaust manifold for small block Chevelle - Chevelle Tech
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post #1 of 21 (permalink) Old Aug 16th, 19, 12:02 AM Thread Starter
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2 1/2in exhaust manifold for small block Chevelle

There was never such a thing as a log style 2 1/2in ex manifold for Chevelles with a small block, correct???

Well, maybe.
In Alan Colvin's CHEVROLET by the numbers, 60-64, pg 160, there is a GM engineering drawing of a LEFT side ex manifold that supposedly has a 2.5in outlet, casting number 3840715. It is shown to be used on 64(ONLY) 300-365hp/327 engines---------------and supposedly the 365hp/327 was not installed in 64 or 65 Chevelles.

He states that it is very rare. I have never known of such a manifold, but after seeing this information in his book, I'm not ready to dispute it's existence.

No Right side is shown in his book.


Soooooooo, here is my question, has anyone seen, does anyone have, OR, does anyone know of the existence of such a manifold, left, and/or right side???

Tom Parsons
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post #2 of 21 (permalink) Old Aug 20th, 19, 12:31 AM
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Re: 2 1/2in exhaust manifold for small block Chevelle

Here is what you can do, I did this on a 409 RH manifold as I could not find one that was not cracked (I can fix them now). You can take a 2" dump and open it up to the 2.5" with some die grinding but it works. The next step is getting a gasket to seal, the ones I used were from Sanderson.
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post #3 of 21 (permalink) Old Aug 20th, 19, 7:21 AM
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Re: 2 1/2in exhaust manifold for small block Chevelle

Back in the early 70's, I bought a complete '64 327/300 hp engine at an auction. I wanted the engine for a street rod project and didn't care about the stock exhaust manifolds. After my high bid, the guy that was bidding against me came over and asked if I would sell him the exhaust manifolds. That was all he wanted. At first I said no, but he offered almost as much as I paid for the whole engine, around $400 IIRC. So he got the manifolds and I basically got a free engine. Always wondered why he would pay so much for stock exhaust manifolds.

I think he knew something about those part numbers......

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post #4 of 21 (permalink) Old Aug 20th, 19, 12:47 PM Thread Starter
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Re: 2 1/2in exhaust manifold for small block Chevelle

Quote:
Originally Posted by augy View Post
Back in the early 70's, I bought a complete '64 327/300 hp engine at an auction. I wanted the engine for a street rod project and didn't care about the stock exhaust manifolds. After my high bid, the guy that was bidding against me came over and asked if I would sell him the exhaust manifolds. That was all he wanted. At first I said no, but he offered almost as much as I paid for the whole engine, around $400 IIRC. So he got the manifolds and I basically got a free engine. Always wondered why he would pay so much for stock exhaust manifolds.

I think he knew something about those part numbers......

And I bet you do not know where those manifolds are today, right?

Tom Parsons
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post #5 of 21 (permalink) Old Aug 20th, 19, 3:02 PM
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Re: 2 1/2in exhaust manifold for small block Chevelle

Quote:
Originally Posted by DZAUTO View Post
There was never such a thing as a log style 2 1/2in ex manifold for Chevelles with a small block, correct???

Well, maybe.
In Alan Colvin's CHEVROLET by the numbers, 60-64, pg 160, there is a GM engineering drawing of a LEFT side ex manifold that supposedly has a 2.5in outlet, casting number 3840715. It is shown to be used on 64(ONLY) 300-365hp/327 engines---------------and supposedly the 365hp/327 was not installed in 64 or 65 Chevelles.

He states that it is very rare. I have never known of such a manifold, but after seeing this information in his book, I'm not ready to dispute it's existence.

No Right side is shown in his book.


Soooooooo, here is my question, has anyone seen, does anyone have, OR, does anyone know of the existence of such a manifold, left, and/or right side???
Does it have to be a "log style" manifold ? Are you sure none of the Corvette 2-1/2" manifolds will fit a Chevelle chassis ?

These look like they would fit :

67 Chevelle convertible, 138 vin, original 4 speed, bucket seats, Marina Blue with black interior
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post #6 of 21 (permalink) Old Aug 20th, 19, 6:07 PM Thread Starter
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Re: 2 1/2in exhaust manifold for small block Chevelle

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Originally Posted by nashville beth View Post
Does it have to be a "log style" manifold ? Are you sure none of the Corvette 2-1/2" manifolds will fit a Chevelle chassis ?

These look like they would fit :
Corvette-Style 2 1/2-Inch Ram Horn Exhaust Manifolds - YouTube

No, no and no.
Ram horn style manifolds-------------OF ANY KIND-----------will not work in a Chevelle. The front cross member of a Chevelle is IMMEDIATELY below where the outlet of the manifold would be-------------Ram Horns WILL NOT work in a Chevelle.

The ONLY (stock style) exhaust manifolds that will work for a small block in a Chevelle are the log style. Period.
In the history of first generation Chevy small block engines that got manifolds (Ram Horn style) with a 2 1/2in outlet were 62-65, Corvettes and full size cars.
UNFORTUNATELY, the log manifolds were never made with 2 1/2in outlets. BUUUUUUUUUUT, as I have mentioned above, in Alvin Colvin's book, Chevrolet by the numbers, 60-64, pg 160, he shows an engineering version of the log manifold for a 1964 Chevelle, with a 2 1/2in outlet-------------so far, no one seems to have seen one, have one, or is aware of such a manifold.


These are log style manifolds that will only work on a Chevelle with a small block engine. There are NOT any versions of these manifolds with a 2 1/2in outlet.
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post #7 of 21 (permalink) Old Aug 20th, 19, 7:40 PM
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Re: 2 1/2in exhaust manifold for small block Chevelle

First I thought one of the 90s truck manifolds were larger, but more important, what about Extrude Hone? Is that worth it for guys running cast manifolds? The reason I ask is that they are near me and I wanted to check them out for kicks.

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post #8 of 21 (permalink) Old Aug 20th, 19, 8:29 PM
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Re: 2 1/2in exhaust manifold for small block Chevelle

I did have some late model truck manifolds that looked like the Chevelle ones that were 2.5" two circle track guys bought them and I made $400 for all 4 of them. I only had $20 into all of them. I do know they came of a 1 ton truck, would not be original but definitely would work.
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post #9 of 21 (permalink) Old Aug 20th, 19, 11:27 PM
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Re: 2 1/2in exhaust manifold for small block Chevelle

Another option if hen's teeth are uncommon in your neck of the woods:

https://www.castheads.com/manifolds-...ust-manifolds/

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the more you realize there is that you don't know." - Dad


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post #10 of 21 (permalink) Old Aug 20th, 19, 11:47 PM Thread Starter
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Re: 2 1/2in exhaust manifold for small block Chevelle

There is another option, but I have ZERO interest in it.
In the mid 60s, a Chevelle with a hi-perf small block (ie L79), came with 2 1/2in exhaust pipe. BUUUUUUUUUUT, the end of the pipe which attached to the ex manifold had been REDUCED down to 2in to fit the manifold opening (yes, this was FACTORY installed pipe).

Thus, you still have the 2in bottle neck at the manifold outlet end. I see ZERO benefit with that!


For the hard core purists, this style of pipe (2 1/2in with 2in on the end), is available from the aftermarket vendors.

Tom Parsons
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post #11 of 21 (permalink) Old Aug 21st, 19, 2:51 AM
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Re: 2 1/2in exhaust manifold for small block Chevelle

do we have anybody with a mill here who might be will to make a few passes at a log manifold to see what they might be opened up to? Without hitting air? Would this be too tough? a guy could make a fixture for bolting the manifold to to hold it on the table?

Let me see if I can scare up a couple of these manifolds experiment on. I know a guy with a Bridgeport and an SF-600 but he's hell to get any work done.
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post #12 of 21 (permalink) Old Aug 21st, 19, 3:23 AM Thread Starter
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Re: 2 1/2in exhaust manifold for small block Chevelle

Quote:
Originally Posted by Tom Mobley View Post
do we have anybody with a mill here who might be will to make a few passes at a log manifold to see what they might be opened up to? Without hitting air? Would this be too tough? a guy could make a fixture for bolting the manifold to to hold it on the table?

Let me see if I can scare up a couple of these manifolds experiment on. I know a guy with a Bridgeport and an SF-600 but he's hell to get any work done.

One thing I might add to this suggestion, is that a 2 1/2in Ram Horn compared to a 2in Ram Horn, has larger internal passages as well as a larger outlet than a 2in manifold. Thus (theoretically) it will flow better.

Even if the outlet of a 2in log manifold is increased in size, the internal passages will remain the same size and flow MAY not be improved that much.
I have headers on the 70, ONLY because I feel the log manifolds just will not flow enough for the 400 engine.
I sure wish Chevy had made 2 1/2in log manifolds!!!!!!!!!! Hell, they sure made 2 1/2in manifolds for every other kind of engine they made------------327, 348, 409, BB.

Tom Parsons
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post #13 of 21 (permalink) Old Aug 21st, 19, 7:27 AM
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Re: 2 1/2in exhaust manifold for small block Chevelle

Quote:
Originally Posted by DZAUTO View Post
And I bet you do not know where those manifolds are today, right?
No idea. Don't even know who the guy was. Just another guy at an auction.

I'm sure you already know this, but some Ram's Horns curve rearward at the outlet. Late 60's Chevy PU, I think. Don't know if they would work on a Chevelle, and they are probably not 2.5" either.

I'm sure a lot of help, right?......
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post #14 of 21 (permalink) Old Aug 21st, 19, 10:10 AM Thread Starter
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Re: 2 1/2in exhaust manifold for small block Chevelle

The pickups and the very early V8 Chevy II manifolds were Ram Horn that angled back. Although, they were not the same and NONE of those were 2 1/2in.
The LAST year for 2 1/2in Ram Horn was 65, and ONLY on Corvettes. The last year for full size pass cars was 64.
Good used originals are almost non existant now. The Chinese replacements (I won't call them copies) are good manifolds if functionality is all that matters. They are just plain with no casting numbers or features that the originals had.
Here is a comparison of left and right originals and Chinese. They are cheap (usually less than $50) and actually, they are VERY GOOD manifolds for functionality, I've bought a few pairs and been quite pleased with them. The last pair I used was about 1yr ago on a SB400 that we built for my son's 73 Vette.
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post #15 of 21 (permalink) Old Aug 21st, 19, 10:17 AM
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Re: 2 1/2in exhaust manifold for small block Chevelle

Chevrolet was releasing over the counter 901s and 902s into the '90s. I've got a couple sets of NOS manifolds with those dates. Dates are on the side facing the block so they can't be seen.
Just more info on a set of ramhorns tht won't fit a chevelle.
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