2 inch or 2 1/2 inch exhaust manifold

[Michael M.]

Is there any benefit to using a 2 1/2 exhaust manifold on a 66 L79? Obviously, the drawback is that it's incorrect for that year if being judged. Maybe a better question would be: do you lose any performance with the 2 inch manifold when compared with the 2 1/2 inch? I'm replacing a badly rusted set of manifolds, so I could use whatever I want. The car is not being judged and it has incorrect side exhaust as it is (which I really like, so I'm not switching back) and that needs replacing as well. Any thoughts on that? Was the 65 L79 a "hotter" engine with the 2 1/2 inch exhaust all the way out? I wonder why Chevy bothered switching? The pipes expand to 2 1/2 inches anyway, what was the point?

Thanks in advance for any replies. Always enjoy reading everyone's posts.

Mike

[Gene M.]

Re: 2 inch or 2 1/2 inch exhaust manifold

Mike
The side pipes are so restrictive what would be the gain? You will need 2 1/2" front sections on the side pipes. The old neck down 2" will not fit. You end up with hi horse 65 side pipe system on your 66.

Side pipes all 2 1/2" custom made with low restriction mufflers (oh but the noise) would wake things up.......

[Michael M.]

Re: 2 inch or 2 1/2 inch exhaust manifold

Thanks Gene. Actually, I'm replacing both anyway. I'm not sure whose side pipes are on the car now. They are pretty loud though. I'm not thrilled with the noise but I love the look. I am contemplating something not so loud. I was told that the original GM side pipes were fairly quiet when compared to some reproductions on the market now. Maybe If I use something quieter it'll have a bit more restriction and maybe the 2 1/2 inch manifolds will help it some. Not exactly sure which way I'm going yet. Just gathering some info while I figure it out. Are side pipes actually more restrictive than the exhaust out the rear? Don't the side pipes expand to 2.75 inches through the baffle area? Maybe that's not enough in such a short run. Through the back it's 2.5 inches all the way through, except for the 2 inch section on the 66-67.

[Gene M.]

Re: 2 inch or 2 1/2 inch exhaust manifold

The under car 2 1/2" system is less restriction compared to the side exhaust. One would think not but that is the case. Be advised anything bigger than the 2 1/2" will be louder.....

Years ago I had custom stainless side exhaust on my 68 with cherry bomb mufflers (red bullets) less restriction but loud as hell. They didn't stay on long.

I suggest after market stainless 2 1/2" '65 system with the 2 1/2" manifolds as the stainless does seem to be less noise.

[William C.]

Re: 2 inch or 2 1/2 inch exhaust manifold

Back in the day, GM engineering guys used to liken Sidepipes as like "Putting a cork in the exhaust" from a performance perspective.

[Michael M.]

Re: 2 inch or 2 1/2 inch exhaust manifold

I did contemplate restoring it back to the exhaust out the back. The hanger on the trans mount is still there and I have an original rear lower panel and NOS bezels, but the thought of dealing with the cut off rocker panel brackets and the restoring the tabs on the front fenders (they left the rear ones alone) kinda snapped me out of that. I'd rather drive it than do all that work just to hang the rocker panel. I knew what it was when I bought it. I love the look.

[John H.]

Re: 2 inch or 2 1/2 inch exhaust manifold

Don't the side pipes expand to 2.75 inches through the baffle area?

Mike -

Nope. The straight section core pipe diameter on GM sidepipes is slightly less than 1-7/8"; sidepipes were promoted by Styling and Sales, not by Engineering.

[Michael F.]

Re: 2 inch or 2 1/2 inch exhaust manifold

IMHO The only sidepipes worth a performance damn. Are the 4 into 1 Hooker style headers with a large empty tube exit. Back in the 70s I loved those on a red 65-71 convertible in flat black.With wide aluminum slot wheels.

If you are going to use the 2 1/2 inch manifolds? Run the N-11 off road system. CCentral has nice units. It's 2 1/2 inch to the muffler inlet. Great sound and perfect performance. Not for your yr. But none the less factory designed.

[Timothy B.]

Re: 2 inch or 2 1/2 inch exhaust manifold

Michael,

IMO, and you get lots of them here, if you have a original motor car why not continue putting it back the way it left the factory. Do you really think you are going to notice a difference driving around on the street?

The money spent is dollars toward the correct parts for the car.

[Michael M.]

Re: 2 inch or 2 1/2 inch exhaust manifold

Mike -

Nope. The straight section core pipe diameter on GM sidepipes is slightly less than 1-7/8"; sidepipes were promoted by Styling and Sales, not by Engineering.

Ah, I see. I guess the 2.75 diameter refers to the outside diameter of the pipe, including the baffles. Too bad, now I'm leaning toward restoring it to original configuration along with the extra work involved. I guess I could drive it around without the rocker trim while i work on it. Although, she goes real well as it is, 2 inch manifolds and tight side pipes not withstanding. The L79 is great little engine. I'm very happy with it.

[Michael M.]

Re: 2 inch or 2 1/2 inch exhaust manifold

Michael,

IMO, and you get lots of them here, if you have a original motor car why not continue putting it back the way it left the factory. Do you really think you are going to notice a difference driving around on the street?

The money spent is dollars toward the correct parts for the car.

I agree about restoring the car toward the way it left the factory. I'm restoring the front suspension now with NOS springs, rebuilt parts instead of replacement parts, the correct finishes (pretty much), etc... I just wasn't looking forward to the problem with the rocker panel brackets. They've been cut short by an inch or more and the lower front fender was cut as well. I'm not afraid of the work, I rather enjoy it. A few more parts and a little more frame prep and I'll be putting her back together. It's more about the time. The weather is great and I want to go driving. Like I said, I guess I can go driving without rocker trim as I work. I did go out and buy the rear lower panel and the bezels with an eye toward the correct configuration.

My original post was a question about the performance of 2 vs 2.5 inch manifolds and exhaust. Hey, I live in New York but we do have highways here and my driving is not only on the street. I don't drive on racetracks, but there are some great roads north and east of where I live, so she gets to stretch her legs quite a bit.

[Joe L.]

Re: 2 inch or 2 1/2 inch exhaust manifold

I agree about restoring the car toward the way it left the factory. I'm restoring the front suspension now with NOS springs, rebuilt parts instead of replacement parts, the correct finishes (pretty much), etc... I just wasn't looking forward to the problem with the rocker panel brackets. They've been cut short by an inch or more and the lower front fender was cut as well. I'm not afraid of the work, I rather enjoy it. A few more parts and a little more frame prep and I'll be putting her back together. It's more about the time. The weather is great and I want to go driving. Like I said, I guess I can go driving without rocker trim as I work. I did go out and buy the rear lower panel and the bezels with an eye toward the correct configuration.

My original post was a question about the performance of 2 vs 2.5 inch manifolds and exhaust. Hey, I live in New York but we do have highways here and my driving is not only on the street. I don't drive on racetracks, but there are some great roads north and east of where I live, so she gets to stretch her legs quite a bit.

Michael-----


You won't perceive any performance difference with the 2-1/2" manifolds. You could change to them if you wish but you'd be exchanging originality for no benefit whatsoever. So, why do it?

[Duke W.]

Re: 2 inch or 2 1/2 inch exhaust manifold

The under car 2 1/2" system is less restriction compared to the side exhaust. One would think not but that is the case. Be advised anything bigger than the 2 1/2" will be louder.....

Years ago I had custom stainless side exhaust on my 68 with cherry bomb mufflers (red bullets) less restriction but loud as hell. They didn't stay on long.

I suggest after market stainless 2 1/2" '65 system with the 2 1/2" manifolds as the stainless does seem to be less noise.

Agree. Given the OE sidepipes' restriction, the lower restriction of the 2.5" inch manifolds is probably meaningless.

Though I've never been able to quantify the improvment, the transistion from 2" to 2.5" in the head pipes may be the biggest offender on post-65 engines that only had 2" manifolds.

If you bolt up a 2.5" head pipe to a 2.5" manifold and compare it to the later 2" configuration, any rocket scientist can tell that when dealing with high subsonic flow velocities the early 2.5" manifold/2.5" pipe joint is clearly superior to the post-'65 configuration.

Duke

[Michael M.]

Re: 2 inch or 2 1/2 inch exhaust manifold

Thanks everybody. So now I have some good information and I just have to make a decision. Not so easy. But thanks for the info and opinions. It's great to have this forum. I have much better info than this morning and I've changed my mind about what to do about 3 times today alone! I can't imagine why they went with 2 inch manifolds just to increase the diameter of the pipe back to 2.5 . I can understand the sidepipes, they may be restrictive but they look sweet. Why go to the trouble of redesigning a cast iron part? They had what they needed on hand already. Even switching the alternator to the left side, they already had the right parts.

[Michael M.]

Re: 2 inch or 2 1/2 inch exhaust manifold

Michael-----


You won't perceive any performance difference with the 2-1/2" manifolds. You could change to them if you wish but you'd be exchanging originality for no benefit whatsoever. So, why do it?

You're right Joe and so is Tim, why bother? Although Duke makes a good argument that the 65 version has to be superior, I probably won't notice the difference.