Exhaust Manifold Bench Racing

[Patrick H.]

Hello All,

Last evening while working on the motor for the "High Tech 327" 1960 fuelie as well as another 350 for a 72 LT-1, a few of us got to thinking:

Is it possible to have a "full" 2 1/2 exhaust sytem on a car such as my 71 350 or a 72 LT-1 350, and can it be built with new GM parts? Seeing the 2 1/2 LH 1965-style exhaust manifold got us thinking that it could be easily installed on a "shark" car, and given the chassis similarity the 2 1/2 exhaust pipes from, say, a 65 fuelie could be used from the manifold to the transmission. This would then eliminate the restriction of the manifold and the proximal exhaust. The complicated side MIGHT be the right hand side, where we do not know, off hand, of 2 1/2 inch exhaust manifolds available from GM without any bracket on the front that a 71 or 72 wouldn't need. Reproductions without a generator bracket or similar are likely available, but $$$.

The other thought was, is this worth it? On a car that needs new exhaust, the manifolds from GM at trade price might be $200 for the pair, and the front pipes could be part of the exhaust pipe kit. For the extra $200, would the performance gain be worth it?

Any thoughts from those who may have tried this?

Thanks for the input,

Patrick Hulst

[Michael R.]

Re: Exhaust Manifold Bench Racing

Actually, the sharks did come with 2.5" exhaust..at least the hi perf small blocks, and big blocks. You can get the 2.5" rams horn manifolds thru the GMPP catalog. A 68-71 C10 truck cyl head alternator bracket will allow you to use manifolds that don't need brackets for mouting the alternator directly onto the head. Since it doesn't sound like keeping original is a concern, headers and side pipes are always a good combo. STW, Mike

[Jon W.]

Re: Exhaust Manifold Bench Racing

Patrick, I had a sweet 383-incher built last year to switch places with the original L-46 in my 1970 A/C coupe. Didn't want headers. Repro 2.5" manifolds were used...Right side = Paragon PR-1239 with the choke tube hole closed off. A/C compressor mount bolted up nicely. Left side was Paragon PR-1287 with alternator mounting to this manifold. A complete 2.5" exhaust system for mid-year small-block cars was used with minor tweaking. Open headers would undoubtedly yield more horsepower, but I think this set-up makes for a great high-performance street system.

[Patrick H.]

Re: Exhaust Manifold Bench Racing

Mike,

The 68-72 small block cars ALL came with 2" manifolds, and the exhaust is actually 2" for the first several inches after leaving the manifold until it widens up to 2 1/2 inches. This is the system used on LT-1's, but I am looking for 2 1/2 inches all the way. They did not come from the factory with "full" systems like this, so thus my inquiry in to maifolds and head pipes that will fit.

Headers and side pipes sound good, but most people will never be able to tell my 2 1/2 inch swap if I use GM pieces.

Patrick Hulst

[Michael R.]

Re: Exhaust Manifold Bench Racing

I know this, hence my suggestion on the 2.5" GMPP manifolds. As far as the pipes, either a custom bend or an earlier pipe, but I haven't actually looked at the difference in a mid-year vs shark pipe. STW, Mike

[Joe L.]

Re: Exhaust Manifold Bench Racing

Patrick-----

There is currently NO 2.5" outlet exhaust manifold available from GM which does not have the pre-66 alternator brackets. So, here are your options as I see them:

1) For the 1972, you could use reproduction manifold GM CASTING #3797942 for both sides. Use 63-65 Corvette with SHP 2.5" front exhaust pipes and 68-72 2.5" rear pipes and mufflers. For the 71, use the above manifold for the right side and use reproduction manifold GM casting #3846563 for the left side. This manifold is a 2.5" outlet with the alternator bracket required for a 68-71 application. Use exhaust pipes mufflers as above.

2) Use manifold GM PART #3814970 (casting #3797902) for the LEFT side of your engines and GM PART #3797901 for the RIGHT side. This will position the brackets at the rear of the engine where, other than their funky appearance, they should cause no problem. If there is any sort of interference (which I doubt), you could trim off enough of the brackets with a bandsaw to resolve it. DON'T trim off the brackets down to the manifold contour, though, or you'll weaken them to the point of imminent failure. For the 71, you'll also need to add alternator bracket GM #14015539 (iron) or GM #14015510 (aluminum) to the left side forward cylinder head. These manifolds currently GM list for $119.00 each, but you should be able to buy them for $100 or less, each. Use exhaust pipes/mufflers as described above.

Option #1 will offer a cleaner installation, but at a greater cost. Option #2 will be just as functional, considerably less expensive, but not as clean looking.

If I were building a small block with no concern or need for AIR fitings, option #1 is POSITIVELY the way that I would go. I would NEVER, EVER use headers on a street car and this option would provide a clean installation and the ABSOLUTE BEST exhaust flow characteristics that you could achieve with cast iron exhaust manifolds.

[Michael R.]

Re: Exhaust Manifold Bench Racing

You can use a small block truck alternator bracket on the head, and pick up one for about $15 at an truck parts supply store..lot cheaper than a Vette piece, since this a street app anyway.. NEVER, EVER use headers on a street vehicle...I have to ask..why? STW, Mike

[Joe L.]

Re: Exhaust Manifold Bench Racing

Mike-----

You can buy a brand new aluminum Corvette alternator bracket that I mentioned for about $24 and a brand new cast iron Corvette version for about $19.

The primary problem with headers is the generation of excess heat into the passenger compartment. I know of many folks that have removed headers from their Corvettes for this reason alone. Secondary problems include excessive heat transferred to the steering box and starter, fitment difficulties, burned wiring, exhaust leaks, and the need to pay constant attention to keeping the header-to-cylinder head bolts tight.

In my opinion, headers are just not the thing for street operation, irrespective of the fact that they offer significant performance gains. GM has NEVER installed headers on a PRODUCTION vehicle. They've supplied them over-the-counter or as a customer or dealer installed option, but they've never installed them on a production car. And, that includes L-88s, ZL-1s, LS-6s, ZR-1s, ZR-2s, or even LT-5 DOHC engines.

My opinion on headers has never wavered. When I was an 18 year old guy with a brand new Chevelle SS-396 I felt the same way about headers as I do today. So, whether I'm right or wrong, I'm consistent.

[Michael R.]

Re: Exhaust Manifold Bench Racing

Consistency counts, especially in this day and age. Personally, I prefer them, and am willing to put up with the minor gotchas(which usually only take one go round to rectify) for a better scavenging and freer flowing exhaust. I'm not sure about the General, but other late model performance oriented models, ie 5.0 Mustang(yeah, I know, throw the guy out) certainly did. I was under the impression the that Grand National and 1LE Camaro's did as well. As far as older GM muscle not having them, I'll betcha cost was more of a factor than anything else...why produce one special part for an already small production run when you share them across multiple brands... STW, Mike

[Jim T.]

Re: Exhaust Manifold Bench Racing

Patrick if you are going to follow through with the installation of the complete 2 1/2 exhaust or just add the 2 1/2 inch pipes to the original cast iron exhaust manifolds there is one more piece you should use. The exhaust pipe bracket at the rear of the transmission for the 2 1/2 inch exhaust pipes is different in that it is made for the 2 1/2 inch pipes. I made the conversion to 2 1/2 inch pipes years ago when I took the 69 factory style side exhaust off of my 70 and changed out this bracket for the installation.

[Dale Pearman]

Re: Exhaust Manifold Bench Racing

All this discussion about manifolds, 2 1/2 inch exhaust and headers is very interesting BUT nobody has said a word about exhaust gas scavenging or placing the pole, (in Laplacian space) at the lowest possible point in the power band. The idea is to achieve a FLAT, WIDE torque curve for the street, not to peak the torque, (at a much higher absolute value) but at a very narrow RPM. These kinds of engines are NASCAR long track gems and an absolute pain in the ass on the street! I've always understood that the intake side defines the upper rpm pole and the exhaust side the lower rpm pole. The torque curve is the superimposition of both. BUT, what do I know?

I know this: GM engine development engineers did thousands of dyno runs to optimize ALL the components of their mass produced cars for the street. The incredible performance of the Corvette is the result of using THE RESULTANT COMBINATION OF GM OPTIMIZED COMPONENTS IN COMBINATION. Any High School Harry can open Summit's catalog and "improve" the work of hundreds of graduate engineers and thousands of dyno runs by buying "Super-flo" whatevers, and "Stealth" mufflers with "magnum" cams and "street fighter" this and that. It's a blast to watch the performance of their engines deteriorate with each new bolt on upgrade. I suppose the GM engineers worked in a vacuum without knowing anything about these aftermarket goodies.

So, I've been there (with chromed oil pans, etc.) done that, (spent two tons of money) and decided that before I improve on the work of all these engineers, I'm gonna really think hard about what I'm doing.

The manifold design and exhaust system on your Corvettes was tweaked by GM to provide the best possible synergy in the overall congeneric of your engines. I spent a lot of bucks having Brezeninski hog out a pair of GM 901 and 902 manifolds to empty into my complete-through 2 1/2 inch exhaust system. The result was LESS bottom end torque! The torquiest 327 I EVER put my foot into was an iron intake with an AFB, stock 901/902 manifolds, Rhodes lifters, and a 151 cam advanced 4 degrees!

If I were you I'd save my money and leave things alone.

Varooom!


CLICK HERE

[Bill Becker]

72 LT1 with mid year side exhaust

FWIW, back in the Winter of 75 I was living in Omaha, and had just purchased a used 72 LT1. I wanted to improve the performance of the exhaust and loved the looks of the 69 side exhaust, but liked the sound and idea of chambered exhaust. While working at an automotive warehouse at the time I ordered a new set of 2 1/2" side pipes for a midyear through a local GM dealer. Then went to a local corvette shop who had a lot of salvage parts and purchased a set of what I think were 65 2 1/2" manifolds. Not so sure I didn't use the alternater bracket on the manifold. Worked great except I seem to remember blowing donut gasket quite frequently.

[Dave W]

I'll 2nd Joe L

I've had plenty of street vehicles with headers installed (alumakote, stainless steel, safety wired bolts, reinforced flanges, locking washers, super gaskets from planet Krypton etc). I could never keep them from rusting or warping, loosening and leaking etc...In every case I either sold the car first, or ripped them out and put in good old cast iron manifolds. The HP gains were not worth the hassles and could be made up elsewhere. Joes right, headers are thinwall compared to iron manifolds and generate a tremendous amount of heat into an already tight engine compartment (midyears) not to mention being complicated by the extra BTUs a big block would be throwing off. Add fiberglass firewalls/floorpans transfering the heat into the inside of the car and you develop a whole new respect for a well designed cooling system. as a side; cast iron manifolds can be extrude honed to improve their flow characteristics (not to the extent of headers), while retaining stock appearance.

No matter how much I rigged them what I did and the gains he significant HP gains achieved with headers, but the hassles involved in maintaining them never warranted using them (..not when those same gains can be made up in airflow, at same

[Michael R.]

Re: I'll 2nd Joe L

The one thing I keep hearing is "significant HP gains achieved" and how you guys are willing to forego that. If headers are going on a car that sees weekend only or trip driving, why would you be willing to forego this? If the car's a daily driver, than that places a different emphasis on the driving style, and perhaps not having some of the header hassles is more important. I drive my '69 about 5k miles a year, and love the power and sound associated with the headers. They've been on for about 7 years, are HPC coated which has also held up very well, better than I remember the ram's horn manifolds which I had on the car in the early days..of course there weren't the coatings options for those at the time either, at least as far as I remember. I realize of course that since this is an NCRS discussion board this is probably off topic since I don't think any NCRS cars would get judged with them! IMHO, Mike

[Michael R.]

Re: Exhaust Manifold Bench Racing

Let's see..last I checked, Holley, Edelbrock, Comp Cams and the like had their own R&D engineering staff as well as dynos, flow benches, etc..But you're correct, anyone who buys one performance piece like headers without reviewing the ancillary effects on the engine such as changing cam timing, air and fuel flow changes, lower back pressure etc is asking for trouble. This is the same person who'll slap a Holley 850 on a small block and wonder why the car won't run. Precisely why the vendors felt the need to offer integrated, dyno-tested packages to avoid this kind of component mismatch. Let me ask you a question: Do you think if GM said to the R&D staff, OK guys, here's a blank checkbook, design any components you need and don't worry about cost, the cast iron manifolds would have been the result? I tend to doubt it. As much the engineers wanted to boost performance, they had the financial constraints, as well as regulatory concerns such as emissions standards to contend with, as well as design issues such as the Vette's low hood profile. Can you see an LS6 with a true high rise intake? Wow! My hat goes off to them for producing the power that they did. Of course, all these points are in reference to sixties/early seventies iron...Newer gen motors with fuel injection, computer controls etc are an entirely different animal. Whew, I need to switch to decaf.. STW, Mike