Joe, I have been looking for a right side 2.5" exhaust manifold on ebay for the last few days for my 67 SB car. From my earlier post you recall that I am looking for the part without any bracket mounting points, which you indicated was only the part with casting number 3797942 on it. I am seeing a few manifolds posted with part number 3797902, that appear to be 2.5" and WITHOUT bracket attachments. Do you think they are just reading the numbers wrong? That is actually the p/n of the manifold I have now, and it certainly has the brackets on it.
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Re: Joe - Exhaust manifold part question
Clarification: It appears they are 3797901 parts listed as left side manifolds, but they have not apparent brackets. Isn't the right the same except with longer studs? -
Re: Joe - Exhaust manifold part question
Tim-----
The 3797942 manifolds usually also have the 3797902 casting number on them, too. Sometimes, they just have the derivative numbers (i.e.'902' and '942'). I don't know why both numbers were used, but it may be that the '942' manifold was made from a modified '902' mold or pattern. In any event, if you have a manifold with no alternator mount and 2-1/2" outlet, then it's a '942' or equivalent. It may be possible to grind the mounting boss off of a '901' or '902' manifold, but I don't think that it could be done cleanly, if at all, without ruining the manifold.
The '901' and '902' manifolds are identical except for the relative position of the alternator mount.In Appreciation of John HinckleyComment
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Re: Joe - Exhaust manifold part question
So if I understand correctly, I can use 2 # 3797901 manifolds on both sides of the engine and they will fit and bolt to the exhaust pipes properly?
I realize the alternator 'ear' is not required on the right side, but if no other 2.5" manifold is available from GM(?) for the right side, then this is a workable solution?
I have a 69' Coupe with a ZZ3 crate engine and I'd like to get a little more exhaust flow without going to headers.
Any advice would be greatly appreciated-
Thanks!
ElmComment
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Re: Joe - Exhaust manifold part question
So if I understand correctly, I can use 2 # 3797901 manifolds on both sides of the engine and they will fit and bolt to the exhaust pipes properly?
I realize the alternator 'ear' is not required on the right side, but if no other 2.5" manifold is available from GM(?) for the right side, then this is a workable solution?
I have a 69' Coupe with a ZZ3 crate engine and I'd like to get a little more exhaust flow without going to headers.
Any advice would be greatly appreciated-
Thanks!
ElmComment
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Re: Joe - Exhaust manifold part question
Elm even with the 2 1/2 outlet exhaust manifolds from GM, they will still be less than a 100% match of the D-type exhaust port of the ZZ3's aluminum heads.
With the 2 or 2 1/2 inch exhaust manifolds there will not be a smooth flow out of the heads. I have a ZZ motor in my 70 and used the original 2 inch exhaust manifolds even though I had a pair of the 63 2 1/2 inch exhaust manifolds on my spare parts shelf. I did not use the 2 1/2 exhaust manifolds because my factory air conditioner bracket and original alternator bracket could not be used with them. What I do have and is the 2 1/2 inch exhaust pipe system on my 70 which helps with exhaust flow. The 2 1/2 inch exhaust pipes start out as 2 inch system connected to the 2 inch exhaust manifolds. To install this system I only had to change the steel bracket under the rear of the transmission that the exhaust pipes connect to accomodate the larger 2 1/2 inch pipes.Comment
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Re: Joe - Exhaust manifold part question
Elm even with the 2 1/2 outlet exhaust manifolds from GM, they will still be less than a 100% match of the D-type exhaust port of the ZZ3's aluminum heads.
With the 2 or 2 1/2 inch exhaust manifolds there will not be a smooth flow out of the heads. I have a ZZ motor in my 70 and used the original 2 inch exhaust manifolds even though I had a pair of the 63 2 1/2 inch exhaust manifolds on my spare parts shelf. I did not use the 2 1/2 exhaust manifolds because my factory air conditioner bracket and original alternator bracket could not be used with them. What I do have and is the 2 1/2 inch exhaust pipe system on my 70 which helps with exhaust flow. The 2 1/2 inch exhaust pipes start out as 2 inch system connected to the 2 inch exhaust manifolds. To install this system I only had to change the steel bracket under the rear of the transmission that the exhaust pipes connect to accomodate the larger 2 1/2 inch pipes.Comment
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Re: Joe - Exhaust manifold part question
Elm-----
Yes, you can use 2 of the GM PART #3797901 (CAST 3797901) manifolds. Using this manifold, the unused (for your application) generator mounting boss will be on the rear of the right side manifold and the forward end of the left side manifold. You could also use the GM PART #3814970 (CAST 3797902) manifold on both sides. In this case, the generator mounting boss will be on the front of the right side manifold and the rear of the left side manifold. Or, you could use one of each and, depending which side you put them on, you could have botth bosses to the front or both bosses to the rear. Which manifolds that you use depends only upon where you want the unused bosses to be. Depending upon other installed equipment on your particular car, the choice of mounting boss locations can be important.
1964-71 Corvettes are best fitted with the GM PART #3846563 left side manifold. This has the exact forward generator mounting boss that is required. Then, use either of the above 2 manifolds for the right side, depending where you want the unused boss.
If you want a manifold with NO boss and 2-1/2" outlet, then you need to get a reproduction of the GM #3797942.
As Jim mentioned, these manifolds will not exactly port match a fast burn aluminum head. However, they will be no different than using any pre-95 cast iron exhaust manifold. So, the port mismatch will exist regardless of which manifold that you use. You can eliminate some of the mismatch by grinding and "port matching" the exhaust manifold to the head. I wouldn't do it, though. You won't gain much by doing this.In Appreciation of John HinckleyComment
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Re: Joe - Exhaust manifold part question
Elm-----
Yes, you can use 2 of the GM PART #3797901 (CAST 3797901) manifolds. Using this manifold, the unused (for your application) generator mounting boss will be on the rear of the right side manifold and the forward end of the left side manifold. You could also use the GM PART #3814970 (CAST 3797902) manifold on both sides. In this case, the generator mounting boss will be on the front of the right side manifold and the rear of the left side manifold. Or, you could use one of each and, depending which side you put them on, you could have botth bosses to the front or both bosses to the rear. Which manifolds that you use depends only upon where you want the unused bosses to be. Depending upon other installed equipment on your particular car, the choice of mounting boss locations can be important.
1964-71 Corvettes are best fitted with the GM PART #3846563 left side manifold. This has the exact forward generator mounting boss that is required. Then, use either of the above 2 manifolds for the right side, depending where you want the unused boss.
If you want a manifold with NO boss and 2-1/2" outlet, then you need to get a reproduction of the GM #3797942.
As Jim mentioned, these manifolds will not exactly port match a fast burn aluminum head. However, they will be no different than using any pre-95 cast iron exhaust manifold. So, the port mismatch will exist regardless of which manifold that you use. You can eliminate some of the mismatch by grinding and "port matching" the exhaust manifold to the head. I wouldn't do it, though. You won't gain much by doing this.In Appreciation of John HinckleyComment
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Re: Joe - Exhaust manifold part question
Jim/Joe,
Thank you for the information!
Of course, now I have several more questions ;-)
Is GM # 3814970 also a 2.5" outlet manifold?
The GM Performance Parts site doesn't specify the 2.5" outlet like the they do on the # 3797901 manifold.
My 69' is all manual- No PS, PB or A/C so I don't need to worry about too much other than the alternator mounting (which is on the left side).
Jim, you had mentioned that you have(had?) a 70' with the ZZ motor in it and you ran the factory 2" manifolds into the 2" to 2.5" exhaust pipes as per the factory. I'm not an exhaust flow expert by any means, but it seems odd that Chevy would have 2" manifolds that open into 2.5" pipes. In my mind, I don't see any advantage since the overall flow is pretty much dictated by the biggest restriction, no?
It would almost seem like having a 1" garden hose on a 5/8" water valve- It looks impressive, but it doesn't flow any more water.
Any thoughts?
Thanks again for your time!
Elm.Comment
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Re: Joe - Exhaust manifold part question
Jim/Joe,
Thank you for the information!
Of course, now I have several more questions ;-)
Is GM # 3814970 also a 2.5" outlet manifold?
The GM Performance Parts site doesn't specify the 2.5" outlet like the they do on the # 3797901 manifold.
My 69' is all manual- No PS, PB or A/C so I don't need to worry about too much other than the alternator mounting (which is on the left side).
Jim, you had mentioned that you have(had?) a 70' with the ZZ motor in it and you ran the factory 2" manifolds into the 2" to 2.5" exhaust pipes as per the factory. I'm not an exhaust flow expert by any means, but it seems odd that Chevy would have 2" manifolds that open into 2.5" pipes. In my mind, I don't see any advantage since the overall flow is pretty much dictated by the biggest restriction, no?
It would almost seem like having a 1" garden hose on a 5/8" water valve- It looks impressive, but it doesn't flow any more water.
Any thoughts?
Thanks again for your time!
Elm.Comment
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Re: Joe - Exhaust manifold part question
Elm-----
Yes, the GM #3814970 (cast 3797902), GM #3797901 (cast 3797901) and GM #3846563 are all 2-1/2" outlet manifolds. However, I just checked and the 3814970 and 3846563 are now discontinued. So, the 3797901 is the "sole survivor" of the 2-1/2" outlet manifolds. I should also emphasize that NONE of the 2-1/2" outlet manifolds ever had provisions for A.I.R. All 1969s used this system so, if you want to keep that intact, you can't use these manifolds.
2-1/2" outlet manifolds for small blocks were last used in PRODUCTION for the 1965 model year. After that, ALL small blocks got 2" manifolds. Many of these had 2-1/2" exhaust, though. There was a reduced diameter section on the forward exhaust pipe to allow connection of the 2-1/2" pipe to the 2" manifold. Did this make sense? I don't know, but that's how it was done.In Appreciation of John HinckleyComment
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Re: Joe - Exhaust manifold part question
Elm-----
Yes, the GM #3814970 (cast 3797902), GM #3797901 (cast 3797901) and GM #3846563 are all 2-1/2" outlet manifolds. However, I just checked and the 3814970 and 3846563 are now discontinued. So, the 3797901 is the "sole survivor" of the 2-1/2" outlet manifolds. I should also emphasize that NONE of the 2-1/2" outlet manifolds ever had provisions for A.I.R. All 1969s used this system so, if you want to keep that intact, you can't use these manifolds.
2-1/2" outlet manifolds for small blocks were last used in PRODUCTION for the 1965 model year. After that, ALL small blocks got 2" manifolds. Many of these had 2-1/2" exhaust, though. There was a reduced diameter section on the forward exhaust pipe to allow connection of the 2-1/2" pipe to the 2" manifold. Did this make sense? I don't know, but that's how it was done.In Appreciation of John HinckleyComment
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Re: Joe - Exhaust manifold part question
I just picked up a GM #3797942 manifold on ebay, and I occasionally see them there. Also, there apparently were some manifolds with casting number #3797901 that have no mounting bosses on them at all, as they have shown up on ebay too.Comment
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Re: Joe - Exhaust manifold part question
I just picked up a GM #3797942 manifold on ebay, and I occasionally see them there. Also, there apparently were some manifolds with casting number #3797901 that have no mounting bosses on them at all, as they have shown up on ebay too.Comment
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