2 inch vs 2 1/2 inch manifolds
08-14-2001, 02:30 PM
#1
Burning Brakes
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2 inch vs 2 1/2 inch manifolds
There was a thread awhile back talking about switching earlier year manifolds to a C-3. About getting better
exhaust using bigger. I thought it was a 65-67 stock manifold. I was wondering if anyone had done this and what
modifications were necessary. If one were to buy new 2 1/2 inch exhaust pipes would they bolt right up or does
the car have to go to BUBBA.com? New exhaust is coming up on "my list" and I was doing some prep work.
SEMPER FI--1970 Dave
[Modified by 1970 Dave, 12:48 PM 8/14/2001]
exhaust using bigger. I thought it was a 65-67 stock manifold. I was wondering if anyone had done this and what
modifications were necessary. If one were to buy new 2 1/2 inch exhaust pipes would they bolt right up or does
the car have to go to BUBBA.com? New exhaust is coming up on "my list" and I was doing some prep work.
SEMPER FI--1970 Dave
[Modified by 1970 Dave, 12:48 PM 8/14/2001]
08-14-2001, 04:00 PM
#2
Drifting
Re: 2 inch vs 2 1/2 inch manifolds (1970 Dave)
Nope. No bubba required. Pick the right parts and it looks dead stock. I have that system on my 69 convertible. Clearance next to the starter is a little tight but then again I have the 350HP 11" clutch (with a bigger bell housing - mid-years never had the big clutch). It DOES fit but an aftermarket mini starter wouldn't be a bad move.
You want:
64-65 2.5" manifolds (you'll need to grind the generator bracket off one of them)
64-65 2.5" headpipes. I got mine from Corvette Central but I'm sure any of the stock manufacturers can get you the same parts.
1970 LT-1 exhaust pipes from the trans crossover back.
Here's what it looks like:
http://www.photoloft.com/view/Image....9565&i=3040969
(OK, so I don't have one of the mufflers on yet, you get the picture...)
[Modified by 69L71, 2:02 PM 8/14/2001]
You want:
64-65 2.5" manifolds (you'll need to grind the generator bracket off one of them)
64-65 2.5" headpipes. I got mine from Corvette Central but I'm sure any of the stock manufacturers can get you the same parts.
1970 LT-1 exhaust pipes from the trans crossover back.
Here's what it looks like:
http://www.photoloft.com/view/Image....9565&i=3040969
(OK, so I don't have one of the mufflers on yet, you get the picture...)
[Modified by 69L71, 2:02 PM 8/14/2001]
08-14-2001, 05:59 PM
#3
Drifting
Re: 2 inch vs 2 1/2 inch manifolds (69L71)
Dave and "69L71"------
The manifolds which you want if you want to go with 2-1/2" outlet are as follows:
left side----GM #3846563--------this manifold has the forward generator bracket which 68-71 require
right side----GM #3814970 (casting #3797902) or GM #3797901----these will have the unused pre-66 style alternator bracket. If you use the '970' the bracket will be positioned at the front; if you use the '901', it will be at the rear. I don't recommend grinding these off; you will weaken the manifold. If you want a right side manifold with NO bracket, buy a reproduction of the GM #3797942 manifold which has no bracket, but costs about $250. The 3814970, 3797901, and 3846563 manifolds currently GM list for $123.17/each.
None of the above-reference manifolds have provisions for air injection reactor (AIR) fittings. This is fine for a 1970-71 (except LT-1) since, otherwise, no 1970 Corvette used AIR. However, for 68-69 the AIR has to be omitted if these manifolds are used. That, of course, is emissions-illegal. GM NEVER offerred a 2-1/2" outlet manifold with AIR provisions. That's because 1965 was the last year that 2-1/2" outlet manifolds were used on a Corvette small block and AIR was not used on ANY 1965 or earlier Corvette.
By the way, on the clutch issue, mid years DID use a 14" flywheel with 11" clutch. However, they were used only on 66-67 big blocks. No mid year small block ever used one.
The manifolds which you want if you want to go with 2-1/2" outlet are as follows:
left side----GM #3846563--------this manifold has the forward generator bracket which 68-71 require
right side----GM #3814970 (casting #3797902) or GM #3797901----these will have the unused pre-66 style alternator bracket. If you use the '970' the bracket will be positioned at the front; if you use the '901', it will be at the rear. I don't recommend grinding these off; you will weaken the manifold. If you want a right side manifold with NO bracket, buy a reproduction of the GM #3797942 manifold which has no bracket, but costs about $250. The 3814970, 3797901, and 3846563 manifolds currently GM list for $123.17/each.
None of the above-reference manifolds have provisions for air injection reactor (AIR) fittings. This is fine for a 1970-71 (except LT-1) since, otherwise, no 1970 Corvette used AIR. However, for 68-69 the AIR has to be omitted if these manifolds are used. That, of course, is emissions-illegal. GM NEVER offerred a 2-1/2" outlet manifold with AIR provisions. That's because 1965 was the last year that 2-1/2" outlet manifolds were used on a Corvette small block and AIR was not used on ANY 1965 or earlier Corvette.
By the way, on the clutch issue, mid years DID use a 14" flywheel with 11" clutch. However, they were used only on 66-67 big blocks. No mid year small block ever used one.
08-15-2001, 11:02 AM
#4
Drifting
Re: 2 inch vs 2 1/2 inch manifolds (Joe Lucia)
Joe:
Thanks for filling in the details.
I didn't sweat the AIR provisions because my 69 long since lost the system. Here in Maryland classic cars are emmissions exempt so I don't have a problem. In CA and other states everything you said is true. I suppose someone could consider buying a cheap 901 manifold and seeing if the AIR provisions could be machined in.
I wouldn't worry about machining off the generator bracket. The 901 and the 942 have the same design genesis according to Chevrolet By The Numbers. Just don't be too zealous and ground away more than you need to. When I did mine I had a machine shop to a sonic check and there's still plenty of meat there.
I don't have a listing here, but I'm not sure all big block 65-66 cars had the 14 flywheel. I have a friend with a REAL, documented 427/425 HP '66 convertible and he has a 10.4 inch clutch. Its very possible that the large clutch was available on big blocks, but I agree, no small blocks had em; and I'm convinced (after looking under a freind's 65 with 2 1/2" pipes) that the flywheel/bellhousing has to be the reason for the tight starter clearance
Thanks for filling in the details.
I didn't sweat the AIR provisions because my 69 long since lost the system. Here in Maryland classic cars are emmissions exempt so I don't have a problem. In CA and other states everything you said is true. I suppose someone could consider buying a cheap 901 manifold and seeing if the AIR provisions could be machined in.
I wouldn't worry about machining off the generator bracket. The 901 and the 942 have the same design genesis according to Chevrolet By The Numbers. Just don't be too zealous and ground away more than you need to. When I did mine I had a machine shop to a sonic check and there's still plenty of meat there.
I don't have a listing here, but I'm not sure all big block 65-66 cars had the 14 flywheel. I have a friend with a REAL, documented 427/425 HP '66 convertible and he has a 10.4 inch clutch. Its very possible that the large clutch was available on big blocks, but I agree, no small blocks had em; and I'm convinced (after looking under a freind's 65 with 2 1/2" pipes) that the flywheel/bellhousing has to be the reason for the tight starter clearance
08-15-2001, 10:42 PM
#5
Drifting
Re: 2 inch vs 2 1/2 inch manifolds (69L71)
69L71-----
While it might be possible to install AIR fittings on the 65 2-1/2" outlet manifolds, I don't think that it would be too practical. Manifolds which have AIR fittings have a boss added which is drilled and tapped for the straight thread, tubing fittings. These fittings must seat on cast iron at the base of the fitting in order to seal. Without the thick boss, I don't think that properly installing these fittings would be possible.
With respect to the 14" flywheel and 11" clutch on mid year big blocks, 1965 L-78 (425hp/396) DID use a 10.4" clutch and 12-3/4" flywheel. Also, 1967 L-88 used a 10.4" clutch and 12-3/4" flywheel. However, all other 66-67 L-36, L-72, L-68, and L-71 used an 11" clutch with the 14" flywheel. In fact, 1966 and very early 1967 used a unique bellhousing, GM #3872444. This bellhousing is relatively rare and brings big $$$ from 66 Corvette big block restorers. Most 1967 big blocks used the GM #3899621 bellhousing. Both the '444' and the '621' are nearly identical bellhousings for 14" flywheel, 11" clutch applications. They differ only with respect to the configuration of one of the external reinforcing "ribs".
While it might be possible to install AIR fittings on the 65 2-1/2" outlet manifolds, I don't think that it would be too practical. Manifolds which have AIR fittings have a boss added which is drilled and tapped for the straight thread, tubing fittings. These fittings must seat on cast iron at the base of the fitting in order to seal. Without the thick boss, I don't think that properly installing these fittings would be possible.
With respect to the 14" flywheel and 11" clutch on mid year big blocks, 1965 L-78 (425hp/396) DID use a 10.4" clutch and 12-3/4" flywheel. Also, 1967 L-88 used a 10.4" clutch and 12-3/4" flywheel. However, all other 66-67 L-36, L-72, L-68, and L-71 used an 11" clutch with the 14" flywheel. In fact, 1966 and very early 1967 used a unique bellhousing, GM #3872444. This bellhousing is relatively rare and brings big $$$ from 66 Corvette big block restorers. Most 1967 big blocks used the GM #3899621 bellhousing. Both the '444' and the '621' are nearly identical bellhousings for 14" flywheel, 11" clutch applications. They differ only with respect to the configuration of one of the external reinforcing "ribs".
