67 exhaust manifold question - 2" or 2.5"??

[Timothy B.]

I asked this earlier in the year but had some of my data wrong. My 67 327/300 HP manual transmission car originally should have had 2" rams horn manifolds and 2.5 inch exhaust. Somewhere along the line it got 65 manifolds with 2.5 exits and 2.5" undercar exhaust, which from a performace viewpoint should be better. I need to replace the front undercar pipes and the right exhaust manifold anyway, and the question is should I replace the left also and take this opportunity to switch to 2" original manifolds and correct 2" to 2.5" pipes or keep the 2.5" manifolds for performance reasons??

I know all 67 small blocks had 2" manifolds. I am considering switching my cam and intake over to those from a 350HP car. I saw a post from Clem earlier in the year which mentioned that with 2.5" manifolds I might see a few extra HP at higher rpms but actually loose some low end torque. Is this correct? I don't currently show the car, but I am generally trying to move towards original, but at the same time I don't want to do something that will noticeable hurt my performance and fun to drive factor either.

[Jim T.]

Re: 67 exhaust manifold question - 2" or 2.5"??

What is wrong with the right 2 1/2 inch installed exhaust manifold?

[Timothy B.]

Re: 67 exhaust manifold question - 2" or 2.5"??

Nothing is "wrong" with it, but it has an attach bracket on it for some accessory or maybe right side mount alternator that isn't supposed to be there on my car. I am considering an add-on (Vintage Aire) a/c system maybe this summer, and the bracket is in the way I believe. The bracket makes the manifold immediately noticeable as not being right for my car. Both 2.5" manifolds are in perfect condition.

[Jim T.]

Re: 67 exhaust manifold question - 2" or 2.5"??

I am aware of the extension of the exhaust manifold on the 2 1/2 in exhaust. I sold a pair that I had been storing for over 33 years recently. GM still has the 2 1/2 exhaust manifolds available, but there is a market for originals. The 65 Vette with 327/365 and air used the 2 1/2 exhaust but I have no knowledge what brackets were used to mount the air conditioning compressor. Adding the air would depend on which type of compressor you are going to use.

[Rob A.]

Re: 67 exhaust manifold question - 2" or 2.5"??

I had a '66, 300HP with incorrect, '65, 2 1/2" exhaust manifolds. The car ran smooth and strong. I put the correct 2" on and there didn't seem to be any noticeable decrease in performance.

[Joe L.]

Re: 67 exhaust manifold question - 2" or 2.5"??

Tim-----

The 2-1/2" outlet GM PART #3846563 manifold will work perfectly for a 1967 small block on the LEFT side. It has the exact same alternator bracket as was originally used for your car, but with a 2-1/2" outlet size. It's still available from GM for about 100 bucks, although it may vary from the originals with respect to nuances of configuration.

For the right side, it sounds like you have manifold GM PART #3814970 (casting #3797902) or GM PART #3797901 (casting #3797901). Both of these manifolds have a side mount for the alternator. On your car, these mounts would be unused and, with A/C, they MAY interfere with the compressor mounting bracket. The compressor, nor any other part of the A/C system, ever mounted to these mounts on the manifold.

The only 2-1/2" outlet manifold ever made which had no brackets was the casting #3797942. This manifold hasn't been available from GM in many years, if it was ever available in SERVICE. However, it is available in reproduction (for about $250/each).

[Timothy B.]

Re: 67 exhaust manifold question - 2" or 2.5"??

Yep Joe, you are exactly right about what I have. My left manifold fits fine, and the right side is the casting #3797902 with the generator mount. I do see the casting #3797942 show up on ebay every once in awhile. As I figured, the responses are mixed as to performance. I have to decide if I want to move towards originality or ...