Duke or Clem 2-1/2 ex manifold ???

[Michael F.]

Duke or CLem or anyone have any horsepower comparisions of 2-1/2" exhaust manifolds for a small block vs headers...horsepower, performance in general. Thanks.

[Duke W.]

Re: Duke or Clem 2-1/2 ex manifold ???

In one recent lab dyno test of a 40 over 327 with nicely pocket ported heads and LT-1 cam, headers made about six percent more peak torque, but only one percent more peak power (SAE gross) than the 2.5" manifolds. In both cases peak power was achieved at 6500 and was still climbing, but 6500 was selected as the limiting test speed.

I was surprised that headers did not show more improvement in peak power. On the car with an exhaust system the overall difference might even be smaller. The 2.5" manifolds and exhaust system are quite efficient. My recommmendation is to stick with the manifolds and install the lowest restriction mufflers possible - something on the order of the OE off-road mufflers.

Duke

[Timothy B.]

Re: Duke or Clem 2-1/2 ex manifold ???

Along these same lines, my 67' 327 300HP car originally had 2" manifolds and 2" exhaust. Many years ago the car had aftermarket headers on it, and when they were removed we ended up with what I think are '65 manifolds that are 2.5" and we installed a 2.5" undercar exhaust system. Knowing that my car is "supposed" to have 2" exhaust, I had it is my plan to pick up new manifolds and switch it back. What difference should I expect in power from the engine going back to 2" exhaust from 2.5"? I don't expect to show the car, but I am generally switching things back to stock where I can. Maybe I should just leave the exhaust as-is?

[Duke W.]

Re: Duke or Clem 2-1/2 ex manifold ???

If your car originally had 2" pipes that would indicate it was originally a Powerglide as all 300 HP with manuals had the 2.5" pipes.

I can't give you a feel for how much difference there would be by reverting back to 2" manifolds and pipes, but it will probably be noticeable SOTP, especially in the upper end of the rev band. The 2.5" manifolds and matching front pipes will bolt-on with all the original shielding, etc. so the car will appear relatively "original" to casual observation if it is done right.

In your case were all the original heat and RFI shields (assuming it had a OE radio option) installed along with the manifolds? If so, unless you are interested in pursuing a high NCRS award, I would just leave it as is.

Duke

[Clem Z.]

Re: Duke or Clem 2-1/2 ex manifold ???

the large size exhaust may gain a few HP at the top but you will lose bottom end torque.