Inliners International Forums General Discussion Hi Performance My '78 250 cid build

I haven't taken a set of calipers to measure my head pipe, but it does look pretty stout. The common thought on pipe size is Chevy put 2" on their non-performance engines and 2.25" on the performance motors. But this rule probably only applies to the pre-1975 systems. When Chevy added the catalytic converter and L6 integrated head, they probably swapped everything to a 2.25" header pipe which is why the 250 1bbl integrated head had a 2.25" outlet on the exhaust. Looking at the diagram for the L6 & low-perf V8 exhaust, there is a 2.25" clamp on the cat end of the muffler, while the tailpipe end is a 2" clamp. This might mean the intermediate pipe between the cat and muffler is 2.25" too.

The Z28 head pipe is the same as the L6 and low-perf V8s, so the Z28 catalytic converter probably has the same inlet size of 2.25". But if it has a 2.5" outlet to the Y-pipe, it is probably not the same cat from the L6 and low-perf V8s, which the Walker part numbers being different seems to support that. So the Z28 exhaust starts with the cat on back.

So if you have a '75 to '79 1bbl 250 L6, you could swap everything after the header pipe on back with Z28 parts. Or you could delete the cat with a custom 2.25" to 2.5" pipe and hook it up to the cat back of the Z28. Then later if you swapped the integrated head to a non-integrated head with a 292 HD exhaust manifold and just hook the 2.25" header pipe to the new manifold so you can drive it down to the exhaust shop to fabricate a new 2.5" header pipe that can hook up to the Z28 cat-back. The setup would be identical to the '77-'81 305/350 Z28 setup with the same size L6 header pipe bend at the same angles, but it would be even more free flowing for the smaller L6 than the V8.

I could possible set this system up myself on my Camaro as the header pipe is cut with a turbo muffler stuck on the end of it, by adding a 2.25" straight pipe to the end of the muffler and cutting it off where it meets the Y-pipe. But I'm not sure how to connect up a regular 2.25" pipe to one with a 2.5" fitting for a catalytic converter?

The stock 2.25" L6 header pipe is not a great design, as it goes straight down and then under the oil pan at a 90 degree bend and then around the starter back to the catalytic converter. A much better design would be 2.5" pipe going back under the manual linkage and then crossing under the transmission to the Y-pipe if that is possible.