Originally Posted by Cincinnati Slim
I think the thing to try is a low-buck "port-match" of the factory manifold followed by a trip to the "Extrude Hone" folks. That will max out the flow of the factory manifolds yet retain a factory look and keep the costs down. This process works really well on "resto-mod" muscle cars. It avoids all the cost/fit/noise/leaks issues associated with headers. A decent factory manifold moded in this fashion will make almost as much power as a set of headers. The headers don't really show much gain until high RPMs very close to red-line. There are some exceptions; some engines have just horrible factory manifolds. The M-Benz 3.2 set look pretty decent !
Slim
Porting and Extrude Honing won't work on these headers. They are made of stamped stainless steel and lazer welded together. You can't port something made of sheet type metal. You'll end up putting hole in them. If you tried Extrude Honing 1 of 2 things might happen. The material would just get squeezed though and no change would occur because of the already smooth interior finish. Or the pressure would be so great that you would literally split the manifold at the seams.