I had a question about fabricating an exhaust. I'm looking at building a 3" exhaust for my project. I have a header with a 2.5" collector/flange. I'm considering using a pre-bent tubing kit, 2 resonators (no cat - running one in place of the cat), 3X6" flex pipe, and a muffler. How important is it to have a 2.5"-3" transition after my header? Would it be detrimental to just weld 3" tubing to a 2.5" flange w/o a transition? Should I run a 2.5" flange --> 2.5-3" transition --> 3" Flex pipe...., or is the transition even necessary? What would be your suggestion?

 

i would say from what i know of flow(not much) you'd want a transistion to be about 4-6 inches of pipe. id run the transition then the rest of your setup.

but to be fair i do have a flex pipe shoved into my 2.75 collector and just welded in the 2.5 fp, but you do have that extra quarter inch.

 

That's actually the exact exhaust I am building currently as well. Right after the collector is probably debatable as a jump like that might appear to the manifold as somewhat of an "open" header (good for resonance effects), but it could also cause the exhaust gas to lose significant velocity when it encounters such a large volume change. I personally am going for a transition, but that would be an interesting comparison to run.

 

Dave Stadulis (SMSP) recommended a transition. Considering his experience, I'd tend to go with his recommendation. I just thought I'd share his advice.