Ok first off this isnt your typical open header question. not is it good or not.

I work at a mechanic shop and was talking to my boss about running open header on my 12.5:1 ls/vtec what he said they use to do in their hot rod days. when they would run open header they would " tune them" cut the flange off and weld an ext pipe on it and spray paint it crank the car and where the paint burns off cut the pipe there. I'm wondering if this could be good advice for when i bring my car to the dyno my header is a 4-1 that collects 3" then steps down 2 1/2.

 

If memory serves, I think this is how they use to determine where to put the cross over pipe to make it an "H" pipe in the dual exhaust. I don't think this was used to determine total header length or where to put the step down etc. Maybe don F could chime in on this one?

 

can anyone else shed some light on the subject?

 

Yeah, read this thread and it will explain it for you: https://honda-tech.com/zerothread/2306105