Originally posted by drift
no one makes a 2.5" collector on their header in the aftermarket except the Kamikaze system (but their short primary system is anti-productive). plus, it would require a catalytic converter changeout.
a stepped system, i.e. 2.25" collector and catalytic converter combined with a 2.5" exhaust, will cause exhaust flow turbulence.
like i said, do all the research first as the aftermarket already has.
The common aftermarket has produced bolt on parts, not horsepower. Good dyno gains have been seen moving to 2.5" exhaust systems on B series engines. I can't see how an engine with a larger displacement and high valve lifts can possibly benefit from a smaller exhaust system.
AN-R produces a header with 2" primaries and a 2.5" collector. Hytech, SMSP, and Burns Stainless are other good choices.