RE: Pipe Setup Question
Is it the MAC's airplane pipe or marine pipe? I ran the marine pipes all the way down to 12.5" on my boats. They have shorter/steeper cones and are designed for higher RPM setups.
In a perfect world your like the header and pipe to be the same size at the joint and have a perfectly smooth transition, but they will work just fin if you start cutting into the cone slightly. I think the reliability penalty from having them mismatched would be higher than the performance penalty. You'll burn through couplers more often if they are mismatched.