[JAM1775]

I'd rather go with the 3" ( longest length) and let the rifling shape of the inside louvers do their job by facilitating exhaust velocity. Accept the added noise if you were looking for power over your old exhaust. To do otherwise by reversing the louver direction so the cups in the louvers face the motor must create turbulence. This cannot help scavenging. When I blew out the glasspacks in my FFR 2.5" ID inner tube mufflers I swapped them for the RACE LOBAKS. These mufflers have 1.5" ID center tubes surrounded by the augers which easily flowed as well if not better than my old 2.5" tubes. They idled quieter but really can bark when you stab it and scream at the top end. Lobak is 510-783-3714. $63 each I think at 26" L.
When I pulled my blown glasspacks I first installed 18" L Dynomax Race Bullets with
3"ID tubes. The motor seemed to rev faster, especially above 4500-5000rpm at WOT ...but.....but...my ears were killing me ! OMG ! One week they lasted.....before I hit them with a sawzall and installed the Race Lobaks. But they sure did make power !