I've heard of straight through exhausts breathing better and let the bike run lean.
I've not been real hip on why, I can understand if you change the way the air enters, but get fuzzy on why once past the exhaust valves.
09 Aprilia RS125 plated
06 Paul Smart Sport Classic NFS ever.
08 S4RS Tricolore #081 NFS ever.
01 M600, aka, The Pink Monster
97 900SS CR
72 Norton Commando 750
03 KX60 with S4R rear shock,Honda CB350F, Hodaka Ace90, 3x Yam-YGS1, Yam-CS3 200, Vespa small frame, Gilera 106, Puch Sabre, Puch 50 Boy Racer, Benelli 250, Benelli 360, Honda CB350T, and many more.