Here is a thread that provides some good info, especially on the BTG single axle-back exhaust--a favorite of many forum members:
http://www.ptcruiserlinks.com/forum/...ms=btg,exhaust
From what I've read, dual exhaust is for "looks only" (on the PT Cruiser) as there are no performance gains on four cylinder engines including the GT Turbo. If you are planning to eventually add the
Stage 1 Turbo Upgrade then you may want to just put in a K&N (drop-in) free-flow performance air filter rather than a CAI as members of this forum have indicated that
Stage 1 was designed to achieve maximum performance with the stock air-box. Just my two cents. Good luck. [8)]