Inlet diameter for the exhaust?
 

Does anyone know the inlet size of the stock boxster exhaust for the 01 base 2.7ltr? I'm thinking its 2.25" but I'm not certain. Anyone know? I haven't had the chance to crawl underneath to measure it yet. I plan on building my own custom exhaust and need to know before I start ordering parts. :D

The outer diameter of the outlet of the catalytic converter is 1.8" on all 986 Boxsters. This is roughly the same size as the inner diameter of the muffler inlet pipes.

As I say to everyone who starts building their own exhaust: don't. The factory muffler is tuned with length and chambers so that it doesn't have any unpleasant tones. It's extremely difficult to acheive this with off-the-shelf parts. I can guarantee that whatever you construct will drone horribly between 2,000 and 3,200 RPM and that you'll tire of it quickly.

Good luck...