[Andy748]

Hi, Briefly (if we ignore the 52-57mm race type systems) there are 3 main types; 45mm dia full system, 45mm headers to 50mm dia end cans with a 45-50mm centre pipe and finally a full 50mm system (as fitted to 998s I believe). I hope this helps. Depending on your model, state of tune and intended use be careful not to over size the exhaust. Over sizing pipes esp the headers will reduce the efficiency of gas extraction at low rpm and therefore effect tractability in most 'real world' situations.

[Edited on 21-6-2005 by Andy748]