Thinking about trying out this test pipe to replace my cat with. My question is, will i have to disconnect my o2 with this test pipe, and what is the stock diameter of the exhaust?

http://cgi.ebay.com/ebaymotors/86-92...spagenameZWDVW

 

no it just replaces the cat converter or at least it should

 

...which will remove the function of your auxilary ports and decrease the top end performance of your car more than the test pipe will benefit (compared to a high-flow cat). You'd have to fabricate a port and attach a tube for the split-air piping, in which event it would be cheaper just to build the whole thing from scratch yourself anyway since you don't have the benefit of just buying and dropping in.