what size is the downpipe that comes from the the exhaust mani to the cat? i wanna get a 3" cat but im not sure if it will hook up to the downpipe, and why doesnt the stupid downpipe have a flange on it..thats so dumb....thanks

 

the dp is 2.5"... biggest that will fit w/ ac... and yes any muffler shop can weld a flange onto it to fit a 3' cat...