Am i correct by saying that the stock y pipe on my 01 5.3 silverado which has the two catalytic convertors is 2.5". And if thats true would it also mean that my headers, where the flanges meet, would also be 2.5"?? And would it also conclude that if i were to change out my cats for high flows would i need to get a 2.5" inlet and 3" outlet since the rest of my exhaust is 3" from that y's back??? Thanks guys

 

Not much to be gained with hi flow cats on a 5.3 and to big of pipes (dual 3 inch tail pipes) can actually reduce low speed torque and low speed part throttle responce. I little back pressure is not bad at all.

 

I dont have duals though, this is the swept side exhaust in the stock location!! its still two into one. But i'm merely talking about the y pipe only. The y pipe is 2.5" - 3" outlet which then goes to my swept side exhaust!