OK I've got 2 cats...one is at the Y pipe and the other is a regular big ole cat, I have a clogged y-pipe so I'm having a stock one just put on in it's place...I'm going to then cut off the reg cat and put a magnaflow high flow cat on and just clamp it...my question in what size of an inlet/outlet will I need. Also what type of replacement high flow will I need? 2-way, 3-way, or 3-way + air. What do they mean when stating Dual Oxygen Boss , or single...or no air injector?
This is for a 96' 4.9L.

 

I dont know if it is any different on the 4.9, but on my 5.0, the pipe exiting the front cat/ y-pipe is 3". The big cat then reduced it from 3" to 2 1/4".

 

5.0L here.

You AIR injection tube might not fit the new cat. It might not reach and connection size might be different. I tried the high temp silicon hose to temporarily fix my a.i.r. tube but it always burnt off in a week.

You will have a hard time finding a cat that starts at 3" then exists 2 1/4"...but you can go 3" & 3"(meaning replace everything all the way back). Most exhaust shops can step your exhaust also if need be. Some exhaust shops will work with you to get it to work well. The other thing to consider is leaving the second cat off and have the AIR tube just go into the side of the exhaust tube...not doing anything but pumping air into the ehast stream.
Did this help or make it worse?

 

I forgot you have a 96 (OBDII) so you'll need to keep the second cat and second O2 sensor. You have two O2's right?

 

On mine, the air tube didnt go into the rear cat. I never looked for it, but it must go in the front one, or somewhere along the piping beefore the rear cat.

 

Well anything with the cat's and tube size exhaust shops won't touch around here because of Cali laws so I have to do it my self...I have 3 o2 sensors all together..1 on each manifold and one behind the first cat(inbetween the first and second) so that's why I'm just leaving the front one stock and playing with the second.
I don't see any hose's or anything going to the second cat so I guess I don't need to worry about the 2-way, 3-way, or 3-way + air, and Dual Oxygen Boss , or single...or no air injector stuff I have no clue what it means.
Is there a way I can measure the tube? I want to keep the stock size pipe and just be able to clamp the high flow cat on because if I get in trouble, and when it comes time to get smogged again I just want to be able to un-clamp the high flow and clamp on the stock cat.