What exhaust size is to big? I want to do a cat back system with long tube headers and Y pipe and hi flow cats. would i be safe with a 3.5" cat back system? My stock system is 3" now. This is on my 01 yukon 4x4 5.3L motor. Thanks everyone!

 

Stock is actually closer to 2 3/4" on that year truck. Stock piping will flow more than adequate for what you've got, but going to a full 3" catback wouldn't hurt. Also, what muffler are you going to be running?

As far as 3.5" goes, since you're running cats it might not be as big of an issue, but it's definitely overkill for a basically stock 5.3.

 

3.5 would probably be the biggest I'd go (IMO from what I've heard from other people around here). Even if you went boosted, I think most of the guys think that 3.5 is pleanty.

 

im not sure wwhat muffler it is but the kit is a single 3.5" kit brand is MBRP

 

There is a formula you can use to get you close to what you need.
It is (2.2 x HP goal) and that will be the CFM flow the exhaust needs to be.
Figure out what your HP goals are and times it by 2.2
If you go to big you will not get good flow and scavenging and hurt performance. The CFM you get from your calculation is for the whole exhaust system, not just muffler or headers.
Hope that helps.