Good morning everyone, ive been looking into getting an exhuast but unfortually the ones ive found online are only catback systems. So my question based on you guys that have experince there set up, would a full exhust from pre cat to exit 3in to 2.5in split from muffler be better than just the gibson setup? Or does those flatten out parts of the exhuast piping neccerary for back pressure? Also to add in i plan to have a custom tune done either by pcmforless or here in houston. Current mods are volant cai looking into doing port and polish tb. Seeing how im going to be doing a tune afterwards i figured if i leave the second cat off the tune should take care of the ck engine light n if not i would have them install a high flow cat. Thanks for any advice n comments in advance. By the way im mainly looking for low end torque n hopefully fuel economy. A shop here in houston would do a full custom exhuast for $400 3in to 2.5in split so it would split similer to the gibson exhuast.

06 hummer 3.5l
Volant cai
Port n polish tb (pending)
Tuned pcm (pending)
Exhuast (pending)

 

First the flat spot you are talking about is for clearance issues not so much as back pressure. If I were to do it all over again, I would buy a long tube header and build my exhaust from there. There is no need for a 3" exhaust on the H3. the Gibson is a 2.5" with a 3" tip. I would use a hi-flow cat and eliminate the precat. The precat is just there to keep temps when cold, and in Texas like here in Oklahoma, it does not get cold enough to worry about.

I currently have a stock replacement header which bolts right in like the manifold, but gets rid of the pre cat. I am also running a Gibson extreme dual exhaust. I have A k&n, ported tb, and tuned.

I installed the Gibson, and then had the manifold crack. I then found the header for the same price as the manifold.

 

CAT back vs custom exhaust doesn't mean much with an I5, neither is going to significantly increase performance................. more about sound than anything else.

Now if you do a "skeeter" and delete the front "precat or mini cat" in the exhaust manifold and/or install a header instead, there is a bit of performance boost to go along with some sound.

So, IMHO go with what is easy for you. I went with a MagnaFlow CAT back years ago when prices were better than they seem to be now. If you have a good local exhaust shop, that could be you. Forget 3", you have an I5, 2.5" is plenty.

 

Thanks doc, i found a local shop that would do a full exhaust the way i mentioned for $400 with split 3in diameter. They mention they would gut the pre cat n small cat making it straight free flow. Now they did mentioned it would be cheaper if they went smaller diameter. So all in all i feel going custom would be better money wise. Would too big of an exhaust also kill the performance.

 

I know it will kill your fuel mileage, but with exhaust you can only let out as much air as the head will allow.