||04-26-2013 12:51 PM
there are all kinds of tricks and "games" you can play, but the long and short is don't destroy the original if you are looking for a performance upgrade. Theoretically you could build a 3 inch system using the same basic setup, using 2- 2 1/5 inch cats feeding a 3 inch system as long as you locate your upstream and down stream O2 sensors properly. The big question is will this do what you are looking for, theory and reality are not always spot on. The 4.0 engine has historically been known for substantial efficiency increases when the exhaust is opened up. If you are running a dual O2 sensor system it's easy to get around this setup as the first O2 sensor is near the manifold (in some setups the first O2 sensor is actually IN the exhaust manifold) and the second is right behind the cat tailpipe side. these are easy to deal with. The CARB based emissions systems ( thank you California) have four sensors and these are an issue because readings need to be in balance as well as with in range...
In the end you may just end up putting the original back in place. with a cat back upgrade. Yes there ARE better flowing cats. no question there. the question however is will you gain, as the exhaust flow is engine speed specific, you will only gain significant power if you are running engine speeds over 4 grand... but by allowing easier exhaust flow you do allow a cooler running engine which theoretically will increase fuel mileage, but again theory and reality don't always go hand in hand.