Over on Coloradofans dot com, JLaudio29 posted a graph of the dyno results after removing the first cat.




The word is that the removal does not cause any check engine lights or other codes, completely transparent. Looks like it would be the same for the Hummer I-5.

 

I didn't get a good comparison for my mileage this past weekend. I was following the wife down the highway doing 10 mph over my normal speed and playing catch-up at times instead of maintaining a steady speed and going with the flow. That and I finished up the second half of the tank towing a trailer and sofa back home. I still averaged 16.6 mpg for that which is better than expected when compared to doing the same with my F150.

 

the 5-10 hp isnt worth messin with,,but i always wonder about what matters,,MPG??

 

I haven't done the Cat rmvl, but I can tell you the HP bump I got from the new intake is noticeable on the hwy and well appreciated! If the engine flows more air and can make better power I expect I will see a slight increase in MPG on the hwy as I'm using less throttle to maintain speed through the hills. (if I can keep my foot out of it!)