ok so i replace my old magnaflow cat which was 2.25 single/ in and out and i was getting around 330 to a tank and now i have a 3" single in/ and out and now im getting around 240 per tank...any ideas why?

so i think im going to switch to a 2.5 single/ in and out to get my mileage back up...

 

Something else happpened in the mean time? or maybe the new cat is deffective? My brain is telling me a more free flowing exhaust can only help efficiency.

 

i have no idea the only thing is that the new brand of the new cat is not magnaflow, its called catco.

 

There is a pressure differential test that can be done to verify if a cat is working right or not. If that is the only thing that changed I'd be highly suspicious of that cat.

 

maybe the magnaflow was a highflow cat and the catco one isn't?

what was wrong with the magnaflow? I wanna put one on mine to replace my gutted cat.

 

There is such a thing as too much flow. If you do not have enough back pressure then yes it can happen. The exhaust no longer scavanges the hot gasses out of the chamber, in short Exhaust Tuning. Track and strip guys do not worry about back pressure because of the rpms they run. If you did nothing else to your exhaust then it could be what Maxx suggested.

 

what made you change out your cat ?

 

Agreed but does it result in inefficiencies or just an undesireable powerband RPM? I can't seem to fathom how reducing pumping losses could possibly reduce efficiency.

 

Maybe the precat o2 got damaged in some way. They won't always throw a code.

 

Would that happen if the cat was installed backwards?