The factory has to meet smog regs first then they work on mileage and the other subtleties. The O2 cheater replaces the one behind the converter and tells the computer what it wants to hear so it's not trying to make up for the converter that isn't there any more. Mind you, you could get in a heap of do-do if they caught you using one in some jurisdictions.
As for the mileage increase, going from 22 to 26.4 is a 20% increase. If a 460 starts at 10, then 12 is a 20% increase. Take a 460 with a 2.5" single plus cat and go to 2.5" duals with no cat and I'd bet you'd top that increase. As for power, 10% on a 4.3 is around 18 where on a 460 it's over 30.
Oh, and by the way, I've run vehicles with dual straight pipes and it's a bit much for noise, especially in crowded areas.
1949 Mercury coupe
Nosed, Decked, Frenched
58 T-Bird interior
472 w/toploader 4 speed OD
Performer RPM w/Holley 3 barrel
Isky 304 degree solid cam
Narrowed 3.25 9.5 inch with Detroit Locker
Last edited by D Pope; 04-13-2012 at 06:14 PM.