[jim_howard_pdx]

You cannot measure a car's performance by how it feels driving it. "I cannot break the tires loose, or not as easily." You either need a chassis dyno, or take it to the track.

Mustangs will typically pick up about .1 to .2 seconds with the X Flow, proper pipes and mufflers. You can pick up .3 by going to a 2,500 RPM stall converter on your AOD. Add a better upper and lower intake, bigger injectors, and set the timing right and you can lower the ET by a full second. Add 3.73 or 3.90 gears and you are in the 13's for sure, and many are running 12 second times with this combo with an E cam and 1.7 roller rocker arms.

If we all set up our cars to break the tires loose, we would have huge monster torque engines efficient from 600 rpm to 2,500 rpm. This is not where races are won or lost.

The reason why your low end torque went away is that CATs are highly restrictive. They showed the engine a lot of backpressure which improves low end torque. You could gain that back by going with a 2" exhaust but then if you add the CAT back in, you will have NO POWER.

My Mustang has 2.5" exhaust tubes, and of course it is pre-CAT. I would have more HP if I went to a 3" exhaust, but it would not be as good coming out of a signal, and it would get poor gas mileage. So there is give and take with any choice you make. Unfortunately in exhuast systems you cannot have your cake and eat it too. You chose the rpm where you want the most torque, and you size the exhaust to tune into that max torque.

If you add HI FLOW CATS back on your H tube exhaust, you will regain some of the torque you lost off idle. For the same money, I would just bump up your stall converter to 2500 RPM and enjoy the extra HP and the half second faster time slips.