I had a 91 SC fox body with 550 at the wheels. I put highflow cats and it was a HUGE loss. I never had it dynoed, but I swear it robbed 150hp. I wonder if this will be the same?
The biggest difference I noticed (other than sound) was during deceleration. Huge difference. The best way I can describe it is like driving a diesel and turning the exhaust brake off. The car does not want to slow near as much with the O/R pipe as it did with the stock unit with the four cats. Didn't really notice a gain or loss on the low end, but on the high end it feels like a little more pull. Did not have my car re-dynoed though to verify where the power changes actually were.
Mustang Enthusiest recently did a test with the stock H pipe, a number of aftermarket X and H pipes with and without cats. I don't remember the exact numbers, but it was less than 10 rwhp and 10 rwtq difference between any of them, and IIRC it was more like 2 rwhp/rwtq. There is an obvious difference in sound, but very little difference on the dyno.
That X pipe that you can swap out the converters looks like a nice option....
Mustang Enthusiest recently did a test with the stock H pipe, a number of aftermarket X and H pipes with and without cats. I don't remember the exact numbers, but it was less than 10 rwhp and 10 rwtq difference between any of them, and IIRC it was more like 2 rwhp/rwtq. There is an obvious difference in sound, but very little difference on the dyno.
That X pipe that you can swap out the converters looks like a nice option....