The hi-flow cats flow some pretty good numbers. I'm running them and I just dyno'd 277 RWHP. I guess I could have gotten 10 more (maybe) without them, but then I've heard you run into problems with not enough back pressure for the O2 sensors.
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86 GT, GT40P heads, Crane 2031, 1.7 RR's, Cobra intake, 65mm TB, 73mm Vortech, 24# injectors, MAC GT40P headers, 2-1/2" BBK H-pipe (w/cats), Flowmasters, 3.55s, Groundpounder subs and lower control arms, 277 RWHP.
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