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Speaking from my own experience on this issue, I can advise that the fitting of a Hirsch cat-back exhaust system gave me a bhp gain of 10bhp.
Car felt "livlier" but not significantly so. The power gain was only 5% given that the car started from 200bhp.
Fitting a 3" downpipe and sport cat at a later date gave a significant improvement of around 15-16bhp and the car defininitely felt quicker.
This is my own experience using rolling road readings on a 200bhp Saab 9-3 with a standard Saab ECU. I suggest that you may get a percentage gain, rather than an absolute bhp gain, based on original bhp.
Car felt "livlier" but not significantly so. The power gain was only 5% given that the car started from 200bhp.
Fitting a 3" downpipe and sport cat at a later date gave a significant improvement of around 15-16bhp and the car defininitely felt quicker.
This is my own experience using rolling road readings on a 200bhp Saab 9-3 with a standard Saab ECU. I suggest that you may get a percentage gain, rather than an absolute bhp gain, based on original bhp.