exhaust hp
 

How much hp is expected to be gained at the fly and the rwh when adding long tube headers and a hpipe with coverters to a 99 gt with flowmasters?

In my latest MM&FF mag. they dyno'd a stock Bullit, then put BBK longtubes, and H-pipe, and it added 30RWHP! So I'd expect similar gains with your car.

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-1989 GT, not stock.

-2000 Camaro SS-A4,
13.4's bone stock.

-1989 Chevy S-10, EX. Cab-383(500+hp on motor), tubbed, trick flow heads, trick flow pistons, etc... autometer phantom gauges, "built" 700R4, roll cage, lexan back window, corbeau seats, R.H.S. 5 harnesses, Fuel cell, convo pro wheels, 15x14's with 29x18.5 M/T'S on back, 15x4's on front, nitrous, and so much more!

but, in the mm&ff story, they were using an uncatted h, along with a computer retune.

i have those items with catted h, and no computer tune. 20hp seems more likely to me.
but maybe my car was a factory hound to begin with.

Well the real benefit will be seen when you add a power adder - supercharger.