i got a 91 z28 with a stock exaust... (1 muffler splitting into two 1" pipes splitting out the sides)... im putting an actual dual exaust with two mufflers and 3" pipes... how much horse should i get with this

 

Putting full dual exhausts on this car can be done, but it is a PIA andcreates ground clearance problems. The HP gain will be the same as anygood cat back system single pipe system, about 10-15 HP. IMO unless you have a higly modified engine, the two big pipes may actually reduce low end torque.