hi, my S is with vortech SC. just want to know if it's fine to custom a 3 inch dual exhaust from head? is it too large ?
sound is not a problem.

 

I ran a 3-1/2 split into 3 with low resistance mufflers. Just changing out the cat back from the Amuse dual to this one upped the rw torque through the entire power range from 2250 rpm to 9400 rpm. Torque gains were largest above Vtec where they averaged about 10 ft lbs. Horsepower gains were present too, peaking out at 467+ rwhp.

That's 15 rwhp and 10 ftlbs torque just from opening the exhaust from the header back. The header chokes down dramatically at the collector and then opens up to the 3-1/2 inch main pipe.

This is putting out 160 rwhp more than just a stock Vortech setup. I would expect a car in the 300 rwhp might see a small increase. As the power goes up the impact of exhaust system resistance increases.