How much faster will I go?
 

I have recently decided to start modifying my car a little mre and the first place I plan to start is the exhaust. I currently have stock headers and H-pipe with 4-cats. The only thing I have done is the Cat-back and I read another post that said the cat-back in stock form is the least restrictive and will give the least gains.

As for the parts I plan on 1 5/8" GT-40P headers, and a 2 1/4" H-pipe with high flow cats. Will the 2 1/4" be enough for my near stock car, I have read that the 2 1/4 is plenty for up to 300 hp and I was wondering if anyone has had experience between a 2 1/4 and a 2 1/2 pipe.

The real question is, how much faster should I expect to go. My previous best wa 13.6 and this was accomplished at sea level in 40 degree weather with 3.73 gears. Recently at 800-1000 elevation in 75 degree weather with 3.55s the car has gone 13.8. What should I expect with 3.73s in 40 degree weather at sea levl, and what should I expect with 3.55s in 75 degrees at 1000 feet above sea level?

Why are you getting GT-40P headers? Do you have GT-40P heads?

I would get a set of long tube if I were you. They may be a bit of a pain to install, but the power you will gain will be worth it. Besides shorty headers aren't worth that much on a stock to mildy modded stangs. I would also go with a 2 1/2" off-road H-pipe.

Long tubes, a off-road H-pipe, and a cat-back should yeild around 30-40hp, IMO.

Your car as it sits would benifit most from 1 1/2 inch headers and 2 1/2 inch pipes.

1 5/8 headers would probably slow you down.