My plan is to run straight from my headers 3" pipes connected to Flowmaster 40 Series Mufflers and before the mufflers the outer casing of cats wenlding surving no purpose but a look to fool cops if ever stopped and after the mufflers continue the 3" piping curving towards the outside coming out before the rear tires. I should save about 2 feet each piping and I'm wondering how much performance I could achieve with this and how much noise i should expect coming from this set up?

 

if your header primaries are big 3 inch piping all the way back is loud as hell. My friend has it on his mustang and it gets annoying after a certain point. I'd go with 2.5" piping, and weld in exhaust cutouts, that way when ur at a show or something when you really wana be loud un cap em

 

if i remember correctly, i read somewhere that a single 3" exhaust can flow enough air for up to a 450hp motor. so 2 2.5" pipes should be good for wuite a bit more than that

 

The mufflers will be the choke point in your system. Carrying the 3 in pipes back to true 3 inch inside diameter straight through mufflers like Dynomax Ultraflow Welded should make more RWHP than the Flowmasters.

Mufflers not only surpress noise, they take heat out of the exhaust stream; maybe 300*F or so at high power. The cooler exhaust has a lower flow, so smaller pipes after the muffler don't hurt much. A smooth transition from 3 into 2.5 inch after the muffler should work.

As pointed out, 3 inch pipes can be noisy, especially if you dump them sideways ahead of the rear wheels. Carrying two mandrel-bent 2.5 in. pipes over the axle won't hurt much at all.

Remember, the muffler is the big restriction. Make sure it's a high flow one (or 2).

Unless you plan to use your cats as Terminator Boxes, run a pipe straight thru them.