I am looking at a new catback system many of them are three inch but i found mufflex makes an exact fit system for my car with a 3.5 or 4.0 inch intermidiat pipe and dual three inch pipes after the muffler. I read the post on restriction vs exhaust velocity and is this pipe too big or will it work well and leave plenty of room for more power?

 

By the time it gets to the cat, exhaust velocity is a moot point. From there on back, you don't want to induce restriction. Exhaust velocity is an issue in the primaries.

Having said that, dual cats & 3" cat-back is great for the typical 350 TPI with mods. Anything bigger will hurt most in the back pocket.

 

Any bigger than three inches is too big for a street car. Now if you were running a built up big block, three and a half or maybe even four inches would be okay, but for your application three inches should be plenty. I run two two and a half inch pipes on my car (second gen) and it is easily enough for the motor.

 

One of my friends has a modified 305 IROC-Z with a 4 inch pipe that he swears by.

Does the lower velocity of the larger diameter pipe hurt low down torque?

At what RWHP levels does a 3 inch pipe cease to be adequate?

 

This is a single exhaust setup, not a dual exhaust setup. 3" minimum for a 350 imo.

 

A 3.5" single exhaust is equivelent to a dual 2.5" if you figure the cubic inch inside diameter of the pipe. With that said there is a theory which is to long to get into here again. Do a search and it has been written. A simple analogy would be to hook a 2" fire hose to your 1/2" garden outlet. What will the water do at the end? It will trickle out with no velocity. Now put a 1/4" hose on and the water is going to squirt out with great force.

Exhaust gas has the same basic theory. To small of pipe is not good and to big is not good. There is a window for the optimum pipe size vurses horsepower for your application. For the 2 sizes you are in question about I would go with the 3.5" if you have mods or going to do mods.