2.5" or 2.25" Cat Back
 

What will be best?

I know you'll loose backpressure going to 2.5" and many say 2.25" is the best for a N/a car.

But I've also heard people say you don't need backpressure. Which sort of makes sense, because really the only time backpressure is an issue is on the bottom end of RPM band. Correct me if I'm wrong..

So once you're rollin, backpressure shouldn't matter. It's kinda like increases the diameter of wheels/tires.. Will take more to take off but easier once moving to get speed. Right?

let me hear your suggestions..

you don't need back pressure, EVER... it's a myth

either one is good, just make sure that they use as few bends as possible, and be sure to go under there soon and sand down the welds some and add some weld paste, that you can get from auto zone,, prevents rust
Bruce

Re: 2.5" or 2.25" Cat Back
 

This has been covered before in other threads, but I shall rehash it...

You're not looking at the entire picture. In a normal N/A piston engine, the airflow through the exhaust pipe helps to pull fresh intake air into the engine. This effect is known as "scavenging" and is vital to decent engine performance, particularly at low rpm.

If you increase exhaust pipe diameter, you generally will decrease backpressure, but you will also decrease airflow velocity (you will also generally increase noise) :). Decreased backpressure is good, but not when it comes at the expense of decreased velocity and poor scavenging. Therefore, you need to strike a balance between the two.

Generally speaking, a stock to mildly modified Protege engine needs 2" to 2.25" exhaust pipe for decent velocity and good low-end power. More diameter will decrease backpressure, but will hurt velocity and make more noise. The backpressure reduction from 2.5"+ exhaust pipe is theoretically good, and it may increase peak power by a couple of horsepower, but it will hurt overall speed and power because Protege engines aren't capable of high enough rpm to move enough air to make 2.5"+ pipe pay off.

I wish I woulda known that when I had my muffler welded on.