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This has been discussed on other forums and various articles, but tuning noobs, and even some very experienced tuners sometimes need reminders:

This is a summary of exhaust diameter and velocity. And what changing the diameter really means:

This gets more complicated with turbos, though the principles are the same.

Velocity - High exhaust velocity acts as a vacuum to clear the area directly after the valves and pull combustion byproducts down the exhaust stream. The quicker you can clear the exhaust, the easier it is for the cylinders to fill with fresh A/F, and the more complete a burn you can achieve.

Volume - The bigger the diameter pipe, the higher potential volume that you can flow. The more you can flow, the more the pipe can hold at any instant.

RPM - Low RPM means a low exhaust volume, high RPM means you're pumping a high volume of exhaust. Exhaust does not move faster just because RPMs are higher. Exhaust velocity is also determined by the 2 principles above.

Back pressure - At high RPM, high volume, with small diameter pipe, the potential of the exhaust system hits a point of diminishing returns. As velocity increases, and passes it's peak efficiency, the engine begins to build back pressure. back pressure means that the exhaust can no longer evacuate the gas as quickly as it's being fed into the pipes. This hinders the ability for new A/F to get into the cylinders.

Torque Curves - Why does everyone say to get a fat exhaust, and what is it really doing? And why does the OEM use such small diameter pipe?

Average drivers don't accelerate to, or drive at redline, so exhaust is tuned for lower RPM driving. A low volume exhaust will hit optimum velocity at a low RPM. However, for true performance, you need to get the engine into its powerband, which is almost always in the upper third of the RPM range. Enter the big diameter, free flowing exhaust. Which will maintain a higher velocity with more volume, meaning your engine can pump and clear more byproduct without building backpressure. But at the same time you're sacrificing low end torque as you increase volume, due to the loss of velocity. So while you're increasing peak torque and HP, you're losing low end response and performance.

Tune your system accordingly. Back pressure is bad, Velocity is good, Volume is the means by which to attain peak velocity and minimum back pressure.

here are a couple good links (one of them is from Honda, but even they have good ideas sometimes):
Auto Exhaust Science Page 2

Backpressure- - Automotive Forums .com Car Chat

Backpressure vs. Exhaust Velocity Explained - Honda-Tech
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