Aftermarket Downpipe Differences

['97boostinGST]

Are all aftermarket downpipe's diameter equal?
I know some neck down to 2.5" if I am correct, at the O2 to match the stock housing.
But when it comes to name brands like; Apex'i, Megan, Punishment, are there any differences in them at all?

 

[jukematt]

I actually just did a paper for school on this about choosing an exhaust system.

There are differences in materials used, like different steel types (or not even steel) for example. As far as sizes go, my friend has a Megan which is 3" from the O2. But like RoadRace has 2.5" (because of the O2) then widens to 3" at the flex. They also have a special one for the Evo III O2 since it is at a different angle.

The other thing to consider is how they are made. Stock, our cars have crush bent pipes meaning they aren't smooth inside the bends. The best way to go is mandrel bent pipes which are made so they are smooth inside to provide better airflow.

From what I found, 321 stainless steel would be the best choice for longevity, strength, and corrosion resistance. Second is 304 stainless steel which is the most common and has properties like 321 just not as high of strength, etc.

 

['97boostinGST]

Any info on the Apex'i? I found a deal on one, brand new.
Is that one 2.5" at the opening or 3" all the way?

 

[Boosted98gsx]

Honestly, don't get a downpipe. Get an 02 eliminator. Better flow than the OEM unit, and a true 3" downpipe.

From what I found, 321 stainless steel would be the best choice for longevity, strength, and corrosion resistance. Second is 304 stainless steel which is the most common and has properties like 321 just not as high of strength, etc.

T409 is a very good high temp stainless as well, with great strength properties, just less nickel, so not as much magnetic resistance (what do you need that for anyways?)