Front and Rear rotor housings
 

What are the differences in front and rear rotor housings?? I have one that has no stamp on it (or it's impossible for me to see).

far as i know, there is none.

Then Maybe someone can shed some light as to why they are stamped. Or maybe what the consequences of running 2 front housings would be.

There is a hole that I think is for air injection that is drilled into the gap around the exhaust sleeve. It is drilled up from the bottom and is drilled to the back of the front housing and the front of the back housing. That is why they're stamped. I think there are some other easy to fix differences like turbo coolant feed

After much searching on both the forum and on actual housings. I found the hole for the air injection difference and it seems that it only affects emissions and not functionality. You could run a motor with the housings swapped and you wouldn't notice a difference. That is until you got to the emissions place and they failed you. Atleast that is the way it is on an S4 engine. Not too sure about S5 or S6...

turbo rear rotor housings have feed for turbo coolant like said above. i used to fronts and just tapped elsewhere, easy fix.

so if your running a S4 with a T4 setup thats oil only then 2 front housings can be used with out problems? most rear housing im coming accross are hit with apex damage.