Cast Iron to PVC

[ksplu13]

I have a shower with 2" trap, I am redoing cast to PVC. I want to know if I can run a 2" pipe to 3" soil drain pipe then go upstream of 2" to 3" soil drain for vent for shower?

 

[Mr_David]

sounds right.
You can increase drain size going downstream and Vents pipe going upstream.

some old homes where built with 3" & 4" vent stacks but a 2" vent is sufficient for a toilet, sink, tub and shower group of fixtures. Check your local building codes.