Concerned About Venting Drain on New Shower

[aopchuck]

I' installing new shower and trying to fit waste in cramped subfloor. This is the view from the basement with the p trap just loosely attached to the shower drain a bit behind the current waste pipes. The top of the main sewer waste stack is on the in the shadows on the right and has an open top to that wye. I'm wondering if I can just run the shower drain through the P-trap over to a straight dump down on the main sewer stack using a 3" to 2" reducer without creating an S trap. The main vent line is about 2 feet away to the left in this photo, and I would be dropping this on the top of the main sewer stack. Or, should I look at adding an air admittance valve if possible before the dump down? Thanks for your help.

 

[Terry]

The venting needs to be on the top side of the trap arm whether it is a sink or a shower.

 

[Sponsor]

 

[Mr tee]

The cleanout on top of the 3" combination is useless because there is not enough clearance to use it. It looks like the CO adapter is not pushed in all the way. If you ate double lucky 1) it is not cemented in and you can pull it out 2) it is possible to run a 1 1/2" vent straight up from the CO and either pop it through the roof or tie into an existing vent higher up - then you could put a 3 X 2 bushing in the top of the combination and a 2 X 1 1/2 X 2 sanitary tee atop that and tie the trap into the side of the tee. Not much room to work and you would need to be double lucky.