Shower p trap need help

[robert40]

This my attempt of trying to go from a 2" drain to a 1.5" p trap. it doesn't drain as fast as it should and is obviously a bad idea. trying to get this draining right before i set the acrylic 36" axis shower base. looking for the best way to make this work as the new drain for the shower will offset from the original location a bit. any help appreciated I realize this wrong in so many ways so be nice this is for a cabin in the woods so trying to get this done without having to spend money for professional help.

 

[Jadnashua]

While a typical showerhead should not approach a 1.5" drain's capacity, realize that a 2" pipe can flow almost twice the volume as a 1.5" one can (1.778x:1, rounded off). Throw in the galvanized pipe that may be rusted and have its interior diameter reduced from new, and it becomes worse. If the line isn't vented, depending on where it is draining, that could be a factor, too. Is there at least 1/4"/foot slope in the horizontal section?

 

[Sponsor]

 

[Cacher_Chick]

You have created a new problem in that the trap arm cannot jog downward between the trap and its vent because it can then siphon the trap.

The 1-1/2" drain will work fine as long as it is unrestricted.

 

[Terry]

When replacing tubs with showers I often used the 2" x 1.5" reducing coupling as you have shown. It works.
Back in the 50's, all showers had 1.5" drains. Current code is 2", but that will work too. You many want to snake the old galvanized will you have things open to clean that up as much as you can.