Diameter of drain from watercloset

Most codes allow it, but it's between you, your local code and the inspector's interpretation.

I don't know if this will enter into your decision making process, but it is my understanding that cast iron pipes muffle the sound of flushing water much better than PVC. If the pipes run near any type of living space, such as a finished basment, you might want to consider retaining the cast iron.

If the PVC pipes are hung with the right hangers the sound coupling is minimized. Application of adhesive sound damping mats can get rid of most of the rest of the noise. Second story baths are most likly to make noise, its not always a problem.

boy we just replaced our bath plumbing above kitchen, just sink and tub.

its very noisey, you can hear the water running...........

a toilet would make more noise

Yes, 3" is the minimum drain size for toilets. I have heard that modern low flow toilets may even work better with a 3" drain, of course that may be a myth.

Cheers, Wayne

How big are your TURDS ? ? ? ? ? ?

I have a 3" drain system on a septic and everything works fine.

The thing that separates a 3" drain from a 4" drain is a flushed Q-tip. It can tumble down a 4" without getting stuck.

So if a man drops his 3 inch penis in the toilet it will go down, but not a 4 incher......

Damn, I got a 9 incher. Glad it can be retrieved without going down the pipes !!!

Now we all know why blondes like big dick men ~~~~

This, and the one about sound of water rushing, is one of the most reasonalbe points made here. Thanks to all for the technical explanations and advice.