Water closet arm size

[tbucket]

I am starting a major project that will involve replacing my existing 4" cast iron drain system with pvc, in order to correct some existing problems and later add a basement bathroom and kitchen. I have done a lot of research on the internet and through plumbing books, and think I know how to do it the correct way. I had intended to use 4" pipe for the existing wc arm to the stack, and later use 4" for the new basement wc to the new horizontal drain. I will install low-volume toilets (probably Toto) in the existing and new bathroom. I read in one of the plumbing books that 3" pipe for the wc would be better that 4" to insure that there is sufficient velocity for the solids to move along with the liquid. Is this true? The same books say that "bigger is better" when installing drain lines and encourage going to larger than minimun required pipe sizes when possible.

[pushkins]

3" lines are most common in nearly all residential homes, these will transition to 4" as they near the septic lines and sewer lines.
If your replacing existing 4" cast iron then you will need to use a Fernco (rubber boot with bands) to tie into this line, this would be a good place to reduce from the 4" down to 3", if that is indeed what you what to do.

[tbucket]

Thanks for the reply. My main concern was that the 4" pipe wouldn't drain well with the new low-volume toilet because of lower velocity in the horizontal section of the arm and for that reason it would be better to go with 3". If newer homes are plumbed with the 3" toilet arms, this could be why. In addition, the smaller size arm would not hang down as far and allow more headroom in the basement space below. I had planned to use a Fernco for the connection from pvc to cast iron. I appreciate that you took the time to address my concern.