Scott Townsend wrote:
I'd have Cleanouts at either ends of the 90.

Is that the only reason? What about the flow of the liquid getting away
from the Solid? Am I too worried about it?


In hopes of putting this myth to rest once and for all, I found another
article specifically on the subject from Plumbing & Mechanical
Magazine. Here's an excerpt from a Google cache of the article:

***
Lessons Learned

One of my favorite stories about George occurred at a Plumbing
Exposition. George was on the Board of Directors of the ASPE Research
Foundation. He helped set up a demonstration model for the show. The
model had four of these new fandagled 1.6-gpf water closets. There was
a 3-inch drain line in clear plastic pipe. (Installed by a UA
contractor, of course.) The model had different segments of pipe to
demonstrate different flow parameters. One section of pipe was pitched
about 3 inches per foot.

As the show was about to begin, George was speaking to a few of us
standing around. He looked at this steeply pitched pipe and said, "We
might have a problem with stoppages in this section of pipe. I'm afraid
the liquid will be running away from the solids." I just smiled.

As the demonstration began, they flushed carrots, beets and peas. When
the vegetables hit the steep section of pipe, they took off down the
pipe, flying at a high speed. George looked over at me and said, "Well,
I'll be. You knew that was going to happen, didn't you? You see, my
apprenticeship instructor always taught us that if you pitched the pipe
too much, the liquids would run away from the solids." I said to
George, "My plumbing instructor said the same thing, but my engineering
professor explained why this is a plumbing myth and would never
happen." For the next hour we spoke about engineering principles. Here
George was in his 70s learning new concepts so he could relay the
information to others.
***