John Kunkel wrote:

I'm surprised the arc is red (prohibited), it should be yellow
(caution) to
warn against continuous operation in that rpm range.

On most of the original planes with this marking, I remember that it
was a yellow marking. Over the last several years, I've noticed the
red arc popping up in the same range. I just figured that either
someone got the color wrong, or the spec. was changed by Piper.


It's pretty hard to accelerate from idle to max rpm, without passing
through
the range that is red arced on the tach.

That's correct. The only reason I could figure they'd change it to
red would be to emphasize that you really don't want to spend time with
your engine running in this range. I could see pilots confusing it
with the yellow arc on the airspeed indicator. In smooth conditions
it just fine to operate in the yellow arc.
John Galban=====N4BQ (PA28-180)