Interesting that many have not defined the origin of the placement of
the "red line" on the tachometer.

I don't know where to look it up, but I suspect it has something to do
with prop tip speed (as one poster mentioned).

Shorten the prop, lengthen the prop, change the pitch and you will
necessisarily change the placement of the redline on the tach.

Change the pistons (bore); change the rods (stroke); change the rings
(compression); change the induction system; change the exhaust (back
pressure) and you change the torque of the engine, again altering the
redline.