Here is the piece of the artical that I mentioned.
"Advance affects the expansion chamber primarily by altering
exhaust gas temps. More advance generally reduces EGT, to a
point. Retarding the timing makes the mixture burn later, and
more heat escapes into the exhaust pipe. Higher EGTs raise Vs,
and remembering the equation for Lt, the peaking RPM of the
motor varies directly with Vs.
Simple version: retarding the timing at high RPMs will give you
more overrev. This is no big secret, and my motor will pull
hard for an extra 500 rpm or more simply by retarding the timing
4 degrees. The trick is to keep a good amount of advance
through the upper-mid rpms, and retard timing after peak power
to extend the rpm range of the pipe. Being able to rev another
500 rpms may save a racer several shifts per lap, so retard is
an easy way to tune the exhaust system on the fly."