memix:
Since we're re-creating some info for the moment, here's how the interplay with timing advance and gearing works:

When you're setting up a new power system or stepping onto a new track with anything faster than a 10.5 motor, always start with the timing advance LOW. If you're too slow, start to gear up to whatever the track can handle or whatever you can handle. Remember that low lap times are what you're looking for, and that blazing speed down the straight that everyone likes may end up giving you SLOWER lap times in the long run

Once you've got the gearing in the right range, then you can experiment with some higher timing advance for a bit more straight speed and punch out of the corners, but remember - adding timing advance is very much like adding a tooth or two to the pinion - The car will be faster, and the motor will get hotter. Watch your temps while you're doing it and experiment with how you like the feel of a few teeth down on the pinion combined with a few settings up on the timing. Generally low timing and faster gearing will give you a smoother softer feel in the infield - generally good for tight technical tracks. Higher timing and slower gearing gives you a punchier hard hitting feel (which some prefer on any track).

For 10.5, 13.5, 17.5 and 21.5 motors:
Racing setups for these motors has generally evolved into "max out the timing advance, and adjust gearing to keep motor temps in check".

Your temp gun and lap time sheet are your best friend here. With the timing maxxed out, a very good driver can recognize the "breaking point" of the motor very quickly. This is the point when you are adding a tooth, seeing laptimes go down and the motor is not overheating yet, and you add another tooth and repeat. At some point when you add one more tooth, usually two or three things happen all at once: You notice you have lost a lot of rip out of tight corners (the motor is maxxed out on torque), your laptimes have gotten slower, and the motor is at or beyond it's safe max temp. This is the breaking point, and for Nationals level drivers - they go down one tooth and call it a day.

For club level, I would recommend going down 2 or 3 teeth to give you some extra security from blowing motors, and here's why:

Traction conditions can change during the day, and added traction = more motor heat. Ambient temps change during the day, and higher ambient temps = more motor heat. Even something like a slight drivetrain bind, or a wheel bearing starting to go bad will add friction to the drivetrain and thus add motor heat. If you are just barely one tooth under the breaking point, there are many many factors that might come into play that will shoot you right over the edge in an instant, and then you've smoked a motor.

Max temps:
21.5 = 140F
17.5 = 145F
13.5 = 165F
10.5 = 175F
Anything faster than 10.5 = 185F