Could someone explain how shimming, nitro percentage, and glow plug heat affects timing.

My understanding is that higher nitro percentage advances timing, while lower nitro percentage retards timing. Also shimming an engine retards the timing by increasing head clearance while reducing head clearance advances timing. So, the overall affect would be: advanced timing increases acceleration, while retarding timing decreases it. Also, I heard that using higher nitro percentage will increase acceleration at the expense of top end speed while lower nitro percentage would lessen acceleration while increasing top end speed.