1. Why do stock SA tables dip a few degrees at 2800-3200 RPM 70Kpa and up and then ease the timing back in?

2. Why do 305 tables have more advance than 350s with iron heads?? Compression I would assume?

3. Why does advance fall off as the map goes up when you would think it would need more advance to make more power?

These may sound like stupid questions, but I want to understand why before I modify something like that.

 

#1 - Most times if spark dips when going from low rpm to high rpm, it's because of VE being high = torque is high = more air in cylinder = faster burn = less spark actually needed for best torque with a side effect of being easier to knock.

#2 - No good single reason - just whatever makes best torque without knock. Combustion chamber, swirl, compression ratio, cam - all important.

#3 - Flame speed in a combustion chamber is highly dependent on amount of air (density) and swirl. Best power does not come from "advance". It comes from having the peak cylinder pressure from combustion occur at the best time - actually near 15 degrees AFTER TDC. For this to happen, spark must occur some time before that in order to have enough of the charge burned to actually get peak pressure in that position. SO, when more air is in the cylinder, it burns faster, which needs less advance in order to hit the magic balance point of peak cylinder pressure at 15 degrees AFTER TDC.