[Barry_R]

That graph is actually very similar to the one from the MSD billet distributor instructions. The combination of blue and silver springs is one of my personal favorites. Whats happening is that at cranking speed you have minimal advance so it cranks easily.

At idle speeds of +/- 1000RPM you have a modest amount of advance because of the light springs that you can set to the desired 20ish for light throttle off-idle driving.

After that 1300or so RPM it advances in a non-linear fashion because the real light silver spring gets stretched out sooner than the blue one. You limit the max amount of advance by using different stop bushings - the black one gives the least amount of distributor advance - another favorite...