Just wondering because need help with this.

If somebody can explain the differences between these distributors(mechanical advance vs vacumm advance). I have crane cams iginition syst, XR3000 and PS91 coil.

Thank You.

I don't know the Unilites per say but I would hazzard a guess that both have mechanical advance, and one also has vacumm advance in addition to the mechanical. Vacumm advance is only used during no load condishions of the engine, coasting, idle, and will have no effect during WOT application. Vacumm advance typically will give you a smoother running engine and better MPG.