Posted by Daryl in Oz [162.158.2.109] on Thursday, November 01, 2018 at 18:42:07 :
In Reply to: ok. but wait a second, Jim posted by DDD [172.69.68.240] on Thursday, November 01, 2018 at 15:43:28 :
What Jim said, vac helps with "normal" running and mileage, while the mechanical part looks after the "load" side by retarding the ignition as the revs come down. The military and farming community used to prefer things simple, so mostly mechanical distributors and these have a different advance curve to vac units.
As you increase rpms the mechanical dist advances the timing until it reaches a factory determined maximum, all this at a fairly constant rate, there's a bit of science involved with how fast the advance is vs rpms as the engine starts to speed up based on springs and weights in the dist.