vacum advance general question

[ronnie]

hi all , from standing start i can understand how the vacum advance is needed and works , but if sitting about 30 / 40mph if you accelerate hard do you still need and use the vacum advance ??

[Alec]

Hello Ronnie,

it is much more about the amount of throttle, wide open throttle and the vaccuum advance goes back to zero, partial throttle and with higher vacuum the distributor advances due to the vacuum module.
The whole purpose of this is to allow the car to be set up for a weaker mixture at cruise, and as a weaker mixture requires more ignition advance the vacuum does this automatically. This is done to improve fuel efficiency, i.e. mpg. This is all separtate from the mechanical advance which is purely on engine speed.

Alec

[bmcecosse]

It also operates at idle......thus the static advance can be set just slightly before TDC - and as soon as the engine starts it pulls the advance up to the 'best' position for a smooth idle. It also 'brightens' the engine so when you press the pedal it accelerates away nicely - an engine with a failed vac advance is noticeably flat when the pedal is floored.