Smoke in the Wires April 2016

Lucas Distributor Vacuum Control
By Jeff Simpson
What follows is the fourth of a series of articles reprinted with permission from The Register, the
newsletter of the Tucson British Car Register. All words and photos copyright Jeff Simpson.
Why do we need a vacuum control on the distributor anyway? They don't even use one on racing
engines so it can't be very important. Based on the number of variations and part numbers, it
seems that even the manufactures where not quite sure what they were trying to accomplish with
the vacuum control. But they actually do have a purpose. While the mechanical advance adjusts
the spark advance based on engine speed, the job of the vacuum control is to adjust for engine
load. It should provide better gas mileage and better drivability. When you are cruising along at
part throttle, the air/fuel charge into the cylinders tends to be leaner. A lean mixture burns slower
and therefore requires a little extra advance in order to get the most power from the least fuel.
Back to getting the maximum push on the piston at just the right instant. Not an issue with race
Smoke in the Wires April 2016 Page 15