by Todd Ryden
(Taken from High-Performance Ignition Systems by Todd Ryden)
The vacuum advance canister that you see hanging off the side of many distributors is another form of advancing the timing, but it does not really play a performance role.
Vacuum canisters were introduced to improve economy during times of high vacuum, such as moderate cruising speeds. The advance only happens under these conditions, and once you accelerate, vacuum drops and the timing returns to the mechanical amount. You can see why they’re not important to race engines.
Some engine builders have noted that a big-cubic-inch engine, such as an Oldsmobile or Pontiac 455, actually runs a little cooler on long moderate drives when it is equipped with a vacuum advance. When it comes to vacuum advance, check your engine specifications or with the engine builder for recommendations. There are too many variables; and remember that excessive advance can result in detonation.
A vacuum advance canister generally advances the timing by 10 to 15 degrees. You can also get vacuum canisters that allow you to adjust the amount of advance. Vacuum advance works as an economy boosting tool and isn’t required on performance applications.