Vacuum Advance (And Why You Want It)


An often-asked question from many callers relates to whether they really need a vacuum advance mechanism on their distributor. I think this question stems from their observation that many “high performance” distributors do not incorporate vacuum advance and the resulting implication is that it is not desirable or necessary for a “good” ignition. There are a handful of applications where vacuum advance is not of significant benefit:

1) Pure racing engines
2) Severe duty very large trucks
3) Constant speed and load applications (airplanes, generators, pumps)

Other than the above, for normal automotive applications the vacuum advance will benefit the engine as follows.

1) Improved idle cooling
2) Improved idle quality
3) Improved fuel economy
4) Improved throttle response
5) Improved drivability
6) Enables improved spark knock control under full throttle accelerations
7) Enables leaner fuel jetting at light load to further improve fuel economy.

The basic reason for all these improvements is that the vacuum advance mechanism allows the distributor to supply a more optimum spark timing proportional to the load and speed output. Without the vacuum advance the distributor can only vary spark timing in proportion to speed and ignores its need for approximately 20 additional degrees of spark timing (“advance”) at light loads: (idle and cruise conditions)

The basic reason for the change in optimum timing at light loads is that when operating at light loads, the mixture is leaner for fuel economy and less dense because of light load. These conditions cause the charge to burn slower, and thus, to reach peak pressure at optimum point in the cycle, the spark must be initiated earlier. Failure to do this will result in “retarded” spark timing and all the aforementioned losses.

All engines are different, and have different spark timing requirements, but they are all the same in that as load is decreased, additional spark timing is required for optimum combustion.

Do yourself a favor – 1) make sure your distributor has a vacuum spark system and 2) experiment to find out what your engine “likes” for timing at idle, light load, and heavy load. Then change the vacuum can to achieve a result closer to the optimum.

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