george68hemirr
I think you guys are full of shit.
Distributor Vacuum Advance
Do you really need a vacuum advance on your distributor? Many high performance distributors do not incorporate vacuum advance and so many people think it is not necessary for a good ignition system. There are a handful of applications where vacuum advance is not a great benefit:
Pure racing engines
Severe duty very large trucks
Constant speed and load applications (airplanes, generators, pumps)
Other than above, for normal automotive applications the vacuum advance will benefit the engine as follows.
Improved idle cooling
Improved idle quality
Improved fuel economy
Improved throttle response
Improved drivability
Enables improved spark knock control under full throttle accelerations
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.
You should: make sure your distributor has a vacuum spark system and 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.
Do you really need a vacuum advance on your distributor? Many high performance distributors do not incorporate vacuum advance and so many people think it is not necessary for a good ignition system. There are a handful of applications where vacuum advance is not a great benefit:
Pure racing engines
Severe duty very large trucks
Constant speed and load applications (airplanes, generators, pumps)
Other than above, for normal automotive applications the vacuum advance will benefit the engine as follows.
Improved idle cooling
Improved idle quality
Improved fuel economy
Improved throttle response
Improved drivability
Enables improved spark knock control under full throttle accelerations
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.
You should: make sure your distributor has a vacuum spark system and 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.
I used to race an 11 sec Plymouth Scamp with a 383 in it and a friend of mine showed me this trick a long time ago. 
