Spark Advance Engine POWER!

[jfaubion]

>   Also, for peak power and
> performance, the vacuum and centrifugal advances should be in good working 
> order and should advance the spark under load or RPMs.  However, in
> American cars, the vacuum advance is (and has been since about 1972) a
> vacuum *retard*.  This does reduce NOX emissions but it is detrimental to 

Actually most American cars DO have vacuum advance. The main purpose of the 
vacuum unit is to allow the engine o start with a modest amount of advance 
to reduce the loading on the starter motor. Once the engine fires and 
manifold vacuum builds it advances the timing to ensure a smooth idle. 

When you open the throttle plates, manifold vacuum drops, and this will 
effectively retard the timing but this helps prevent knock. When the vacuum 
drops in a wet flow manifold the fuel can deposit on the manifold walls and 
will cause a momentary lean condition until the fuel from the accelerator 
pump can arrive. This would cause knock if the timing were not retarded. 
Once the engine speed increases the mechanical advance comes in which 
compensates for the loss of advance from the vacuum unit. 

Some cars do use a dual canister vacuum unit to provide the advance using 
ported vacuum which basically does the same job as the mechanical advance. 
Those vehicles that use it usually have either no mechanical advance or a 
small amount of mechanical advance.

John Faubion
jfaubion at cs-smtp.tyndall.af.mil