what exatly does vacuum advance do

[wolds]

Set my timing on my 83 SC RoW yesterday and used my vacuum gauge to plug distributor vacuum line while I was setting it per manual. Noticed that the only time I saw a vacuum signal was when I blipped the throttle. Does this seam right? So I am guessing that the port for this line is located on the airbox side of the throttle plate. A little theory would be good if someone could enlighten me.

[fasterlaster]

My understanding of the vacuum advance is that it works in tandem with the centrifugal advance. The vacuum advance will advance the timing when the engine is under partial load (partial throttle). This helps provide a bit more power when it's needed. This really helps out at idle to take off, and helps to advance the timing just off idle when the engine isn't spinning fast and the centrifugal advance has not had an opportunity to advance the timing yet. As far a location, the port should be on the airbox side of the throttle plate, until the plate is partially open, when it will be on the other side of the plate and receiving a vacuum signal that is passed to the distributor. That's my understanding at least.

[Chuck.H]

Here's my take on it - it helps fuel ecomony and drivability, in that it advances the timing whenever there isn't heavy load on the engine (high load = low vacuum). If it didn't remove the advance during high load it might lead to knocking.

Chuck.H
'89 TurboLookTarga, 346k miles

[wolds]

OK, that makes sense and that is why when I blipped the throttle I saw a short spike in vacuum. So the vacuum advance is basically filling the lag time between idle and when the centrifical advance begins to feed in. Thanks

[mreid]

Not quite. While mechanical and vacuum advance both advance the spark, they operate independently.

There are actually three advances at play: base, mechanical, and vacuum.

Base: is the advance in degrees before TDC (Z1) at idle.

Mechanical: advances timing bases on rpm. Adds to base on a predetermined rate with a maximum. The rate and max are determined by springs, weights, and bushings in the distributor.

Vacuum: advances timing based on engine vacuum. Since vacuum is highest under cruise, advance serves to optimize fuel combustion hence gas mileage. Most vacuum advance uses a "ported" signal. This means the vacuum signal is above the throttle plate(s) and only sees vacuum above a certain rpm. This keeps vacuum advance at zero at idle.

EFI uses the computer controlled spark to apply an exact mix of base, mechanical, and vacuum for every engine operating condition.

[wolds]

Thanks for the info mreid, your explanation makes sense.