Page created by T June 26th 2011:
- 1 Vacuum Advance:
- 1.1 Why Have Spark Advance:
- 1.2 Mechanical Advance:
- 1.3 Why Have Vacuum Advance?:
- 1.4 The Throttle Butterfly Controls The Vacuum Spark Advance:
- 1.5 The EGR Port:
- 1.6 Checking the Vacuum Advance:
- 1.7 Vacuum and Mechanical Advance in Operation:
- 1.8 Spark Ported Advance:
- 1.9 Vacuum Ported Advance:
- 1.10 Full Throttle:
- 1.11 Links:
- 1.12 Terms:
Why Have Spark Advance:
The Burn Rate of Fuel is a constant for a given mixture. When the Engine RPM speeds up it needs more Advance because there is less time available for the Fuel to burn. When the Engine RPM slows down it needs less Spark Advance because there is more time available for the Fuel to burn.Spark Timing is made to change to match the Burn Rate of the Fuel to a particular RPM and load.
Mechanical Advance (the Balance Weights inside the Distributor) is responsible for simply increasing the Spark Timing as the RPM increases and decreasing it as the RPM slows.
Why Have Vacuum Advance?:
The Burn Rate of the Charge also changes depending on whether the Mixture is Rich or Lean. A Rich Mixture burns faster than a Lean Mixture.An Engine under Load causes the Mixture to be Rich.An Engine not under Load causes the Mixture to be Lean.Vacuum Advance retards the Spark Advance when the Engine is loaded and running Rich by sending less Vacuum to the Distributor. Vacuum Advance advances the Spark Advance when the Engine is unloaded and running Lean by sending more Vacuum to the Distributor.
The Throttle Butterfly Controls The Vacuum Spark Advance:
The Throttle Butterfly acts as a Control Valve. It exposes Vacuum to the Distributor when it reaches a particular point of opening. This occurs at around 1000 RPM with the Engine unloaded. The vacuum is continuous as long as the Butterfly doesn't move from that location. After reaching the activation point, the Throttle Butterfly provides increasing Vacuum the further it opens. This is true if the Engine is unloaded.
The EGR Port:
The EGR Port is controlled by the Throttle Butterfly and opens at 1800 RPM in an HZ Six with the Engine unloaded. The Cannister Connection is opened at the widest Throttle opening to make sure the Engine has enough RPM to purge the Canister without gagging.
Checking the Vacuum Advance:
Don't forget that when Timing a red/blue/black Engine you remove and plug the Vacuum Advance Line. You then set the Timing and Idle RPM to spec.
Next you open the Throttle and make sure the Timing advances. This process checks the Mechanical Advance. If all is to Spec, you reconnect the Vacuum Advance and do the same again. The Spark should be unchanged at Idle (because the Butterfly has the Spark Port closed). As you open the Throttle the Timing advances further than previously and the Total Advance is significantly higher than when the Advance Line was Plugged. At Full Throttle the Advance figure is the sum of both Vacuum and Mechanical Advance.
Vacuum and Mechanical Advance in Operation:
Consider the Engine loaded case which is where the Dynamics of Vacuum Advance occurs. If a Car is cruising at a steady Speed with the Engine at 2000 RPM, the Advance will be steady.
The amount of Advance will be the Mechanical Advance Figure for 2000 RPM, plus the Advance for the Amount of Vacuum the Throttle Butterfly permits. If the Car hits a slight Grade and the Driver doesn't move the Throttle, the Engine will slow slightly causing the RPM to drop and the amount of Vacuum sent to the Distributor to drop. The result is less Spark advance. The idea is to couple the amount of Advance to the Burn Rate. The Engine is turning slower, there is more time for the Fuel to burn so less Advance is needed. When the Car is back on level Road again, as long as the Driver has not moved the Throttle, the RPM will return to the original figure and the Spark Advance will be back the original figure (Mechanical plus Vacuum Advance).
Spark Ported Advance:
This is when the Distributor Vacuum Advance Line is connected above the Throttle. The further the Throttle opens, the more Vacuum is applied to the Distributor causing more Spark Advance. The amount of Vacuum also changes with the RPM. The lower the RPM, the less Vacuum. The higher the RPM, the more Vacuum. Spark Ported Vacuum Advance provides the best Power and Economy for Street use. HZ and later use Spark Ported Advance.
Vacuum Ported Advance:
This is when the Distributor Vacuum Advance Line is connected below the Throttle (plugged directly into the Inlet Manifold). The Vacuum is at a maximum at Idle and decreases as the Throttle opens. Grey Motor Holdens and some later Automatics used this method.
With both Spark Ported and Vacuum Ported Advance, the Vacuum disappears at Full Throttle meaning that only Centrifugal Advance is present.