Item 9 from Outer Space

[Speedy Squirrel]

THE FOLLOWING INFORMATION APPLIES TO '78 AND '79 USA CARS WITH COUNTERCLOCKWISE ROTATING DISTRIBUTORS ONLY.

Please refer to the diagram posted by Billjam on this thread:

vacuum lines help

You can see a device labeled item 9.

Item 9 has a port on the left that is connected to the upper intake manifold. This port can only see pressure (no vacuum).

Item 9 has a port on the top that is connected to the port at the bottom of the throttle body. This port can see both pressure and vacuum.

Item 9 has a port on the bottom that is connected to the vent line on the warm-up regulator (hard to make out in billjam's scan). This port is open to the atmosphere.

The way it is supposed to work is as follows:

When not in boost, vacuum is applied to upper distributor port in the diagram to advance the timing at part load. There is no vacuum or pressure on the other port.

As boost builds up pressure is applied to both the upper port (from the throttle body), and the other Port (from the upper intake manifold). The balance between these pressures starts to retard the timing, but not too much.

At full boost, the pressure from the lower port on the throttle body is high enough on the top port of item 9, and is enough to activate it, which dumps the pressure from the lower port on the Distributor to atmosphere through the port on the bottom of item 9. This allows the pressure at Port 1 on the distributor to provide maximum retard.

And that is what item 9 does.