Vacuum advance

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Vacuum advance is a spark timing mechanism. It advances the timing under light load, thereby making more power and better fuel economy. It lets the timing go back to normal under heavy loads, thereby preventing detonation (pinking/pinging).

Overview

This is a good system that adds both extra performance, better fuel economy, and smoothness to the driving experience.

The only downside is for high-RPM A-series racing engines, where the advance mechanism adds some mechanical instability to the spark timing of the Distributor.

Connection

The rubber hose on the Distributor goes straight over the 'vacuum advance port' on the Carburetor. That's it.

Emission controlled vehicles have a Tee in this line for the TCVAS system. This system cuts vacuum under some conditions, lessening performance but improving pollution (less pollution).

Degrees

For a factory stock engine, including 1.5" exhaust this a typical factory advance curve:

Vacuum Advance - part D412-80 for manual transmission

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Maintenance

The only maintenance is to periodically make sure the diaphragm is not leaking.

See: Vacuum Leaks

Vacuum Retard

Some newer USA engines (e.g. 1981 B310) are fitted with a vacuum retard diaphragm. This is for emission control reasons, it actually retards the spark timing. This results in improved emissions (less pollution).

Note that this does not hurt performance. It is activated via the Throttle Opener vacuum control line, so only works when Decelerating.

Vacuum Advance - To use or Not?

Yes, use it. It is good. It advances your engine. More spark = more power.