Because less dense fuel mixtures (as you would find under high vacuum cruising conditions) burn at a slower rate than denser fuel mixtures (as you would find at wide open throttle), the spark needs to occur sooner in the combustion cycle. Engines were provided with vacuum advance to provide this additional advance at high manifold vacuum levels. Vacuum advance was simply a vacuum-operated piston that pulled the breaker plate or magnetic pickup assembly around the distributor shaft. Engines came with a variety of vacuum advance units from the factory satisfy the sometimes conflicting requirements of emissions and fuel economy.
The specifications of the various OEM vacuum advance units may be found in Motor or Alldata manuals. However, these are now often considered obsolete by OEMs so exact replacements can only be found by searching junkyards. Failing that, aftermarket suppliers still offer replacement units but they often consolidate their parts so that one part fits a wide variety of applications. For Delco HEI ignition systems, Blue Streak lists the following vacuum advance units suitable for propane applications:
|Blue Streak PN||Advance Starts(" Hg)||Maximum Advance(Distributor Degrees)||Maximum Advance(@ "Hg)|