Regarding setting timing with a vacuum gauge, I have found that adjusting timing to the highest vacuum level results in overly advanced timing. I have found, after much experimentation, that using a tachometer and vacuum gauge together will give a better result using the following procedure:
1. Disconnect the vacuum connection from the distributor.
2. With the tach and vacuum gauge connected, advance the distributor until either the RPM or vacuum stops increasing.
It doesn't matter which one peaks first; stop advancing the timing when one reaches a peak.
3. Retard the timing to reduce the vacuum 1 "Hg.
When using this procedure, make sure the vacuum gauge is connected to manifold vacuum, not a ported source.
I have read, but not tried, other procedures that advance timing to the highest manifold vacuum, then retard to lose 1.5 "Hg. While I haven't tried that procedure, I think it will result in retarded timing, especially if the engine is operating above about 3000 feet elevation.
Regarding the change in atmospheric pressure/barometric pressure, that pressure does, indeed, change at about 1 "Hg per 1000 feet of altitue above sea level. The change is fairly consistant up to about 10,000 feet altitude.
Because of that change, the power developed by any internal combustion engine diminishes as its operating altiude increases above mean sea level. The change is not especially evident below 3000-3500 feet elevation. The reason power diminishes with an increase in altitude, assuming proper fuel mixture and ignition timing, is that the cylinder compression pressure diminishes as atmospheric pressure/manifold pressure decreases. That in turn results in reduced combustion pressure (from fuel burn) with resulting lower thrust on the crankshaft (lower torque).
A rule of thumb that is often used to determine compression pressure is: Atmospheric Pressure X Compression Ratio. Applying this approximation to an engine at 6000 feet elevation, 7:1 compression ratio and atmospheric pressure of about 9 PSI (15-6), gives a compression pressure of about 60 PSI at normal engine cranking speed.
There is much more that can be said on this topic; compression pressure is not as simple as the approximation impies, but this is enough for now.
Harvester
