Quote:
Originally Posted by Bill Hanlon
I'll bet you are not pulling 14.75-16.75 in/hg at 4600 RPM. Probably not enough vacuum at that engine speed to give ANY vacuum advance except when you close the throttle completely
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The ignition vacuum advance is an engine load ignition regulator/sensor.
At 4600 rpmīs you may have the pedal to the metal (WOT). High load on engine means low vacuum signal in intake and no vacuum advance needed since A/F mixture is enriched.
Or, you may drive fast at 4600 with part-throttle. Less load on engine means high vacuum signal in intake that activates the vacuum advance helps burning the leaner mixture at proper time in the combustion cycle for best efficiency
And, the load on engine (and speed of course) at 4600 "cruising" is different using 2.56:1 or 4.33:1 gears in the rearend and anything in between.
The ignition vacuum advance regulates the amounts of advance needed depending on load on engine (intake vacuum signal at all times the engine is running).
If you want to know what is going on with vacuum advance at all rpmīs and driving situation just connect a vacuum gauge to the intake at a full manifold source via a long hose so you can watch wile driving.
FWIW