Originally Posted by Straight-line-69
Thanks NB,..yes, very low vacuum conditions remove vacuum advance functionality.
But the question above wasn't how VA was 'turned off', but why.
John Z believes it's necessary for differing burns speeds of differing AF ratios.
I thought it had more to do with combustion temps and pressure caused by WOT coupled with load, which produce pre-ignition.
I'm thinking we both may be right but wanted his angle.
John Z,..you still out there?
There are lots
of factors influencing what the optimum timing should be under many diverse operating conditions, most of which have been mentioned in the last few posts above; the challenge in the 60's was to develop simple and reliable mechanical systems that although separate, would complement each other and manage ignition timing based on both engine rpm and load without resulting in engine damage. The result was the combined centrifugal and vacuum advance systems, which worked amazingly well for decades considering their simplicity. Those systems are also relatively simple to tune, IF
you understand how they work and what they're supposed to do, and understand that the rules are different between street cars and race cars.
The onset of emissions requirements began to complicate the ignition equations, and subverted the centrifugal and vacuum advance systems into doing things they were never intended to do, most of which reduced driveability and efficiency.
Catalytic converter technology overcame many of those problems, and the ultimate solution that we enjoy in our daily drivers today came as a result of computer technology that senses, measures, and changes both A/F ratios and ignition timing maps hundreds of times per second and compensates instantly for temperature, altitude, load, onset of detonation at any level, and self-learns the driver's habits, and you can tune them with a laptop without getting your hands dirty. That's progress.