Quote:
Originally posted by Stallion
the way I always saw it is when the distributor is called a "mechanical" distributor, they are indirectly referring to the vaccuum advance on it. In other words, when the crank is at a higher RMP, the vaccuum is greater therefore advancing the ignition and timing for the distributor and ignition system.
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No. Although the two discrete advances work in concert with one another ... they are separate functions and work independantly.
The mechanical advance consists of a pair of springs & flyweights that increasingly swing outward from centrifugal force. This motion begins approximately upon startup and continues as rpm increases ... until flyweights hit a stop ... ideally at or near cruising speed. This swinging motion translates to a rotation (advance) of the points’ cam or HEI’s reluctor. This mech advance motion can be recurved using different springs or flyweights or both. Note the mech adv motion is entirely a function of shaft speed; not load nor manifold vacuum.
The vacuum advance consists of a vacuum diaphragm (vac can) connected via a short rod to a rotating plate whereupon the points or pickup (HEI) is affixed. As engine load decreases, manifold vacuum increases. As manifold vacuum increases the diaphragm pulls on the rod which in turn pulls on the rotating plate; then the affixed points or pickup are rotated (advanced) in relation to the cam or reluctor. When at cruising speed with little or no load, there will be high manifold vacuum and vacuum advance is ideally fully-advanced. Given same condition, but driver stabs accelerator to floor; throttle plates open wide (load increases) and manifold vacuum drops dramatically ... then vacuum diaphragm-rod-plate returns to zero/near-zero degrees vacuum advance. This vacuum advance motion can be recurved using a different vac can, an adjustable vac can & by adjusting the rod stroke with a bushing. Note the vac adv motion is entirely a function of load and manifold vacuum; not shaft speed.
Some late C3 (about 81-82 I’m not sure) use similar big cap HEI; although it has NO vacuum advance can. With those models, I understand the vac can advance function is replaced with a computer-driven or electrically-driven function. Not simply a matter of GM tossing a needed advance function; the function was replaced but not deleted. Again, I’m not too familiar with late C3.
Lars archived papers on setting ignition timing and recurving are highly recommended. His are comprehensive, clear & concise; replete with part numbers & instructions.
JACK
vacuum system
Linear Mode
