The vacuum advance mechanism consists of a diaphragm which automatically advances and retards the ignition timing according to the engine speed and operating condition. The diaphragm forms two chambers, one of which is connected to the induction manifold and the other is open to the atmosphere. A linkage connects the diaphragm to the distributor. When the diaphragm is in its normal position, the contact breaker is held fully retarded.
When the engine speed increases the induction manifold depression is high and the diaphragm is pressed up by atmospheric pressure. The movement of the diaphragm moves the contact breaker in the opposite direction to rotate and thereby advancing the ignition. A decrease in vacuum allows the diaphragm to return back to its original setting retarding the ignition.
It is seen that the centrifugal advance mechanism takes much care of speed only and not the load conditions, whereas the vacuum advance mechanism takes much care of load conditions. Therefore, a combination of the two mechanisms is applied to the distributor. In this design, part of the ignition advance is due to the centrifugal force and part of the vacuum produced in the intake manifold. The combination of the two mechanisms gives the practically perfect spark timing for all driving conditions.
Explaining Vacuum & Mechanical Advance Mechanism.
Racecars don’t run vacuum advance systems, why ought to I?
It’s true. If you headed right down to your local drag strip and checked out most of the distributors delivering spark to their hopped-up engine hosts, you’d notice a definite lack of vacuum canisters. the reason for this is often simple: racecars operate primarily at full throttle conditions – mutually may expect of a “race” automotive. not like streetcars that require to start out cold, get acceptable fuel economy, and idle in traffic while not warming, race cars don’t expertise the maximum amount of variance in operation conditions.
And, like we have a tendency to lined earlier, vacuum advance is nonexistent at low manifold vacuum, the system would bring nothing to the table on a automotive that’s driven at wide-open throttle. Would a vacuum canister cause issues on a racecar? No, it’d not, however within the name of simplicity, most billet, race-oriented distributors leave them out of the combo.