How does the ignition timing is modified with the help of the manifold vacuum?

Question

i know how mechanical Timing Advance but what does the vacuum?

Answer 1

When the engine is under a load the vacuum changes, which in turns sends a vacuum signal to the vacuum advance on the distributor, which adjusts the timing accordingly.

Answer 2

Please read this: The vacuum advance mechanism works through a vacuum diaphragm mounted on your distributor. Many different vacuum canisters are available. They all provide advance to the distributor BUT if the vacuum source is above the throttle plates there is no vacuum to the canister until the throttle begins to open. If the source is below the throttle plates you have full vacuum at idol. If the source is below the throttle plate the vacuum to the canister drops as you accelerate causing the canister to pull the advance backward or completely off. As the intake manifold gains vacuum the vacuum advance begins to work again. You do not want vacuum advance during full throttle acceleration or pulling away from dead stops or pulling loads up hill. In either case you will hear terrible spark knock and can beat your valves and hammer pistons to death. While accelerating and pulling the mechanical advance is doing the work. In cruising conditions with no load the vacuum advance does nearly all of the work as the distributor shaft is not spinning fast enough to swing the spring resisted weights outward.

Answer 3

Engine vacuum is highest at an idle.
As the engine RPM increase the spark must be delivered to the cylinders sooner.
BTDC [Before Top Dead Center]
The vacuum hooked to the distributor moves a plate with either the pick up coil or points. At a idle the plate is pulled all the way forward. As the engine RPM increases the vacuum goes down allowing the plate to move back, thus delaying the spark delivery.

Answer 4

manifold vacuum will rotate the plate (where the POINTS and Condenses attach or the Magnetic Pick up is mounted to) and cause the "spark timing" to begin earlier. This is usually needed because during acceleration, you need something to help get past the "BOG" point. Without some type of advance, your engine would almost always stumble during acceleration.