Whats the difference between mechanical and vacuum advance?
Mechanical advance is the centrifugal weight and spring assembly located underneath the rotor that provides the largest part of the timing curve. The vacuum advance canister, located on the side of the distributor, provides additional timing only when the engine is pulling vacuum. The mechanical advance along with the initial advance (base timing set at idle) is what gives you total timing. An example of total timing would be 12 initial plus 24 of mechanical which equals to 36 total. Vacuum advance is not figured into total timing because it will only function when the engine is not under a load. Example: If your vehicle is cruising on a flat stretch of road or going downhill, vacuum advance will come in and add as much as 15 to the total timing for increased fuel mileage and cleaner spark plugs. It is not uncommon to see as much as 50 – 52 of timing with vacuum advance. But don’t be alarmed by this being too much timing as the vacuum advance will decrease once you accelerate and put t
