lets revisit timing

A couple of basics that many of you know, but some may not:

• Fuel does not "explode" in the combustion chamber, it burns.
• The burnt fuel creates gasses that take up more space and creates force that pushes down the piston.
• It takes some time for fuel to burn completely it is not an "instant burn".
• If you ignite the fuel too late, like at top dead center (TDC), the piston is already moving down and not much force is created.
• If you ignite the fuel too soon, the force of the expanding gasses pushes against the piston moving up towards TDC, and you get pinging. This also takes away power from the engine and can damage the engine.
• So for an engine to idle well, it needs some static or initial advanced timing to optimize the way it runs due to its idle speed. This is set by how you turn your distributor.
• As the engine speeds up, the fuel has to be ignited sooner so the fuel has time to burn. So centrifugal (mechanical) advance is added as the engine speeds up. This is controlled by small weights and springs in the distributor.
• A rich mixture burns more quickly and a lean mixture burns slowly. When an engine has a high vacuum, low load situation the fuel mixture becomes lean. So Vacuum advance ignites the fuel sooner so it has time to burn effectively. By the way, since a drag race car never does this, vacuum advance is not needed in these cases, but it is very beneficial on the street.

(The above assumes an "old fashioned" car with a distributor like our old Mopars.)
How much initial, mechanical and vacuum advance any car needs is based on complex factors such as compression, design of the combustion chambers, cam shaft profile, octane in the fuel, etc. What works for one car may not work well for another.

Hope some of this is helpful.