MSD Ignition LS1 LS6 Timing & Rev Control - Build An Ignition Curve

The springs and weights in a mechanical-advance distributor determine the rpm at which the advance curve begins and ends. Basically, heavy weights with light springs begin and end the advance curve at a lower rpm. The same weights with heavier springs require more rpm to eventually create the same total mechanical advance, which requires a higher rpm to achieve maximum timing. The total mechanical is determined by the length of the slot and diameter of the pin. For most street engines, a good curve should have achieved total mechanical advance by 2,800 to 3,000 rpm.

This is a typical mechanical-advance mechanism on the top of an MSD small-block Chevy distributor. The weights and springs combine to determine the rate at which timing is added to the engine based on rpm.	This pin-and-slot mechanism establishes the amount of mechanical advance added to the engine. MSD uses a replaceable bushing that determines maximum advance. A smaller bushing allows the pin to travel farther and create more mechanical advance.
When engine vacuum is applied to the canister, a rubber diaphragm pulls the plunger (arrow), which moves the mount plate, advancing the timing. Changing the distance the plunger travels affects total vacuum advance. Less travel equals less advance.

Vacuum advance is used to create additional ignition timing at part-throttle, light load conditions when there is very little cylinder pressure. Low cylinder pressure requires advanced ignition timing because the air and fuel in the cylinder are less dense, which means it burns slower. A vacuum-advance canister located on the outside of the distributor is connected to a ported manifold vacuum source on the side of the carburetor that sends manifold vacuum to the canister only after the throttle is opened slightly. This prevents creating vacuum advance at idle. The travel created by the vacuum-advance canister pulls on a plate in the distributor that advances the ignition timing. Adjusting the amount of travel is one way to adjust the amount of vacuum advance. This timing is only added at part-throttle. As the throttle opens and load is applied, manifold vacuum drops, reducing the amount of vacuum advance. At WOT, there is minimal manifold vacuum and no vacuum advance.

Each engine is different in terms of when and how much timing it needs. This is where tuning comes in. By messing with total advance for maximum power and vacuum advance to optimize cruise efficiency and fuel mileage, you can create a very happy street engine that runs better and uses less fuel, all without spending much money.