The vacuum advance distributor has both vacuum and mechanical (centrifugal) advance. For street use, the vacuum advance unit works great. It will have low intitial timing to make cranking the engine easier. Once the engine is running, the vacuum chamber pulls the advance plate to get the engine to idle smoothly. The centrifugal advance starts coming in aound 1000 RPM (depending on the springs and arms used) and should be full in by 1800 to 2000 RPM,