Non-Synchronous vs Synchronous Shifting

Non-syncronzed automatic transmission

A non-synchronous automatic transmission uses a one way driving clutch that overruns as the next gear engages. This prevents binding and allows the output shaft to spin faster increasing vehicle speed. This is different from more precise synchronous shifting. Synchronous shifting requires precise timing between apply devices.

Syncronous shifting

In this three speed transmission, in first gear the ring gear is driven while the planetary carrier is held in reaction. This drives the sun gear providing gear reduction. The shift to second gear requires the sun gear to be held and the planetary carrier to be driven. The release of the sun gear and the application of the planetary carrier must be precisely timed. This timing is known as shift overlap. Inaccurate timing will result in binding or overspeed.