Synchromesh Transmissions

Most modern cars are fitted with a synchronized engagement gearbox from the factory to deliver smooth, reliable, and quiet operation, which is paramount for a daily driven vehicle. Synchromesh transmissions operate using a collar that applies force to a cone-shaped clutch attached to the gear. The collar allows the shaft speed of the gearset and the input shaft to be matched or “synched” to the output shaft prior to the collar locking into place and initiating a shift.

Synchromesh gear engagement is best at lower engine speeds and requires a bit more time compared to a dog box, to facilitate shifts. Limitations of synchromesh gearboxes in high-performance applications include slow upshifting at very high engine speeds—e.g. 9,000 rpm—and slow downshifting, as well as the need to fully use the clutch.

Dog Box Transmissions

Dog engagement is normally used in racing applications where fast, precise shifting is needed. Dog gear engagement is facilitated by numerous large teeth (dogs) that mate into matching openings machined into the opposite surface of the drive gear. Unlike the synchro engagement, there is no synchronizing mechanism to assist in equalizing speed. Ideal gear selection—e.g. minimal clashing and wear of the dog rings—is achieved by quick shifts; the motto here is “the quicker the better”, so bang away.

There is no depressing the clutch in the conventional sense like with the synchromesh transmission. A momentary break in engine load until the shift is achieved by a quick throttle blip or clutch depression. The driver will then experience the dog ring engaging with the next gear and the throttle can be reapplied. With practice this can be done in milliseconds. In fact, a driver can preload the stick shift in the direction of the next shift, and then when he either blips the throttle or clutch the shifter will quickly click in the desired gear.

With all else equal, dog-engagement gears are much stronger than synchro-engagement gears because without needing to make space for synchro rings, the gears themselves can be made thicker. The number of dogs (teeth) and the size of the openings determine the window of opportunity that the dogs have to engage during the shift event. Rings with a smaller number of teeth provide a more efficient, smoother shift quality. The downsides to this easier engagement are increased noise and abruptness on the shift.