What types of engagement are available? This is a common source of confusion when searching for information on transmissions. There are two types of engagement: Synchro and Dog.
Synchro engagement (also known as synchromesh) is what most passenger cars use.
a. Smooth operation on the road
b. No special driving style required
a. Synchros and shifting forks can break when abused and shifted aggressively
b. Slower to shift vs. dog engagement
Dog engagement is what most race prepared vehicles use.
a. Engage at any rpm
b. Allows for wider gear design
c. Will flat shift (Flat shifting is a practice where the clutch or gas do not need to be totally lifted in order to shift. Users will slightly lift one or the other to unload the dogs to allow a very fast shift)
a. Difficult to drive on the road
b. Recommended only for race use
c. Special care required under normal street use
What is a dog box? A dog box is the term for a transmission utilizing a dog engagement. A dog box can have either helical or spur gears or both.
What is a half dog box? A half dog box utilizes a combination of synchro and dog engagement. It depends on the manufacturer and customer preferences. Generally speaking, the first and second gears are dog engagement and the rest of the gears are synchro engagement. A half dog box can have either helical or spur gears or both.
What is synchronizer (synchro) engagement?
Synchros are synchronizers inside the transmission. These are the actual parts that move when you move your gear shift from side to side and back and forth. Their job is to connect the gears of the transmission to the shafts that they ride on and lock them together. This must be done at a gradual rate or the gears will grind. The synchronizer drives a cone shaped metal piece against the gear and starts the gear spinning. It accelerates it to the speed of the output shaft. When the gear reaches the speed of the output shaft, the synchro meshes completely with both of them and directs drive through its splines from the gear to the output shaft or vice versa.
What is dog engagement? Dogs are basically no more than cogs on a slider. The shifter pushes them into a receiver ring which engages the gear it is attached to. There is a separate dog and receiver for each gear in the transmission. On a racing transmission, there is a lot of "slop" (the gaps in the receiver are a lot larger than the teeth on the dog), which makes it easier to move into and out of the gears at higher RPMs without fully engaging the clutch.
Can a dog box be driven on the street? Though generally reserved for race applications, many users drive dog boxes on their daily driven vehicles. To do so, you need to learn proper shifting techniques and rev matching skills to decrease wear on the dog gears. The Shifting FAQ, this article, local personnel, or your transmission professional should be able to provide assistance with proper shifting techniques. This is one decision that should not be made lightly. If there is the slightest doubt in your mind as to your technique or attitude towards a daily driven dog box, you should opt for a synchro engagement transmission. Another thing to keep in mind is that a dog box is definitely not an option for a vehicle that sees occasional use by others such as your spouse, friends, visitors, valet, etc.