The downside to helical gears is that their shape creates a lot of thrust load along the axis of the gears or down the shaft they're mounted on. This thrust has to be controlled via thrust bearings and greater sliding friction provided by the transmission fluid or gear oil. The thrust load generated by helical gears also puts a lot of stress on the transmission housing or case, so OE transmission cases must be designed to handle this load. However, when thrust load is increased thanks to that big turbo you just installed, you can max out the holding capacity of the case - not to mention wear out the synchros quite rapidly. That's when it's time to consider upgrading to a set of straight-cut dog-engagement gears like those manufactured by Magnus Motorsports, PPG, Hollinger, KAPS or Quaife.
Unlike a synchromesh gearset, a dog box gearset uses large square-shaped blocks or "dogs" on one side of the dog-engagement rings that serve to lock the ring to the adjacent gear. These dogs are much bigger and stronger than synchronizer teeth, allowing the dog box to handle a higher horsepower engine, and the dogs are also typically spaced such that they'll more easily align with the indentations on the side of the gear or engagement ring they're being locked to. The straight-cut gear teeth provide complete contact along each tooth's mating surface at all times, providing added strength over helical gears, and their straight profile also means that there's no thrust load to stress the transmission case.
Because there's no synchronizers helping to match the speed of the gears and the input/output shafts, a dog box does require a strong and direct shifting action when changing gears and it also requires skillful rev-matching. As Marco from Magnus said while showing me some destroyed dogbox gears, "In the hands of a savage, these gears can be destroyed pretty quickly despite the fact that the dogs are much stronger than synchros and the high-grade material we use for our gears is hardened and very strong. If you shift fast and with conviction you'll have no problem finding the next gear. But if you shift like a sissy, the dogs will not fully engage and they will start to chatter against each other. This is the only time you will start to see excessive wear. Basically, premature wear is caused by misuse, either from granny shifting or from trying to force the gears in after missing a shift. It's also important to keep in mind that although you can shift without using the clutch, there must be a momentary release of power to shift [because the dogs are back-cut so they'll stay engaged once you take your hand off the shifter] so you can either tap the clutch or lift off the gas a bit when shifting. When done right, you can't beat the speed and solid engagement you get with a dog box."
As for that special high-pitched whine, that's caused by the straight-cut design of the gear teeth. In fact, the teeth for the reverse gear in standard transmissions are straight-cut, so that's why you hear that high-pitched whine when reversing in a manual transmission vehicle. As cool as this soundtrack may be, the real reason to upgrade to straight-cut dog-engagement gears is for greater strength and durability in high-horsepower vehicles where the OEM transmission has been overwhelmed by torque and thrust load and has become a source of repeated parts failure. In fact, if driven correctly, a dog box will not only improve reliability thanks to its superior strength, but the dog-engagement system makes for quicker and more solid feeling shifting action. Just look at the way Van Os bangs through the gears in his Dutch Supercar BMW and tell me you don't want to do the same!
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A synchro-engagement helical gear (left) next to Magnus Motorsports' dog-engagement straig
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On the left is a regular S2000 transmission, on the right is the Quaife dog box. It's much
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The dog-engagement blocks (left) have beveled edges and are smaller than the indentations