Here's a Muncie four-speed (right) compared with a Richmond six-speed. When you can visual
Dan Schultz, Layton, UT: Could you please explain how to figure out how much torque a manual transmission can handle compared with engine output? To handle the torque of different motors, I read that Ford's T10 transmissions used lower low gears in First gear for big-blocks compared with small-blocks. How much torque will a T10 handle?
Jeff Smith: This appears to be somewhat of a black art within the aftermarket transmission business as to how it rates its transmissions, and there is no standard within the aftermarket as to torque capacity. There are several levels of testing, and frankly the waters are a bit muddy. But what we can do is look at some basic engineering and design concepts with manual transmissions that you can use to judge the torque capacity of a transmission. A quick way to estimate torque capacity is to measure the distance between the centerlines of the input shaft and cluster gear. The greater apart the centerlines are, the larger the gears and therefore the greater the torque capacity. As an example, here are several transmission center distance figures:
This is a good place to start, but even within the Tremec T56 trans lineup, the company lists boxes that are rated anywhere from 330 to 550 lb-ft capacity, so the center distance is just one variable. As for the Super T10, Richmond Gear lists torque capacity ranges from 375 lb-ft for the 2.43:1 First gearbox down to 286 lb-ft capacity for the 3.42:1 First gear transmission.
The Richmond capacity points out that a big factor is the First gear ratio. As the First gear ratio becomes deeper (from 2.20:1 to 3.26:1, for example), the input shaft gear becomes smaller (much like a pinion gear becoming smaller as you move from a 3.08:1 rear gear to a 4.11:1). A smaller pinion or input gear offers less torque capacity than a large gear. Gear material also plays a role in this torque capacity game. There are some World Class Ford T5s, for example, that are built with 9310 nickel steel gears that are significantly stronger. This will help capacity, but keep in mind that the higher the steel's tensile strength, the more brittle it becomes, so that becomes a trade-off. Finally, the angle of the gear teeth has an effect on ultimate strength. A straight-cut spur gear, for example, is far stronger than a helical-cut gear. The problem with spur gears is they are incredibly noisy. Helical- or angle-cut gears drastically reduce the noise but at the cost of strength. The old Muncie rock crusher transmissions were stronger but also noisier than their M20 or M21 production cousins. This is a rough outline of how to judge torque capacity.
Within the manual transmission aftermarket, however, I am not aware of a standardized test procedure. Many aftermarket transmission companies rate the trans at what they compute as the torque it can handle under sustained, continuous duty. This is not any kind of impact load test. There is a big difference between the two. As an example, the next time you're at the state fair with your buddies and see one of those old-time grip strength test machines, let your friends go first. When it's your turn, hit the machine with an impact load as opposed to a continuous load-you'll win every time.
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