A855 - 5 Speed
Somewhere I read that a major determinant of transmission torque capacity is the distance between the countershaft and mainshaft. So boxes with higher (lower numerically) first gears tend to be stronger. Can't find the source though.
Physics. It's a matter of leverage. But that's also an 'all things equal' kind of comparison. It's similar to why larger ring gears are stronger. It's possible, to a point, to be 'stronger' with a closer spacing, but at some point the engineer runs out of super-alloys on their available materials list and has to resort to increasing the lever-arms to reduce the point loads.
Think of it this way: if you're transmitting 100 ft-lbs of torque from the pinion to the ring gear, and the ring gear is 1ft in diameter, the force applied at the ring gear interface is 200 lbs (200 lbs * 1/2 ft = 100 ft-lbs), if the ring gear is 1/2 ft in diameter (6"), then the force applied at the interface is 400 lbs (400 lbs * 1/4 ft = 100 ft-lbs). Note that the force is multiplied the lever-arm distance, or 1/2 the diameter which is also the radius. Obviously the numbers are all made up, but it gives you an idea of the maths.
@Rat Bastid is also right about the torque 'rating' of a box. The method used to establish that rating is too important to ignore it. So is the rating for a static load? For a dynamic load? Is it a CYA number so that they can 'exempt' themselves from any warranty on anything more than a stock motor?
Driveline weight, and not to mention car weight, would have a significant impact on how much load can even be placed on a gearbox. A dyno may be able to get a motor to generate 500 ft-lbs, but that same torque is going to accelerate a car quicker than the reaction force can be generated or sustained.
Engine masters even did some tests about how RPM rate (300/s vs 600/s) can affect engine output. All things equal, you can make a mundane engine look like a hero by slowing the rev-rate.
But in either case, most cars are going to rev far faster than even 600rpm/s - think about it: if you did only rev at 600rpm/s it would take 10 seconds to go from idle to 7k! Most higher powered cars are going from 4k to 7k 3-4 times (or more) in that same 10 seconds - or about 900rpm/s at least! Lots of motors are going to see a major dip in their rated output at that kind of rev rate, and the corresponding load on the gearbox.
All that to say: a 300ft-lb box MAY survive behind a 500ft-lbs motor in a 2k pound car, but put it into a 4k boat with 305 ft-lbs and it might shatter. Fix the clutch and it can survive behind both, but that's a different matter..
