Where the curves stop is called the "redline" because the car's tachometer has a red line painted there. In the old days, if you took a car past its redline it would just blow up. Nowadays, the computers cut the gas and don't let you blow up the engine (it would cost BMW a lot in warranty replacements otherwise). The redline of my old car was 6500 RPM, which is fairly standard. The redline on the M5 is 7200, which is very high for a street car, but not at all high for a race car (F1 cars rev up to 18,000 RPM's - but they use up 4 engines every racing season - my M5 has to last me ten years).
The torque curves really reflect the engine design. The 545i has a definite plateau between 3500 and 4000 RPM, and drops off on either side of that. The turbocharged M5 is engineered to have a broad flat torque curve all the way from 1500 - 5750 RPM.
In terms of driving, in the E60 if you want to accelerate you need to get the car up to around 3600 RPM, which is high enough to scare dogs and little children (they tend to turn around, look at you, and think "what an idiot"). By contrast, in the M5 you feel the torque available to you whatever your RPMs, in whatever gear you happen to be in.
When accelerating hard, you generally want to run the car out to its redline in every gear. On the other hand, you might think that shifting earlier, before the torque drops off would be better? You would be wrong. Even though the torque curve starts dropping off, its made up for by the torque division penalty you would have to pay to shift to the next gear up.
For example, if we accelerate the M5 hard out to its 7200 redline in 2nd gear, we are down to making "only" 400 ft-lb of engine torque which turns into 3200 ft-lb at the wheels due to the 8:1 gear ratio. If we then shift to 3rd gear, the engine gets dragged down to 4500 RPM (because the new gear ratio is 5:1, and the inertia of the speeding M5 is such that the wheels are not going to slow, so the engine must).
You might think, "Awesome! back up to maximum torque at 500 ft-lb, now we should get a burst of acceleration!" Nope, because the wheel torque suddenly drops to 500 ft-lb * 5 = 2500 ft-lb from the 3200 ft-lb it was at only moments ago (when it was at the "bad" part of the torque curve, but we were at 8:1 torque multiplication). And there was no point shifting any earlier than redline, we just would have taken that torque multiplication penalty sooner and enjoyed less torque from an earlier point than holding out to redline.
The curve that is actually telling you this story is the power curve. The rule is that you should shift early only if doing so gets you more power in the new gear than in the old, otherwise, hang on. Because the M5 dips so little towards the end, this situation does not really come up at all, as is the case in most good cars where the gear ratios are well-matched to the engine.
Some people swear by the torque curve, and say it is all important. Others swear by the power curve. Both camps holding any such opinions are just plain dumb, because the two curves represent exactly the same thing. One can be fully derived from the other. They cannot be varied independently at all.
- Power (in HP) = Torque (in ft-lb) * RPM / 5252
- (I demonstrate this in the blog article Horsepower is Torque)
In the case of the M5, it has a relatively high redline of 7200 RPM, and it holds its power right up to it. When compared to a car with a lesser redline it means the BMW will be torquing along at high multiplication factor long after the other car has had to shift up and lost its accelerating power to the new gear ratio, even if its engine torque is higher (goodbye Cadillac CTS-V!).
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