Because they are at different gear ratios from the wheels, that sum is a completely meaningless number.
I think it is 400 ft-lb from a stop at the wheels; 153 ft-lb at the motor times a reduction gear of 2.636 gives 403 ft-lb. Like most electric motors, the peak torque is at a standstill. The engine hits peak torque close to its software-enforced "redline". I'm still not certain what's going on in that chart, though, because obviously the engine can't run at all below about 900-ish RPMs, and the MGs can go over 5,000. I think the Prius's transmission makes it rather hard to say what the torque is at a given RPM without applying some other constraint(s) as well; there are too many variables to be able to solve for torque off of just a single fixed variable (RPM).
Rated "redline" of the ICE is at or close to 5000 RPM. The car will try to not exceed that, and that "shutdown" also applies to the electric motors. 400 ft-lbs sounds high but is the oft quoted value. Why do you think they need "traction control"? Not all electric motors have peak output at 0 RPM. Most DC ones do, and the three phase electric motor in the Prius acts like a DC motor with a commutator. A two phase electric motor has low torque at 0 RPM, and uses a capacitor to derive the "second phase" when starting. For example, many table saws use this type of motor. It can be held at 0 RPM without too much trouble (don't do it though), but can be hard to stall once at operating RPM. The Prius motors loose torque due to "back EMF", which opposes the input power. It increases with RPM.
Are you sure you can use the 400 lb.ft of the MG2 like that? Since there is a reduction in the PSD between the MG2 and the ICE, the 400 lb.ft of the MG2 does not appear like that at the wheels. European Prii get quotes as having 207 Nm (152,6 lb.ft) of torque from the electric motor. Also 100 bhp at 2000 rpm seems a lot, seeing how the car accelerates.
Mg2 is coupled directly to the wheels through the reduction gears, and it's torque output of 153 ft-lbs at zero speed is multiplied by the reduction gears to 400 ft-lbs. This has nothing to do with the ICE.
Nobody seems certain what's going on with that chart, because the variables are not well defined. Torque from a a stop at the wheels must be more than 400 ft-lb for the car to be able to climb a steep hill. Maybe that number is maximum torque at the input to the final reduction gear?
Yes, a Search finds a couple threads for that: World's first 2010 Prius Dyno 2005 Prius on my chassis Dyno!!
