Motor controllers typically have a maximum current rating (sometimes specified both as a short-term < 2 minute rating and a long-term 60 minute rating).
I believe the stock controller in the 2012 Zero S / DS bikes is the
Sevcon G8035, or the G80 Size 4. This controller is rated for 420A "boost" (<10s) output, 350A 2 minute output, and 180A 60 minute output.
I believe the 2013 bikes also use this controller, which is questionable to my mind.. the Size 4 controller is already a weak point thermally for the 2012 bikes, and the 2013 bikes make double the power (although at a higher voltage).
The upgraded controller is the Sevcon G8055, or the G80 Size 6. It's a physically larger controller rated for more current: 660A "boost" output, 550A 2 minute output, and 220A 60 minute output.
I believe the G8055 is the stock controller on the Empulse. The Empulse should be much happier under continuous load than the Zero bikes.In electric motors, current is roughly proportional to torque. Voltage is (roughly) proportional to RPM. A 50% increase in peak current produces roughly a
30% increase in peak torque. Edit: this may be running into motor limitations.
As to the "4" and "6" nomenclature.. it's possible that internally the controller are modular, and the Size 4 uses twice as many modules as the Size 2, and 2/3 as many as the Size 6.. there's also a Size 8 controller (used by Brammo for the Empulse RR in 2011 I believe), and a Size 10.