No, there probably will be no speed unlock, at least not a significant one. The HEV drive units appear to close to "maxed out" both mechanically and electronically.
From what I've been able to reverse-engineer, the Ultium motors appear to "redline" at 14,000- 16,000 RPM. The Hummer's front drive unit, based on tire size and drive unit gear ratio (13.3:1), hits 14K RPM at about 106 mph. (The Lyriq's motors at its max rated speed appear to be closer to 16K rpm.).
Yes, EV drive units, similar to ICE engines, have "redlines".
The redlines are based on a combination of the following:
- structural issues beyond a certain motor RPM, (centrifugal forces on the rotor and magnets, bearing speed issues, and/or dynamic rotational balance issues). Tesla's Plaid, which can crank out almost full HP at over 20K RPM, has a carbon-fiber-wrapped rotor (conventional rotors use steel), which both helps to reduce balance issues, contain the magnets from the centrifugal forces, and reduces high-frequency electromagnetic losses (see #2 below).
- high-speed power sag issues: Once a motor passes its peak power/torque point (usually around 50-60 mph), the power curve slowly drops. Once it drops below the shaft-horsepower required to continue to be able to accelerate the EV, you are at max speed. This sag is due to a) electromagnetic inefficiencies within the rotor/stator building from the increasing AC frequencies required to push the motor to ever-higher RPM's and b) increased switching losses at the inverter. The Gen 1 Ultium drive units are using conventional silicon MOSFETS, which have a steep decreasing efficiency curve as the switching speed goes up. (Note that GM is coordinating with Wolfspeed to probably use silicon-carbide (SiC) MOSFETS for their power transistors in Gen 2. These allow 800 VDC power systems, reduce inverter losses by 30% or more, and allow higher switching speeds that could drive motors to higher RPM with less power sag. Tesla, Hyundai, and Lucid are using SiC in their drive units, which allows them to have slightly more-efficient drives and run their motors at higher RPM's. Also, SiC is required for any 800 VDC EV power architectures)
- Shifting to the braking-at-106 mph situation, remember that a vehicle's embodied kinetic energy increases with the square of the speed. The EV has 75% more kinetic energy at 106 mph than at 80 mph. There is a speed and energy level where conventional brakes will prematurely fail from the massive dissipated heat rate before they will stop a vehicle in an emergency situation. It is very possible GM found that kinetic energy limit for a 9,500 lb truck was somewhere near 106 mph.
