Drive Unit

The L410 series Rolls Royce V8 engine used in the SY/SZ series produces, depending on version, about 200-400 bhp and 200-400 lbf⋅ft of torque. In order to achieve the goal of maintaining or exceeding a 'sufficient' performance, it was therefore necessary to source an electric motor with these characteristics. The only motors currently widely available that meet these specifications are those fitted to Tesla vehicles, which from 2017 onward started appearing on the secondhand market at reasonable prices.

Apart from their high power output, Tesla drive units are also characterised by their inclusion of a high ratio (~9:1) final drive, and a high max rpm of up to 18000 rpm. These features, together with a flat torque curve up to about 6000 rpm, are what allow the unit to directly drive the wheels without the need for a traditional transmission.

In addition, the inverter electronics, that produce polyphase AC for the motor from battery DC, and also generate DC during regenerative braking, are integrated into the motor/diff housing in a very compact unit, that is not much larger than the standard Rolls Royce final drive unit.

Tesla Drive Unit Options

Several Drive Unit designs have been produced by Tesla, differing in power, size and layout:

Model S

Excluding Plaid, the Tesla Model S uses 3 separate Drive Unit (DU) designs depending on configuration

  • 'Large DU'

  • 'Small DU'

  • Front DU

As can be seen from the drive layout diagram, all of these DUs are mounted rear of their differentials.

Model 3/Y

The Tesla Model 3/ Y is available with

  • Rear DU

  • Front DU (optional)

Note that the rear DU is mounted forward of its differential and, whilst being less powerful than the Tesla 'large DU', is much more compact. This DU has the additional benefit of being a PMSR design that provides higher efficiency than an induction motor, helping to improve range for a given battery size. It also has a slightly 'longer' final drive ratio of 9.0:1 vs. 9.73:1 for the Large DU, which, whilst it does reduce the maximum attainable acceleration, also reduces the tendency for wheel-spin.

Rolls Royce SY/SZ Rear Suspension

The SY and SZ series of cars both employ a similar rear suspension and final drive layout, consisting of rear trailing arms attached to a crossmember mounted to the monocoque on 'compliant' mounts.

The spare wheel is mounted rear of the final drive, as is the battery in earlier models, meaning that this space is not usable without cutting the existing bodywork.

The final drive weight is supported by a separate cross member, which is connected to two chassis rails that form part of the monocoque. It is also braced by a torque reaction arm to the main crossmember ( The SZ series provides additional bracing for the higher performance engine fitted to those cars, in the form of a triangulated tubular space-frame).

Design Options

Based on the above, and in order to make EV technology available to as many vehicles as possible, two designs were created:

  1. The 'Standard Edition' design which uses the Model Y/3P drive unit, and which fits within the safe performance envelope of all SY and SZ vehicles without structural modifications (estimated availability, December 2021)

  2. The 'Performance Edition' which uses the Tesla Model S Large rear motor, and is only suitable for the SZ based vehicles, due to their improved chassis performance, and available space (estimated availability, July 2022)

TeslaRR 'Standard Edition'

Earlier cars, such as Silver Shadow and Corniche, require bodywork modifications in order to fit the Tesla 'large' motor, as it sits rear of the final drive, using space occupied by the battery and hood hydraulics. Since one of the project goals was to not make any structural changes to these historic cars, this poses a problem as it necessitates cutting the battery compartment or boot floor of some donor cars. In addition the SY chassis and braking system was not designed for the amount of power and torque this motor provides.

For these reasons, the Tesla Model Y/3P rear motor was selected as the most suitable drive unit for those cars, as it fits within the space between the rear trailing arms, while still providing 'adequate' power.

In fact, the Model Y/3P Drive Unit produces 233kW at the wheels at 90% SoC, which is sufficient to a propel a Silver Spirit from 0-60mph in well under 8s, and to a maximum speed (even with its less than optimum aerodynamics) of over 115mph. Most converted vehicles will see a significant improvement over factory performance, while maintaining the famous Rolls Royce 'waftability'.

TeslaRR 'Performance Edition'

Although bearing a strong family resemblance, the SZ chassis was significantly upgraded over the SY that preceded it, and later in life became the basis of the Bentley Azure. With its improved torsional stiffness, uprated suspension, upgraded braking system with ABS, and stronger drive shafts with CV joints at both ends, it is a platform that has proven itself capable of safely handling high power drivetrains.

As a result of the uprated SZ chassis, it is safe and possible to use the Tesla 'large' motor, and this is installed in a custom cradle that mates with the existing rear crossmember mount points and braces. The motor is driven directly using spoofed CAN bus messages, using technology from 057tech.

The Tesla 'large' motor, with its power output in Performance guise of 400kW and 660Nm of torque, is sufficient to a propel a Silver Spirit from 0-60mph in well under 5s, and to a maximum speed of over 135mph. This comfortably exceeds the Bentley Turbo RT , which was the pinnacle of SY/SZ series performance.

In order to get maximum power transfer with such a powerful drive unit without excessive wheel-spin, some form of traction control is highly desirable. Tesla vehicles employ selective wheel braking which involves direct interaction with the ABS braking system. For safety reasons, the TeslaRR braking system is kept as close as possible to the Rolls Royce factory installation, and specifically the ABS system remains completely standard. Therefore, if traction control is required, a limited slip diff from Quaife can be fitted in place of the standard Tesla open differential.