A tech-centric website is reporting that a number of Tesla Model S P85D owners have dyno’d their cars only to discovery they fall short of the claimed 691 hp they were promised. We also saw that it proved slower than claimed to sixty, as well…
According to Digital Trends, the best they could achieve with a freshly topped off battery was 557 hp, and they reportedly complained that in-gear performance from 70 to 90 mph was nowhere near as good as that of an Audi RS7.
The automaker has not yet provided an official explanation for this, but JB Straubel, the company’s technical boss, did offer up a preliminary explanation via a blog post.
His reply suggests it’s difficult to “directly correlate horsepower ratings in petroleum burning vehicles to horsepower in an electric vehicle,” adding that “the physics of an electric vehicle propulsion system are very different from a gasoline one.”
He concludes by explaining that when they supplied the total power figure for the P85D, all they did was add up the shaft horsepower ratings of the two electric motors.
Here’s what he said in detail:
“Attempting to directly correlate horsepower ratings in petroleum burning vehicles to horsepower in an electric vehicle is a difficult challenge. The physics of an electric vehicle propulsion system are very different from a gasoline one. In an EV, electrochemical reactions in the lithium ion cells create electricity. That electricity flows through power electronics that control the voltage and current, then it flows to electromagnets in the motor that create powerful magnetic fields rotating the shaft to turn the wheels. The power required to rotate this shaft has the most correlation to traditional measures of horsepower. However, the chain actually begins in the electrochemical reactions that happen in the battery pack. Depending on the battery’s temperature, state of charge and age, the amount of electricity extracted can vary widely.
There is some confusion about our methodology for specifying “equivalent” horsepower ratings for our all-wheel drive, dual motor vehicles – the “D” versions of Model S. This document will hopefully answer those questions.
Electrical “Horsepower”
Defining electric power in terms of horsepower is not very intuitive. Kilowatts or Megawatts are a much more useful unit. Electricity alone can’t generate physical motion the way a horse or a fuel-burning engine does. An electric motor converts electricity into motion. Think of electric power as flowing much like fuel flows from a tank to an engine. Various situations (low state of charge, cold temperatures, etc.) can reduce this flow of electrons below the ultimate capability of the electric motor. In other cases, the potential flow of electricity may exceed the capability of the electric motor (warm battery, short duration accelerations, etc.). Since the battery electric horsepower rating varies it is not a precise number to use for specifying the physical capability of an EV. The motor shaft horsepower, when operating alone, is a more consistent rating. In fact, it is only this (single or combined) motor shaft horsepower rating that is legally required to be posted in the European Union.
Dual Motor vs. Single Motor (P85 vs. P85D)
The shaft horsepower rating of the rear wheel drive single motor Model S is straightforward and roughly 360-470 hp depending on the variant (60, 85 or P85). Also, it is generally similar, but not the same, as the battery electrical “horsepower” output. The difference is most obvious to drivers when the battery is at a very low SoC. In this state, the chemical reactions generate less voltage and less equivalent horsepower, even though the physical electric motor hasn’t changed. The maximum torque the electric motor(s) are capable of is nearly unchanged as the battery horsepower changes even though the maximum shaft horsepower is reduced as the battery horsepower reduces.
When we launched the all-wheel drive P85D, we took the straightforward and consistent approach of specifying the combined capability of the two electric motors, front + back. The torque from the two motors comes together resulting in a huge boost in acceleration, the “g’s” you feel in a P85D. This is why Insane Mode is so delightful. The vehicle takes off slightly faster than 1g of acceleration delivering the amazing 3.1 second 0-60 mph (96.6 kph) performance. This acceleration was verified by Motor Trend using a base vehicle and medium weight driver. It should be noted that a larger occupant and additional options that increase weight will reduce the acceleration. Also, the Motor Trend standard excludes the first 28 cm of rollout. Including this rollout adds approximately 0.2 seconds to the acceleration.
One additional note is that, while gasoline cars get worse with altitude, electric cars actually get faster. All cars experience reduced air resistance, but gasoline cars become increasingly oxygen-deprived the higher they go. The Motor Trend test was done at approximately sea level, so the Model S will outperform a combustion car of the same nominal acceleration as altitude increases.
With the shaft horsepower coming out of the motors the situation is not always as simple as front + rear. As we have pushed the combined motor horsepower higher and higher, the amount of times where the battery chemical horsepower is lower than the combined motor horsepower has increased.
Also, the all wheel drive system in the dual-motor cars distributes available electrical horsepower to maximize torque (and power) in response to road grip conditions and weight transfer in the vehicle. For instance, during hard acceleration, weight transfers to the rear of the vehicle. The front motor must reduce torque and power in order to prevent the front wheels from spinning. That power is fed to the rear motor where it can be used immediately. The opposite happens when braking, when the front motor can accept more regenerative braking torque and power.
All Wheel Drive 85D and 70D
Where some confusion occurs is that in the 85D and 70D vehicles the combined motor shaft power is very similar to the battery electrical horsepower under many normal conditions. With the P85D the combined motor shaft power can often exceed the battery electrical horsepower available. The dual motors utilize the battery horsepower in the widest variety of real world conditions. The true measures for any performance EV driver are acceleration times and driving performance of the vehicle.
JB Straubel”
Note: the following video is from DragTimes’ May 2015 dyno attempt, which didn’t go so well…