Ever since the release of the high-performance Tesla P85D, its combined horsepower of 691 has been quoted liberally and loudly.
CEO Elon Musk confirmed the rating on “60 Minutes.” It was printed in a Tesla Model S brochure (see below). And a multitude of automotive magazines and websites listed it.
But one group of Tesla owners questioned whether this number is accurate. In an open letter to Musk last month, the owners challenged Tesla’s statements on the P85D’s horsepower.
“We are writing to you as a group of concerned Tesla owners, investors, and perhaps most importantly, supporters … because the Tesla Model S P85D falls considerably short of actually making 691 horsepower,” the letter stated, which was signed by 71 Tesla owners.
An excerpt from the letter:
“The basic facts are as follows:
“The P85D was marketed as making 691 horsepower. It doesn’t. We believe, based on various testing methods and tools including dynamometer testing and testing with professional performance data loggers that the maximum energy output is 415 [kilowatts], which results in a maximum horsepower of 557 before any drivetrain losses on a fully charged battery. Even without factoring in drive train loss, this is almost 20 percent less than the advertised horsepower.
“The missing horsepower is quite noticeable at highway passing speeds. For example, from 70-90 mph, the P85D should perform like a car with a power to weight ratio of one HP for every seven pounds. Instead it performs like a car with one HP for every nine pounds. The result of this is that from 70-90 mph the P85D is easily outperformed by an Audi RS7 with a power to weight ratio of only one [horsepower] for every eight pounds.”
Yesterday, Tesla’s Chief Technical Officer JB Straubel discussed horsepower ratings for electric vehicles (EVs) on the company blog, though he didn’t mention if his post was in response to the letter.
“There is some confusion about our methodology for specifying ‘equivalent’ horsepower ratings for our all-wheel drive, dual motor vehicles,” Straubel said. “Attempting to directly correlate horsepower ratings in petroleum burning vehicles to horsepower in an electric vehicle is a difficult challenge.”
“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,” Straubel explained.
“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.”
Straubel also noted that, to calculate the overall horsepower of a dual-motor EV, the rating of the front motor was added to the rating of the rear motor.
“The shaft horsepower rating of the rear wheel drive single motor Model S is straightforward and roughly 360-470 [horsepower] depending on the variant (60, 85 or P85),” he said. “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.
“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.”
In his article, Straubel offers plenty of technical specifications for the P85D. He talks about the g-forces during acceleration, He lists how many seconds it takes the P85 to reach 60 mph and the length of the rollout that’s excluded. But nowhere in the article does he either confirm or correct the 691 combined horsepower rating.
Instead, he offers a final impression:
“The true measures for any performance EV driver are acceleration times and driving performance of the vehicle.”
P85D owners that feel cheated out of the promised 691 horsepower will likely not feel appeased by Straubel’s closing thought, or by a letter that doesn’t directly declare what the combined horsepower is.