Tesla’s J.B Straubel Discusses Model S AWD Power & Torque Specifications

SEP 23 2015 BY ERIC LOVEDAY 26

Tesla Model S Dual Motor

Tesla Model S Dual Motor

Tesla Model S P85D

Tesla Model S P85D

Tesla’s chief technical officer, J.B. Straubel, authored a blog post on Tesla Motors’ website titled “Tesla All Wheel Drive (Dual Motor) Power and Torque Specifications.”

In this post, Straubel attempts to clear up “…some confusion about our methodology for specifying “equivalent” horsepower ratings for our all-wheel drive, dual motor vehicles – the “D” versions of Model S.”

Straubel says that his post “will hopefully answer those questions.”

Here’s the post in its entirety:

Tesla All Wheel Drive (Dual Motor) Power and Torque Specifications

JB Straubel, Chief Technical Officer September 21, 2015

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.

Motor Trend's Tested Specs/Test results

Motor Trend’s Tested Specs/Test results

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.

Tesla Model S P85D

Tesla Model S P85D

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

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26 Comments on "Tesla’s J.B Straubel Discusses Model S AWD Power & Torque Specifications"

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Tesla cannot run from things that can be tested. 4-wheel dynos exist, to prove horsepower, yet they choose to neglect the “battery chemical horsepower” weak link, and instead over-represent with descriptors, like 691HP, and “259HP front HP, and 503 rear motor power”.

The cars don’t make this HP, and it is that simple.

Still wicked fast. Just no reason for the slippery slope that is beginning to catch others.

Meh. As long as the car can output that HP in the optimal situation (fully-charged I guess?), then I’m fine with it.

It’s not like the gas car companies don’t exaggerate their claims a lot.

Between Hyundai/KIA on MPG, GM on ignition switches, Toyota on acceleration, the airbag fiasco, Ford on MPG, and VW on dieselgate . . . the entire industry seems like a cesspool.

Anyone that thinks industry can ever self-regulate has been thoroughly disproven.

Yeah, ICE cars don’t get the rated horsepower when they are driven in sub-optimal conditions such as at high altitudes (thinner air).

ICE cars never get the manufacture’s reported horsepower or torque on a wheel dyno either. Because the manufacturers come up with their engine ratings at the crank, without any regard to drivetrain loss, or other factors from the rest of the automobile.

The engine’s output that car makers report has always been measured separately from all the rest of the components in the car. That is the same way Tesla is reporting their electric motor ratings.

Yes. It’s SAE net at the crank. However, that SAE net now includes losses due to pumps and other accessories.

Seems the issue some folks have is that they used a different method for the other models.

There are some really annoyed (and many very annoying) folks on TMC that either came from other fast highend cars or that spent/wasted a bunch of money to upgrade their P85 or S85 to P85D with the hope of crazy horse power. They don’t seem to care about 0-60 but only about like 50-90. Some are just looking for a free ludicrous upgrade it would appear. As some pointed out it brought a new/diff breed of consumers to Tesla. For better or worse.

Overall I think Tesla’s 0-60 numbers are getting a lot of positive attention/respect in the car world. Of course the above TMC folks will go ad nauseum about the “1 ft rollout” issue.

What it really suggests is that key performance measurement metrics (or more accurately marketing comparison metrics) are likely to evolve to something different for all-electric cars.

Battery and controller power curves might replace engine torque & power curves, for example.

Even the one foot roll-out was really an evolution created to offset the throttle lag caused by electronic engine management for emissions. In the old days one just opened the barrels and poured in a gallon of fuel with double pumper carburetors… Dirty, but fast!

Uhhh… No.
1ft rollout is a holdover from the staging beams at a drag strip. The strip uses two parallel beams to measure start time. As a result, the car gets a ‘free’ foot of movement before the wheel clears the beam and the timer starts.

You literally pulled your ‘throttle lag’ explanation out of your rear end.

J.B Straubel’s post appears to be “damage control”, attempting to justify Tesla posting, for the twin-motor Model S, horsepower ratings which significantly exceed the amount of power that the battery pack is capable of providing, and thus significantly exceeding the amount of power available to push the car down the road.

Citing the horsepower ratings of the two motors individually is useful, and it is appropriate to list those numbers in specs for the car. Giving the sum of the two — as Tesla did in its advertising — is highly misleading, whether or not Tesla actually intended to mislead potential customers.

As they say: You must not only avoid impropriety, you must avoid the appearance of impropriety. Tesla is guilty of flagrant impropriety in hyping its accomplishments, so it appears this was a case of deliberately misleading the public, whether or not Tesla actually made a decision to do so.

I am a big fan of Tesla’s vision, its accomplishments, and its goal. That doesn’t mean I give them a pass on their often highly misleading hype.

“The true measures for any performance EV driver are acceleration times and driving performance of the vehicle. — JB Straubel”

The 0-60 are easily measurable and “feelable ” by normal people driving the cars and those were accurate by general standards.

All Teslas previous cars were measured “street style”, but now they say: Oh by the way, the numbers marketed for a year for the P85D was measured “drag race style”.

Is the battery incapable or just incapable when it is not fully charged?

The P85D, P90D battery is incapable, as setup by Tesla, to reach “motor power”, under any condition.

The implied HP, of running the 18650’s Tesla uses to a C-rate of about 6, is something others have said would achieve closer to the 691HP. “Ludicrious”, for both 85 and 90kwh batteries, replaces the 1300 amp fuse, with a 1500 amp fuse. By doing so, it uncorks much of the additional C-rate (discharge), that is needed.

Glass is definitely half-full. I just hate to see the word play.

Epa didn’t care about that but the VW scandal got tesla nervous obviously 🙂

VW was about pollution and physical trickery.

That looks crasy fast!
Hope i will not get to disappointed, when i will get my 85D on monday

I’m so jealous! 😉

The lesson is, if you’re going to drag race, make sure you are fully charged first.

Actually 85-90% charged should be best.

The read I get, is that JB’s indirectly telling people that their cars will get better performance, as newer batteries become available. We’ve already seen a capacity jump and a retrofitted pack for a car they’ve not built in years (Roadster). SO, the precedent is certainly there for such an expectation.

This is probably the first time I’ve heard there there is physical “Headroom” still left in the drivetrain for better performance as the vehicle ages.

What kind of Tesla Owner are you? Is your glass half full, or half empty?

I guess I can see a grey area of subjectivity to the issue. But no one can deny the amazing performance of the current P90D’s. Nothing in that price range, with zero emissions and almost no sound– touches it.

But people are weird. And I find it surprising that some folks are still dissatisfied with their amazing EVs. *shrugs*

Weird.

This comes down to people not understanding how automotive items are rated. The aftermarket performance industry runs into this all the time. For example, if a 600 CFM carburetor is installed on a small displacement engine with a relatively low RPM redline, it will never flow 600 CFM. Is it still a 600 CFM carb, if the rest of the motor can only flow 450 CFM? Yes, the carb is still a 600 CFM carb. Tesla rates their electric MOTORS at 221-hp/244 lb-ft front, and 470-hp/433-ft-lb rear. When the car does not flow enough electricity to run both motors at their peak rating, are they still 221-hp/244 and 470-hp/433-ft-lb motors? Yes they are. The motor ratings are always what they are, because that is the rating of that part. That part will always keep that rating regardless of anything else. Just like a 600 CFM carb is always a 600 CFM carb, even when operating at 200 CFM or 450 CFM. If a car company sells a car with 250 liter/hour fuel pump, or a 600 CFM carb, or a 200 MPH speedometer, and the fuel pump never flows over 150 liters an hour, and the the carb never flows… Read more »

I don’t know why JB is spouting this nonsense since he must know better. AT least you’d THINK he’d know better.

1). One Horsepower is 746 watts.
2). Higher elevation reduces motor cooling.

I think the issue here is that when ICE HP Is measured, it is clearly defined at crank shaft and we assume that engine will make that HP given at sea level with reasonable temperature/pressue and fuel delivery which are pretty standard. But with electric powertain, you have 3 components here. They can be rated differently and it is the overlap of the 3 that is the “system level hp at the drive shaft”. If battery can only deliver 150HP, Inverters can only handle 120HP and motors can handle 200HP, then the system is only 120HP at best at the drive shaft before the gearbox… What Tesla has shown is the spec of the motor or motors. You can argue that is misleading or NOT. Tesla took the aggressive interpretation that ICE is rated by its engine capability, so I am going to rate the electric motor capabilities… So, there is some truth to that. But the difference is that Engine’s fuel and air delivery system is actually tunned to match that HP claim (except for evlevation). In the case of Tesla, it is battery max output that is limiting the total power delivered. It is like an engine with… Read more »
Factory ICE motor HP and torque ratings aren’t de-rated for accessory draw. In fact, when they are tested in the stand, outside of the car, they don’t even have stuff like A/C, or cooling fans hooked up at all. They usually don’t even have a full exhaust system, a full intake system, or any draw on an alternator. It is just a motor on a stand, with the coolant being forced through the motor from an external source, air blowing over the motor from an external source, and electricity being provided by an external source. No transmission losses. No transfer case losses (AWD/4WD). No torque converter losses. No driveshaft losses. No differential losses. Heck, Tesla’s don’t even have half of those components. I think the biggest problem with folks trying to understand electric vehicle torque and HP ratings, is that folks don’t even really understand how their gas car’s HP and torque ratings are measured. Factory ICE motor HP and torque ratings really don’t take into account how much of that power can be put to the ground at all. Take Pickup Trucks for example. Ever notice that factories rate the HP and Torque for their truck motor exactly the… Read more »

Well, Tesla and other EV makers also don’t rate their electric motors clearly either.

They are NOT rating it according to battery temperature or motor temperature or SW that would detune the inverter power to protect battery.

Those 1200 A or 1500 A are ideal situation for the battery as well…

So, it is all “loosely” defined for everyone.

HP should be rated at wheel.

All EV motor makers (and all ICE motor makers) rate their motors according to long-standing ANSI rating standards. The fact that you are not familiar with these long-standing, scientific, ANSI standards that have been peer-reviewed and are the scientific gold standard for rating motors doesn’t make their ratings unclear. The ANSI standards do not rate “at the wheel” for ANY power source. Not for AC motors, not for fuel cells, not for ICE motors, not for rotory motors, not gas turbines, etc. You have a significant misunderstanding of how devices are rated. There is no possible way to give a motor a single HP or torque rating while at the same time accounting for multiple battery temps, multiple motor temps, etc. That would result in a range of different numbers. That is why ANSI standards exclude such factors for hp and torque ratings. If you disagree with ANSI standard testing, please feel free to join ANSI and become a full member, and submit your proposals. If they accepted, then Tesla and everybody else will follow your new standard. Until then, they will follow the current ANSI standard. And if you are unable/unwilling to learn and understand what the ratings mean,… Read more »