Dragtimes P90D Ludicrous 0-60mph and 0-100mph video

[sorka]

What's the offset from calculated wheel+drag power to the REST KW?

Not sure I understand the question. Are you asking what the loss is from battery to wheels? After factoring in drag and before 90 MPH, it's about 10% which is remarkable for an AWD vehicle until you consider how little drivetrain mass there is from motor shafts to the wheels compared to a typical ICE AWD.

If that's not what you meant, then clarify and I'll try and answer.

 

[lolachampcar]

sorka,

Nice work!! and yes, 10% is very nice when you consider inverter/motor losses. I guess it is no surprise that Tesla produces an efficiently engineered drive system.

 

[sillydriver]

Zextraterrestrial -- thanks for the link to the Wikipedia article on Back EMF. That motivated me to do some more searching and I found two very interesting early blog posts from Tesla. This one

AC Induction motor. Why? | Forums | Tesla Motors

gives great info on how they work and why Tesla chose them, although it doesn't answer my question. This one

Roadster Technoloy - Motor | Tesla Motors

describes the motor used in the roadster. It shows that motor's curve of torque v RPM.

The fall from 300 at ~4.5k to 150 at ~9k is 1 / RPM, so constant power, but the further fall to 75 at ~13k is faster than 1 / RPM, so lower power. I assume the model S motor has the same high-speed fall in power. I guess lower power/acceleration at high speeds, where the motor is above it's power band, is the tradeoff for the simplicity of 1 gear and no transmission. The question I haven't answered is whether there is inefficiency. The Back EMF article implies not: the battery voltage would drop, so less power would come out of the battery at high RPMs. I need to study induction motors more, especially power factors, reactive loads and the like -- I should know this stuff. I was a physics major 35 years ago, but I slept through electromagnetism, which was my least favorite subject.

Sorka -- you said a 10% power loss. In the text of the roadster article it says 88% efficiency, so a 12% loss. Very good estimate on your part.

 

[lolachampcar]

I've always looked at BackEMF as rolling a ball off a table. At the start, the table top is three feet off the floor so the ball is moving a certain speed when it hits the ground. Increasing rpm is like lowering the table; there is less ball speed when it hits the ground. Eventually, the table is level with the floor at which point there is no ball speed. This is why things like brushless DC motors (similar to AC induction) have a RPM/V rating which is partially a function of how the motor is wound and how many polls it has.

Tesla can alter the design of their motors to change the BackEMF equation. They've indicated the front and rear motors on the dual drive cars have different characteristics which allows for better overall efficiency as you use the best motor combination for any given road speed.

Lastly, I too found the whole BackEMF does not affect efficiency thing strange until I considered what BackEMF actually meant. Say you start with 100V source and zero rpm. The motor "sees" 100V across its terminals. Spin that same motor up until the BackEMF is 50V and now the motor only "sees" 50V and, assuming the impedance has not changed, only takes half the current. Motor power output is falling but so is power being drawn from the battery.

All the above is a simplification. Please feel free to chime in and correct things.

 

[WarpedOne]

assuming the impedance has not changed,

BackEMF actually increases "effective" impedance.
Faster it spins, higher it is. Higher impedance == less power at certain input voltage (that can only be as high as battery voltage).

 

[qwertzy]

Hey guys, in this video the Rs7 stage 2 with awd has 700hp and reaches the 1/4 mile in 11.38 seconds. So its safe to say the P90D has 691hp just based on that.

 

[Andyw2100]

I thought people following this thread might be interested in this relatively recent post:

P90D Ludicrous 0-100 mph x 3 runs

It includes links to three 0-100 runs in a P90D with Ludicrous using a Racelogic Performance Box.

 

[sorka]

Sorka -- you said a 10% power loss. In the text of the roadster article it says 88% efficiency, so a 12% loss. Very good estimate on your part.

I'd like to take credit for that but I'm using tools that pretty much give me the answer.

- - - Updated - - -

Hey guys, in this video the Rs7 stage 2 with awd has 700hp and reaches the 1/4 mile in 11.38 seconds. So its safe to say the P90D has 691hp just based on that.

How so?

 

[qwertzy]

That was actually a bad comparison, as in the second part of the video the 700hp rs7 does 10.8 seconds in the 1/4 mile.

The rs7 weighs 4475 lbs, so thats 461 lbs less than the P90D at 4936 lbs. For the Tesla, the extra weight does help put more power to the wheels faster along with the instant torque. But if the rs7 had the extra weight, that benefit is negated with the lack of instant torque. Aerodynamics is in favor of the Tesla's .24 coefficient vs .30 from the rs7.

If the P90D actually does 10.9 in the 1/4 mile, it would be pretty close in performance to a 700hp awd vehicle that weighs less.

 

[P85DEE]

That was actually a bad comparison, as in the second part of the video the 700hp rs7 does 10.8 seconds in the 1/4 mile.

The rs7 weighs 4475 lbs, so thats 461 lbs less than the P90D at 4936 lbs. For the Tesla, the extra weight does help put more power to the wheels faster along with the instant torque. But if the rs7 had the extra weight, that benefit is negated with the lack of instant torque. Aerodynamics is in favor of the Tesla's .24 coefficient vs .30 from the rs7.

If the P90D actually does 10.9 in the 1/4 mile, it would be pretty close in performance to a 700hp awd vehicle that weighs less.

The trap speeds will tell a better story.

 

[sorka]

That was actually a bad comparison, as in the second part of the video the 700hp rs7 does 10.8 seconds in the 1/4 mile.

The rs7 weighs 4475 lbs, so thats 461 lbs less than the P90D at 4936 lbs. For the Tesla, the extra weight does help put more power to the wheels faster along with the instant torque. But if the rs7 had the extra weight, that benefit is negated with the lack of instant torque. Aerodynamics is in favor of the Tesla's .24 coefficient vs .30 from the rs7.

If the P90D actually does 10.9 in the 1/4 mile, it would be pretty close in performance to a 700hp awd vehicle that weighs less.

The P90D's 1/4 mile time is front loaded because it puts down way more power at lower RPMs (i.e insane torque) giving it an insane 0-60 time. This is why the P85D and P90D have much better ETs than their exit speed would suggest. You don't find any ICE cars that can pull off an 11.6 second 1/4 mile that only exit at 115 MPH.

The exit speed is a much better indication of power than exit time.

The RS7 has a power to weight ratio of 1 hp / 8 lbs. The P85D was speced at 1 hp / 7 lbs but in reality is 1 hp / 9 lbs. The P90D is actually 1 hp / 8 lbs, about the same as the RS7. If you put a P85D next to a stock RS7 at sea level and both punch it at 60 MPH, the RS7 will pull away at a rate suggested by their respective power to weight ratios. I imagine the P90D, with it's 611 hp (456 KW) would be about a tie. The problem with that guess though is that a stock RS7 still exits the 1/4 at 122 MPH compared to the P90Ds 115 MPH. With equal power to weight ratios, the P90D should be exiting around 122 as well.

A lot of folks are scratching their heads wondering how the P90D can only have the same exit speed as the P85D.

 

[AWDtsla]

The trap speeds will tell a better story.

How does it matter if the Tesla still won? Would you prefer a car that did 10.9 at 160 mph? It would be completely miserable to drive.

 

[sillydriver]

The RS7 has a power to weight ratio of 1 hp / 8 lbs. The P85D was speced at 1 hp / 7 lbs but in reality is 1 hp / 9 lbs. The P90D is actually 1 hp / 8 lbs, about the same as the RS7. If you put a P85D next to a stock RS7 at sea level and both punch it at 60 MPH, the RS7 will pull away at a rate suggested by their respective power to weight ratios. I imagine the P90D, with it's 611 hp (456 KW) would be about a tie. The problem with that guess though is that a stock RS7 still exits the 1/4 at 122 MPH compared to the P90Ds 115 MPH. With equal power to weight ratios, the P90D should be exiting around 122 as well.

Speaking of power to weight ratios, I think this post is relevant.

P90D Ludicrous 0-100 mph x 3 runs

Hope I managed to copy the URL correctly -- I am new at this. It's in the thread Andy pointed to in post 167 of this thread.

 

[P85DEE]

How does it matter if the Tesla still won? Would you prefer a car that did 10.9 at 160 mph? It would be completely miserable to drive.

I was speaking in response to qwerty's post #169.

Indeed, Sorka answers it the same way, when he says "The exit speed is a much better indication of power than exit time", however he elaborates a bit further.

BTW, would you expect a car with the ability to go through the traps at 160mph to run an ET of only 10.9 seconds???????

 

[AWDtsla]

BTW, would you expect a car with the ability to go through the traps at 160mph to run an ET of only 10.9 seconds???????

This is precisely the misunderstanding. Is it possible? Absolutely. The lower the trapspeed, the quicker the car has output a given amount of energy. You guys would prefer high trapspeeds because it proves this "horsepower" value, I would prefer lower trapspeeds because the car drives better. Horsepower was always the wrong measurement. Anyone that had a tuned turbocharged car already knew this. My real reaction to all of this is "whatever". The car is fast as sh**. It's only slow when you specifically pick a scenario in which it is disadvantaged, i.e. give the gasoline car time to upshift to the right gear, all mechanical parts up to speed (turbo's spooled, driveline lash taken out, driveline momentum fed). Anyone that's tried to upgrade a turbocharger should have figured out that you can get massive horsepower from a bigger a turbo, but 50-70, 70-90, etc times can even get worse because how long you're waiting for the power to arrive. Even high-response high-revving european v8's had the same damn problem, you're not making rated power until you're near redline.

I'm out of this thread, I'll tune back in when some does or no one can reproduce the 10.9 quarter in a P90D.

 

[sorka]

This is precisely the misunderstanding. Is it possible? Absolutely. The lower the trapspeed, the quicker the car has output a given amount of energy. You guys would prefer high trapspeeds because it proves this "horsepower" value, I would prefer lower trapspeeds because the car drives better. Horsepower was always the wrong measurement. Anyone that had a tuned turbocharged car already knew this. My real reaction to all of this is "whatever". The car is fast as sh**. It's only slow when you specifically pick a scenario in which it is disadvantaged, i.e. give the gasoline car time to upshift to the right gear, all mechanical parts up to speed (turbo's spooled, driveline lash taken out, driveline momentum fed). Anyone that's tried to upgrade a turbocharger should have figured out that you can get massive horsepower from a bigger a turbo, but 50-70, 70-90, etc times can even get worse because how long you're waiting for the power to arrive. Even high-response high-revving european v8's had the same damn problem, you're not making rated power until you're near redline.

I'm out of this thread, I'll tune back in when some does or no one can reproduce the 10.9 quarter in a P90D.

You've just haven't driven a fast car so your perspective is different. At any highway speed, the RS7 downshifts in milliseconds and the turbos are spooled up almost as fast. If you've ever driven one you'd know the 50-70 and 70-90 passing speed kills the P85D. The P85D might have a slight 1/4 car lead for 100 ms but after that, it's all over. This is for any passing maneuver above 50 MPH.

That said, I agree with you if you're talking speeds below 50 MPH. In stop and go driving on expressways, I'd take the P85D over the RS7 in a heartbeat. On the open highway, it's the other way around.

 

[lolachampcar]

"You've just haven't driven a fast car so your perspective is different"

As much as I disagree with others on issues, and I use that disagree in the kindest possible way, I can understand a vast majority of the positions expressed. Those that I do not understand, I use a mirror on myself and think they must view me in the same light.

sorka, I think you missed the mark on the quote above when it comes to me. I believe I have enough seat time to comment with conviction. The real issue here for me is that a motor is not an engine. Applying engine standards to a motor so you can be upset is not taking responsibility for knowing the difference. It is not that the PDL can not perform like an ICE up top but that an educated buyer should know that it never had a chance to in a production BeV with a long life battery. This is why I can not get upset over the issue. By the same token, I've heard arguments about buying the car in Europe based solely on the hp numbers and being disappointed. I can not logically dispute that position.

 

[dsm363]

You've just haven't driven a fast car so your perspective is different. At any highway speed, the RS7 downshifts in milliseconds and the turbos are spooled up almost as fast. If you've ever driven one you'd know the 50-70 and 70-90 passing speed kills the P85D. The P85D might have a slight 1/4 car lead for 100 ms but after that, it's all over. This is for any passing maneuver above 50 MPH.

That said, I agree with you if you're talking speeds below 50 MPH. In stop and go driving on expressways, I'd take the P85D over the RS7 in a heartbeat. On the open highway, it's the other way around.

So what gas powered car under $250,000 accelerates like the P85D below 50 and like the Audi above 50mph then? That looks to be the car people were expecting with the P85D. Each kind of engine or motor has an area it excels at. Some people seem to have bought the P85D expecting both but getting only one.

 

[yak-55]

So what gas powered car under $250,000 accelerates like the P85D below 50 and like the Audi above 50mph then? That looks to be the car people were expecting with the P85D. Each kind of engine or motor has an area it excels at. Some people seem to have bought the P85D expecting both but getting only one.

I was expecting both. It was advertised as 3.2 for 0-60 and 691 hp. I knew enough about EVs to expect the excellent throttle response, but not enough about Tesla to expect the hp is not achievable by the car.

 

[sorka]

So what gas powered car under $250,000 accelerates like the P85D below 50 and like the Audi above 50mph then? That looks to be the car people were expecting with the P85D. Each kind of engine or motor has an area it excels at. Some people seem to have bought the P85D expecting both but getting only one.

I think people were "expecting" a car that had the specs that were advertised. They advertised a peak torque which they met and and results in low end grunt off the line consistent with their advertised torque. They advertised a peak horsepower which was severely and grossly overstated by 136 to 211 hp (depending on state of charge 30 to 90%) and as such such only accelerates like a 5000 lb car with 480 to 555 hp at anything but slow speeds. I do most of my driving on the freeway. If someone else who bought a P85D does most of their driving stop and go, it's their prerogative to be satisfied. That doesn't mean everyone else is expected to be and it doesn't excuse what Tesla did.