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lolachampcar
Well-Known Member
I think the place where we differ here is not the engineering definition of a horsepower but the willingness or unwillingness to understand that ICE and BeV are not the same. We can come up with example after example of where a BeV does not perform as expected from an ICE vantage point. Tesla plays fast and free with advertising motor horsepower all the time knowing what the battery can and can not deliver. I have no idea if the JB's suggestion that there may/is a point where the system is motor limited and if that point equates to the total possible output of the motors. Unlikely as it is, there may actually be an instant where the car produces large amounts of hp. Without Tesla's information or highly sophisticated analysis of the whole system, I simply do not know. Likely, it does not.
From my perspective, it simply does not matter. Tesla told me how quick and how fast my car would be and the car they delivered exceed those specifications. The big difference is I knew it would not perform like a monster ICE above 60 mph and did not assume, just because they provide a hp figure, that the whole system's nature had been redesigned so as to have low and high end performance.
perkiset
... this one goes to 11
FCOL. You can't hardly go anywhere on this forum anymore without this argument.
So back to the OP - awesome. And yeah, I also love it that it's *nearly* silent .. the tiny whine I get, which I can only hear well when the AC is off, is extremely satisfying and SciFi.
So back to the OP - awesome. And yeah, I also love it that it's *nearly* silent .. the tiny whine I get, which I can only hear well when the AC is off, is extremely satisfying and SciFi.
Not disputing that BEVs have different performance characteristics that ICEs. But the P85D accelerates at freeway speeds exactly as it should for the weight and the horsepower it has. If it had 691 hp, it would accelerate the way I'd expect a 691 hp car to accelerate on the freeway.
At lower speeds, the BEV does better than the ICE where those speeds allow the BEV to produce the power that the ICE can't make at much lower RPMS giving it a tremendous torque advantage. From a pure acceleration standpoint, it's not the that the BEV is limited at freeways speeds, it's that it has a big advantage at low speeds where the ICE at those low speeds can't match the torque (i.e. power at low RPMS) with the BEV. It's not until the ICE reaches peak power and it's multi speed transmission can keep it near it's peak power that the acceleration is similar to the BEV hp to hp and lb to lb.
Tesla's cooling of the DU and battery is insufficient to sustain high power for a long time. Should Tesla have disclosed this too? Perhaps, but I've already pointed out the applications where owners might care about that. Not my beef. I'm not as demanding. I'd take 691 hp for short periods of time to effect highway passing maneuvers and I'd be satisfied even if it required cooldown periods in-between.
Despite the differences, my beef is that the P85D does't produce 691 hp for even short periods. It never even comes close and is in fact between 480 and 555 hp depending on SOC. A 52 hp difference, at highway speeds, between the 85D and P85D and even that difference vanishes as both cars drop below 60% SOC. Yet there was a 315 hp listed difference between the 85D and P85D.
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This argument, with slightly different players and methods of persuasion, has been going on for 7 months now without stop. In multiple threads, often OT. Has anyone been convinced of anything yet? Are we close to a resolution? I haven't seen anyone on either side budge from their position, so what is the point of this?
scott jones
Member
Ok, correct me if I am wrong but I always thought horsepower related to potential top speed and torque to acceleration and yet everyone seems to be trying to translate horsepower to acceleration. Additionally, none of this takes gearing into account i.e. you can take a 500 HP ICE engine and change acceleration based on gearing vs. the torque curve. Hitting max speed requires careful gearing to have the engine at max HP right when you hit max speed. Aren't these tradeoffs precisely why electrics are so quick off the line but lose to ICE at higher speeds?
Correct, but that hasn't stopped the argument that effectively says "If the car had 691HP, highway acceleration would be so much better". Not true, as all BEVs tend to perform better off the line, with a single gear ratio. Maybe the "agree to disagree" part can become isolated to how Tesla should represent power, is it miss-leading or not, etc.?
Electrics are quick because it's not horsepower that matters, it's how much potential energy you can translate into kinetic energy. Power is a measure of rate of energy output, but since an ICE can only utilize it's crank power in limited scenarios (i.e. engined revved to launch rpms, gearing allows engine to run at optimal horsepower output), it can't convert as much kinetic energy as an electric car like a Tesla. Where-as a Tesla has reached maximum power output as soon as we exit the traction limited region of acceleration.
stopcrazypp
Well-Known Member
Let's get back to the original topic, which is that the P90DL does not appear to get its advertised 1/4 mile time and this is the first time this has happened in a Tesla. We have enough threads about the whole power argument (where we are pretty much arguing in circles), so let's not turn this one into another one of those.
Andyw2100
Well-Known Member
Torque is a function of hp at a specific RPM.
hp = torque * RPM / 5252
torque = hp * 5252 / RPM
Saying an engine has a lot of torque is just saying that it produces a lot of horsepower at lower RPMs. The amount of power it takes to accelerate from one velocity to another velocity is a function the amount of power under the curve, not peak power. So a car that accelerates really fast from a stop is producing a lot of power at low RPMS and more area under that curve up to some speed.
So from a start, the P85D has a massive advantage because it's able to produce lots of power at very low RPMs and hence accelerates faster from low speeds. The ICE has to get into it's power band first which is *FAR* narrower than the P85D's power band. But once the ICE gets there, if it's multi speed transmission is geared right, will keep it it right in the middle of it's peak power band.
Once both the ICE and the P85D are producing maximum power, the playing field is essentially level and the car that accelerates faster from that point will be the car that has the highest power to weight ratio. This is why both the M5 and the RS7 will out accelerate the P85D from 70-90 even though all 3 cars are making the same power but the lighter weight of the RS7 and M5 given them a higher power to weight ratio.
sillydriver
Member
I think you explained this accurately and very well. I would add that it seems like there is something about Tesla's induction motor system that loses efficiency at high RPMs. Acceleration is force over mass, and leaving things like air drag aside, that force should go as 1 / RPMs or 1 / vehicle speed, given a single gear and assuming constant motor power. The power the battery produces is constant in an acceleration run, but it seems like Tesla's acceleration in Gs falls off faster than 1 / speed at high speeds, even accounting for air drag. It seems like the horsepower Tesla is sending to the wheels falls at high speeds because of some kind of loss of efficiency transforming electric power into mechanical power at high motor RPMs. If so, the high electrical power coming out of the battery has to go somewhere, and that must be heat. Dissipating the heat must make improving high-speed acceleration even harder. My question for the group is whether any people knowledgable of electric motors here know whether efficiency does decline at high RPMs.
Back EMF at high RPMs surely makes efficiency decline - pretty sure the motor begins trying to generate as well as use energy? must produce lots of heat at that point too
https://en.wikipedia.org/wiki/Counter-electromotive_force
not sure if this works as an analogy -
running down a hill you get to a speed where you can potentially go faster but your legs will only end up slowing you down, probably catastrophically
https://en.wikipedia.org/wiki/Counter-electromotive_force
not sure if this works as an analogy -
running down a hill you get to a speed where you can potentially go faster but your legs will only end up slowing you down, probably catastrophically
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I agree. When you correlate the "power" field in KW from the REST API to the vbox calculation of power at the wheels and then add the cost of drag at various speeds back into the curve, the REST KW reading is nearly flat with power at the wheels but it starts to diverge after 90 to 95 MPH. After 100 it starts to become obvious. Then after 120 MPH, it it seems to diverge rapidly at that point even after you account for the acceleration lost due to drag.
I'm in the process of producing some new graphs for my 0-60 thread and should be ready in a few days:
Playing around with VBOX Sport.... 0-60 times...
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