920 lb feet at the wheels is really low at those speeds at least.
In this case it really can't be at the wheels. Usually the dyno uses the gearing ratio to calculate torque at the shaft.
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P100D on the Dyno....
- Thread starter fiksegts
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In this case it really can't be at the wheels. Usually the dyno uses the gearing ratio to calculate torque at the shaft.
Torque at the wheels is measured, but that's not what your graph shows. 920lb-ft of torque at the wheels is incredibly low. It would be about 4.1kN of force, assuming wheels with a 24 inch diameter, which would accelerate the vehicle at about 0.2g, resulting in a 0-60 time in 14 seconds. Just keep in mind that the Hellcat has over 8,000lbs-ft of torque at the wheels in 1st gear. Even a Camry 4 cylinder has over 2000lbs-ft of torque at the wheels, in first gear. Torque at the shaft can be calculated using the gear ratio and I think that is what your dyno guy tried to do, but failed.
Agree R.S. The math doesn't add up.
They quoting "torque is what matter not hp" don't understand how it works.
HP tells more about the performance.
A torque number alone the engine can make at the shaft tells nothing without knowing gearing ratio and rpm.
Its only important information for the guy designing the transmission/gearing
When a car’s performance is tested on a dyno, its torque is measured The measure of an engine’s performance is torque. Horsepower is an additional number that’s attained by multiplying the torque by the RPMs
Horsepower = (Torque x RPMs) / 5252
They quoting "torque is what matter not hp" don't understand how it works.
HP tells more about the performance.
A torque number alone the engine can make at the shaft tells nothing without knowing gearing ratio and rpm.
Its only important information for the guy designing the transmission/gearing
When a car’s performance is tested on a dyno, its torque is measured The measure of an engine’s performance is torque. Horsepower is an additional number that’s attained by multiplying the torque by the RPMs
Horsepower = (Torque x RPMs) / 5252
If they dyno it in 1st gear, maybe. But no chassis dyno can measure that high.
I don't know the gear ratio on the Hellcat. But it was not possible to dyno the 510 hp Subaru STi I had before on a Dynapack in first gear. I tried, just got red screen and to much torque error Engine Torque was 680nm or 501 ft lbs.
Gear ratio on the Subaru was 1st: 3.636 and final drive ratio: 3.90. 680nm x 3.636 x 3.90 = 9642 nm (7111 ft lb)
So I have no problem to believe the Hellcat puts down more than 8k in first gear.
The Dynapack was a DAQ54. Max limit on the Dyno is 2625hp and 8500 nm (6269 ft lb) for a 4wd car. Max for a rwd car is 1390hp and 4500 nm (3319 ft lb)
That is the reason we dyno in the 4 or 5 ft gears because they are closer to 1:1.
Did try a Tesla P85 in that dyno but did as expected get the same to much torque error.
600nm x 9.73 = 5838 nm (4305 ft lb) So above the torque limit for rwd on the Dynapack.
If the torque numbers Tesla did specify for the P85D and P90D are correct they should put down about 9058nm at the wheels.
931nm x 9.73 = 9058nm (6680 ft lb)
The front drive unit is 9.34:1 so its not 100% correct.
Engine Torque was 680nm or 501 ft lbs.Gear ratio on the Subaru was 1st: 3.636 and final drive ratio: 3.90. 680nm x 3.636 x 3.90 = 9642 nm (7111 ft lb)
So I have no problem to believe the Hellcat puts down more than 8k in first gear.
The Dynapack was a DAQ54. Max limit on the Dyno is 2625hp and 8500 nm (6269 ft lb) for a 4wd car. Max for a rwd car is 1390hp and 4500 nm (3319 ft lb)
That is the reason we dyno in the 4 or 5 ft gears because they are closer to 1:1.
Did try a Tesla P85 in that dyno but did as expected get the same to much torque error.
600nm x 9.73 = 5838 nm (4305 ft lb) So above the torque limit for rwd on the Dynapack.
If the torque numbers Tesla did specify for the P85D and P90D are correct they should put down about 9058nm at the wheels.
931nm x 9.73 = 9058nm (6680 ft lb)
The front drive unit is 9.34:1 so its not 100% correct.
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Just listen to me, 920 lb-ft is just way too low to be wheel torque. If you refuse to to the math I'll do it for you. Let's see how quick the Model S would be, if it really had just 920 lb-ft of torque.
We are going for best case here, no inertia and maximum torque from 0-60.
torque: 920lb-ft = 1250Nm (let's get it to nicer units, first)
wheel diameter: 24 inch = 61 cm
weight of the car: 4650lbs = 2100 kg
Force at the wheels: 1250 Nm/(61 cm/2) = 4100 N
Acceleration: 4100 N / 2100 kg = 1.95 m/s^2 = 0.2g
60mph = 26.88 m/s
0-60: 26.88 m/s / 1.95 m/s^2 = 13.78s
If we included inertia it would be even worse. So tell me, does it still seem like you "measured" wheel torque?
I will approach this from another direction relative to what R.S has posted above. I will also use common "American" units instead of metric. Of course both will give the same equivalent results; I am using non-metric for those who feel metrically challenged.
We know that the P100D can reach 60 MPH in under 2.5 seconds from a standing start. I assume that we can all agree on this base point. Note that I am completely ignoring the 1 foot rollout that is commonly used in 0 to 60 MPH times. Given an assumed weight of 5,000 pounds with driver (possibly slightly low) and the 2.5 seconds to reach 60 MPH, that requires an *average* thrust of right at 3,000 pounds. Since the effective tire radius (corrected for compression, 751 revolutions per mile with 245/45R19 tires) is about 1 foot 1.5 inches, that thrust has to come from an average four wheel total torque of 3340 pound-feet.
This represents the *average* torque, the peak torque would be much higher. All of this is quite trivially calculatable. If anyone can provide the fairly continuous time to speed data for a P100D, along with the vehicle plus driver weight, the wheel torque and HP as a function of speed is easily plotable. Data every 0.1 second or less would be quite desirable. Determining the front and rear motor torque and HP is more difficult since the front and rear gear ratios differ. However, a reasonable approximation could be made by incorporating the front and rear motor HP ratings along with the known gearing ratios.
As far as I am aware, Tesla has not released to front and rear motor torque limit and HP ratings for the P100D.
We know that the P100D can reach 60 MPH in under 2.5 seconds from a standing start. I assume that we can all agree on this base point. Note that I am completely ignoring the 1 foot rollout that is commonly used in 0 to 60 MPH times. Given an assumed weight of 5,000 pounds with driver (possibly slightly low) and the 2.5 seconds to reach 60 MPH, that requires an *average* thrust of right at 3,000 pounds. Since the effective tire radius (corrected for compression, 751 revolutions per mile with 245/45R19 tires) is about 1 foot 1.5 inches, that thrust has to come from an average four wheel total torque of 3340 pound-feet.
This represents the *average* torque, the peak torque would be much higher. All of this is quite trivially calculatable. If anyone can provide the fairly continuous time to speed data for a P100D, along with the vehicle plus driver weight, the wheel torque and HP as a function of speed is easily plotable. Data every 0.1 second or less would be quite desirable. Determining the front and rear motor torque and HP is more difficult since the front and rear gear ratios differ. However, a reasonable approximation could be made by incorporating the front and rear motor HP ratings along with the known gearing ratios.
As far as I am aware, Tesla has not released to front and rear motor torque limit and HP ratings for the P100D.
There are two ways to measure horsepower. Dynamic and static. Static gets your horsepower at a fixed speed against a PAU that prevents acceleration like an eddie current. The PAU puts out enough power to counter the power put out by the wheels. Measuring power this way will factor out power loss due to drivetrain mass since the drivetrain mass isn't accelerating. Since this only measures power at one speed, you don't get a power curve out of it. If you want a power curve, then you need to do this test at multiple speeds.
An acceleration run will give you a power graph. The heavier the drum or the mass, the longer it will take to accelerate to a given top speed and the less the power due to drivetrain loss will be. The lighter the drum, the faster the acceleration and the more the drivetrain mass factors into it because the ratio of drivetrain mass to drum mass is now larger.
All the math isn't needed to demonstrate the point, it's actually quite simple. If the car weighs 5000 lbs, and the wheels are approximately 1 foot radius, you need 5000 ft/lbs to propel the car at 1G. I think someone graphed it at 1.4G peak? OK you need 7000ft/lbs at the wheel to pull that off. Anyone know of a dyno that reads even over 2000ft/lbs?
The most accurate way to dyno an engine or motor is via a shaft dyno. In this way you are directly measuring the torque the produced, literally "at the shaft". As you can also directly measure shaft RPM, you can derive HP. No gears, clutch, torque converter, tire slippage, etc... That, however, requires removing the power unit from the vehicle, which is impractical for most average folks.
A chassis dyno is a more convenient method to take measurements. However, it's doing so at the wheels, which is after all of the drivetrain items mentioned above. That drivetrain includes gear reduction. A reducing gear ratio decreases RPM, but increases torque, as a direct function of the gear ratio.
Thus, a chassis dyno must calculate torque at the shaft by measuring both the engine/motor RPM, and the wheel RPM. It can then determine how much gear reduction has occurred, and divide the measured torque appropriately to estimate shaft engine/motor shaft torque. Correction factors may also be applied to compensate for driveline friction losses, torque converter losses, environmental temp/humidity, etc...
If possible, the transmission is put in a 1:1 (direct) ratio, to eliminate as much variability as possible. You still, however, have a final drive ration present in the differential (often in the 3:1 - 4:1 range). That means the torque will have been multiplied by 3-4 after the differential. You then have another factor for tire diameter before you actually transfer the power to the dyno rollers.
As just about every car will have a final drive ratio that includes gear reduction, the actual measured torque on the rollers will be greater than the engine/motor shaft torque. However it will also measure a lower RPM than the shaft. By dividing by the ratio of the RPM difference (and applying the other corrections), you estimate shaft torque.
So yes.. although a chassis dyno will not report an estimated engine torque of a few-thousand ft-lbs, it may very well be measuring that much at the wheels as an input to it's calculations.
A chassis dyno is a more convenient method to take measurements. However, it's doing so at the wheels, which is after all of the drivetrain items mentioned above. That drivetrain includes gear reduction. A reducing gear ratio decreases RPM, but increases torque, as a direct function of the gear ratio.
Thus, a chassis dyno must calculate torque at the shaft by measuring both the engine/motor RPM, and the wheel RPM. It can then determine how much gear reduction has occurred, and divide the measured torque appropriately to estimate shaft engine/motor shaft torque. Correction factors may also be applied to compensate for driveline friction losses, torque converter losses, environmental temp/humidity, etc...
If possible, the transmission is put in a 1:1 (direct) ratio, to eliminate as much variability as possible. You still, however, have a final drive ration present in the differential (often in the 3:1 - 4:1 range). That means the torque will have been multiplied by 3-4 after the differential. You then have another factor for tire diameter before you actually transfer the power to the dyno rollers.
As just about every car will have a final drive ratio that includes gear reduction, the actual measured torque on the rollers will be greater than the engine/motor shaft torque. However it will also measure a lower RPM than the shaft. By dividing by the ratio of the RPM difference (and applying the other corrections), you estimate shaft torque.
So yes.. although a chassis dyno will not report an estimated engine torque of a few-thousand ft-lbs, it may very well be measuring that much at the wheels as an input to it's calculations.
For the real world, isn't it more useful to know how much power you're putting down to the wheels? The AWD MS has a lot less drivetrain loss than a typical AWD car which has to go through a transmission, flywheel, transfer case, and then finally two drive shafts before hitting the differentials. This is why typical AWD cars lose 25% in the 1:1 gear(typically fourth) while the tesla only loses 10%. And if you're comparing first gear on an ICE AWD to the MS from a start, the ICE AWD power train loses even more at lower gears with the highest losses occurring in first gear.
jcaspar
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In an ICE, you use engine RPM not wheel RPM to calculate HP. If you used motor RPM in the Tesla the HP would be massive as it is turning up to 16,000 rpm. That is how formula 1 engines get such high HP numbers with small displacements and relatively low torque; high rpm.
For a Tesla:
920 ft/lb X 16,000 RPM/5250 = 2803 HP!?!
For a Tesla:
920 ft/lb X 16,000 RPM/5250 = 2803 HP!?!
"do all the calcs you want, I'm posting what the dyno results were"
My point, which you seem to have missed, was that at least 3,340 pound-feet of wheel torque is required to reach 60 MPH in 2.5 seconds from a standing start with a 5,000 pound vehicle. Hence, 920 pound feet is *NOT* the wheel torque but rather the computed motor torque using an unstated gearing ratio since the front and rear motors have different gearing. Unless the engine / motor is directly coupled to the dyno, the measured wheel torque is used to compute the engine / motor shaft torque and HP by knowing the gearing and estimating the gearing losses (the gearing losses are not measured by the dyno). Hence, the calculations that both R.S and I performed both show that the 920 pound-feet of torque was *NOT* what was measured at the wheels by the dyno, but rather that value represents the estimated shaft torque of both motors combined.
My point, which you seem to have missed, was that at least 3,340 pound-feet of wheel torque is required to reach 60 MPH in 2.5 seconds from a standing start with a 5,000 pound vehicle. Hence, 920 pound feet is *NOT* the wheel torque but rather the computed motor torque using an unstated gearing ratio since the front and rear motors have different gearing. Unless the engine / motor is directly coupled to the dyno, the measured wheel torque is used to compute the engine / motor shaft torque and HP by knowing the gearing and estimating the gearing losses (the gearing losses are not measured by the dyno). Hence, the calculations that both R.S and I performed both show that the 920 pound-feet of torque was *NOT* what was measured at the wheels by the dyno, but rather that value represents the estimated shaft torque of both motors combined.
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