How come my calculated engine torque is so much less than the manufacturer's specifications?

[Ardy]

Here are my calculations. Am I doing anything wrong?

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[17Magnetic5.0]

Here are my calculations. Am I doing anything wrong?

Where did you get the equations from I feel like something might be slightly off

 

[17Magnetic5.0]

https://x-engineer.org/automotive-e...hicle-dynamics/calculate-wheel-torque-engine/

Equation 12 gave me 379 ft-lbs I think you’re missing a multiplication by 2 somewhere in the there

 

[Ardy]

Where did you get the equations from I feel like something might be slightly off

I didn’t get the equations from anywhere. Just common sense. I know the calculations I did are correct. Maybe I am missing a factor that I didn’t account for.

https://x-engineer.org/automotive-e...hicle-dynamics/calculate-wheel-torque-engine/

Equation 12 gave me 379 ft-lbs I think you’re missing a multiplication by 2 somewhere in the there

379 ft-lb torque at the wheels would be impossible since the engine torque is 350 and there’s expected up to 25% loss through transmission.

 

[ice445]

Where did you get the 6.570 m/s figure?

 

[vdin]

This video can probably explain which calculation to use. Look at the whiteboard calculations he's using.

Wheel torque = Engine torque x gearing

Gearing = 4.696 x 3.15

Engine torque = 420 ft/lbs

Wheel torque = 5917 ft/lbs x (whatever ratio you want to use for transmission loss)

The point of the video is to show that wheel torque is a useless measurement however since it can be changed with gearing.

 

[17Magnetic5.0]

I didn’t get the equations from anywhere. Just common sense. I know the calculations I did are correct. Maybe I am missing a factor that I didn’t account for.




379 ft-lb torque at the wheels would be impossible since the engine torque is 350 and there’s expected up to 25% loss through transmission.

Just because a manufacturer says 350 doesn’t mean it is. Sometimes cars are underrated by a lot and sometimes overrated. I’m no expert on the subject as I’m an aerospace engineer and haven’t worked with torque and ratios in a while so I had to look the equation up. Best way would be to look at a dyno and verify. I looked up a dyno sheet for a 19 EcoBoost and it dynoed at 288 and 380 x 0.25 = 285 so it seems spot on.

 

[engineermike]

I’m a little lost as to where the input came from, namely the acceleration.

Assuming the acceleration came from the dash accelerometer:

6.57/9.81 = 0.67 G

mass of car * accel = thrust
4200 lb * 0.67 = 2800 lb of thrust

thrust * tire radius = axle torque
2800 * 14/12 = 3300 ftlb

3300 / 3.15 / 4.69 = 223 ftlb

The 10r80 only absorbs about 10%, so
223 * 1.1 = 245 ftlb

This is still low but I think a few things will account for it:

Traction - Depending on tire, it might not be holding max torque. I know my PS4S will only hold about 0.75 G on most surfaces. A lesser tire won’t hold as much.

Torque limiters - The cal has dozens of torque limiters. I don’t know that the factory cal will even allow max torque to be produced in 1st gear.

Drivetrain inertia - In low gears, a lot of power is absorbed by accelerating the rotating parts of the engine and transmission. This is the difference between “brake” and “inertial” power. This is also why cars make more rwhp on dynojet-style inertial chassis dynamometers in higher gears.

I would reset the accelerometer and try again in a higher gear like 6th and be sure to start at a low enough rpm to capture the torque peak.

 

[Stang 19]

Uhhhhhh, Ixnay on the 25% drivetrain loss! You would be cheating yourself there since it is probably no more than 15%, And quite possibly closer to 10 .

 

[engineermike]

In looking back, I think the OP calcs are correct, save the commie units ;)

The car weight is a bit low at 3670 lb. The mass should be the entire weight of the car with passenger and fuel.

 

[GreenS550]

I think your calculation is incorrect. You are dividing by the overall ratio to determine the engine torque. This is incorrect. You cannot assume that the gear multiplication worked backwards will be precise. Then you have the torque converter. The math would work with no parasitic losses, but the dyno that gives, say, 380 pound/feet is the only and best way to determine the actual torque. You are also using the acceleration figure based on what acceleration? Acceleration can vary based on many, many factors.

 

[Ardy]

Where did you get the 6.570 m/s figure?

From my dash.

It’ll do 0.74g with brake boost but that’s using the torque converter to multiply torque. To eliminate that, I did no brake boost launch and got 0.67g on the dash. I think it’s pretty accurate since the same sensor is used to deploy airbags I’m assuming

 

[Ardy]

I’m a little lost as to where the input came from, namely the acceleration.

Assuming the acceleration came from the dash accelerometer:

6.57/9.81 = 0.67 G

mass of car * accel = thrust
4200 lb * 0.67 = 2800 lb of thrust

thrust * tire radius = axle torque
2800 * 14/12 = 3300 ftlb

3300 / 3.15 / 4.69 = 223 ftlb

The 10r80 only absorbs about 10%, so
223 * 1.1 = 245 ftlb

This is still low but I think a few things will account for it:

Traction - Depending on tire, it might not be holding max torque. I know my PS4S will only hold about 0.75 G on most surfaces. A lesser tire won’t hold as much.

Torque limiters - The cal has dozens of torque limiters. I don’t know that the factory cal will even allow max torque to be produced in 1st gear.

Drivetrain inertia - In low gears, a lot of power is absorbed by accelerating the rotating parts of the engine and transmission. This is the difference between “brake” and “inertial” power. This is also why cars make more rwhp on dynojet-style inertial chassis dynamometers in higher gears.

I would reset the accelerometer and try again in a higher gear like 6th and be sure to start at a low enough rpm to capture the torque peak.

Thx for the reply. There are a lot of good points here.

 

[Ardy]

380 x 0.25 = 285

he means 380 - 25% of 380

 

[Semp1]

If the gen 3 coyote has less than 200lbs of Tq than every other car must have 10. Because the gen 3 pulls. But this is definitely incorrect.

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