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Torque vs Horsepower explained

Started by chargerdon, May 31, 2023, 12:53:56 PM

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Saw this article on MotorTrend on demand.

If you read down past the first couple of pages, it goes on to explain that a Dyno only measures torque and that the HP numbers  are calculated based on the torque.   It also goes on to give the formula which is HP=torque x rpm /5252   and because it is calculated HP and Torque curves always cross at 5252 RPM.   Why 5252 ?   

I did some more research and found that back in the 1700's the equated 1 HP to 550 Lb moved 1 foot in 1 one avg horse could do.   So in 1 minute a horse could move 550lbs x 60sec = 33,000 lbs.   Now divide 33,000 by 6.28332 (amount of inches on a 1 ft torque wrench moved and you get the magical 5252 which is then used in the horsepower equation.   WOW...   


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No idea how true it is, but..... I read several years ago that the person that determined how to measure torque was told that no one would understand it and he needed to have something that people would understand. Since it was during the period when horses were still widely used, he developed the formula for calculating horsepower.


A simple way to understand it:  Torque pushes you into the seat, horsepower keeps you there.


Thanks for posting the info.   The weightlifting example was very helpful for me in understanding the difference. 


Torque measures the twisting FORCE or rotational power produced by an engine.

Horsepower measures the RATE at which (work is done) the torque is delivered to the wheels.


Horse power was a measurement of moving weight over a time period. It is a measure of work produced by the motor. Torque is rotational as mentioned above. It is your rear wheels ability to turn and launch the car. Of course the two are related but you could have more torque and less horsepower and vise versa. Torque is critical when launching since it has to fight a tremendous amount of inertia. But once you are moving less torque is required as the car is accelerating. I'm guessing you can have the same motor that has say 350 horsepower, but I would think your torque will change if you swap out your rear from a 2.73 with 4.10 gears. Atleast when testing in the car. Correct me if I am wrong.


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I always thought people buy horsepower, but drive torque.
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Forgive me if this explanation is VERY geeky.  I am a retired electrical engineer who specialized in motors and motor controls.  Torque vs speed was my life for MANY years.  I will try to explain it for non-geeks.

Torque is the number that REALLY matters.  I think it was Carol Shelby who said "Horsepower sells cars but torque wins races."  Torque is a force times the length of a moment arm.  If I turn a wrench with a given amount of force, the torque applied to the bolt is the force multiplied by the length of the wrench.  If I use a wrench that is twice as long and apply the same force, twice the torque is applied to the bolt.  The units for torque are pound-feet, also said foot-pounds.  This is pounds multiplied by the moment arm in feet. (Newton meters for metric lovers). 

Power is torque multiplied by the rotational speed.

A dynamometer measures the torque produced at a given speed.  Then it changes the speed and measures how much torque is produced at THAT speed.  Then it changes the speed and again measures the torque produced.  The result is a chart that shows the torque produced at each speed.  Then power is calculated for each speed by multiplying the torque produced at that speed times the speed.  The result is then shown as a power vs speed chart.

At a given speed, the torque produced by the engine is multiplied by the gear ratio of all of the gears between the engine and the wheels.  For example, if the engine is producing 300 ft pounds of torque and the car is in first gear (2.65:1) then there is 795-foot pounds of torque coming out of the transmission (300 times 2.65).  Then the rear end gears multiply it again.  If the rear end is a 3.23:1, then there is 2568 foot pounds of torque being applied to the rear wheels (795 times 3.23).  A 205/75R14 has a diameter of 26.1" (call it 26")  This gives us a radius of 13"  or 1.08 feet (13/12=1.08)  2568 foot pounds divided by 1.08feet = 2377 pounds of thrust applied by the wheels to the pavement.  Sir Isaac Newton says that acceleration is force divided by mass.   The actual equation is given as F=ma.  Algebra gives is a=F/m. This means that the thrust applied to the pavement by the wheels directly causes acceleration.

The thing that is misleading about showing the horsepower for an engine or motor is that since power is torque multiplied by speed, you can get high power with very little torque if you spin it fast enough.  Because in this case, the torque produced is low, so is acceleration.