Applying Torque vs Horsepower

[GT Pony]

HP wins races. Look at an F1 car ... low torque, but high HP due to RPM (recall that HP = T X RPM/5252). Match the HP curve to the proper gearing and you win races.

I missed this earlier. I don't disagree with your formula, obviously, but the horsepower didn't win the race. That's the part people need to get our of their head when evaluating a vehicle. The horsepower isn't what is making the car move. The torque alone doesn't win the race, but regardless of the RPM horsepower doesn't exist without torque. The engine creates torque and it achieves RPM, both by design. It doesn't create or achieve horsepower. It's the reason a new SS beats a new GT--the engine makes 50 lb/ft higher in torque at similar rpm. That's a big advantage.

This is why the statement (based on the two primary power numbers used to brag on or sell a vehicle) that torque wins races is more accurate.

Yes, "torque" makes the vehicle move ... but HP tells you how fast that vehicle will accelerate because HP has the RPM component. Torque and HP are strictly related to each other through the HP equation, as the HP formula shows. So if engine A has more HP at X RPM, it also has more torque at that same X RPM.

Only time "torque wins a race" is if the one engine makes more torque at the same RPM as another engine (all other variable held constant). And that also means that same engine makes more HP at that same RPM - that's what your statement in red means.

When the statement "torque wins races" is made, it's in regard to the same engine/car ... not comparing different engines/cars to each other - that's a whole different subject matter.

Anyone who thinks that peak torque in an engine wins the race is wrong, and all they have to do is go down the race track and in one run shift around the peak torque RPM and then in the next run shift around the peak HP and tell me which one got you there faster.

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[TexasRebel]

Excuse my ignorance but is there any reason that formula uses 5252 and would that number be the same regardless of engine specs?

1 HP = 550 ft lb/s

There are 2 Pi radians in one revolution, and 60 seconds in a minute.

550 * 60 / 2 Pi = 5,252.1131

or approximately 5,252

 

[dgc333]

Point is - if you want to go fast, low rpm torque will be of little help. If you want to grunt around town at low rpms in higher gears, yes, big torque is by far better.

Arguably the fastest car in the 1/4 during the muscle car era was the Buick GSX. It made 510 ft-lbs of peak torque at 2800 rpm and 360 peak HP at 4600 rpm, redline was 5500 rpm. It had the highest torque and lowest HP of all the big dog muscle cars yet is generally recognized as the fastest and is still winning in Pure Stock drag racing.

I guess you haven't seen a full size diesel pickup making 900 or more ft-lbs of torque in the 2000 rpm range and about 500 HP at less than 4000 rpm turning low 10s in the 1/4.

Your generalization gets proven wrong all the time.

 

[dgc333]

1 HP = 550 ft lb/s

There are 2 Pi radians in one revolution, and 60 seconds in a minute.

550 * 60 / 2 Pi = 5,252.1131

or approximately 5,252

And it all came to us via James Watt observing horses lifting weights. He came up with the formula so he could compare his steam engines in terms people understood, the power of a horse.

 

[Dominant1]

Arguably the fastest car in the 1/4 during the muscle car era was the Buick GSX. It made 510 ft-lbs of peak torque at 2800 rpm and 360 peak HP at 4600 rpm, redline was 5500 rpm. It had the highest torque and lowest HP of all the big dog muscle cars yet is generally recognized as the fastest and is still winning in Pure Stock drag racing.

I guess you haven't seen a full size diesel pickup making 900 or more ft-lbs of torque in the 2000 rpm range and about 500 HP at less than 4000 rpm turning low 10s in the 1/4.

Your generalization gets proven wrong all the time.

Not quite:

Hemi Cuda 13.10 @ 107mph
70 Chevelle SS454 13.12 @ 107mph
66 Hemi Satillite 13.25 @ 110mph
69 Hemi Road Runner 13.32 @ 108mph
70 Hemi Road Runner 13.34 @ 108mph
70 Buick Skylark GS455 13.40 @ 105mph
68 Hemi Charger 13.50 @ 105mph
68 Hemi Road Runner 13.54 @ 105mph
69 Mustang Mach 1 428 13.60 @ 106mph
70 Challenger 440 13.62 @ 104mph

 

[GT Pony]

Arguably the fastest car in the 1/4 during the muscle car era was the Buick GSX. It made 510 ft-lbs of peak torque at 2800 rpm and 360 peak HP at 4600 rpm, redline was 5500 rpm. It had the highest torque and lowest HP of all the big dog muscle cars yet is generally recognized as the fastest and is still winning in Pure Stock drag racing.

I guess you haven't seen a full size diesel pickup making 900 or more ft-lbs of torque in the 2000 rpm range and about 500 HP at less than 4000 rpm turning low 10s in the 1/4.

Your generalization gets proven wrong all the time.

It probably won 1/4 mile races because the high low end torque (which means more HP at low RPM) helped it get off the line much quicker - plus other cars he was racing probably didn't have 360 peak HP. But once it hit near redline the engine was most likely kept there for the rest of the race. So the massive low end torque only helped for the first 60 feet, but after that is was all HP related.

If the shifting happened when the engine was at 3000 RPM instead of 5000 RPM from 60 ft to the end of the 1/4 mile, he would probably have lost most races.

It's actually the area under the HP curve that wins races ... but in general it's still HP that "wins races".

 

[Norm Peterson]

Excuse my ignorance but is there any reason that formula uses 5252 and would that number be the same regardless of engine specs?

Because HP is expressed in units equal to 33,000 lb*ft/min, which is a force (lbs) times a linear velocity (ft/min). To convert that to a torque (lb*ft) times a rotational velocity (in radians) you need to divide by 2*pi. 33,000 / (2*3.14159) = 5252.1

It's a units conversion constant completely independent of engine specs.


Norm

 

[Norm Peterson]

Crudely . . . I'm leaving out aero and rolling drag and rotational inertia effects. You do need to have some idea what the engine's torque curve looks like. Not just its peak value.

[Acceleration in g] = ( { [EngineTorque@RPM consistent with speed and gearing] * [TransmissionRatio] * [AxleRatio] * [Efficiency] } / [TireRadius] ) / [CarWeight]


Norm

 

[Coolmanfoo]

Because HP is expressed in units equal to 33,000 lb*ft/min, which is a force (lbs) times a linear velocity (ft/min). To convert that to a torque (lb*ft) times a rotational velocity (in radians) you need to divide by 2*pi. 33,000 / (2*3.14159) = 5252.1

It's a units conversion constant completely independent of engine specs.


Norm

Thanks Norm! I've really appreciated your posts on this thread! That chart you posted early on, is incredible!

 

[PJR202]

Horsepower. Doesn't. Make. The. Car. Move.

Nevermind. I give up.

 

[GT Pony]

HP is torque at speed, so yeah HP makes the car move because torque is part of HP. You can have torque without RPM, and in that case the HP equals zero and the car goes nowhere ... oh, never mind.

 

[Dominant1]

My car is faster then yours not because of hp and torque, its because i said so...lol

 

[5LITER]

I was told long ago. Hp is how fast an engine can spin and torque is the force it makes.

 

[GT Pony]

RPM is how fast the engine can spin ... HP is what is produced when torque is produced with RPM (HP = T x RPM/5252).

You could have 1/4 HP and the engine could spin 20,000 RPM (ie COX 0.49 2-stroke model airplane engine), so HP is not how fast the engine can spin.

 

[Trackaholic]

I like to ask questions along these lines to engineering candidates I interview. Here are some areas to ponder:

Let's say I have a shaft spinning at 10 RPM producing 10 lb-ft of torque. Now put a 10 tooth gear on that shaft and connect it to another shaft that has a 20 tooth gear. How fast does that second shaft spin? How much torque does it produce? How much power does the first shaft make? Assuming 100% efficiency, how much power does the second shaft make?

Going through these calculations will highlight that torque and speed can be multiplied and reduced with gearing, such that any torque can be achieved with the associated change in speed, yet power is conserved. To very accurately model acceleration it makes logical sense to convert engine torque to force at the wheels and integrate using F=ma over the RPM range in question. However, knowing that power is conserved through these conversions means that looking at peak power/weight should give a good idea of overall performance, especially when talking about rolling acceleration.

When talking about drag racing, the launch is a huge part of the time, so low end torque at the wheels can have a significant effect. As can things like a well calibrated launch control and rapid shifts, not to mention peaky torque curves (either due to big low-end numbers like most modern turbo cars, or due to small low-end numbers like the GT350 and VTEC YO). This can make the peak power/weight simplification less applicable.

In short, you need both torque and the rate at which the torque is applied to know anything about performance. Or, you need torque and the associated gearing. I can take that 1/4 HP RC airplane engine and gear it to make 1000 ft-lbs, but it will still accelerate a car like a 1/4 HP engine.

-T

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