HP VS Torque

[Bluelightning]

Torque gets you going, Horsepower keeps you going.

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[07S281E]

You buy horsepower. You drive torque..

 

[Jaymar]

It's a constant that only works with feet, pounds, and minutes.
1 HP = 33,000 ft*lb/min
1 REV = 2π
HP = ft*lb*2π/min/33,000

33,000 / 2π ≅ 5252

If you switch to inches it becomes HP = in*lb*rpm/63,063

Sure, and if you want to get real nutty we could use gram lightyears per hour too! But I have a feeling the exponential notation will start getting a little unwieldy to use.

 

[TexasRebel]

Sure, and if you want to get real nutty we could use gram lightyears per hour too! But I have a feeling the exponential notation will start getting a little unwieldy to use.

those units don't work out.

gram is a unit of mass. You need an acceleration to turn this into a force.

newton-lightyear per hour? ...maybe?

then again, lb-in is not as uncommon a unit of measurement as N-ly

 

[Jaymar]

those units don't work out.

gram is a unit of mass. You need an acceleration to turn this into a force.

newton-lightyear per hour? ...maybe?

then again, lb-in is not as uncommon a unit of measurement as N-ly

Damn SAE system giving me bad habits! You're right, it would be newton-lightyear per hour. We use inch-lbs more frequently than ft-lbs in my industry but after many years using the latter it still feels like a weird unit to me. Then I'll get documentation in metric units and throw the whole thing off. The real clincher comes when you work at a site overseas and have to work out what tools are what. I start mixing my units up sometimes.

 

[dirtwarrior]

So how close is my analogy from post 1?

 

[Jaymar]

So how close is my analogy from post 1?

You have all the right pieces, just needed to put them together.

 

[barron64]

In a car, torque gets you rolling… HP accelerates you through the gears.

 

[Meatball]

I like to think of hp vs torque like this: Bicycle analogy. When you ride your bike, how much pressure you put on the pedal is torque...essentially, pounds of pressure you press down times the length of the crank arm gives torque "at the crankshaft". Then it's multiplied by the gearing to give you multiplied torque at the wheel (diameter of rear wheel also has an effect).

In any one gear, lets say not a low gear so you can press down just as hard on the pedal while moving slow as moving faster. Same torque at the crank (and wheel), but when you can pedal faster while applying the same pressure on the pedal, you can move the bike faster and farther...doing more work....more "horsepower". Maybe you can even press a little less hard (diminishing torque at higher rpm) yet still move the bike more than you could at slower speeds (higher hp at the higher rpm despite less torque). In any one gear, the torque, however, will determine instantaneous acceleration.

I think that's the key to the reason hp is important. Even if you're applying less torque, if you can spin the engine faster you can stay in a lower gear and take advantage of the higher torque multiplication for longer. That's a major advantage...thrust drops big time between gears so even if an engine is making less torque/power it can have more aggressive gears and stay in gear to higher rpm. That's why a stock V6 S550 mustang with 280 ft-lbs can destroy a stock foxbody with 300 ft-lbs and 200-400 pounds less weight, even if the fox had the same transmission and lets say it could rev as high (it would make much, much less torque at high revs). I think gearing and revs makes hp the more important stat to pay attention to. Just my opinion.

The interesting thing is, our engines don't make that much torque....400-420 ft-lbs for stock coyotes. If there were no gearing it would be like two 205 lb guys standing on 1 ft long crank arm pedals, one on each rear wheel. The 3800 pound car would just ooze forward, not light up the tires and accelerate away. The engine's ability to rev and use gearing is what does it. Any engine but esp a coyote can spin like crazy while applying that torque...with torque peaks at about 4400rpm, they're pushing max on the pedals while applying 293 power strokes PER SECOND. The gearing at most speeds in lowest possible gear for that speed has the engine spinning MUCH faster than the wheels, so the tranny and diff are gearing the engine crank speed way down with huge torque multiplication.

If a big and more torquey pushrod V8 makes more torque lets say even at 4400rpm, it probably can't sustain reasonable torque (or even survive) at 7000 rpm like a coyote can, and if the gearing and redline/fuel cutoff is right, the coyote can make more peak hp. That's why our physically huge engines with giant heads yet relatively low displacement are effective. Everything else being equal, hp wins races but the torque/gearing/wheel diameter will say how much instant acceleration you'd feel. Pick what's important to you.

 

[ay1820]

This is both simpler and more complex than it seems.

Simply put, it takes power to maintain speed. It takes torque to change speed.

As others have said, power is defined by Torque x RPM x C (where C is some constant that makes the units work out right). The units we use are irrelevant to the concept and just a way of measuring the same thing (e.g.; distance between two cities is the same weather we measure it miles or in kilometers). Power can be measured in HP, Watts, calories/min, etc. The constants would change, but the actual power remains the same.

It takes a certain amount of force (torque) to move the mass of car up hills, push it against the wind and overcome rolling resistance. The amount of force required is also a function of speed (more force at higher speeds due to exponentially higher wind resistance). The gear ratio between the motor and the wheels (inclusive of the wheels) determines the RPM required to move the car at a particular speed. So the ability of a car to maintain a particular speed is determined by its ability to produce torque at the RPM required for the vehicle to travel at that speed. There a variety of efficiency factors that get thrown into the equation too, but that will only complicate the discussion and can be ignored for now.

If you want to accelerate, you need to apply more force (more torque) both to push the mass of the car to the higher speed and to overcome the higher wind/rolling resistance. So the ability to accelerate is determined by the difference between the torque being output by the engine at its current RPM and the maximum torque the engine can produce at that same RPM. If you already at or near the peak torque for a given RPM, you won't be able to accelerate.

So an engine that has a fairly flat torque curve (i.e.; one that makes close to peak torque over a wide RPM range) will tend to feel like it can accelerate faster since you can produce more torque at almost any engine/vehicle speed. But an engine with a narrow torque band could potentially accelerate faster if you keep changing gears and keep the engine at the optimal RPM to produce near max torque.

My old '89 5.0 Mustang had a very flat torque (making 80% of peak torque between 2,000 and 6,000 RPM, IIRC) and it felt like it could accelerate at almost any speed in any gear. But was limited by the lower overall torque produced. My Coyote 5.0, on the other hand feels like a dog at lower RPMs, but once you get the revs up, you get that kick in the pants that my old pushrod 302 could never deliver.

So neither horsepower or torque as single numbers tell you the whole story, which is why this is so confusing. You really need to know the torque/HP curve as a function of RPM to make a truly meaningful judgement. But all that said, if you want a simple number the higher the HP the bigger the smile factor

Sorry for the long lecture, hope this makes at least some sense.

 

[AvalancheSVT]

Torque is simply force applied at a distance.

Power is energy per unit time.

In terms of automotive performance, torque is how far back in your seat the car can push you, power is how long the car can continue to accelerate and keep you there.

The more torque a car has, the more acceleration it can impart (momentarily). The more power a car has, the further it can maintain that acceleration.

Power is preferred because torque can be achieved through mechanical machine and multiplication (aka gearing). Higher torque simply reduces the amount of multiplication required to be practical/effective.

this. all of this. one million percent this.

 

[WildHorse]

Another tidbit is that in any ICE engine the torque and HP lines on a graph will always cross at 5250 rpm.

Covert it to kilowatts and it'll no longer crosses over. BTW, kilowatts is a more accurate rating than horsepower.

 

[ay1820]

Covert it to kilowatts and it'll no longer crosses over. BTW, kilowatts is a more accurate rating than horsepower.

It's neither more or less accurate. It is just a different scale.

 

[GregO]

Covert it to kilowatts and it'll no longer crosses over. BTW, kilowatts is a more accurate rating than horsepower.

We’ve owned several SAAB Aero cars over the years. Tuned the first 9000 Aero by Maptun advertised power, was always in watts !
Did I see someone mention the name James Watt ?

 

[WildHorse]

It's neither more or less accurate. It is just a different scale.

KW is more accurate cause horsepower was based on flawed formula 200 years ago.

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