HP VS Torque

[GregO]

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

Flawed in what way ? That not all horses have the same strength on any given day.

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[ay1820]

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

Horsepower is just a unit of power measurement and is no more or less valid than a watt or any other (just like both a mile and a kilometer are units of distance measurement). How it was originally conceived of doesn't matter.

Horsepower is just a defined measurement scale (like inches on a ruler) that has a specific mathematical definition based on imperial measurement units. Any of the units can be converted into their SI equivalents, so you can go back and forth between watts and horsepower with no lost of accuracy simply by converting the units (within the decimal precision of your calculator that is).

Now whether a unit of horsepower really represents a meaningful measure of the power a horse can deliver is a different question.

 

[WildHorse]

Now whether a unit of horsepower really represents a meaningful measure of the power a horse can deliver is a different question.

Well originally Watt surmised that each horse pushed with a force that Watt estimated at 180 pounds. Whereas with kilowatts it can be precisely measured. But I guess it's whatever the end user wants to go with.

 

[Bullitt0819]

All the important automotive terms:

- Understeer is when the front end hits the wall
- Oversteer is when the back end hits the wall
- Horsepower is how fast you hit the wall
- Torque is how much of the wall you take with you

 

[TexasRebel]

Well originally Watt surmised that each horse pushed with a force that Watt estimated at 180 pounds. Whereas with kilowatts it can be precisely measured. But I guess it's whatever the end user wants to go with.

3kW will get you 4HP.

 

[Pony2015]

My dad was part of the "real" mechanic from years ago which could diagnose a car by sound or feel. There are so very few left.
On thing I do remember from him I asked the difference between HP and Torque. Th answer is so simplistic and beautiful. Torque definition is a twisting motion but when it applies to HP and torque. torque means the amount of wortk an engine can do. HP means how quickly it can do it.

Apologies for replying to an old thread, but I had the same question and this video provides a crystal clear answer:



Bottom line, horsepower (relative to vehicle weight) wins races and achieves top speeds. Max horsepower is at or near redline, and that's where you want to keep the engine for max acceleration. Torque anywhere other than at redline (eg, low end torque occurring at lower RPM) can make your engine feel powerful at lower RPM, and perhaps is more fuel efficient (hence trucks are high torque low RPM) -- but you don't win races at low RPM. When predicting who will win the race, place your bet on the car with the higher horsepower at or near redline, not the higher torque (weight being equal).

 

[dirtwarrior]

The video is good and hew says same as dad says

 

[Angrey]

The maximum a an automobile can accelerate forward is with the greatest torque applied to the driveline, then the wheels and tires, then the tires to the ground. You can not PHYSICALLY create forward acceleration (using a wheel) without torque, which is the force applied at a distance/radius.

To MAXIMIZE forward acceleration, you must maximize the overall torque received at the tires. To SUSTAIN that maximum acceleration is where power (the expenditure of work/energy over a length of time) comes into play.

We can overcome insufficient torque created by the motor with gear multiplication (hence the use of transmissions and differentials). But there are practical limits to this (more gears = more space and more weight).

The bottom line is if you want to maximize acceleration, assuming adequate traction, you maximize the area under the torque curve.

The two are also inexorably connected using a chassis dyno because in order to MAINTAIN a torque output, the motor HAS to expend more and more work (so when you see a table top flat torque curve, the reason you see an ever increasing power curve is because the motor HAS to put in increasing work in order to maintain that torque level.)

Where this confuses people is on the top end of the rpm range where power may indeed be high, but if the torque is dropping the available force to apply to moving the vehicle forward is dropping.

I can't be any clearer, to maximize acceleration over a set of different gears, the optimal approach is to maximize the area under the torque curve, NOT the power curve.

 

[K4fxd]

All I know is my 72 SS Nova Big Block runs slower 1/4 mile times than my Coyote mustang and feels 7 times faster.

 

[WildHorse]

Assumptions:
Car A : 1000 hp / 100 ft-lbs
Car B : 100 hp / 1000 ft-lbs
Both cars weigh 3,000 lbs.
Both cars have identical gearing and tire size.
We'll use the equation for acceleration (a = F/m) to calculate the average acceleration during the 1/4 mile run.
The race starts from a standstill (0 mph).

Time and Speed:
Time = Distance / Speed
Speed = √(2 * Acceleration * Distance)
Car A:
Speed = √(2 * 1.6 ft/s^2 * 1,320 ft) ≈ 44 ft/s
Time = 1,320 ft / 44 ft/s ≈ 30.0 seconds
Car B:
Speed = √(2 * 16 ft/s^2 * 1,320 ft) ≈ 137 ft/s
Time = 1,320 ft / 137 ft/s ≈ 9.62 seconds
Conclusion:
Based on these calculations, Car B with 100 hp and 1,000 lb-ft of torque would be significantly faster in a 1/4 mile race, completing the distance in 9.62 seconds. Meanwhile, Car A with 1,000 hp and 100 lb-ft of torque would take 30.0 seconds to cover the same distance.

 

[K4fxd]

Based on these calculations, Car B with 100 hp and 1,000 lb-ft of torque would be significantly faster in a 1/4 mile race, completing the distance in 9.62 seconds. Meanwhile, Car A with 1,000 hp and 100 lb-ft of torque would take 30.0 seconds to cover the same distance.

I didn't check your math but if it is accurate car a will be going 130Mph and car B will be going 50 Mph at the 1/4 mile mark.

Torque is quickness and HP is top speed.

This is why some cars will run 11 seconds at 105 and others will run 11 seconds at 117 Mph

 

[WildHorse]

I didn't check your math

Pretty sure the math is correctomundo.

 

[Jaymar]

Assumptions:
Car A : 1000 hp / 100 ft-lbs
Car B : 100 hp / 1000 ft-lbs
Both cars weigh 3,000 lbs.
Both cars have identical gearing and tire size.
We'll use the equation for acceleration (a = F/m) to calculate the average acceleration during the 1/4 mile run.
The race starts from a standstill (0 mph).

Time and Speed:
Time = Distance / Speed
Speed = √(2 * Acceleration * Distance)
Car A:
Speed = √(2 * 1.6 ft/s^2 * 1,320 ft) ≈ 44 ft/s
Time = 1,320 ft / 44 ft/s ≈ 30.0 seconds
Car B:
Speed = √(2 * 16 ft/s^2 * 1,320 ft) ≈ 137 ft/s
Time = 1,320 ft / 137 ft/s ≈ 9.62 seconds
Conclusion:
Based on these calculations, Car B with 100 hp and 1,000 lb-ft of torque would be significantly faster in a 1/4 mile race, completing the distance in 9.62 seconds. Meanwhile, Car A with 1,000 hp and 100 lb-ft of torque would take 30.0 seconds to cover the same distance.

This is the mathematical equivalent of plugging an extension cord into itself.

 

[Angrey]

I didn't check your math but if it is accurate car a will be going 130Mph and car B will be going 50 Mph at the 1/4 mile mark.

Torque is quickness and HP is top speed.

This is why some cars will run 11 seconds at 105 and others will run 11 seconds at 117 Mph

In all transparency, the car with 1000 hp and 100 ft-lbs of torque will be faster. Why? Because we can multiply the torque through gearing. The torque curve I'm referencing is net/resultant with the final multiplication accounted for (not the engine torque) so for instance, if you have a car that has a very small displacement and revs to the moon and creates a ton of power but not a lot of torque, you can overcome this with transmission (aka Formula 1) multiplication (and rear end).

The total torque multiplication that the tires receive and put to the ground is what propels the vehicle. The POWER that the motor creates is what's necessary to either increase or maintain that forward acceleration.

When we see a Dragy graph, you can observe the changes in acceleration based upon the shift and the drop in total torque output (the multiplication drops and therefore the total torque to the ground drops). This is of course assuming perfect traction (if you're making more torque than the tires can handle, the tires break static friction and spin). But the motor's torque output has remained consistent, as well as the power, but per my overall point, the TORQUE to the ground has dropped and therefore the forces to accelerate the vehicle have dropped, thus we observe a drop in acceleration.

In order to maintain a constant acceleration, the motor must employ ever increasing amounts of work.

The entire point of this discussion is that in rare instances, there MAY be cases where reving all the way to redline is not as fast as shifting earlier and keeping the vehicle in the most amount of torque (aka acceleration).

We see these instances where the torque curve drops off the table at the top (even though the power is still high because it's trying to keep the torque up, it's just faltering).

In those situations, it will result in faster acceleration (and elapsed time) to shift earlier and keep the motor more in it's optimal torque output range.

It's rare, but when you see a torque curve that peaks and drops sharply (like in low end off road builds as an example) reving it out to redline will result in less force and less acceleration.

Of course there's other factors at play here as well, like with an auto trans vs a manual. Short shifting an auto is obviously less impactful than a manual where not getting the full benefit of the entire rpm range is much more detrimental.

There are also situations where simply staying in the same gear (manual) at the very end of a run results in the faster time, not because the car accelerates faster but because losing acceleration to the shift is the more dominant issue.

 

[Angrey]

And per my previous posts, there's ANOTHER benefit to this analysis. You might find that with falling torque curve on the top end, you can maintain the same acceleration (and elapsed time) by slightly shorting shifts. When that's the case, you should ask yourself if reving an extra 1000 or 1500 rpms is worth the abuse or risk to the motor (for either no gain or potentially negative performance). All things being equal, if the motor torque is dropping rapidly in the last 2k rpms, why would you want to contend with all that extra stress on the motor if you can shift earlier and end up the same speed?

Again, in order for this scenario to come into play, it's with output curves that drop off the table at high rpm. Which was the entire observation that I pointed out on a dyno graph that looked particularly excessive in that regard up top.

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