The relation of horsepower vs torque

[Falcon]

First, I understand that horsepower is derived from torque (insert some equation here), and based on the formula used, the torque and horsepower lines will always cross at 5250 rpm. I have heard a lot of information about horsepower vs torque, but I've been wondering about the real story. Everyone I hear talking about the two, says that torque is more important, or horsepower.
Forza example: Ferrari 355 Challenge. 391 hp 275ft-lbs of torque. This car is fast, no two ways about it, and many cars can't hold a candle to it, despite having greater torque but lower horsepower.

So from what I've gathered over the years:

Torque is what you feel (of course)
Horsepower is something (how long you can use that torque given an rpm?)

I suppose a better question would be: how does this relation apply to Forza motorsport?

[racerfink]

I've often heard it described that horsepower is how fast you hit a wall. Torque determines how far you go through it.


I find that racing in the real world, topping out first or second gear is pretty useless. I usually like to shift them a little early, and let the torque do the work. Once I get in the higher gears (near 1:1 or overdrive even), rpm's are needed more than torque. More rpm's will equate to a higher top speed for a given gear (if you have enough power to overcome coefficient of drag).

I realize this doesn't really answer your question like you'd like, but hopefully it gives a little insight. As to whether it's true in Forza, well..... without that seat of the pants feel, it would be pretty subjective.

[ZO6fan]

Since we've established that horsepower is derived from torque through an equation, (torque x rpm)/5250 = horsepower, we are able to caculate horsepower or torque as long as we know one or the other and the rpm at which it was made.

Now for example, say engine A is makes a peak 400hp @ 6000rpms and engine B is makes a peak 400hp @ 8000rpms. Using the formula above engine (in reverse), we know that engine A makes 350ft.lbs of torque at 6000rpm and that engine B makes 262ft.lbs. of torque at 8000rpm.

Here enters the complicated part called gearing. Since engine B has a higher peak horsepower, it can use a lower gear ratio (higher numerically) to multiply the amount of torque getting from the wheels to the ground. Example, engine A is in a car with a rearend gear ratio of 3.42, engine B is in an identical car except that the rearend gear ratio is 4.56 so they are both at peak hp at the same speed. Now say both cars are traveling at the same speed and are both at their peak horsepower. If all other variables are equal (and for the sake of arguement lets say they are) then while engine A is applying 350ft.lbs of torque to the ground (no drivetrain loss for arguements sake) at that same speed engine B has through gear multiplication 133% more torque (465ft.lbs) to the ground at that same speed. (for you math and engineering guru's I know my calculations probably aren't correct but these are just examples to explain the point). To achieve this percentage I just took the reargear ratio of the car for engine B and divided the reargear ratio of engine A into it which equaled 133% increase.

Now lets say both cars have the same gear ratio's throughout. Since engine A has peak power at 6000rpms it will have to shift before engine B which peaks at 8000rpms so engine B can remain in a given gear longer giving it that torque multiplication from the lower gear ratios. Example, both cars are going 75mph and through gearing both engines are turning 6000rpms. While engine A will have to shift to, lets say third, engine B can continue to 99mph before going to third (again using that 133% difference). Most manufacturers will not do this because they can take advantage of gearing which as previously mentioned increases torque output as horsepower can not be multiplied since it is derived from torque. This increase in torque is what helps accelerate the car quicker, so while having an engine that produces copious amounts of torque is nice an engine that is suited to take advantage of gearing will accelerate better.

One might say well then why don't we all drive cars with formula 1 engines. It may seem great to get all that torque through gearing but reliablity comes into play as well as drivability. Many engines that make high rpm horsepower suffer from low rpm torque, but not all (alla, increased displacement). Weight also plays a roll in that its takes more low end torque to get a heavier vehicle moving. So most manufacturers compromise on this depending on what they want the engine to accomplish.

Hope that made sense as I am not great at explaining things.

[Area 47]

torque gets you moving, hp keeps you there.
simple figure

and contrary to popular belief. hp and tq do NOT always cross at 5250

[zerofritz]

and contrary to popular belief. hp and tq do NOT always cross at 5250

Now, see - i've seen chart's that show that very thing, different crossover points, but can you explain why? Because mathematically it *has* to, doesn't it?

[Loafer]

The Ferrari 355 Barchetta is a light car

[bripab007]

The only reason hp and tq would not cross at 5250 RPM is if the two sides of the graph used different figures for the hp and tq peaks, like this one:

Otherwise, yes, they must meet at that RPM. However, other than that, they are not really related in the way that is popular belief (at least on internet forums). Here's something that might explain it more clearly:

Torque and horsepower are directly related but distinctly different. Torque is a measurement of force applied to a lever at some distance from a pivot point (a moment arm), expressed in units of Pound Feet, Newtom Metres, Dyne Centimetres, etc.

However unlike torque, which a a class of measurement, horsepower throws time into the equation and is a unit of measurement of the class Power--other units of power are KiloWatts, Joules per Hour, Calories per Minute, etc. This particular unit of power, horsepower, was made up by a fellow who wanted to sell steam engines and needed to provide an easily understood (from the perspective of the era) definition of his engine's capabilities.

What should be obvious from all of this is that the various "torque vs. horsepower" debates engaged in by many are meaningless, even beyond the fact that that one is a class of measurement and the other is a unit of measurement. The big "gotcha" in the debate is that the two are directly related by a single factor, time--which so far has proven to be an immutable constant.

[Falcon]

Z06: Okay, I think I know what you're getting at now. So engine B makes 262 ft-lbs of torque at 8000 rpm. So why is it when an engine is engine dyno'd, most engines (in a car at least) have their torque somewhere in the middle of total engine rpm available? Taking engine B and it's 8,000 rpm (we can assume this would be redline, correct?) in the real world, peak torque would be at 3-5000 rpm for most production engines while horsepower would be a higher rpm value.
So would the statement 'torque is what you feel, horsepower is how long you feel it' be accurate if it were applied to your explanation? Hmm, that kind of makes sense really. As far as I know, horsepower and torque were both discovered when horses indeed did the work. One horse can pull x amount of weight per x amount of feet. 1 hp. Using two horses, the same amount of weight can be pulled the same amount of feet, but it can also be pulled longer before the horses get tired. 2hp.
I don't know how else to grasp the concept. I'm sure there's a large gap of information in my head when it comes to explaining this concept. Yeah, I got to stop thinking about it for so long, makes my head swim.

Area 47 and Zero: Mathematically, the lines do have to cross at 5250 rpm (I am %90 sure of that), but as far as I know, that rule stands most notably when an engine is engine dyno'd. As Z06fan stated, gearing will drastically change the torque per gear used. In which case the engine is chasis dyno'd, showing wheel horsepower and torque.
As for the discrepencies of that rule of 5250, I too have noticed that the lines cross every now and then. So assuming the dyno is working flawlessly, what is causing that? Frictional losses? Bad wire? Is the dyno sheet SAE corrected numbers or is it raw numbers?

Hrm....

[Area 47]

if an engine makes peak hp at 8200 rpms, and peak torque at 7200 rpms, they will not cross at 5250.


honda engines are known for this. and some diesel engines don't cross period. the number above were taken from an old car of mine.

hell my car now doesn't cross at 5250. the rule is stating that it has too, but rules can be bent and/or broken.

[ZO6fan]

So engine B makes 262 ft-lbs of torque at 8000 rpm. So why is it when an engine is engine dyno'd, most engines (in a car at least) have their torque somewhere in the middle of total engine rpm available

I did not state the peak torque of this engine only that it makes that much torque at its peak horsepower. The torque peak while possibly relavant doesn't mean as much as the torque curve or possible lack there of depending on engine design.

Taking engine B and it's 8,000 rpm (we can assume this would be redline, correct?

For the sake of arguement and this example sure.

So would the statement 'torque is what you feel, horsepower is how long you feel it' be accurate if it were applied to your explanation?

My understanding is that torque is what does the work horsepower is how much of that work is being done within a given amount of time.

if an engine makes peak hp at 8200 rpms, and peak torque at 7200 rpms, they will not cross at 5250.

Even if the torque peak is higher than 5250, unless there is something wrong with the testing equipment or an unforseen variable, hp and torque will cross at this mark on all engines (even an Honda S2000)

... and some diesel engines don't cross period

The main reason many diesel engines torque and horsepower lines never cross is many never reach 5250 rpms.

For those that want what appears to be an indepth look at the subject of this topic follow this link http://www.mazda6tech.com/articles/d...d-gearing.html as it should clear up much confusion. (has graphs for visual help explaining it)