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Old 10-03-2012, 09:28 PM   #1
Elemento1991
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Default Why does horsepower matter, does anybody know?

Ok first off I sadly no longer have a suby but I know the members here are extremely knowledgeable. This question pertains to my Cummins but I am still very curious. This is a question I've been wondering about a lot lately. Horsepower isn't a true measurement, it's calculated mathematically from torque and engine speed.

HP = Torque x Engine speed/5252

So if torque peaks around 4k why do we rev all the way to redline, there must be something else that comes into play that I am missing.

If this is the case it seems like my truck would destroy my buddies GMC 1500 since it has over double the torque. This however isn't what happens since we're pretty much neck and neck. The cummins does weight a lot more and has a lot more to spin too but we are talking two times the power here.

Just something I've been curious about for a while...

Anyone know the answer?
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Last edited by Elemento1991; 10-03-2012 at 09:34 PM.
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Old 10-03-2012, 09:34 PM   #2
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Because racecar...... Or to see who can piss further
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Old 10-03-2012, 09:36 PM   #3
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Have you tried googling it? I've always sorta wondered the same thing since a lot of old tractors are like 5hp but you could rip a house off its foundation with one. The difference between your trucks could be gearing as well.
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Old 10-03-2012, 09:36 PM   #4
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Last edited by 6lug; 10-03-2012 at 09:42 PM.
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Old 10-03-2012, 09:42 PM   #5
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Some what makes more sense now. I guess the diesel has potential to do a lot more work, but it does the work at a slower rate. My truck pulls 60-70 foot trees like nothing but I guess that doesn't mean it's gonna do it quickly. Makes sense. High HP would probably have to work a lot harder to pull a big load, but with out it it moves faster.
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Old 10-03-2012, 09:52 PM   #6
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We go to redline because in the end, it's torque at the wheels that matters, not torque at the crank. For example at 6k rpm in 1st gear, you might be putting down X torque to the pavement, but if you shift to 2nd, you'll drop down to 4k rpm, which is more torque at the crank, but it's 2nd gear so it results in less overall torque to the wheels. Something like that.

I think depending on your car's dyno graph, it may be advantageous to shift before redline in some gears.
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Old 10-03-2012, 09:54 PM   #7
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Yup those threads helped. Makes sense to me now why formula 1 cars are so fast but have about 1/3 of the torque as they do hp and that's why they need to rev so high. Now I wanna look up the Audi Diesel Le Mans cars that rev to 7k or so rpms but still have impressive hp and torque values.
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Old 10-03-2012, 09:55 PM   #8
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You want the product of torque x rpm to be as high as you can.

work = torque x rpm. and hp is work. 5252 is just a constant to make the numbers look right using imperial system. There's another constant for use with metric system.

An ideal 200 hp engine would drop in torque linearly with RPM and produce 200hp across the entire useful RPM range, but there is no such engine in existence.
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Old 10-03-2012, 10:02 PM   #9
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Originally Posted by rdecker20 View Post
Because racecar...... Or to see who can piss further
lmao beat me to it! my friend has a 320whp e30 bmw, and if i had a dollar for everytime i've heard that phrase id be driving an sti instead of an obs haha
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Old 10-03-2012, 10:06 PM   #10
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Quote:
Originally Posted by Elemento1991 View Post
Ok first off I sadly no longer have a suby but I know the members here are extremely knowledgeable. This question pertains to my Cummins but I am still very curious. This is a question I've been wondering about a lot lately. Horsepower isn't a true measurement, it's calculated mathematically from torque and engine speed.

HP = Torque x Engine speed/5252

So if torque peaks around 4k why do we rev all the way to redline, there must be something else that comes into play that I am missing.

If this is the case it seems like my truck would destroy my buddies GMC 1500 since it has over double the torque. This however isn't what happens since we're pretty much neck and neck. The cummins does weight a lot more and has a lot more to spin too but we are talking two times the power here.

Just something I've been curious about for a while...

Anyone know the answer?
HP IS the true measure. When we're talking about energy out, we have to talk in terms of energy. HP is energy. Torque is an instantaneous entity meaning we can measure it at any one point in time and know the engine is outputing 200 ft-lbs. However, we have no clue what that 200 ft-lbs means in terms of energy without also tying it to time and rotation. It isn't until we convert that torque to HP that we get a number we can use to tell us car acceleration and top speed. We don't use torque to measure against wind resistance. We use HP. For example, a Subaru may use around 40HP to travel at 65mph just to continuously counter wind resistance.
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Old 10-03-2012, 10:19 PM   #11
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I'll give some of these a try.

Quote:
Originally Posted by Elemento1991 View Post
Horsepower isn't a true measurement, it's calculated mathematically from torque and engine speed.
Torque is calculated too. Many things are calculated, but to imply they have no value because of that is wrong.

Quote:
HP = Torque x Engine speed/5252
Power at a given rpm is related to the torque at that same rpm. But, when you see figures quoted for engine power and torque, they are usually maximum values calculated at different rpms.

Quote:
So if torque peaks around 4k why do we rev all the way to redline, there must be something else that comes into play that I am missing.
Gearing is the reason: http://www.datsuns.com/Tech/whentoshift.htm

Quote:
If this is the case it seems like my truck would destroy my buddies GMC 1500 since it has over double the torque. This however isn't what happens since we're pretty much neck and neck. The cummins does weight a lot more and has a lot more to spin too but we are talking two times the power here.
Weight is the enemy of speed Plus, chances are that your engine doesn't rev as high and may be designed for lots of low end torque, but the torque curve will drop steeply at higher rpms. This helps explain the problem.

Quote:
Power and torque
For commercial uses requiring towing, load carrying and other tractive tasks, diesel engines tend to have better torque characteristics. Diesel engines tend to have their torque peak quite low in their speed range (usually between 1600 and 2000 rpm for a small-capacity unit, lower for a larger engine used in a truck). This provides smoother control over heavy loads when starting from rest, and, crucially, allows the diesel engine to be given higher loads at low speeds than a petrol engine, making them much more economical for these applications. This characteristic is not so desirable in private cars, so most modern diesels used in such vehicles use electronic control, variable geometry turbochargers and shorter piston strokes to achieve a wider spread of torque over the engine's speed range, typically peaking at around 2500–3000 rpm.
While diesel engines tend to have more torque at lower engine speeds than petrol engines, diesel engines tend to have a narrower power band than petrol engines. Naturally aspirated diesels tend to lack power and torque at the top of their speed range. This narrow band is a reason why a vehicle such as a truck may have a gearbox with as many as 18 or more gears, to allow the engine's power to be used effectively at all speeds. Turbochargers tend to improve power at high engine speeds; superchargers improve power at lower speeds; and variable geometry turbochargers improve the engine's performance equally by flattening the torque curve.
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Old 10-03-2012, 11:25 PM   #12
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Weight is the enemy of acceleration. Lack of aerodynamics is the enemy of speed.

Gearing is based on power output. You can't just look at torque alone. When you change gears, you change the torque at the wheels (independent of the torque of the engine). When you shift from 1st gear to 2nd gear, you lose 40% torque at the wheels. You will not drop 40% torque by the time you hit redline. You will still put more power down to the ground in the lower gear revving higher despite the torque dropping. There's a thread in the naturally aspirated section covering this in more detail.
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Old 10-04-2012, 12:01 PM   #13
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Barring confusing math equations, it's quite simply work over time. For reasons I won't go into and as noted by leecea, diesels tend to have narrower powerbands. The power is abundant and it comes on quickly, if we're talking about turbo diesels. The diesel can do a lot of work, but it can't do much over a given amount of time. It's generally why semis have tons of gears. Most turbo diesel powerbands are from 1,800rpms to 3,500rpms with power plummenting by 4,000rpms.

Gasoline motors, while making less overall torque, do more work over time, generally making more average power across a much wider powerband. Carroll Shelby coined the phrase "Horsepower sells cars, torque wins races." That's far from the truth and if it were really case, diesels would be in all race cars. What really wins races is the power under the curve. The more average power there is under the acceleration band or "power band", the quicker and faster the car will go. It's why gasoline turbo motors are so effective. Rather than having a linear and somewhat peaky HP curve in relation to torque, the turbo significantly broadens the powerband in both HP and TQ. HP comes on much earlier and the torque curve is elevated significantly. Though an NA car may make the same HP, there's a ton more of it under the powerband with the turbo motor.
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Old 10-04-2012, 01:05 PM   #14
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Quote:
Originally Posted by MaddMax View Post

Gasoline motors, while making less overall torque, do more work over time, generally making more average power across a much wider powerband. Carroll Shelby coined the phrase "Horsepower sells cars, torque wins races." That's far from the truth and if it were really case, diesels would be in all race cars. What really wins races is the power under the curve. The more average power there is under the acceleration band or "power band", the quicker and faster the car will go. It's why gasoline turbo motors are so effective. Rather than having a linear and somewhat peaky HP curve in relation to torque, the turbo significantly broadens the powerband in both HP and TQ. HP comes on much earlier and the torque curve is elevated significantly. Though an NA car may make the same HP, there's a ton more of it under the powerband with the turbo motor.
The power under the curve is only true because of the transmissions we generally are putting into cars. If you take a vehicle that doesn't shift gears and the transmission is happy with no variance in input rpms, peak torque/hp is all that matters, the rest of the curve is worthless.

My specific example would obviously be a CVT, in particular a snowmobile engine/drivetrain. Snowmobiles are set up with a very narrow powerband, so barely crawling along to 100 mph might be 1500 rpms's max difference. The engines are designed to put out a huge amount of power in a very, very small window. (130+ hp in a .6 liter 2 cylinder NA engine that weighs about 75 pounds isn't hard to achieve.)
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Old 10-04-2012, 01:20 PM   #15
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I read an interesting article about torque vs HP. The guy visited a water wheel for an old flour mill. The people had calculated that the mill had a few thousand ft/lb of torque. The author said, hypothetically, why can't we gear this mill and hook it up to the axle of a car, we could accelerate to 60 in no time. Problem being, the wheel only rotate at a few rpm. After a few calculations, I think the wheel only had 2hp despite it's massive torque.

As a physics major, I find it easy to break things down to the bare essentials

What is torque? Simply a rotational force. For torque, we need an axis of rotation, a distance from said axis, and a force applied.

What is Power? Power is force over time. HP is simply a unit used to describe this power.

This is how I like to think of things.




Here's a good analogy.

When you lift things you are doing "work" right? You are lifting a mass working against gravity. Energy must be expelled.

Picture 2 people, one a very hefty weight lifter (Our diesel engine) another, an average fit guy (Teh scub) Our weight lifter can lift a lot, but he can not move very quickly (Lower RPMs than scubman) Our average guy can only lift about 1/2 as much as out weight lifter (A big difference) but can do move twice as quickly as the weight lifter.

We set these two people up by a bunch of blocks.

Our weight lifter can lift 2 heavy blocks at a time and set them on a table.

Our average man can only lift a single block at a time, but can move the block from the ground to the table twice as fast at the weight lifter.

Would these two people not do work at the same rate over time?

*For simplicity sake, assume no work is being done in reaching down to pick up another block*


This relationship is very close to our equation

Torque * RPM
~~~~~~~~~~
5252


Don't disregard RPM or Torque, as both are very important.
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Old 10-04-2012, 01:59 PM   #16
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Because racecar...... Or to see who can piss further
this, its all to see who has the most money and bigger weener
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Old 10-04-2012, 02:28 PM   #17
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Many boring family sedans now have 250-300 hp. Interesting how things have changed. I used to think 300 hp seemed like a lot for an STI. Now you've got Mom and Dad driving their Hyundai something or other with almost as much hp.
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Old 10-04-2012, 05:06 PM   #18
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Originally Posted by nhmtns View Post
Many boring family sedans now have 250-300 hp. Interesting how things have changed. I used to think 300 hp seemed like a lot for an STI. Now you've got Mom and Dad driving their Hyundai something or other with almost as much hp.
Don't forget the car have gotten a bit heavier too, negating some of that power.
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Old 10-05-2012, 02:27 AM   #19
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The best way I've heard described it, tq makes you feel like your going fast, high end hp actually makes you go fast. That's real world results of each.

You can see this in numerous cars, the latest I drove was the 2010 M3, feels slow (slower than the 2010 WRX), but is actually much quicker, running mid 12's in the 1/4. Part of that is no turbo feel, but that's the big tq hit people feel in the car anyway.

I drive big ol' diesels for work (500hp, 1700ft/lbs), and despite how incredably powerful they are, they are definitely not fast. The turbo does sound amazing though...
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Old 10-05-2012, 02:38 AM   #20
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Quote:
Originally Posted by rexblake View Post
The best way I've heard described it, tq makes you feel like your going fast, high end hp actually makes you go fast. That's real world results of each.
No.

Quote:
You can see this in numerous cars, the latest I drove was the 2010 M3, feels slow (slower than the 2010 WRX), but is actually much quicker, running mid 12's in the 1/4. Part of that is no turbo feel, but that's the big tq hit people feel in the car anyway.
having driven both cars, I have NO idea what you are talking about. The m3 feels loads faster, in all aspects.

Quote:
I drive big ol' diesels for work (500hp, 1700ft/lbs), and despite how incredably powerful they are, they are definitely not fast.
you drive "big ol' diesels" on a heavy ass truck. What youre experiencing is power to weight, which has nothing to do with what is being discussed in this thread.
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Old 10-05-2012, 05:31 AM   #21
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Each thier own then, I still feel the M3 doesn't feel fast. Largely because there is no tq kick, it's such a smooth acceleration.

And massive amount of tq in an engine is exactly what's being discussed in this thread. The feeling of tq, and high tq figures, are very different than having massive high end hp. Which is why race cars usually don't have massive tq figures but worry more about hp for top speed.

My statement still stands.
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Old 10-05-2012, 10:05 AM   #22
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hp is a measurement of energy
Torque is a measurement of work

There you go.
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Old 10-05-2012, 03:53 PM   #23
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Quote:
Originally Posted by lavid2002 View Post
I read an interesting article about torque vs HP. The guy visited a water wheel for an old flour mill. The people had calculated that the mill had a few thousand ft/lb of torque. The author said, hypothetically, why can't we gear this mill and hook it up to the axle of a car, we could accelerate to 60 in no time. Problem being, the wheel only rotate at a few rpm. After a few calculations, I think the wheel only had 2hp despite it's massive torque.

As a physics major, I find it easy to break things down to the bare essentials

What is torque? Simply a rotational force. For torque, we need an axis of rotation, a distance from said axis, and a force applied.

What is Power? Power is force over time. HP is simply a unit used to describe this power.

This is how I like to think of things.




Here's a good analogy.

When you lift things you are doing "work" right? You are lifting a mass working against gravity. Energy must be expelled.

Picture 2 people, one a very hefty weight lifter (Our diesel engine) another, an average fit guy (Teh scub) Our weight lifter can lift a lot, but he can not move very quickly (Lower RPMs than scubman) Our average guy can only lift about 1/2 as much as out weight lifter (A big difference) but can do move twice as quickly as the weight lifter.

We set these two people up by a bunch of blocks.

Our weight lifter can lift 2 heavy blocks at a time and set them on a table.

Our average man can only lift a single block at a time, but can move the block from the ground to the table twice as fast at the weight lifter.

Would these two people not do work at the same rate over time?

*For simplicity sake, assume no work is being done in reaching down to pick up another block*


This relationship is very close to our equation

Torque * RPM
~~~~~~~~~~
5252


Don't disregard RPM or Torque, as both are very important.
This is a good way to think of it. Good example.

An engine might be producing it's peak torque at 4,000 RPMs, but it's doing it four thousand times a minute. Increase the RPMs by 25% to 5,000, and drop torque by say, 10%, the total work being done in a minute is more than at the point of peak torque.
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Old 10-05-2012, 04:01 PM   #24
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OMG I had this exact question when helping the gf with fluid dynamics last night!!!!!!!!! ...i'll read up on this later.

From what I got from the PPB section was that torque is through your neck back feeling. I.e. I told my tuner to get the torque curve all the way to the left. on a dyno plot.
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Old 10-05-2012, 04:03 PM   #25
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I guess a better question is how do hp and tq relate to f=ma where f is force m is mass of your car and a is the the acceleration of you car.
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