Torque v. Horsepower

[socalrob]

OK, I know engine output is measured in both torque & horsepower. From what little I understand, big cyclynders = big explosions = more torque, & horsepower is more about, well, something else, as it seems that multi cyclinder engines put out lots of HP compared to engines of equal displacement with fewer cylinders. Maybe HP is a function of air moving through the engine?

What I'm unclear on, among many other things, is seems to me both are a measure of the engine's ability to produce a force moment vector at the contact between the rear wheel & ground. So I'm unclear on exactly what differently is being measured.

One reason this whole topic is of interest to me, I have an 8.1 liter (I think thats about a 500 cubic inch) V8 in my suburban. The extreme of big block engines. I can't recall for sure but think it puts out 350 hp & 450 or 500 ft lbs of torque. What surprises me is that the truck is not really very "quick" off the line. I suspect GM's 6 liter V8 is just as fast. Its like the engine has a huge mass of reciprocating / revolving parts to get up to speed, & it doesn't rev real freely. I assume its like having a big flywheel, those big 1 liter size pistons going up n down. But going up hills or towing, can't really tell the difference from cruising down a flat road.

So thinking about the new Hex Head 1200 boxers, with their counter balancer, is that engine going to be a bit less quick response / rev happy than the 1150 motor with no counterbalancer? Seems almost like we need another measure of engine performance, called "spinibility", that would measure torque or horsepower v. spinning engine / drivetrain / wheel mass. Or is that what horsepower is?

Like I say, what is the difference between HP & torque?

[TorqueMonsterMT-01]

Rob;
Read this first. The "How Things Work" website has the best explanation I have ever seen.

http://auto.howstuffworks.com/horsepower.htm

The short answer is that torque is measured while HP is calculated. HP is a funtion of torque and RPM.

Now, for your Suburban. A motor's torque and HP ratings are only part of the equation when speaking of acceleration. Of course HP and torque make a difference, but the engine's ability to buiid RPM rapidly makes acceleration. Acceleration can be gained by making recipricating engine parts lighter so the engine can rev faster. The trick is to not lose too much rotating mass as torque output can be affected.

Applyiing the power to the ground without losing the RPM requires torque and HP.

Gear ratios make a difference too. Any vehicle needs the ability to apply the power to the ground in an efficient manner. In other words, too high gearing slows the engines ability to produce RPM rapidly, reducing acceleration.

I hope this clears up some things for you.

Regards,
Jeff

[Dallara]

Torque vs. Horsepower...

Always an interesting, and often misunderstood, discussion.

First off, there really is no such thing as "horsepower"... Really. It doesn't exist. It is a simple mathematical construct to try to quantify the engines work over time.

On the other hand, torque is very, very real... And is the engine's ability to apply twisting force on a lever (the crank throw). Dyno's actually only measure torque, period - then a simple equation is applied based on the measured torque at any given RPM, and it the result of this equation is what tries to quantify the engine's ability to do work over time.

The equation is indeed incredibly simple... Here it is:

torque X RPM divided by 5252 = horsepower

If you have an engine that produces 50 ft.lbs of torque at, say, 4,000 RPM it's making incrementally more than 38 HP at that RPM point.

Literally, measured torque is a *snapshot* of the engine's twisting force at a single point in time, while horsepower attempts to quantify how much work the engine can do producing that torque for one minute.

Here's a little page showing where the term horsepower came from:

http://www.sizes.com/units/horsepower_british.htm

Here's another way to look at the subject (and I quote from Brian Beckman)



"Horsepower is a measure of power, which is a technical term in physics. It measures the amount of work that a force does as it acts over time. Work is another technical term in physics. It measures the actual effect of a force in moving an object over a distance. If we move an object one foot by applying a force of one pound, we are said to be doing one foot-pound of work. If it takes us one second to move the object, we have exerted one foot-pound per second of power. A horsepower is 550 foot-pounds per second. It is another one of those historical units that Napoleon hated and that has no reasonable origin in science.


We can expend one horsepower by exerting 550 pounds of force to move an object 1 foot in 1 second, or by exerting 1 pound of force to move an object 550 feet in 1 second, or by exerting 1 pound of force to move an object 1 foot in 0.001818 seconds, and so on. All these actions take the same amount of power. Incidentally, a horsepower happens to be equal also to 745 watts. So, if you burn about 8 light bulbs in your house, someone somewhere is expending at least one horsepower (and probably more like four or five) in electrical forces to keep all that going for you, and you pay for the service at the end of the month!."



Notice his second line - "It measures the amount of work that a force does as it acts over time."

But then look again at the next three lines...

"Work is another technical term in physics. It measures the actual effect of a force in moving an object over a distance. If we move an object one foot by applying a force of one pound, we are said to be doing one foot-pound of work. If it takes us one second to move the object, we have exerted one foot-pound per second of power."

There he is talking about *torque*...

There really is no need for your proposed term - "spinibility" - since it would be largely subjective.

The simple reason some engines rev slower is nothing more than they have more flywheel and reciprocating mass to accelerate. Without question the crankshaft alone in you 8.1 liter engine is far, far heavier than one in a 6.0 liter or 5.3 liter. Indeed the two smaller engine may rev quicker, and provided they carry appropriate gearing they may launch *quicker*... but a they won't be able to sustain hauling a large load up a grade without requiring more throttle, for instance, because they have less torque.

Essentially they have to make their crankshafts cover more distance (spin more RPM) to apply what little torque they have over a theoretically larger slice of *time* (i.e. this equates to the distance mentioned).

Basically the smaller engine has to find a way to apply its smaller amount of torque more often (torque over time), and it does this by revs.

All in all it really is a pretty simple concept, and only appears complex on the surface. Engines don't alter time or physics. You simply have to understand the physics going on and how they are applied.

Hope this helps, but if not, or you are still confused, let me know where you're held up and I will try and clear up any rough spots.

Cheers!

Allan (Dallara)

[DarthRider]

Simple minds tend to think in simple terms. Here's what works for me...
*In general* a real-world way of thinking about horsepower and torque is that Torque produces acceleration while Horsepower produces speed.
This is greatly over simplified of course but it does clarify the physics.
And is all affected by a number of variables such as gearing, weight, power-to-weight ratio, traction, aerodynamic drag, load, etc.
Maybe a better way of stating it in simple terms is that with all other things being equal, Torque produces acceleration while Horsepower produces speed.

Dave

[Bones]

From a bioengineering standpoint, there is a corollary:

If you put on a louder aftermarket exhaust, all the blood flows to your genitals when you twist the throttle. This redirection of blood flow from the brain to the gonads causes a change in perceived acceleration.

Sound Pressure Level (dB) = (# of Testicles) X ( Rod Turgor)

This can be demonstrated very easily in your driveway or at any motorcycle gathering.

Estrogen seems to negate the effect.
Tatoos negate the effect of estrogen. Testosterone amplifies all variables.

Further:

The above equation is inversely related to the number of cylinders, starting at 4 until you get to 2 cylinders. Then, like the twist of a mobius, the phenomenon reverses again. Above 4 cylinders, the denominator disappears.

Hope that helps.

Jeff

[Deans BMW]

What the Bonester said.

[DarthRider]

Exactly what the Bonetser said...but what about singles?
Want wood? Catch a ride on the little single I have in the shop!

[Deans BMW]

I have and it scared me to death trying to keep up with you on Darth.

[DarthRider]

Yeah, but I didn't notice you having any trouble keeping up when you were on Darth and I was on the Super Retard!

Dave

[Deans BMW]

:028: Bro Dave, now that I can see again, I couldn't then, and on my own bike, you will need Hoochie Mama.....:058:

Looking foward to seeing you again this spring in Kerrville.

By the way,....am in the process of cooking up something good for Bonneville, Sept 3 thru 7...:ip:

[Boxerboy]

I guess it's too simple to say torque = acceleration and HP = top speed?

[socalrob]

Thanks everyone, it actually is more clear.

Dallera, your post helped alot. My questions in large part were mostly due to my trying to figure out what differently was being measured, makes since that HP was a time component, and that its derived from an equation based on Torque. I think I see it now. Pretty sad, guess I completly forgot my physics I had in college.

I liked the link to the British HP unit. Reminds me of the Longitude (those north/south lines on maps that pass through the poles) problem in the 1700 & 1800's. Where on the planet to put the zero longitude line was totally arbitrary, maps made by British cartographers used London (Greenwich Observatory to be exact), by American cartographers used Washington, and French cartographers used Paris. Took a long time, but finally everyone settled on London, I assume thats why the world also settled on the meter (French) as part of the compromise.

In the world of Land Surveying, a concept that a number of students have difficulty with is the difference between Accuracy & Precision. You can take measurements with extreme precision, but if your measureing devise has systematic errors (say your 100 ft. measuring tape is in reality 101 ft long), then you will not be as accurate (close to the truth) using a device of lower systematic errors (say you use a 100 ft measuring tape that really is 100.001 feet long, but you only measure each 100 ft segment with a precision on plus /minus 1 ft, measuring out a one mile distance will be statistically pretty close to the truth). A simple concept that nevertheless confusses many.

I'm still wondedring if the engine counter balancer in the new Hex Head 1200 engines causes that motor to rev a bit slower at lower RPM's, and maybe is the cause of some riders not liking the low end response, even though that engine has higher HP & Torque output than the Oil Head 1150 engine. However, when you get the Hex Head spun up to higher RPM's, no doubt it is much faster.

I was taken off guard by my 8.1L 4x4 3/4 ton suburban. I bought it to replace a half ton 96 4x4 suburban with a 350ci engine (5.7L). The 96 was too light IMO, & was not nearly the heavy duty truck my 89 3/4 ton suburban is. So I ordered the biggest / most heavy duty 2001 model I could, not realizing that the 8.1L engine is so big that its very size has certain limitations, and that a 6.0L engine would have been a better choice.

[geechie]

OK, I think I've got it. Life is simple. It all boils down to torque and traction... and I need more of both.

George

[Dallara]

Remember, Folks...


Horsepower does NOT exist, period.

No amount of wishin' and's hopin' will make it exist. It is simply a calculated mathematical construct to make an attempt at quantifying torque over time. Nothing more, Nothing less.

Horsepower does NOT equal speed, or top speed.

Torque is all that really does exist, and it is the only thing that can be measured in relation to an engine's power output other than RPM.

In some ways none of the above is perhaps the best way to measure an engine's power output. A much better method, primarily because it allows you to directly compare engines of different displacements, is BMEP - or Brake Mean Effective Pressure.

With BMEP you can have a 50cc engine and a 1200cc engine (or even a 8.1 liter one) and they can all have, say, 230 psi BMEP.

you can calculate BMEP quite simply with the following equation:

BMEP = 150.8 x TORQUE (lb-ft) / DISPLACEMENT (ci)

An interesting thing to note is that maximum BMEP will always occur at the engine's peak torque. Where an engine produces its mythical *horsepower* peak will always show a lower BMEP (unless the peak torque and peak *horsepower* occur at exactly the same point). This should tell you something.

Here's a couple of examples from EPI Inc. (a design and development center for aircraft, automotive, marine, and industrial propulsion systems):



"One was a 358 cubic inch circle track motor (gasoline-fueled, 2-valve, pushrod V8) which made 531 lb-ft of torque at 6400 RPM (647 HP) and 503 lb-ft of torque at 7600 RPM (729 HP). Those data points represent BMEP figures of 223.5 at peak torque and 211.4 at peak power. Those numbers are truly excellent, and require very-highly developed components.

The best by far that we have seen was a 268 cubic inch drag-race motor (gasoline-fueled, 2-valve, pushrod V8) which made 413 lb-ft of torque at 7800 RPM (613 HP) and 372 lb-ft of torque at 9200 RPM (652 HP) for BMEP figures of 232.7 at peak torque and 209.6 at peak power. The 232.7 number is astonishing at any RPM. The 209.6 figure at 9200 RPM is nothing short of phenomenal for a two-valve pushrod engine."


But back to Torque and *Horsepower*...

If you want to simplify it as some have stated above, a more accurate statement would be:

Acceleration = Torque

Speed (or Top Speed) = Engine's ability to maintain Torque output at higher revs.

If you want to be both accurate, and precise (as SocalRob was illustrating), then suggesting that horsepower = speed (or top speed) really only confuses the issue.

Cheers!

Allan (Dallara)