Torque vs. Horsepower

[tr0ike]

I apoligize if this topic has come up in the past (seems it might have), I'm new to the racing forum here, but you'll probably see me posting a lot as my knowledge increases.

One topic I'm trying to increase my understanding of is the relationships and benefits between torque and horsepower. All you ever hear about from movies, adverts, bragging racers/ricers, etc., is horsepower. Yet it seems to me that HP is only half the equation.

So for the knowledgable, please post your wisdom about any/everything related to horsepower + torque. For the clueless/learning, post up your questions.

[Sticky]

In the end, horsepower is more important, off the line you want torque.

[drftk1d]

horsepower is a function of torque. technically horsepower is an imaginary number and there is only torque.

but for all intents and purposes, torque describes the low end and horsepower the top end.

theres a saying that goes, "horsepower is for racing, torque is for burnouts."

[tr0ike]

To keep my own ill-advised miseducated questions/statements seperate from the original post ... a few things I think I know / wonder about.

*I've heard that torque is merely some simple equation of horsepower / RPM, or something .. don't remember what the factors were.

*I've also been told that horsepower really only comes into affect in higher speeds, i.e. freeway/top gear acceleration. The Top Gear (TV) episode with the Lamborghini Murcialago (sp) vs. the Mitsubishi Lancer Evo VIII (whichever special edition it was), where they place the Mitsu and some Euro econobox in top gear and accelerate - and the econobox pulls away from the Mitsu, which has no HP in lower RPMs/speeds - seems to demonstrate this.
I suppose gearing has a lot to do with it, but is there a general speed / RPM range where HP overtakes torque in importance?

*Why is it that diesel engines put out such massive torque? VW's 1.9L TDI, for example, puts out (IIRC) 90hp but 155 lb-ft of torque

*I also notice that many factory turbo cars have equal HP/torque numbers ... example: IIRC, Subari Impreza WRX STI has 300hp/300lb-ft.

*Why do SOHC engines produce more torque than DOHCs, and have lower power curves? For example, my VW has a SOHC 2.0L producing 115 hp @ 5400(?) rpm + 122 lb-ft of torque @ 3000 rpm. My mother's Civic I drove before I bought my own car has a DOHC 1.6L (non-Vtec); it puts out the same 115hp (at a higher RPM) but I believe only 111 lb-ft torque (likely also at a higher RPM). Does it have to do with the higher compression of SOHC engines somehow?

*What is better? Which do you prefer for various daily/racing applications?
I hear torque is better for average/city driving, as most people don't operate in high RPM ranges. If I were to race against another car with a bit more hp but a bit less torque than my car (i.e. VTEC Civic), I presume my car would be ahead at first - due to quicker shifts and more low-end power - but eventually the civic would gain/pass?

*Why are most engine modifications (intake/chip/exhaust etc.) focused on HP only? Do they actually bump torque a bit as well, but only advertise HP? Is there any way to bump torque? Would it be worth it, or is increasing HP better?

I'm certainly not asking for someone to answer all of these, but any insight/expansion/correction/clarification whatsoever on these, or other topics, would be greatly appreciated

[-The Stig-]

I'll stay quiet to keep people thinking I'm smart...rather than open my mouth and remove all doubt.... so I'll quote a good site for you...

Originally Posted by www.musclecarclub.com

Definition of Horsepower


Horsepower is probably the term most used when discussing muscle cars. However, few people can accurately define what exactly horsepower is. Horsepower is officially defined as "the amount of energy required to lift 550 pounds, one foot, in one second." But what does this mean? To answer that question, we need to know a little history.

The term horsepower was created by James Watt, an engineer famous for his work with steam engines and is most associated with his measure of electric power (i.e. 60 watt light bulb). Around 1775, Watt wanted to create a standard measure of work to compare the output of his steam engines with the horses that were used to move large loads in mines. Watt studied the amount of work that horses did and found that on average, a mine horse could perform 22,000 foot-pounds of work in a minute (for example, move 1,000 lbs 22 feet in one minute or 500 pounds 44 feet in one minute). He then increased the number by 50 percent and came up with a measurement of 33,000 foot-pounds of work in one minute. This measurement eventually became known as horsepower.

Using various physics equations, you can come up with different interpretations of horsepower:

* 1 horsepower is equivalent to 746 watts. So if you took a 1-horsepower horse and put it on a treadmill, it could operate a generator producing a continuous 746 watts.

* 1 horsepower (over the course of an hour) is equivalent to 2,545 BTU (British thermal units). If you took that 746 watts and ran it through an electric heater for an hour, it would produce 2,545 BTU (where a BTU is the amount of energy needed to raise the temperature of 1 pound of water 1 degree F).

* One BTU is equal to 1,055 joules, or 252 gram-calories or 0.252 food Calories. Presumably, a horse producing 1 horsepower would burn 641 Calories in one hour if it were 100-percent efficient.



Types of Horsepower


There are three basic types of horsepower, SAE Gross Horsepower, SAE Net Horsepower, and Wheel Horsepower. Each is the result of measuring the same engine in different ways. These standards were established by the Society of Automotive Engineers (SAE). The SAE is a group responsible for setting various standards within the automobile manufacturing industry. Founded in 1905, the SAE publishes many new, revised, and reaffirmed standards each year in three categories: Ground Vehicle Standards, Aerospace Standards, and Aerospace Material Specifications. Standards allow entire countries to talk to each other in a common language.

SAE Gross Horsepower or Brake horsepower (bhp) was the standard horsepower measurement by the automotive industry up until 1971. Brake Horsepower Power is measured at the flywheel with no load from a chassis or any accessories and with fuel and ignition operations under ideal conditions. An accessory is anything attached to the engine, by any means, which is not required for basic engine operation. By this definition, this would include a power steering pump, smog pump, air conditioning compressor and an alternator. Ideal conditions, often called laboratory conditions, are standardized settings for use during horsepower measurement. During the 1960s they consisted of a barometric pressure of 29.92 Hg and a temperature of 60 degrees F.

SAE Net Horsepower became the standard measurement in 1972, and is still used today. SAE Net horsepower is the horsepower generated by the engine at the flywheel with all accessories attached. This change was made to reflect the numerous energy sapping accessories that cars began to have, such as an A/C Compressor and alternator, and thus was a better representation of the actual power generated by the engine. This number is always lower than the SAE Gross horsepower. Therefore, the same engine could have been rated in 1971 as 360 SAE Gross Horsepower and in 1972 as 300 SAE Net horsepower without any reduction in "power."

Wheel horsepower is horsepower measured at the actual drive wheels, taking into account the load from the chassis and all accessories. It is the most accurate measure of the amount of energy that the car actually generates to move it forward. Wheel horsepower is measured using a dynamometer. This is done by placing the vehicle's driven wheels on a large roller and accelerating the wheels up to redline in first or second gear. The vehicle's ability to turn this roller is measured and calculated (formula below) to come up with a figure that represents how much horsepower is actually available to move the vehicle around. Because a frictional loss between the engine and the driven wheels is unavoidable, wheel-driven horsepower will always be less than SAE Net Horsepower. How much less wheel-driven horsepower will depend on how many mechanical parts exist between a vehicle's engine and its driven wheels. This is usually measured as a percentage loss due to the "friction" of the intermediate components between the flywheel and the actual wheel. For a Rear Wheel Drive car, engine power has to travel through a transmission, driveshaft, rear-differential, and two axle shafts (one for each rear wheel). That's four separate mechanical components taking a bite out of the car's horsepower before the rear wheels even begin to turn. Front-wheel drive cars with transverse-mounted engines usually have a lower frictional loss because horsepower only has to travel from the engine, through the transmission and down two short driveshafts before reaching the wheels. Typical "powertrain" losses run between 15-22% but vary greatly between cars.


Definition of Torque


Torque is a force that tends to rotate or turn things. You generate torque any time you apply a force using a wrench. Tightening the lug nuts on your wheels is a good example. When you use a wrench, you apply a force to the handle. This force creates a torque on the lug nut, which tends to turn the lug nut. Torque is usually measured in English units such as pound-feet (lb-ft), although the international standard is the Newton-meter (1 lb-ft is equal to 1.356 Nm). Notice that the torque units contain a distance and a force. To calculate the torque, you just multiply the force by the distance from the center. In the case of the lug nuts, if the wrench is a foot long, and you put 200 pounds of force on it, you are generating 200 pound-feet of torque. If you use a 2-foot wrench, you only need to put 100 pounds of force on it to generate the same torque.

In a car, the engine converts the horsepower it generates into torque by turning the crank shaft. The combustion of gas in the cylinder creates pressure against the piston. That pressure creates a force on the piston, which pushes it down. The force is transmitted from the piston to the connecting rod, and from the connecting rod into the crankshaft. The point where the connecting rod attaches to the crank shaft is some distance from the center of the shaft. The horizontal distance changes as the crankshaft spins, so the torque also changes, since torque equals force multiplied by distance. Only the horizonal distance is used in determining the torque in an engine. When the piston is at the top of its stroke, the connecting rod points straight down at the center of the crankshaft. No torque is generated in this position, because only the force that acts on the lever in a direction perpendicular to the lever generates a torque.


Measuring Torque


Torque is also measured using a dynameter. The torque generated is measured at different RPMs and the result is plotted on a graph. Then, horsepower is calculated by taking the torque at each RPM, and converting it using the following formula:

Horsepower = Torque X (RPM/5,252)


This formula is the result of combining several formulas into one. First, 1 horsepower is defined as 550 foot-pounds per second. The units of torque are pound-feet. So to get from torque to horsepower, you need the "per second" term. You get that by multiplying the torque by the engine speed. But engine speed is normally referred to in revolutions per minute (RPM). Since we want a "per second," we need to convert RPMs to "something per second." The seconds are easy -- just divide by 60 convert minutes to seconds. Now what we need is a dimensionless unit for revolutions: a radian. A radian is actually a ratio of the length of an arc divided by the length of a radius, so the units of length cancel out and you're left with a dimensionless measure. You can think of a revolution as a measurement of an angle. One revolution is 360 degrees of a circle. Since the circumference of a circle is (2 x pi x radius), there are 2-pi radians in a revolution. To convert revolutions per minute to radians per second, you multiply RPM by (2-pi/60), which equals 0.10472 radians per second. This gives us the "per second" we need to calculate horsepower. We need to get to horsepower, which is 550 foot-pounds per second, using torque (pound-feet) and engine speed (RPM). If we divide the 550 foot-pounds by the 0.10472 radians per second (engine speed), we get 550/0.10472, which equals 5,252. So if you multiply torque (in pound-feet) by engine speed (in RPM) and divide the product by 5,252, RPM is converted to "radians per second" and you can get from torque to horsepower -- from "pound-feet" to "foot-pounds per second."


Putting It All Together


Plotting the horsepower and torque results over a range of RPMs looks something like this:



From the graph, we can determine the peak horsepower and torque ratings - the actual highest value of horsepower and torque, and at what RPMs they are obtained. These values are the most common way of describing the power generated by an engine and is expressed as "320 HP @ 6500 rpm, 290 lb-ft torque @ 5000 rpm." Note that by definition, horsepower has to equal torque at 5,252 rpm and therefore, the lines will cross at this point. Torque will always be higher than the horsepower below 5,252 RPM, equal to horsepower at 5,252 RPM and less than the horsepower above 5,252 RPM. That is why the torque peak occurs at a lower RPM than the horsepower peak.

It is important to realize that the area underneath the curves is just as important (or more so) than the peak value. A fairly flat curve (especially for torque) means that power is available throughout the RPM range and usually occurs with large displacement engines. Smaller engines generally have a drastic peak in output, with dramatic increases and decreases over the RPM range. Two engines may have the same peak horsepower, but vastly different torque ratings and curves.

All material © Copyright 2003 by Musclecarclub.com

I hope that helps.

[drftk1d]

Originally Posted by tr0ike

*I've also been told that horsepower really only comes into affect in higher speeds, i.e. freeway/top gear acceleration. The Top Gear (TV) episode with the Lamborghini Murcialago (sp) vs. the Mitsubishi Lancer Evo VIII (whichever special edition it was), where they place the Mitsu and some Euro econobox in top gear and accelerate - and the econobox pulls away from the Mitsu, which has no HP in lower RPMs/speeds - seems to demonstrate this.
I suppose gearing has a lot to do with it, but is there a general speed / RPM range where HP overtakes torque in importance?

It is called "turbo lag." An unfair comparison, as that evo has a relatively large turbo (its needed to make the FQ's 400hp) so obviously its going to take that long from the highest gear.

[tr0ike]

*head spins*

thanks for that ... highly technical information!

Aside from my lack of understanding in the mechanical engineering field, I came up with one curious question ... considering the difference between measures of horsepower in the past vs. present, does this mean the HP claims of the old muscle cars were actually exaggerated by today's standards?

[drftk1d]

no.

[RedLightning]

Originally Posted by RedNeck383

I'll stay quiet to keep people thinking I'm smart...rather than open my mouth and remove all doubt.... so I'll quote a good site for you...




I hope that helps.

Hey ur back!!! Welcome back homey.



Horsepower sells cars but torque wins races.

[Thourun]

Originally Posted by tr0ike

... does this mean the HP claims of the old muscle cars were actually exaggerated by today's standards?

Not realy since they didn't have as much extra stuff like AC to run. They should do it by the wheels so that people would know how leachy the things like tranny and differential are... but they'd rather advertise biger numbers.

[PWRDbyUNCLEbens]

I think that old Muscle Car HP #'s were decieving. The times they ran at the track didn't seem consistant to me with the claimed HP. Then Again I can't find a site with muscle car dyno graphs to prove it so maybe they weren't off..........by much.

[chevytrucks92]

Originally Posted by PWRDbyUNCLEbens

I think that old Muscle Car HP #'s were decieving. The times they ran at the track didn't seem consistant to me with the claimed HP. Then Again I can't find a site with muscle car dyno graphs to prove it so maybe they weren't off..........by much.

They were decieving, but only becuase they were tested differently. Then, the engines were dynoed with no accessories. Now engines are tested with all their accessories. Naturally, the one with no accessories will dyno higher then one with accessories.

Plus, you also got to realize those old muscle cars had bias-ply tires (which are actually better for straight line performace vs. radials, but really suck for anything else, lol), not the best suspensions, and tremondous weights! Those muscle cars were boats, lol. We're talking over 4000 lbs here.

And for the question: Torque gets you out of the hole, and gives you that "seat in the pants" feel. Horsepower keeps you planted in the seat.

[TheStang00]

torque is a torque arm multiplied by the force applied to that torque arm. so the reason diesel engines have such massive torque is because they have massive torque arms. as you may or may not know diesel engines do not use spark plugs, instead they compress the fuel mixture to the point of combustion. they may have a compression ratio of 23:1. a normal gasoline engine might have something like 9:1... give or take a few.

[Sticky]

If you're road racing you want torque to help you get off the corner, and horsepower to help you get down the straightaways.

Cars like the Integra R's only generate about 130 ft. lbs of torque, but still has over 200 horses because of it's high redline and ability to produce torque at it.

[Steel]

Originally Posted by RedLightning

Horsepower sells cars but torque wins races.

Oh so THAT'S why all the F1 and LeMans racecars use turbodiesel engines! Geewilliker!