Horsepower Information

Just about every car advertisement on TV mentions it, people are talking about their cars always seem to have more than the other guy...

But what is horsepower and what does it mean?

The Definition:

The term horsepower was invented by James Watt, who lived way back between 1736 and 1819.

The story goes that Watt was working with strong draft horses carrying coal and he wanted a way to talk about the power available from one of these horses. Probably wanted to brag to his fellows about how he improved his horses' performance, right? He found that, on average, a horse could do 22,000 foot-pounds of work in a minute. For some odd reason, he then multiplied that number by 50% and pegged the measurement of horsepower at 33,000 foot-pounds of work in one minute. It is that strange, arbitrary unit of measure that has made its way down through the centuries and now appears on your car, your lawn mower, your chain saw, and even your vacuum cleaner and household blender.

What horsepower means is this... In Watt's judgment, one horse at the coal mind can do 33,000 foot-pounds of work every minute (apparently those horses he observed doing 22,000 foot-pounds of work every minute were slackers and were sent out to LePages). A horse exerting one horsepower can raise 330 pounds of coal 100 feet in a minute, or 33 pounds of coal 1000 feed in one minute, or 1,000 pounds 33 feet in one minute, etc. You can make up whatever combination of feet and pounds you like - as long as the product is 33,000 in one minute and you have one horsepower. You can probably imagine that you would not want to load 33,000 pounds of coal in the bucket and ask the horse to move it one foot in a minute because the horse couldn't budge that big a load. You can probably also imagine that you would not want to put one pound of coal in the bucket and ask the horse to run 33,000 feet in one minute, since that translates into 375 miles per hour and most horses can't run that fast. If you have a block and tackle, you can easily trade "perceived weight" for distance using an arrangement of pulleys. So you could create a block and tackle system that puts a comfortable amount of weight on the horse at a comfortable speed no matter how much weight is actually in the bucket.

Horsepower can be converted into other units. For example, one horsepower is equivalent to 746 watts or 2,545 BTUs (British Thermal Units) per hour. So if you took a one-horsepower horse and put it on a treadmill, it could operate a generator producing a continuous 746 watts. If you took that 746 watts and ran it through an electric heater, it would produce 2,545 BTUs in an hour (where a BTU is the amount of energy needed to raise the temperature of one pound of water one degree F). One BTU is equal to 1,055 joules, or 252 gram-calories, or 0.252 food Calories. Presumably the horse would burn 641 Calories in one hour doing its work if it were 100% efficient.

Measuring Horsepower:

If you want to know the horsepower of an engine, you hook the engine up to a dynamometer. Horsepower is pure theory--it's an arbitrary number. A dynamometer places a load on the engine and measures the amount of TORQUE that the engine can produce against the load. Some dynamometers measure the acceleration rate of a known mass at a given rate of speed--like a DynoJet. The basic formula used is:

Force (ft/lbs) = MASS (lbs) X ACCELERATION (ft/second)

Since the MASS of the roller is known, and the ACCELERATION rate can be measured with simple sensors, the resultant force exerted on the roller in "ft/lbs" is then calculated from the known values (mass x acceleration) although not physically measured. With in input from the engine such as RPM--we then input the data and make up "horsepower". There are other correction inputs like atmospheric pressure, humidity, etc...but we won't go into that here. The formula most widely accepted becomes:

HP = Torque x RPM / 5252

To further the initial formula-if you can measure the acceleration rate, you can use a product like a G-Tech (www.gtech.com) to accurately calculate HP and Torque, without ever going to a dyno. "G-Tech" data and track times are normally within a fraction of each other, making this tuning/measuring method very accurate.

But let's first talk a bit more about torque, since it's a tangible measurable force, and we'll explain dyno theory a little bit more.

Torque Imagine that you have a big socket wrench with a 2-foot long handle on it and you it to apply 50 pounds of force to that 2-foot long handle. What you are doing is applying a torque, or turning force, of 100 foot-pounds (50 pounds to a 2 foot long handle) to the bolt. You could get the same 100 foot-pounds of torque by applying one pound of force to the end of a 100 foot handle or 100 pounds of force to a one foot long handle.

Similarly, if you attach a shaft to an engine, the engine can apply torque to the shaft.

A dynamometer measures this torque.

Our MAHA dyno is computer controlled. The computer applies an "eddy current", controlling load to the rollers and actually "holds" the vehicle at a fixed wheel speed, no matter how hard you put your foot into it -- the vehicle axle speed is held there. You can vary the output with the amount of engine load applied at that speed by varying throttle angle. The output to the dyno screen is actual, real world, measured force (torque) delivered from the rear tires to the rollers. It's a pretty cool machine -- I can load the rollers to simulate pulling a load, or driving down the road, driving up an 8% grade, or just hold the vehicle at one speed (really nice for tuning). I've had a low 10 second 1996 Mustang on it which had been engine dyno'd on a Superflow Engine dyno at over 750 hp on alcohol -- tuned with pyrometers instead of air fuel ratios. Although the MAHA software has 500 HP limitations at this point, the MAHA dyno performed flawlessly--holding the Mustang at any speed we chose to maximize the state of tune.

Here's a shot of Mike Williams (a car we previously built parts for and sponsored) '70.5 Camaro from a 2003 run at Mission, B.C. and ran a best 9.42 @ 142 MPH with a BBchevy and Turbo 400 transmission with Ford 9". Needless to say, he usually emptied the pits when he was up to run.

I've personally programmed a different spark map on an engine with a SUPERCHIPS Race ICON, tried the best possible combination on a '99 Nissan Pathfinder Stillen model, and only gained 7 ft/lbs torque at the wheels. That wouldn't even be a tangible, "feelable" gain at the seat of your pants--but this dyno measured it--time after time. Before you think that the ICON doesn't work, I installed one on a 1995 S10 4.3 CPI with a 700R4 transmission and gained 95 ft/lbs of torque by retuning the spark map above 1800 RPM through 5500 rpm.

If you plot the horsepower versus the RPM values for the engine, what you end up with is a horsepower curve for the engine. What a graph like this points out to you is that any engine has a peak horsepower - an RPM value at which the power available from the engine is at its maximum. An engine also has a peak torque at a specific RPM. You will often see this expressed in a brochure or a review in a magazine as "320 HP @ 6500 RPM, 290 ft-lb torque @ 5000 RPM" (the figures for the 1999 Shelby Series 1). When someone says an engine has "lots of low-end torque" what they mean is that the peak torque, like in a diesel application, occurs at a fairly low RPM value.


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Wester's Garage 2016
Updated 11/2015 ru

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