Measure Hp with a stopwatch


Latest update: 8/28/08

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What's this got to do with building a car?
Truthfully, probably not all that much. But people who are thinking of designing or building a car for high mileage or if you are just interested in how much power it takes to drive your car might be curious.

This article wiil show you how to measure the power to drive a car at a steady speed down the road. Actually the technique/formula can also be used to measure the Hp applied to the wheels while accelerating too.



What do you need to take measurement?
To measure how much Hp it takes to drive your car at a steady speed you need nothing more than a stop watch, an accurate speedometer and knowing the weight of your car! I think a lot of people will be surprised how little Hp it really takes.  And also how much overpowered most cars are to have good acceleration.



General idea of how to take measurement
The November 1970 issue of Road & Track magazine had an article that presented several formulas and descriptions of how to measure Hp at the drive wheels in each gear and other interesting data. I'll just present the method and formula for the steady state Hp to move the car.

Basically what you do to take the measurement is drive your car at a constant speed on a level road at a speed faster (the article suggested 10 MPH faster) than the speed you want the measurement to represent. You then put the car in neutral and start the stop watch at the same time. You stop the watch when the car drifts 10 MPH slower than the speed you want to know the Hp needed to drive the car at a constant speed. The results of the various numbers/calculations will give the Hp at the average speed of the two readings.



Taking the measurement
To find out how much Hp your car needs to drive at a constant 60 MPH for instance, find a level road with no wind blowing, no other cars around and drive at a steady 70 MPH. When you are ready to take the measurement, push the clutch in and start the stop watch. When the car drifts down in speed to 50 MPH stop the stop watch and record the time it took.

You should take the measurements several times in opposite directions to cancel out the affects of a non-level road, slight breeze etc.  Do not do the test following another car or truck. You want the air your car is traveling through to be still.



Here's the formula you need to punch the numbers into
Hp = .0001216 * (W+R) * v * (V'/T')

.0001216 a constant derived from the acceleration of an object when acted upon by a steady force (based on Newtons laws) and the conversion of ft/sec to MPH.

Hp = power required to move car at a constant speed.
W  = weight of car in lbs.
R  = a term known as "effective rotational weight" of the wheels, tires etc (use 100 lbs)
v  = average speed, in this example 60
V' = is the speed difference from the start & end of the test (70 - 50 = 20)
T' = time in seconds to drift from the start to end speed of the test.



Here's an example of the numbers taken with my Honda CRX HF
Hp = .0001216 * (1800 + 100) * 60 * (20 / 23) = 12 horsepower at the drive wheels to move the car down the road at 60 MPH.

The max Hp that my car has (60 big ones!) is used for acceleration and better performance etc.  Don't rush out and put a lawn mower engine in your car unless you like 0-60 MPH to be, "Why yes it will do 60 MPH!" ............ (eventually).  Some day I need to take the same measurement on my Honda Insight.

The same formula can be used to measure how much Hp your engine delivers to the wheels while accelerating.



Something to consider
Many people will say that the weight of the car should not affect, to a large extent, the amount of wheel Hp (WHp) to maintain a vehicle at a constant speed.

We all realize that it takes more power to accelerate a heavy car as compared to a light car. While you accelerated the car to the speed you want to drive at you applied force to the mass of the car. The mass had kinetic energy added to it.

When you do the above measurement, the car is slowing down due to aerodynamic drag, friction of the wheels etc. The  kinetic energy is expended to keep the car moving. If the car had zero weight (and hence zero kinetic energy) aerodynamic drag would stop the car very quickly because there would be no kinetic energy to counteract against the aerodynamic drag etc.

If you don't believe this, toss a small piece of cardboard out of the car window into the air flowing by while driving 60 MPH and see how quickly it slows down in comparison to your car if you had pushed the clutch in at the same time when you let the paper go.

To maintain a constant speed you have to apply some amount of constant force to the mass of the car to counteract the force being applied by the aerodynamic and other drag forces that are trying to reduce the kinetic energy of the mass of the car. The heavier the car, the more force (Hp) you have to apply.


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