HP / Highway Dyno Calculator By Bowling

Yes, you can use the open highway to compute horsepower and torque values delivered to the wheels of your car, and use this as a baseline to evaluate the effectiveness of modifications to the car. The actual test is simple: Put the car in the highest gear in which the car can run through the entire RPM range without shifting, like 2nd or 3rd (automatics must use low to stop shifting). Now you must "floor" the accelerator pedal (wide-open throttle) starting at a low speed (or even stopped) and accelerate to a high speed which spans a maximum RPM range, and pay for the speeding ticket later. The trick here is to make a chart of how long it takes to reach certain MPH speeds, taken off of the speedometer. The default test below started at 30 MPH, declared this as zero time, and took readings at 15 MPH increments. One way to get the time is to use a tape recorder and have a passenger shout the MPH readings as the car reaches them, and then later (while not driving) play the tape back and use a stopwatch to obtain the times. Another technique is to use a camcorder which is filming the speedometer and a clock which is attached next to it. Remember that the first time is zero, and each new MPH is relative to this zero (i.e. time numbers keep getting bigger).

Other inputs are car frontal area and coefficient of drag. These can be obtained from past issues of Car and Driver magazine, or the defaults can be used which may introduce a small error in absolute values, but will work fine for relative comparisons of multiple runs.

To aid in the program usage, a chart of drag coefficients is provided.

Also needed are the environmental conditions at the time of the test. This allows the program to convert the HP figures at the flywheel to standard conditions. The advantage of this is to allow "fair" evaluations of HP in the future, and to compare against other vehicles whose power curves are also adjusted to standard conditions, as are most dynamometers and magazine articles.

The calculation also requires you to know the transmission and differential ratios in order to convert MPH to RPM. If these are off or there is a lot of slippage in the drivetrain, this will only affect the positioning of the HP curve on the RPM axis, and not the actual determined values of HP.

User Inputs:

Transmission Ratio:

Differential Ratio:

Vehicle Weight in Pounds:

Coefficient of Drag:

Frontal Area (Square Feet):

Tire Inflation Pressure (psi):

Tire Diameter (Inches):

Engine Cubic Inch Displacement (CID):

Engine Stroke in Inches:

Environmental Conditions:

Carburetor Air Temperature in degrees F:

Test Condition Barometric Pressure (Inches Hg):

Test Location Elevation (Feet):

Test Condition Percent Relative Humidity:

Measured Times and Associated Miles-Per-Hours:

Enter Time - MPH here, an ascending MPH order. Enter up to 9 pairs - leave unused inputs blank

Time(s): MPH:

Time(s): MPH:

Time(s): MPH:

Time(s): MPH:

Time(s): MPH:

Time(s): MPH:

Time(s): MPH:

Time(s): MPH:

Time(s): MPH:

Press this button to submit a computation:

To reset the form input:


The following equations were used in the above calculation:
Horsepower = (Weight(Lbs) * Acceleration (g) * MPH)/375.0
A cubic spline fit was performed on MPH vs. Time values to provide numeric derivatives for determination of acceleration.
Bruce Bowling
Bowling Superior