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Measuring Horsepower
Your Performance Computer can measure your car's
peak and average horsepower, but to do so, you have
to enter the following data using the Setup mode (see
page 20):
1
The weight of the vehicle
2
The speed range over which you want to
calculate average horsepower.
3
Horsepower Factor
We'll discuss each of these items individually.
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Elapsed Time (seconds)
1: VEHICLE WEIGHT
You must enter the weight of your vehicle. This infor-
mation is usually available in your vehicle owner's
manual or brochure. Several different terms are used
by manufacturers. "Curb Weight" is the weight of the
vehicle without passengers and cargo. "Payload" is the
maximum allowable weight of passengers and cargo.
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"GVWR" is the sum of the Curb Weight and the Pay-
load, so it is the maximum allowable weight of the
vehicle including passengers and cargo.
You should use the Curb Weight figure if available, but
add the weight of the driver, passengers and cargo,
plus any additional equipment added to the vehicle.
2: HORSEPOWER SPEED RANGE
You must select the speed range for horsepower calcu-
lations. The Performance Computer will calculate the
average horsepower over this range, as well as the
peak horsepower within the range. In most cases you
should use a speed range in second gear to reduce the
possible effect of wheelspin. The graph to the left shows
a quarter mile run. You can see the 1-2 shift at about
32 mph, and the 2-3 shift at about 56 mph.
The car's flywheel can store energy between shifts,
which will result in optimistic horsepower calculations
immediately after a shift. To avoid this, set your start-
ing point a few mph above the shift point. Then set
your end point a few mph below your next shift.
So in this example, you might set your HP min
speed to 35 mph, and the HP max speed to 53 mph.
NOTE: You don't need to do a graph of your car's
performance to determine these numbers.
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Just set the HP Min Speed a bit above your 1-2 shift,
and set the HP Max speed a bit below your 2-3 shift.
The factory defaults for the HP speed range are
HP Min Speed 30 mph, and HP Max speed 50.
3: HORSEPOWER FACTOR
The Performance Computer calculates your car's avail-
able "Wheel Horsepower," which is affected by drive-
train losses, rolling resistance, and aerodynamic drag.
This calculated horsepower is the horsepower available
to accelerate the vehicle, which is what determines the
vehicle's performance.
However, manufacturers' ratings list a higher
engine horsepower, which is not reduced by these other
real-world factors.
If you like, you can enter a "Horsepower Factor"
to increase the wheel horsepower readings to approxi-
mately correlate to the higher engine horsepower.
For example, if your car is rated at 200 horse-
power, and the Performance Computer calculates your
peak wheel horsepower to be 168 horsepower, you
would divide 200 by 168, to get a correction factor of
1.19
If you adjust the Horsepower Factor in the Setup
Mode to 1.19, your Performance Computer will use that
correction when calculating horsepower.
See the following pages for details on
using the Setup Mode to enter your
settings for these items.
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