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Measuring Horsepower
For the performance computer to accurately measure
your car's horsepower, you must enter information
about your vehicle's weight, drivetrain loss, and aero-
dynamic qualities.
Note: This setup is optional, and is NOT necessary for
accurate Timed Runs or G-Meter function.
In a timed run, the rated horsepower of your car's
engine is consumed in four primary ways:
1) The power to actually accelerate the weight of
the car.
2) The power consumed by frictional losses in the
car's drivetrain.
3) The power used to overcome the rolling resistance
of the tires.
4) The power used to overcome aerodynamic drag.
With the Performance Timer's Setup Mode, you can
enter details about your car that will enable it to cal-
culate the power used by each of these four items. The
Performance Computer will then combine these fac-
tors to calculate the approximate total horsepower
being produced by your engine.
To accurately measure engine
horsepower, the following six
setup items must be correct:
Vehicle Weight • HP Min Speed • HP Max Speed
HP Drivetrain Loss • Rolling Resistance • Drag CDA
To enter Setup, hold the G-METER button.
Then follow the on-screen instructions, which are:
•
To go to the next item, press the START button.
•
To change the setting of an item, press the
UP or DOWN button.
•
To exit the Setup Mode, press the G-METER button.
VEHICLE WEIGHT
You must enter the weight of your vehicle using the
Setup Mode. This information is usually available in
your vehicle owner's manual or brochure.
Several different terms are used by manufactur-
ers. "Curb Weight" is the weight of the vehicle without
passengers and cargo. "Payload" is the maximum al-
lowable weight of passengers and cargo. "GVWR" is
the sum of the Curb Weight and the Payload, so it is
the maximum allowable weight of the vehicle includ-
ing passengers and cargo.
You should use the Curb Weight if available, but
add the weight of the driver, passengers and cargo,
plus any additional equipment added to the vehicle.
20
HORSEPOWER SPEED MIN AND MAX
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.
It is usually most helpful to choose a speed range
and stay with that speed range. This makes it easier to
see improvements as you modify your car, etc.
The Setup Item "HP Min Speed" sets the speed
that the unit starts measuring horsepower.
The Setup Item "HP Max Speed" sets the speed
that the unit stops measuring horsepower.
The default speed range is 30 to 60 mph.
DRIVETRAIN LOSS
A surprisingly large portion of your engine's horsepower
is consumed by frictional losses in the car's transmis-
sion and driveline. Use the following table as a guide.
2 wheel drive with manual transmission
2 wheel drive with automatic transmission
4 wheel drive with manual transmission
4 wheel drive with automatic transmission
Note: If you prefer to measure wheel horsepower,
change this setting to zero.
ROLLING RESISTANCE
This is a characteristic of the vehicle's tires. For most
passenger vehicles, the default of .013 is the proper
choice.
AERODYNAMIC DRAG C
This setting is very important for accurate horsepower
calculation, especially at higher speeds. In fact, the
power needed to push your car through the air increases
with the cube of the car's speed.
For example, if the aerodynamic resistance of your
car consumes 10 HP at 30 mph, if you increase your
speed by a factor of 2 (to 60 mph), the horsepower con-
3
sumed increases by a factor of 2
, or 8 times, to 80 HP.
The CDA factor is actually the product of two items.
"C
" means "drag coefficient," and it is a direct
D
indication of how "slippery" the shape is—how easily
it goes through the air. C
is the measure of the quali-
D
ties of the shape itself, not the size.
In other words, the C
of a small model of a
D
.15
Porsche 911 is exactly the same as the C
D
.20
911.
.20
"A" is the frontal area of the car, in square feet.
.25
The product of these two terms, "C
" times "A," is
D
"C
A," and that's the number you need to enter in the
D
Performance Computer's Setup Mode.
21
A
D
of a full-size
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