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Calculate Horsepower
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Auto Dyno Horsepower Calculator
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Auto Dyno calculates horsepower by measuring your vehicles
increase in velocity, time, and vehicle weight. If you increase
horsepower you will need to measure the base horsepower in brake horsepower
or engine horsepower. Then you make a performance modification and
retest the car horsepower to determine the effectiveness of the modification |
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| Parameter |
Description |
| Vehicle Name |
Name you enter in the Options screen. See
Diagram 1. |
| Peak HP |
Peak Calculated HP from the HP /
TQ / RPM Data |
| Peak TQ |
Peak Calculated TQ from the HP /
TQ / RPM Data |
| 0-60* |
The time in seconds for the 0-60 run. |
| 1/8 Mile* |
The time in seconds it took you to travel 1/8
Mile |
| 1/4 Mile* |
The time in seconds it took you to travel 1/4
Mile |
| Test Duration |
Duration of test in Seconds |
| Calc Max Speed |
Maximum speed is calculated by using your Peak HP
and Aerodynamic Info |
| Timing |
Actual = from the start of movement x
in Rollout = Drag Strip type timing where the first x inches
are not included in the timing |
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| *Your test run must start at 0 for this parameter to be calculated. |
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Diagram 1: [General Section; can be found in the Tools /
Options window] |
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Vehicle Name: The name you choose to describe your vehicle
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Vehicle Weight: The weight of your vehicle including
Driver, Passengers, Water, Oil, and Fuel. This number needs to be relatively
accurate. Weight is used in the Horsepower Formulas. |
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Diagram 2: [Aerodynamic Information; can
be found in the Tools / Options window] |
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Drag Coefficient: This number represents the factory
specified Cd for your vehicle. You input this number. This
number is used to calculate HP loss due to aerodynamic drag. |
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Frontal Area: This number represents the factory
specified Frontal Area of your vehicle. You input this
number. This number is also used to calculate HP loss due to
aerodynamic drag. You can measure this by holding a spotlight behind
your vehicle and measuring the area of the shadow on a wall. The
factory value is usually in the cars specifications. |
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Rolling Coefficient a.k.a. Rolling Resistance: This
value will probably not be available in your vehicle specifications.
It can be calculated using the Coast
Down Calculator method. Default values usually range from
0.008 for small tires on light cars to 0.020 for larger tires on heavier
cars. |
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Include Aerodynamic + Rolling Loss in Calculations:
If this is Checked, the Horsepower required to overcome resistance [drag]
due to Aerodynamic drag and rolling drag will be added to your horsepower
numbers in the HP / TQ Graph and Peak HP / TQ Numbers in the Data
View. If you want to see how much HP is being added for Aerodynamic
Drag and/or Rolling Drag, just select the Custom Graph Tab and graph
(Aero+Rolling Drag HP). This will graph the HP it takes your vehicle
to overcome Aerodynamic Drag and Rolling Drag. You can also graph the
two numbers separately. |
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Diagram 3: [Engine and Transmission; can
be found in the Tools / Options window] |
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Calculate WHP: This calculates actual Horsepower your
wheels are exerting on the road. This will be 10-20% less than the
Horsepower measured at your flywheel [Flywheel Horsepower or Block
Horsepower]. |
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Calculate BHP: Block Horsepower is calculated from
Wheel Horsepower by multiplying it by the percentage mechanical loss you
input in the Mechanical loss box. The average mechanical loss for a 5
speed manual transmission is 15%. |
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Diagram 4: [Climate Information; can
be found in the Tools / Options window] |
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Temperature: This is the outside temperature.
This value is used to properly calculate Aerodynamic Drag and the SAE J1349
Horsepower Correction. |
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Altimeter Pressure: This is the also used to correct
Aero Drag and for SAE J1349 corrections. |
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Humidity: This is the also used to correct Aero Drag
and for SAE J1349 corrections. |
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Elevation: This is the also used to correct Aero Drag
and for SAE J1349 corrections. |
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SAE Correct HP/TQ Data: If this box is Checked the
HP/TQ data used for the HP/TQ Graph and the Peak HP and TQ will be shifted
by this percentage. If you want to learn more on SAE J1349 corrections
for Dynos please do a quick web search. |
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Diagram 5: [Choose Your Car; can
be found in the Tools / Options window] |
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This drop down menu automatically fills in some or all of
your vehicle data including:
- Vehicle Name
- Vehicle Weight [Without Driver, you must add your
weight to this value]
- Drag Coefficient
- Frontal Area
- Rolling Coefficient
- Speedometer Calibration [Always best to calibrate
yourself]
- RPM Calibration [Always best to calibrate yourself]
*Note: If your vehicle is in the list some of the
fields will be populated. You need to make sure that all of the data
is accurate. You should always calibrate the Auto Dyno to your vehicle
and after making modifications to your vehicle. |
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In the following graph:
The Blue
line is the calculated Horsepower. This data has been corrected for
Aerodynamic and Rolling drag loss. It has also been corrected to
standard Horsepower via the SAE J1349 specification.
The Pink line is the
calculated and corrected Torque
[Click
for larger Graph]
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Math:
A vehicles HP is the force accelerating your vehicle F = M*A, where
the mass is the weight of your vehicle, and the acceleration is your
change in speed.
This formula nets the HP required to accelerate your vehicle, but it
does not take into account the HP required to overcome rolling
resistance or to overcome aerodynamic drag.
To measure the force or HP required to overcome rolling resistance
and aerodynamic drag you will need the Frontal Area of your vehicle, its
Cd [Coefficient of Drag], and Weight
Rolling Resistance (LBS)=Weight * (Rolling Coefficient + (Speed
Effect Coefficient*3.24*(Speed/100)^2.5)
Aero Drag = 0.5 * Air Density[kgm3*kgm3_to_slugs[slugs]*Frontal Area*DragCoeff*(speed*1.467)^2
Rolling Resistance [LBS] + Aero Drag [LBS] = Total Drag in LBS
Convert Total Drag [LBS] to HP
HP = Speed * 1.467 * Total Drag / 550
The constant 550 is derived from 1 HP = 550 LBS moving one foot in
one second.
The math is actually far more complicated, this is just a brief
explanation of the mechanics.
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| How does AutoDyno calculate
Horsepower? |
| Auto Dyno calculates Horsepower
by measuring the change in vehicle speed the elapsed time and the
vehicle weight. It then corrects Horsepower for aerodynamic and
rolling drag losses. Lastly it corrects for atmospheric
conditions with SAE J1349 specified formulas. |
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| How does AutoDyno calculate
Torque? |
| Auto Dyno calculates Torque by
its relation to Horsepower and by measuring RPM. Horsepower and
Torque are related by the formula:
Torque = Horsepower * 5252 / RPM
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| How does Auto Dyno differ
from a Chassis Dyno? |
| Great question! I bet
there are more differences than you think. |
| 1. A Chassis Dynamometer
can not measure Horsepower only Torque. It the calculates
Horsepower from Torque. |
| 2. On any fixed dyno you
will not get the same airflow through your vehicle as you will in
motion. With fans you get too much air through your
radiator/intercooler at low "wheel speeds" and too little at
high speed. For Ram-Air systems to produce horsepower it is
typically required to have air velocities greater than 100mph.
The air velocity of fixed fans rarely exceeds 45mph.
If you want to know how your vehicle will perform during a 1/4 mile
run in excess of 100mph, you need to measure the Horsepower at those
speeds, not strapped to a chassis dyno in an air conditioned garage. |
| 3. There are four things
that effect your vehicles performance: Power
[Horsepower/Torque], Drag [Aerodynamic and Rolling], and Weight [Vehicle+Water+Gas+Oil+Driver+Passenger]. The Chassis Dyno can
not measure your aerodynamic drag, Auto Dyno not only measures
Aerodynamic and Rolling Drag it also uses that data to correct your
Horsepower and Torque. Auto Dyno allows you to measure
performance increases due to aerodynamic changes, i.e. body kits, wing
angles, spoilers, etc. |
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For about the price of one Chassis Dyno session you can own an Auto
Dyno.
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| More
Information on AutoDyno |
| View full product details on
AutoDyno. |
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| Purchase AutoDyno |
| Purchase AutoDyno Online. |
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