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Power, Torque and RPM Calculations

VBOX Tools has the ability to calculate BHP at the wheels using any Racelogic Data Logger that can record speed and longitudinal acceleration.To get the most accurate results, it is imperative that:

1. An accurate vehicle weight value is entered - this must include the weight of the vehicle as well as fuel, occupants, and baggage. If possible, getting a weight reading from vehicle weigh pads is the best option. 

2. The test location is ideal - a clear, open, level road is used to capture test data. 

Requirements to ensure the consistent accurate results

  • Open test area: no trees, buildings or bridges nearby
  • A clear, open, level road is used
  • In the chosen gear, accelerate hard from low RPM towards the top of the rev range
  • Open Log file in VBOX Tools

 

Calculating Power

  • Load the logged file into VBOX Tools and open the Graph screen facility in order to view the logged file
  • Click on the ‘Generate New Channel’ button to open the maths channel facility
  • Enter the following equation. For this to be as accurate as possible the user will need the kerb weight of the vehicle and add the weight of anything else that might be in the vehicle (driver, fuel, etc…)
  • Click ‘Generate Channel’ button to apply the calculation, and then close the Maths channel setup window
  • Tick the new Power channel so that it can be viewed in the Graph screen

 

 

To calculate BHP enter the following equation:
 
Weight (lbs) * Long Acc (g) * Speed channel (mph) * 0.003054
 
To calculate kW enter the following equation:
 
Weight (Kg) * Long Acc (g) * Speed channel (kph) * 0.0031107
 
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An example BHP calculation in VBOX Tools​
 
 

RPM Calculation

If RPM has not been measured and logged in the VBOX file, the following guide can be used to generate an RPM channel. This theory is only applicable because the power accel test is only in one gear.
 
  • Drive the test vehicle in the chosen acceleration gear and make a note of the exact speed at an exact RPM point, i.e. 54Kph at 3000RPM.
  • Divide the RPM value by the Speed value i.e. 3000/54 = 55.56, this creates you a "Speed to RPM conversion ratio".
  • Create a RPM channel using the Generate (maths) channel facility in the Graph Screen, Speed x "Speed to RPM conversion ratio"
  • You will now have an RPM channel that you can view in the graph screen, (NOTE it will only be correct for the vehicle when the car is in the chosen acceleration gear)
 

Torque Calculation

To calculate torque the user will need to have calculated the power of the vehicle and RPM. If the user does not have RPM it can be generated in VBOX Tools using maths channels, details below.
 
To calculate Torque in Nm the user will need to create a new maths channel and enter the following equation:
 
Power (kW) * 9549 / rotational speed (rpm)
 
To calculate Torque in ft-lb the user will need to create a new maths channel and enter the following equation:
 
Power (hp) * 5252 / rotational speed (rpm)
 
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09:48, 2 Jul 2015

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