Car profile tips part 1 : weigh your car for more accurate dyno results

on Friday, 03 October 2014. Posted in Using PerfExpert

How to better fill your car profile to obtain realistic results

Car profile tips part 1 : weigh your car for more accurate dyno results

Although PerfExpert can be used to evaluate the mechanical changes of a car by making comparison of measurements before and after modification, PerfExpert can also be used to just check the actual engine horsepower realistically. To do so, not only you must ensure that you perform your tests in the required conditions, but you must also ensure that the car profile parameters you entered are right.

The weight of your car is one of the most important data PerfExpert takes into account when computing dyno test results, as it has a direct effect on the displayed horsepower and torque. In this blog post, we will see how to fill and manage both the 'Car weight' and the 'Additional weight' parameters in the car profile, and how to weigh your car for more accurate dyno results. The other car profile parameters will be covered in future blog posts.

That car used as example here is a Toyota GT86, checkout its PerfExpert Network page here.

Use the 'Car weight' parameter as a contant value

The best way to use this parameter is to consider it's a constant value. That mean you will not alter this value each time you have to test your car.

You don't know exactly how much your car weigh

Ok, don't worry about that, you will be able to start using PerfExpert by considering the advertised curb weight of your car as the 'Car weight' parameter, and later update your test results after weighing up the car.

The curb weight is the "total weight of a vehicle with standard equipment, all necessary operating consumables (e.g., motor oil and coolant), a full tank of fuel, while not loaded with either passengers or cargo" as explained in the Wikipedia definition. This value is provided by the car manufacturer and can be found on the car manual, on car database like ZePerfs.com, and on the PerfExpert support forum, in the dedicated section.

As mentioned in the Wikipedia article, some countries may have slightly different definitions considering that the fuel tank is filled at 90% for example, or that this weight also include a 75Kg passenger. Also, some manufacturer just advertise a base curb weight, that mean the curb weight of the vehicle with only the base trim features installed, not the optional equipment (sunroof for example), as explained in this interesting post

As you can see, although the advertised curb weight is a good value to start with, it's still uncertain and this affects your test result accuracy. So if you want to get the actual engine horsepower realistically, you will need to weigh your car at some point. Then you will be able to modify the 'car weight' on each of your previously performed test, so PerfExpert can re-compute the horsepower/torque and update them.

You have the opportunity to weigh your car

That's the way to go the get more accurate results ! This can be done at a motorsport garage/club using specific weighing scales.

Black Toyota GT86 weighing

You must ensure that you meet certain conditions when weighing your car :

  • The car must not be loaded with either passengers or cargo (not even the driver)
  • All liquids must be level (engine oil, coolant, brake fluid...)
  • Fuel tank must be full

In other words your car must be in the conditions of a "curb weighing" considering the first definition that we saw above.

Black Toyota GT86 weighing

The measured weight must be entered in the PerfExpert car profile as the 'Car weight' and must not be altered until the car is modified.

Toyota GT86 weight

Here we get 1238Kg for a french 2013 Toyota GT86 that has the following modification :

  • Milltek catback resonated exhaust
  • Cosworth air filter in OEM air filter box

Use the 'Additional weight' parameter as a variable value

Each time you will perform some dyno test, you will have to adjust the total weight of the car at the time of the measurement. To do so, you just have to modify the 'Additional weight' accordingly.

Additional weight = (passengers weight) + (cargos weight) - (missing fuel weight)

Here we are considering that the car has not been modified since the time you entered the 'Car weight' parameter, so no need to alter it. Also we are considering that the engine oil, coolant and other fluids are level.

You have to calculate the missing fuel weight from the fuel tank capacity, the actual amount of fuel at the time of the test and the fuel weight per liter :

  • Gasoline : 0.755 Kg / L
  • Diesel : 0.845 Kg / L

For example, if at the time of the test of a gasoline car that have a fuel tank capacity of 50 Liters, we have inside the car :

  • The driver (75Kg)
  • A passenger (82Kg)
  • A tool box in the boot (15Kg)
  • Fuel tank filled at 45% (so it misses 55% of fuel to fill the fuel tank)

Then Additional weight = 75 + 82 + 15 - (55%*50*0.755) = 151Kg

Hope all these tips will help you get more accurate results !

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