Rolling roads are a fantastic way to measure the power of a vehicle. However, they can also lead to many an unhappy and confused car owner as results can often be misread due to a general lack of understanding of the limitations, drawbacks and also benefits of a rolling road.
There are a few fundamental things you should know about rolling roads:
It is impossible for a rolling road to simulate real world driving. Rolling roads provide linear load on the engine and do so with a slow pull from low RPM’s to redline. It can take up to 45 seconds to reach redline. This amount of time and constant load on an engine and turbo charger will inevitably raise intake temperatures well beyond what the real world driving will see. These temperatures will cause the engine to pull back the load values the ECU sees, thus limiting power.
It is very common for a car to have less rolling road performance and yet maintain fantastic power on the street or track.
This can be attributed to several factors.
The fan in front of the car is providing a linear flow of air, usually to a small portion of the frontal area of the vehicle, real world conditions prove airflow is proportionately increased as speed is increased. This isn’t replicated with the airflow on a dyno. Plus, the fans are often pointed at the middle of the vehicle; many intercoolers are placed on the side of the vehicle and receive almost no cooling effect from these fans. This causes the air intake temperature to be well outside of what we will see in the real world. The higher these temperatures are the lower power output will be. The further these values are from the actual road conditions the less accurate the dyno run will be.
Heat-soak occurs in both of the above examples. This is what happens when a car/engine doesn’t get adequate cooling for the given running conditions and the ECU will be forced to adapt to counter the additional heat. When a car suffers badly suffers from heat-soak the ECU will typically go into an EGT(Exhaust Gas Temperature) protection mode; lowering the requested load value and increasing the amount of fuel to cool things down, this results in the curve tailing off much quicker than expected and a lower than realistic power output.
The majority of rolling roads will load a vehicle up during a power run in a certain way, this can vastly effect how the car ‘behaves’ on the rolling road as the ECUs are load based all requests to the engine are calculated based on the feedback the ECU has from the engine and the load it’s under.
This doesn’t even take into account any potential issues that could be inherent with a car, the amount of heat already in the car before it goes on the dyno, or any inconsistencies between operators and rolling road calibration.
Wheel and flywheel figures can be a source of confusion, and there’s a danger of back-calculating flywheel figures from a chassis dyno. Power at the wheels is more meaningful and fairly accurate so long as the ambient and intake temps are reasonable. Certain rolling roads calculate force applied at the rollers, everything from there on is a mathematical equation and as such don’t necessarily give you accurate figures.
One of the biggest mistakes made is to take a figure given from aRolling Roadas gospel. There are so many varying factors between different rolling roads that can affect the output: calibration, temperature, operator, tyre pressures, etc. that vastly different figures can be seen from one rolling road to the next. Realistically a Rolling road can be a great comparative tool to show differences additional components make, but in the guise of a rolling road ‘shoot-out’ for a one off reading they are a waste of time. A figure or power curve only shows what the car is doing on that dyno, on that day in those conditions.
Using a Rolling Road to show the difference between cars or show the increase from software can also be unrealistic without correct preparation. If you have two identical cars running the same quality fuel, tyre pressures, etc. you can still have a variable within the ECU due to differing driving styles and conditions the cars see. One car might have been used much more aggressively than the other and have a much larger correctional factor due to adaptation from excessive heat. This can vastly affect the power output of a vehicle. Something else to be aware of is after programming an ECU the car will take a certain period of driving time to adapt, this period of time is dependent on driving style and conditions.
At the end of the day peak power and torque figures are all pub talk and mean very little in real world conditions. Peak figures don't give you any indication of drivability or even a true indication of increased performance as they ignore the power/torque curve and can’t tell you how smooth, powerful and efficient the power is delivered. This is what makes a car more drivable and thus more enjoyable. For more information about peak figures, click here.