Chassis dynamometer vs 'portable road dyno'

A chassis dynamometer is standard equipment for serious tuners. It is hard to imagine remapping cars without thorough verification of the results. Without true feedback, engine safety and claimed performance cannot be guaranteed.

Measurement accuracy depends on the manufacturer of the dynamometer. It ranges from around 2% error in the cheapest dynamometers up to 0.1% and better in the best dynamometers with high roll weights and fast & precise software.

A so-called 'portable road dynamometer' is a device that measures the acceleration of a car on the road and, based on the declared weight, calculates the supposed power and torque of the engine. The vehicle is only accelerated (estimating the power at the wheels). Later a theoretical rolling and air resistance (e.g. 15% or another guess) are added to the result to get an approximate power on the engine.

It is an inaccurate solution. The car is affected by variable, non-linear air resistance during acceleration, different for different car models. The wind may be blowing. The road is never perfectly flat.

It won't work - too much guesswork.

Also, for the measurement to be accurate, the acceleration of the car must not be too high. You ought to perform the test in high gear (the equivalent of a load on a chassis dynamometer). It means high speeds and therefore the danger of an accident on the road - often in the city. Residents will certainly not approve of car rallies on city streets either.

We do not believe that a so-called "wheel dyno" makes sense. Driving a customer's car on the road to estimate power gain is not professional.