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Dyno Testing Explained: BHP, WHP and How to Read a Power Graph

Key takeaways
  • BHP is measured at the flywheel, WHP at the wheels, so wheel figures are always lower.
  • Comparing a wheel figure with a manufacturer's BHP figure is misleading without allowing for drivetrain loss.
  • Different dynos and calculations give different numbers, so only compare before and after runs on the same machine.
  • Ask for the full graph, not just the peak number.

A dyno graph is the most common proof a tuner shows to sell a remap. It can be useful evidence, but only if you know what you are looking at. This guide explains the main terms and the traps to watch for, with help from two UK motoring sources.

BHP and WHP: what is the difference?

As explained by Car Throttle, brake horsepower (BHP) is measured at the flywheel and reflects the power delivered to the gearbox, while wheel horsepower (WHP) is measured at the wheels. WHP is always lower because it includes the losses through the gearbox, driveshafts and the rest of the drivetrain. Car makers usually quote BHP because it is the higher and more marketable number, but most dynos measure WHP, because driving a car onto rolling rollers is far easier than removing the engine.

How much power is lost?

Car Throttle gives a loose rule of thumb of about 20 to 25 percent, but stresses that it is not a guarantee, because the real loss depends on the drivetrain layout, the gearbox and the weight of components such as wheels and driveshafts. The UK tuning site Fast Car puts wheel figures at roughly 10 to 25 percent below flywheel or crank figures. Either way, a car with 300 BHP at the flywheel will not show 300 at the wheels.

Why two dynos give different numbers

Fast Car explains that the flywheel figure most UK workshops quote is usually calculated from wheel readings on a rolling road, not measured directly. Car Throttle adds that the conversion used by each operator is never completely accurate. In practice, different machines, settings and calculations can show different results for the same car. A before and after comparison is only fair if both runs are done on the same dyno, on the same day if possible, with the car in the same condition.

Units can confuse the comparison

Fast Car notes that BHP is the standard UK unit, while the German PS is about one percent higher and kilowatts give a much lower looking number. Always check which unit a tuner is quoting before you compare it with another car or with the factory figure.

How to read a power graph

  • Look at the whole curve, not only the peak. A broad, smooth gain in the middle of the rev range is what you feel on the road.
  • Check that the before and after lines were measured on the same machine and under similar conditions.
  • Be careful with promises that sound too good to be true, especially on engines that were already close to their limits.
  • Ask whether the figure is wheel, hub or flywheel power, and how it was calculated.

This article is general information, not professional advice. Dyno figures vary between machines and workshops, so treat any quoted gain as an estimate and consult a qualified specialist before modifying your car.

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