The peak figure is the least important thing on the whole chart. We show you what to look at so that a before-and-after comparison actually means something.
A dyno chart has two curves: power and torque, both plotted against engine speed. Most conversations end on a single number — peak power. That is precisely the figure that says least about how the car drives.
Wheel power versus engine power
A roller dyno measures power at the wheels. The figure “at the engine” is the result of adding drivetrain losses estimated from a coast-down run. Those losses depend on the drivetrain type, the gearbox oil, and tyre pressure and temperature.
- single-axle drive: usually 12–18% losses,
- four-wheel drive: usually 20–28% losses.
The practical conclusion: always compare wheel power, and measured on the same dyno at that. Calculated engine power is an estimate, not a measurement.
The area under the curve, not the peak
How a car accelerates is decided by the torque available across the whole rev range you actually use. An engine with a flat, wide curve will beat one with a higher peak that it only reaches in a narrow band just before the limiter.
Look at three things:
- where the rise starts — the lower full boost arrives, the better for everyday driving,
- how wide the torque plateau is — the longer it holds, the fewer gear changes you need,
- what the drop at the top looks like — a sudden collapse usually means a flow restriction or an ECU intervention.
Measurement conditions
Without this information a chart is an illustration, not evidence. A credible report contains:
- ambient temperature and pressure as well as humidity,
- the correction standard applied (DIN 70020, SAE J1349, EEC 80/1269) — the differences run to several per cent,
- the gear the measurement was taken in (usually the ratio closest to 1:1),
- oil and coolant temperature,
- information about the cooling fan and about whether the car was allowed to cool down between runs.
The usual tricks
- Changing the correction standard between measurements — switching from DIN to SAE alone can “add” a few horsepower.
- The “before” run on a cold car and the “after” on a warm one — or the other way round, depending on the result wanted.
- A chart with no axes and no scale — a pretty curve that proves nothing.
- Averaging several runs without stating the spread — it hides the power drop on successive attempts, which is exactly what you want to see after changes to the cooling.
How to compare honestly
- The same dyno, ideally the same operator.
- The same correction standard and the same gear.
- Similar ambient temperature and the same warm-up procedure.
- At least three runs on each side, with the spread between them shown.
- Plus the logs: boost pressure, IAT, ignition correction. The chart says “how much”, the logs say “why”.
What the chart will not show
Throttle response at part load, behaviour over repeated runs on track, refinement and durability. A dyno is a single photograph, not a film of the whole service life. Treat it as a measuring tool for verifying a specific change — used that way, it is invaluable.
