How to read a dyno graph without being fooled

The peak is the least useful number

It is the number that goes on the advert, and it is the one that tells you least. A peak exists for a fraction of the rev range and you will spend almost no time there.

What you actually feel is the area between the two lines across the middle of the range. A car that gains forty horsepower at 6,800rpm and nothing below it will feel identical on the road.

Look at the gap between the lines at 3,000rpm, not at the top of the chart.

Were both runs done the same day?

Air temperature, pressure and humidity all change what an engine makes. A before run on a cold January morning and an after run in a warm workshop will show a gain that is partly weather.

Every honest printout records ambient conditions. If a graph does not show them, or the two runs are dated weeks apart, you are looking at two unrelated charts on one page.

Check the axis

The oldest trick there is. Start the vertical axis at 200 rather than zero and a modest gain becomes a dramatic-looking gap.

It is not dishonest exactly, but it is presentation rather than measurement. Look at the numbers on the axis before you look at the shape of the lines.

Wheel figures versus flywheel

A dyno measures at the wheels. Flywheel figures are calculated by applying a correction for drivetrain loss, and that correction is a choice.

A generous correction factor can add a great deal to a headline number without anything having changed. Ask what the factor was. An honest workshop will tell you without hesitating.

What a good graph looks like

Two runs, same session, conditions printed. An axis starting at zero or clearly labelled. Both wheel and flywheel shown, with the correction stated. And a gap between the lines that is widest where you actually drive.

If somebody is unwilling to walk you through those five things, the graph is decoration.