How to read a dyno graph, sheet or chart
The first thing a person looking at a dyno graph needs to understand is that: All Dynomometer’s read differently. A dyno is not a Ruler where 30cm is always 30cm, a dyno can read high or low and it comes down to a number of factors, things like the actual dyno calibration, the way the car is strapped or tied down on the dyno, the atmospheric conditions or input, the type or brand of dyno and even the tyres on the car. Some of these factors are controllable by the operator and some are not and these factors could change the figures from one dyno to another by over 30%! So, how do you accurately compare dyno figures? The answer is… you cannot.
The dynamometer or ‘dyno’ is a primarily a tuning tool, it enables us to load up the engine at any RPM to simulate the loading the engine would experience on the road, this allows us to tune the engine properly and as such, we are able to measure the engine’s power & torque output at the vehicles driving tyres, this is a lot different to ‘flywheel power’ as advertised by the manufacturer as there is a cost in power from turning the components in the rest of the drivetrain, usually, this is around 30%. This measurement enables us to perform a ‘before tune’ dyno run as the car is delivered to us and then an ‘after tune’ dyno run so we can measure the gain in power and torque from the upgrade. We can then print off the result in the form of a dyno graph as shown below. The graph shows both power in kilowatts (kW), normally on the LH side Y-axis and torque in Newtons of Tractive Effort (N) as measured at the tyres on the RH side Y-axis. Both the Power and Torque are plotted against road speed in KM/H on the X-axis. The road speed on the X-axis can be seen as the ‘rev range’ in one gear, from take off until red-line. Some graphs will show Boost pressure (PSI) or the engines Air Fuel Ratio (AFR:1) in replacement of Tractive Effort (N) on the RH side Y-axis.
Below is a modified graph marked with A-G to highlight some of the things you will need to understand to be able to read the graph.
A/ This Red line is the ‘after tune’ power line, it references kW at the wheels on the LH side of the page.
A1/ This is the peak power number recorded from the ‘after tune’ power run.
B/ The Green line is the ‘before tune’ power line, it also references kW at the wheels on the LH side of the page.
B1/ This is the peak power number recorded from the ‘before tune’ power run.
C/ This Red line is the ‘after tune’ torque line, it references Tractive effort (N) at the wheels on the RH side of the page.
D/ The Green line is the ‘before tune’ torque line, it also references Tractive effort (N) at the wheels on the RH side of the page.
E/ Indicates the start of the ‘power run’.
F/ Indicates the end of the ‘power run’.
G/ This box lists various parameters including the Power run number, BP – Barometric Pressure, RH – Relative Humidity, AT – Air Temperature, IT – Intake Air Temperature, RR – Ramp Rate, Shoot-out mode (a fixed dyno mode), Tyre pressure and the gear the car was tested in.
The main goal of a dyno graph is to show the gain from the modification, the gain is the ‘gap’ between the corresponding power ‘lines’ or torque ‘lines’. In the case above we have picked up 91.4 peak kW, this could also be measured as a percentage and in this case we have picked up 27% more power, again peak. This calculation of percentage can be done at any one km point on the graph.
Other dyno things to keep in mind
- The colours of the runs maybe different to the graph above
- Tractive effort (N) is not engine torque, you cannot measure actual engine torque on any chassis dyno – period! (I could write another whole page on this!)
- Each and every dyno will read different, comparisons of different dyno readings is folly
- Automatic transmission vehicles will read lower than an equivalent manual car
- Automatic transmissions with ‘high stall’ torque converters will read lower peak power figures
- Different tyres can make a massive difference to the power figures measured by the dyno
- The power run is normally performed in one gear, as close to 1:1 ratio as possible
- Running the car in different gears could potentially show different readings
- The average difference between dyno power figures and flywheel power figures, on a two-wheel-drive car, on the HPF dyno, is approximately 33%, so a 100kw engine will show around 75kw on the dyno.