Chassis dynos measure the amount of power being developed at the wheels over a range of engine speeds, recorded at steady-state full throttle. That’s great, but it often tells you little about on-road performance – and invariably, little about the subtleties of good driveability.
For example, what does a dyno chart tell you about the action of a turbo boost control? In short, almost nothing at all.
Here’s why:
1) The dyno acceleration rate in each gear is not representative of what is achieved on the road. This one’s a killer because boost overshoot on transients is hugely affected by the rate of engine rpm increase.
2) Very few people do full throttle gearshifts on the dyno. You know, race up through the gears – to the redline, change gears, to the redline, change gears. Again, it’s in just these conditions that you look for boost overshoots and/or slow increases back to peak boost after each gear-change.
3) I have never seen anyone do a full-bore launch from a standstill on a dyno. And how quickly boost can be brought up in these conditions – ie controlling wastegate creep – is a major aspect of good boost control.
4) On a dyno the intercooler never works as well as on the road, so the actual peak boost is likely to be different in those systems that don’t use boost pressure feedback.
5) On the dyno people never bother trialling all the different combinations of throttle position, load and engine rpm that you’ll find in half an hour on the road.
And it’s not just boost controls. Most dyno operators don’t start a run at very low rpm, so the engine revs being used most often in daily driving are not tested. The ‘area under the power curve’ (especially at part throttle) is ignored, while aerodynamic pressures (eg on the engine intake and intercooler) are all wrong.
A dyno is a great tool. But unless it’s inside a climate-controlled wind tunnel, can replicate all the different acceleration rates and inertial characteristics of the driveline, and operates on real world drive cycles, it’s only a starting point.
What a dyno is good for is engine management tuning for maximum power – and even then, this must always be followed by extensive road testing.
In short, when you’re doing on-road testing, you do not want an ‘on-road dyno’ – instead you want to test road car performance!
Thanks AutoSpeed