Dyno graph spike closing throttle

[Ollies930]

Can anybody explain to me why dyno graphs sometimes show a spike when the throttle is closed? Logic would dictate that the reason this happens is that the throttle bore is too large or that the mixture is lean and when closing the throttle it provides a stronger signal to the carb, thereby enrichening the mixture. But that is not really true, as I have tried bigger jets and lost power. So what does actually cause the spike?

Oliver

[LoganD]

Can anybody explain to me why dyno graphs sometimes show a spike when the throttle is closed? Logic would dictate that the reason this happens is that the throttle bore is too large or that the mixture is lean and when closing the throttle it provides a stronger signal to the carb, thereby enrichening the mixture. But that is not really true, as I have tried bigger jets and lost power. So what does actually cause the spike?

Oliver

It's the electronics in the dyno. For example, if you've got something like an HBM torque meter it will read a bit weird with a large change in angular acceleration rate. This is why you've got to control sweep rate (RPM/sec), and this is why all OEM ratings are done via brake horsepower (bhp) tests. A bhp test has no sweep rate and simply holds the engine at a single RPM point, a static situation in terms of angular acceleration is the best scenario for reading accuracy.

EDIT: I also forgot to mention that on fluid controlled dynos (oil or water) often times the dyno is set up to react to a large load change from the engine by increasing load as a safety measure. This would also cause an unnaturally high reading.