How can I improve dyno repeatability?

REPEATABILITY… the secret to finding performance improvements via dynamometer testing! This is a continuation of the Winter - 2004 DYNOmite News article.

Before you begin making engine configuration or tuning changes based on test results, you need to be certain the results are accurate - or at least something that you can duplicate. It makes no difference if your testing is done on the dynamometer or the racetrack, if you get different results each time (even without making any changes) you don't have control of your experiments. Any conclusion you draw will be based more on your bias than on the results.

If you haven't done it recently, give your system a repeatability checkup. Start by turning on your data-acquisition computer(s) - without even starting the engine. Let the electronics warm up for at least 30 minutes. (You always do that don't you?) This stabilizes components' electrical characteristics, greatly reducing thermal induced drift in readings! Zero out any Torque, EGT, etc. errors… then wait and watch over the next 15 minutes. The drift you observe is an electronic accuracy limit to each channel. You can't find power deviations smaller that these deviations without upgrading hardware.

Assuming the drift meets your requirements, start turning on any accessories, like fuel pumps, cell fans, 60-Hz fluorescent lights, etc. If any of them affect the channel's reading, you should investigate their power supply and ground wiring. Most likely a ground loop exists which is disturbing the offset reference for the channel. (Lights tend to bother frequency readings, not 0-5 volt channels).

At the least, keep in mind that you should be doing your offset calibration of the channel with any offending device in the same state as when you test. Obviously, you do not zero the torque channel, and then turn on a pump, if it adds 2 ft. lbs. to readings!