Unstablr rpm on dyno

Good day everyone,

We operate an Mainline prohub dyno and are beginner in the domain. While testing an Volkswagen Jetta GLI 2017 2.0T on dyno we have an torque reading looking like saw blade before smoothing. Also the rpm look the same.

We were reading rpm through the obdlink so we try to read it on engine and got the same.

Question is since those engine are known to misfire, did light misfire can be seen as rpm fluctuations on dyno? Also the torque reading is to high.

Thanks!

Hi Carl, is this occurring during steady state operation or a ramp run? It could be related to the PID settings of the dyno if the rpm is genuinely oscillating. That would be unusual but not impossible. The torque measured at the hubs is multiplied by the gear ratio and final drive ratio and hence it will read much higher than the expected flywheel torque. What gear are you running the car in? Generally I suggest 4th-5th gear but particularly if you're having issues with rpm oscillation I'd try a different gear and see if that affects the outcome. Ultimately if this continues to be a problem and it is a real rpm oscillation I'd suggest contacting Todd or Craig from Mainline. They will be able to get you sorted out faster than I can.

Thanks for reply!

It happen on the ramp run. We dyno other car and truck without this issue.

We actually try in 4 and 5 gear. Both giving same result.

What lead us to misfire was we replaced spark plug and on the first run after all was stable but went back on next run.

Here is picture of one run rpm graph. All other vehicule we dyno is a straight line