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Jagged Dyno chart

Practical Dyno Tuning

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2018 Flex fuel Wrx.

came in over boosting like crazy and low up top power. Got all of that nice and worked out and finished at great numbers. Dyno operator used gear ratio for rpm instead of a pickup, first time I’ve seen them do that. Dyno is pretty jagged but car feels nice and smooth on the road. Smooth fuel, boost, ignition, and WGDC curves (I can attach a log if needed). Boost error is dialed in as well and there aren’t any massive fluctuations I can find in the logs, what causes this? Some dips are as big as 30whp and I would think this would be felt on the street.

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Hello, I would be asking to see other cars dyno graphs on their dyno if they all look like this then it will be a dyno issue.

Regards Ross

Hi Ross I’ve tuned on this dyno before with no issue. He did say some cars do this on the dyno but no specific reason why.

The usual problem is a misfire, either fuel or ignition, and often down to a bad connection.

However, that tends to be random and that trace seems to be consistent - any chance of logging fuel pressure at the same time, as I was wondering if it may be down to pressure fluctuations as the pump(s) switch or, more likely, the pressure regulator opens and closes slightly to regulate the bleed off flow - if it's slightly sticking it could be cycling between slightly rich and slightly lean?

Apparently, with some very loud vehicles there can be a problem with sound frequencies causing issues, but don't know much about that.

I would bet on electrical noise (coils are a big offender) affecting the dyno speed sensor. Problem could be the front rollers with your WRX.

I can make my SuperFlow output a graph that looks like this if I forget to load/load the wrong profile for an engine config (old dyno, poor adaptability and controls) generally its a sampling speed and filtering issue.

What has been suggested already is correct and definitely something to look into however I'd also go with the low hanging fruit first and see if the Dyno Nerd that you were working with didn't fiddle with something when he was switching to gear/roller speed for your runs.