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I had a strange issue while tuning an Evo IV yesterday running an Evo8 ECU. At about 6250rpm the power curve takes a dive of about 15 hp. At about 6800 rpm it jumps back up by the same amount and continues on until the pull is done. There is no knock retard happening, and the timing curve follows a smooth and logical progression. AFRs and boost are stable too.
You can't hear/feel a misfire but we reduced the plug gap some and the dip is not quite as pronounced but is still very apparent on the graph. Also from my experience with a misfire you normally get a quick series of dips in the graph not a prolonged dip as we have here.
It would seem like the timing being delivered is not what the datalog is suggesting. Maybe a sync/ref issue? Can these show up like this for a defined rpm window?