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There is a deep valley (photo) in my VE table and I wonder If that can be optimized, or could it be caused by airflow reversion?
VE is carefully calibrated on engine dyno. Motor is a Ford V8, four OHC, upper part of intake manifold modified (photo).
Thanks in advance for your comments.
I believe that is a result of the exhaust system. You might try experimenting with the exhaust / intake cam timing in the center of the valley to see if you can find what it wants. I would also look at fuel timing in that area.
Even if the fuel table looks like that, be sure to investigate ignition timing in the same range -- less overall fuel mass usually means you can advance the ignition timing which may bring back the torque (and a flat torque curve is the goal).
As David said, it could be an unfortunate exhaust primary, and/or intake runner, length. The fortunate thing about having a degree of camshaft timing swing available is it allows you to follow the tuning by adjusting the opening and/or closing points to help counter a bad tuned length.
I see it's set up for either EGT or lambda pickups on each cylinder - anything showing up there?
I've seen similar Efficiency tables on other engines, As David and Gord have said, it is usually related to a resonance in the inlet and/or exhaust system that causes the efficiency to drop off. You may be able to work on it with the cam timing (compare the OEM cam timing tables to what you have, it may be that the OEM calibrators faced the same issue and tuned around it) and the relation between the inlet and exhaust cam angles. i.e. you may need to run more overlap to maintain mass flow at that point.
Thank you all for your advice.
@Gord: Those are EGT probes. Initially, 3 cylinders needed slight enrichment (in lower 1-digit percentage range).
Now all temps are within 40degC, near 720 degC under sustained full load.