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Ref Period oscillating at High RPM under load - Help
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Ref Period oscillating at High RPM under load - Help
Just throwing this out there to see if anyone has ever observed the same thing with datalogging.
ECM: 7730
BIN: S_AUJP v6
TunerPro RT
FIRST Intake
383 SBC
HEI Distributor with remote coil
New MSD Coil
New ICM (JEGS)
The reference period (DRP) that is used by the ECM to determine RPM (X25) is erratic above 4000 rpm on WOT acceleration, specially on the 4th gear where the engine is accelerating at a slower rate. In this example, it's about 7 seconds spent in the 4th gear. The OEM "369" ICM and the new JEGS ICM are giving the exact same results. Timing was tested with timing light from 2000 rpm all the way to 6000rpm and it is accurate to the human eye. The test was done in neutral with no load on engine. The reference pulse was accurate during that test and the SA Latency Table was tuned at the same time. It appears that the problem surfaces when the engine is accelerating at a slower rate under WOT (hard acceleration in 3rd or 4th gear). The graph below has capture the issue. It is a hard acceleration, WOT from 2nd to 4th.
The Autometer dash tachometer appears accurate during WOT with no bouncing. The engine is running great, no misfire, no detonation or surge/stumble.
Re: Ref Period oscillating at High RPM under load - Help
Could be crankshaft resonance propagating to the cam drive and through distributor gear.
since it appears to repeat at the high speed and load areas, the tolerances of the timing chain, distributor gear may be showing. Heavy power stroke pulses bouncing the chain?
long shot but tossing ideas,
oil pump relief bouncing may also load/unload the distributor shaft just enough to cause a fluttering that only shows in between power strokes.
Would be good to track over time to see if the amplitude of the oscillation increase to show wear.
Jp
Re: Ref Period oscillating at High RPM under load - Help
Originally Posted by JP86SS
Could be crankshaft resonance propagating to the cam drive and through distributor gear.
since it appears to repeat at the high speed and load areas, the tolerances of the timing chain, distributor gear may be showing. Heavy power stroke pulses bouncing the chain?
long shot but tossing ideas,
oil pump relief bouncing may also load/unload the distributor shaft just enough to cause a fluttering that only shows in between power strokes.
Would be good to track over time to see if the amplitude of the oscillation increase to show wear.
Jp
Yep, thought about something like that. Some sort of a mechanical flaw or limit to GM EST control. Engine is new with new distributor gear. There was a similar problem with the old 305 (LB9) but some how less noticeable. Mind you, I would rarely push the 305 over 5k rpm so I don't have much data. I'm wondering how I could data log the pick-up coil signal (AC Wave) before it goes through the ICM. That way I could see if it's a mechanical issue.
07-21-2021, 04:34 PM
