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Plugged my MUCAR scanner into the 1996 C4 and noticed this odd timing on the livestream data. -13 BTDC? If this were true wouldn’t the car run terrible? It runs excellent during regular driving, I was trying to diagnose a miss/sputter near max rpm acceleration when hot.
After some more googling and browsing the LT1 Camaro/Impala forums I think I found an answer. Some cars apparently will always display the timing as a negative number. When I saw this number I thought maybe my timing was somehow AFTER top dead center. When I plugged the same scanner into my 2003 C5 the timing looked like this, a positive blue number, not a red negative like the C4.
I wonder if the discrepancy is because the LT1's value is for the crankshaft position when the reference pulse is received, not when the EST pulse is sent to fire the coil?
It takes a little thinking to understand the way an electronic distributor ignition system controls timing and spark advance. In a distributor system the ECM receives a reference pulse from the distributor pick-up electronics, applies a time difference, then tells the ICM to fire the coil. The distributor fires the cylinder that the rotor is pointing to. In the distributor system, the ECM does not know, or need to know which cyl is firing. The ECM is responding to the receipt of a reference pulse which occurs every 90 degree of crankshaft rotation.
How is this possible?
