Check your adx file under values, Spark Advance From Hack, then look at bottom for Conversion and if your Base timing at distributor and your initial advance are 0*? Then the conversion should be "(X * 0.3515625) + 0.000000"
If say it was a 7.4L the Initial timing is 3.87 so distributor should be set to 3.87 and the conversion would be "(X * 0.3515625) + 3.870000"
If that is not the cause it is the Spark Latency! None of the stock GM calibrations are exactly correct and they are all different. But that link I gave you before shows how RBob did the math and it can be corrected!
Couple other things to think about are adders and subtracters? Like PE, Coolant comp, Highway spark, Spark Slope, etc...
Interesting note found in that thread that I have verified as well is:
So there is a difference in modules! This is also a reason I always reomend GM EST/ICM modules because I have checked aftermarket EST/ICM modules and have found some to be off by 12* at higher RPM!!!
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