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Thanks, I reviewed that write up a while back, but it comes across as more suggestive than definitive to me. I need a little more detail for a firmer grasp of what exactly is happening particularly regarding a negative value for a timing constant.
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What is happening, particularly, regarding a negative value for a timing constant is that the area between the maximum and minimum constants must not be outside the range of the values typically expected as the result of the equation "desired timing minus reference angle equals delivered timing." If you need a graphical representation of the values typically expected for your engine, make a 3 column chart with the first column labeled "desired timing" and numbered from 0 to 60. The second column should be "reference angle" and in your case should be 60. The third column is "delivered timing" and contains the result of column 1 minus column 2. All but one of the values in column 3 will be negative, with the greatest value (most positive) being 0 and the least value being -60.
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You are probably aware by now that the problem is with timing constants in the code. I changed the ref angle to 59.8 deg which is the constant for the DIS V6, but apparently that is not enough.
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The TWIKI specifically mentions reference angle, max advance, and min advance (max retard). Changing the reference angle alone is not enough.
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The car is at home nearly 500 miles away and it was after leaving that I discovered the following constant mismatches:
________8F DIS vs. 59 distributor
Max adv_ 80.16 __46.76
Max ret __20.04___ 0
Ref ang __59.8____ 0
Initial adv_ 9.1 ____?
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You've been bamboozled. The magnitude and sign of the $8F numbers are not correct Why? The timing values are 16 bit numbers but many of the earlier distributor calibrations ignored the upper 8 bits. It may have started a convention among the early tuning software and hac authors to only read / modify the lower 8 bits. Worse yet, it would likely get even more confusing if $8F were to show max and min timing values as negative numbers.
The max and min values are based on the ignition system and the physical limitations of the engine. They are not in place to prevent tuning errors, but to prevent major catastrophes. You can adjust them closer to the range of timing values you expect to see, but GM does not use them in this way. The easiest way to get the exact values GM used is with a hex editor to view / copy the values directly from $8F into $59. Remember they are 16 bit twos complement numbers, so they will begin with $FF. In $8F they are located at $001D and $001F and are $FFE4 and $FF39, same as those on the TWIKI.
To calculate your own, you can use this method:
Desired limit(decimal) * 256/90 = engineering unit (decimal)
engineering unit (decimal) = engineering unit (hex)
$FFFF - engineering unit (hex) = limit value for calibration. It's about 256/90 off from exact, but it will work with windows calculator in scientific mode.
Also keep in mind that the "max retard" and "max advance" values have to fall on the outer boundaries of the typically expected values from the result of "desired timing minus reference angle equals delivered timing." For a reference angle of 60 degrees and desired timing of 0 to 60 degrees, the maximum value found is 0 for 60 degrees advance and the minimum is -60 for zero degrees advance. Also note that $58 does not use an "initial spark advance" value. As this value is 10 deg in $8F, I believe it's not a bad idea to set the max at 0 and the min at -60.
HTH
