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Thread: Values in tune being changed after saving and reopening tune

  1. #1
    Join Date
    Jun 2009

    Default Values in tune being changed after saving and reopening tune

    Just wondering if anyone else has seen this issue. It's a COS3 tune on 2002 OS. I'm inputting the new injector values, save the tune, reopen and the values have been automatically changed. I.e., minimum injector pulse width set to 0.125 ms and is changed to 0.121580 on its own. I've seen this issue when setting the stoich value, but since i run E85 (stoich AFR 9.76) that had to do with going below the hard limit of the PCM I believe. Any input?

  2. #2
    Joe (Moderator) joecar's Avatar
    Join Date
    Apr 2003


    Due to binary quantization (i.e. can only represent discrete values in binary, can't represent real values), the tunetool is saving the closest representation to what you entered.

    as a crude example, let's say:
    0.5 is represented as 0.100,
    0.25 is represented as 0.010,
    0.125 is represented by 0.001,
    then 0.625 is 0.101 and 0.75 is 0.110... but there is no exact representation for any values in between... e.g. the closest to 0.65 is 0.625 (0.101).

  3. #3
    Lifetime Member 5.7ute's Avatar
    Join Date
    Oct 2006


    Exactly as Joe said.
    In your example, the pcm is using a value of 0.015198 as the scaler.
    When you enter 0.125 as the parameter value (which would be 8.2 from the scaler), the pcm will round this to the nearest value (8 at the parameters address.) So when you next open the file the value of the parameter will be shown as 0.12158.
    The Tremor at AIR

  4. #4
    Join Date
    Jun 2009


    Thank you the responses. Makes good sense.

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