How To Scale a Tune (HP Tuners)

“Scaling a Tune” is a way of “cheating” your way to a better spark table.

Instead of Manifold Pressure to determine spark, GM decided to reference Airflow.  Air Flow is calculated based on the amount of fuel you consumed.  Air Flow is the vertical axis, usually in g/s.

For a Naturally-Aspirated engine, the factory spark table is fine.  In fact, “Full Throttle” works out to be right in the middle of the spark table at around 0.64g/s.

For a boosted engine, the factory spark table will be maxed out at about 7psi – around 1.20g/s.

“Scaling the Tune” is a way of cheating to a more useable spark table at higher levels of boost.  It involves telling the PCM that the injectors are actually half their value, and reducing the entire VE table to half the values.  When the PCM thinks it’s injecting a 37.5lb injector in a “47” VE bin, it’s actually injecting a 75lb injector in a “94” VE bin.  It’s the right fuel, but the computer uses smaller numbers in its calculations.

SOURCE: Tune Scaling 101 – – – Displacement not altered; one Idle table not altered

This is what I did:

  • TUNE SCALED 50% AS FOLLOWS:
    • IDLE – – – Reduce all of these 50%
      • AIRFLOW/ADAPTIVE-IDLE-AIRFLOW
        • Max InGear/ACOFF
        • Max InGear/ACON
        • Max PN/ACOFF
        • Max PN/ACON
        • AC Offset Min/InGear
        • AC Offset Max/InGear
        • AC Offset Min/PN
        • AC Offset Max/PN
      • STARTUP
        • Friction Airflow Decay
        • Friction Airflow Initial
        • Startup Airflow Initial
      • THROTTLE FOLLOWER
        • Decay (PN)
        • Decay (Gear)
        • Airflow
        • Airflow Mult.
      • THROTTLE CRACKER
        • Airflow
    • AIRFLOW  – – – Reduce all of these 50%
      • GENERAL
        • MAP
        • MAF CALIBRATION
          • Airflow vs. Frequency
        • EGR AIRFLOW
          • Airflow
        • MAIN VE
          • Primary
        • CRANKING VE
          • Primary
        • CYLINDER CHARGER TEMPERATURE
          • Bias
            • 0 to 30g/s set to 1.0000, then scaled up to unchanged value at 150
    • FUEL – – – Changes as noted:
      • INJECTOR CONTROL
        • Flow Rate vs. KPA – – – Reduce all of these 50%
      • OFFSET
        • Offset vs Volts vs VAC – – – Duplicate first column throughout (return-style fuel rail)
      • CRANKING FUEL
        • First Pulse Mass – – – Reduce all of these 50%
    • SPARK – – – Changes as noted:
      • ADVANCE
        •  BASE
          • High Octane – Move row data into rows half their original value.
            • ie: Move entire 0.80g/s row into 0.40g/s row, move 1.20g/s row into 0.60g/s row, etc.  (1.20g/s is about max at 7psi boost)
            • Input the timing you want at the desired rows and blend
              • This engine likes 25° at 100kPa ~0.32g/s
              • 20° at 0.48g/s
              • 17° at 0.64g/s
              • 13° at 0.84g/s
              • 11° at 1.00g/s
              • 7° at 1.20g/s
        • BASE CORRECTIONS
          • FUEL
            • Base – Zeroed
          • IAT
            • Base – Move data down so corrections start from a row half it’s original value
              • ie: Correction starts at 0.14g/s instead of 0.28g/s
          • ECT
            • Base – Same as above
        • MBT
          • Max Torque Timing – Move data into rows half their original value as above
            • Input timing you want at desired rows and blend
              • 0.32g/s is now roughly WOT for NA
              • 0.64g/s at now 20°
        • IDLE SPARK ADVANCE
          • In Drive – Move data into rows half their original value as above
            • Input timing you want at desired rows and blend
              • ie: 0.80g/s row is now at 0.40g/s row
              • 0.44g/s and above all the same.
          • In Park – Same as above

While I built the engine for 22psi boost (and had intended to get there), I ended up going back to an un-scaled tune once 8.5psi was a little scary and I knew (since I was driving it) I wouldn’t go over 15.

I drove a scaled tune for almost three years.  I can tell you that the engines drives a whole lot more “polished” with it unscaled, and the better spark resolution (albeit up to 7.5psi) was a lot more tuneable.  The larger VE numbers made tuning a lot more precise.