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How the Variables Interact

This page is a work in progress. The assertions below are not complete and have not been verified. Use at your own risk.

TimingIncreasing timing will, up to a point, increase torque. However, excessive timing increases the risk of detonation. Decreasing timing will increase EGTs (which can be used to aid turbo spool, at risk of shortened turbo life).Ideally, peak cylinder pressure is at roughly 15 degrees after TDC.
AFRDecreasing AFR (richer mixture) helps cool the intake charge, which deters detonation. Decreasing AFR also has a negative effect on miles-per-gallon, for obvious reasons. Best torque is typically 12.0–12.2, but turbo engines often run as rich as 11.0 to allow increased boost and timing without detonation. Increased AFR typically increases EGT as well.Flame speed is fastest at 11.1, which also reduces detonation risk.[1]
RPMIncreasing RPM decreases the time between ignition and 15-degrees-after-TDC, which requires increased timing to maintain idea peak cylinder pressure. 
VEIncreasing VE increases charge density, which increases flame speed, which decreases the timing advance required to achieve peak cylinder pressure at 15-degrees-after-TDC, and correspondingly increases the risk of detonation. 
BoostIncreasing boost increases torque, however it also increases the risk of detonation and thus decreased timing is recommended (unless other steps are taking to avert detonation). Increased boost may also move the turbo to a lower efficiency island, which leads to increased charge temperature (see below).Increased boost should yield increased airflow (i.e. load, MAF signal). If it does not, then the compressor is blowing more air than the engine can inhale - and that air is likely hotter due to decreased compressor efficiency.
Charge TemperatureIncreased charge temperature increases the risk of detonation. Increased charge temperature also decreases torque potential, due to decreased charge density. Decreased charge temperature (e.g. improved intercooler) deters detonation and increases torque potential. 
AVCSOpinions differ on the effects and utility of AVCS changes. 
OctaneIncreasing the fuel’s octane rating deters detonation. This allows increased timing and/or leaner AFR, both of which can increase power. 
Boost control
Target boostIncreasing target boost causes the ECU to vary WGDC to reach the specified boost level for that cell’s RPM and throttle range. 
Wastegate Duty CycleIncreasing wastegate duty cycle adds force to hold the wastegate closed, which yields increased boost. 
Initial WGDCInitial wastegate duty cycle is one of the inputs to an equation that determines how much actual WGDC to use in real time.
Turbo Dynamics (Proportional)Another input to the WGDC equation, this increases or decreases WGDC in proportion to the difference between actual boost and target boost. 
Turbo Dynamics (Integral)Another input to the WGDC equation, this increases or decreases WGDC over time if the difference between actual and target boost remains high over time.Google “PID feedback” for more information.

Unit conversions and air usage to horsepower potentials from [2]

  • 1 crank horsepower requires 1.45 cubic feet of air per minute, or 0.108 pounds per minute.
  • 1 CFM = 0.0745 lb/min
  • 1 cubic meter / second = 35.314 cubic feet / second = 2118.867 cubic feet / minute (CFM)
  • 1 kg/second = 132 lbs/min = 1771.8 CFM
  • 300 grams/second = 531.5 CFM = potential for 366.6chp
  • 50 grams/second = 88.6 CFM = potential for 61chp
  • 1.3405 hp = 1kW
  • 1hp = .746 kW

See also: - Compression, Timing, AFR, and Power - Ignition timing explained - Octane rating explained - AFR and knock


[1] How to Tune & Modify Engine Management Systems, by Jeff Hartman, page 127 [2] hotrod’s post on NASIOC:


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Page last modified on August 04, 2007, at 01:16 AM
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