Understanding the EFI Process


drive by wire componets

 What this means to you continued:

  • Most modern engines will not allow the motor to start immediately after the battery has been disconnected then re-connected or if the ECU has been re-set because the systems needs approximately 5 seconds to fully calibrate the TBW system. If the battery has been disconnected then re-connected or if the ECU has been re-set, please allow the key to stay in the ON position for a minimum of 10 seconds to allow the TBW system to calibrate before you try to start the engine.
  • Providing a constant +12V to the throttle actuator motor will fully open the throttle blade, providing a constant -12V (reversing the polarity) to the throttle actuator motor will fully close the throttle blade.
  • Most modern EMS logic has been calibrated to control the throttle blade opening and closing speed. We have seen instances where the throttle blade actually opens slower if you slap down the gas pedal versus gradually depressing the gas pedal.
  • Most modern EMS programming was established to lessen the emissions output of the motors, so throttle opening and closing delays have been implemented to lessen the emission output of the motors…not to piss you off.
  • Most modern vehicles have some sort of Traction Control System which use the Electric Throttle Bodies to control engine torque output.
  • Most modern vehicles have some artificial throttle response programmed into their control. For instance, if the driver is pushing down on the APP 20% the TPS sensor may report a 50% reading. The ECU is creating artificial throttle response by opening the throttle valve much more that the driver is requesting.

An ECU table is used to control the electronic throttle body system.  The Y-axis breakpoints are engine RPM and the X-axis breakpoints are APP sensor signal inputs. The ECU logic is going to look up what duty cycle it wants to step (square wave) to the electronic throttle body motor based on these two inputs, APP and RPM. It will then use the logic programmed in the on-board PID system to control the mechanical DBW components. Most TBW systems operate in an closed-loop condition where feedback is given to this sub-system and this sub-system will use programmed corrective measures to properly control the throttle blade. Notice how the ECU has been calibrated to shut the throttle duty cycles down to 25% by 7000 RPM, this is done to further protect the motor from an overrev or overrun event. The calibration of this table can dramatically change how a vehicle drives.

 As complex as modern EFI systems are, you can always break systems down to their very basic components which is what we have tried to do with this article. At the very basic level, modern EMS has not changed much since the mid 90’s so have no fears about something being too complex. If you have any particular system that you would like us to address, please let us know with your comments on the MotoIQ web site. We pay attention to those comments and we can easily tear apart another sub-system to help people better understand what is intended on the system basis. EMS consists of several sub-systems, once you understand each sub-system…you understand the entire EMS process. That sounds simple because it is.

We have created a worksheet called “EFI 102 Worksheet” (click here) to go along with this EFI 102 article. The worksheet contains three tabs; one called Injector Calculator which contains a graphical Operating Fuel Pressure Calculator, Fuel Injector Calculator, and a Fuel Flow Calculator. The available fuels for  modern EFI systems will be changing over the next few years, so we have included a basic Octane Mixture Calculator in the Fuel Info tab to help you understand how you can mix various octane ratings of similar fuels. The third tab is the Ignition DA tab which has some additional information specific to the modern Subaru ECUs which use Dynamic Advance logic to tune ignition advance. Please take time to change the worksheet formulas in order to get yourselves familiar with how worksheets function. We hope you have enjoyed the second EFI article from Cobb Tuning.

Cobb Tuning’s EFI 103 article will teach you about the different chassis dynamometers and how they can be used as a testing, calibration, and tuning tool. This class will cover various forms of data analysis and will teach you how to read and understand dyno print outs and various EMS print outs. By then end of class you should be able to see how what changes you make effect the health and power produced by the engine. Class time will be used to demonstrate how to properly tie down a vehicle to a chassis dynamometer and how to operate the dyno and the vehicle during various testing.


Cobb Tuning

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