What is Flex Fuel?

Flex Fuel refers to a system which allows the user to utilize fuel with varying percentages of ethanol. Allowing the vehicles fuel choice to be “Flexible”. This means that without needing to flash a map the car is capable of running and compensating for those varying percentages with no loss of driveability, and typically a performance bump of some kind.

While there are many different ways of establishing and compensating for ethanol content, the COBB system is setup to utilise an added OEM quality ethanol content sensor to provide ethanol content to the ECU so that the ECU can utilise several custom tables to optimize it’s behavior for power and driveability.

When tuning for ethanol there two different things you’re tuning for.

• Adjustments to Engine Parameters for proper operation

  • This usually involves making adjustments to fueling, and cranking. This is relatively easy to get close as the relationship between ethanol and gasoline is quite linear. Some basic math (or using some existing reference material) will get you quite close in a hurry. Adjusting fueling to account for the increase volume of fuel needed without going too rich, and making sure the car has enough fuel when cranking and cold are your primary goals.

• Adjustments for performance.

  • Ethanol has a higher effective octane rating (resistance to ignition/knock) and is much better at absorbing heat from the air. This means that you can typically run either more boost, more aggressive ignition timing, or a combination of both, while meeting the same safety margin you had with gasoline.

Ethanol Content and Stoichiometric Ratio

While there are many good things about ethanol, the fuel quantity isn’t necessarily one of them. Ethanol typically performs ~27% less work than gasoline and has an ideal mixing rate of 9.8:1 versus the 14.7:1 of Gasoline. As Ethanol content rises, and the quantity needed by the fuel system to maintain stoich. rises proportionally, the demand on the fuel system increases. This increased demand means your fuel injectors and fuel pump will typically be working harder or be maxed out in some cases if the power demand gets high enough.

The table above provides some of the basic math for determining the stoichiometric ratio of your current fuel, as well as the percentage increase/decrease of fuel mass versus e0 and e10. It should be useful to rescale various tables such as injector scalar to get things relatively close

Flex Fuel Requirements

To run the COBB NexGen Flex Fuel software you will need (At Minimum)

• COBB NexGen Flex Fuel Sensor

• COBB NexGen Flex Fuel Module

• COBB NexGen Flex Fuel Harness

• Fuel Lines to and from the ethanol content sensor.

The 2015+ Stis however have issues with harmonics in the fuel system (as well as fairly small injectors) so to safely make an increase in power you’ll want to run an upgraded fuel system. Our NexGen stage2 package is designed with parts that resolve the harmonic issue, as well as some large injectors. With that system as a base we added our flex fuel components to make a fairly complete setup to run Flex Fuel (as outlined below)

NexGen Stage2+FF

• Intake Requirements:

  • COBB Redline Intake 725350
    or
    COBB SF Intake SUB00002IA

  • COBB Turbo Inlet 712450

  • COBB Throttle Body Coupler 712455

• Exhaust Requirements:

  • Stock Downpipe

  • Factory Cat-Back

  • COBB Stainless Catback 515132

  • COBB Titanium Catback 515140

• Subaru Spark Plugs 2018 Type RA/2019+ STI spec. D44700

• Fuel System Upgrades

  • COBB 1050x Injectors 312150

  • COBB Billet Fuel Rails 343150

  • COBB Fuel Line Kit 343250

  • COBB Fuel Pressure Regulator Kit 315250

  • AEM In-Tank Fuel Pump 315450

  • COBB Fuel Pressure Sensor 315650
    
    

The Fuel rail, lines and fuel pressure regulator are needed to offset some odd fuel system harmonics that cause issues with fuel delivery on the newer STI engines. The addition of the injectors ensures that the fuel injection system has a reasonable amount of safety room in order to run full power on the map, and have some safety margin for weather/fuel trims etc. This combination should provide a fix for many of the weak points on the stock engines, as well as a good healthy boost of power over stock.

The Fuel Pressure Sensor kit allows you to tweak fueling based off of fuel pressure, as well as provide a margin of safety should your fuel pump fail or not be up to the task of supplying the fuel you’re asking it for. If you’re going a more custom route, the below reference is a general guide as to what injectors should work for you given your power goals.

NexGen Map Setup

Activation of the system is quite straightforward with only a few table changes required

1. Navigate to the Flex Fuel Feature Activation table. This can be found under the folders FlexFuel>Activation/Options (Feature). You’ll want to set this table to a value of 1 to activate it.
   
   

2. Activating the sensor will be done by going to the folder FlexFuel>Ethanol Sensor>Calibration and selecting the table Ethanol Sensor Activation and setting the value to 1.
   
   

Similarly, if you’re wanting to activate fuel pressure it’s the same two steps, activating the feature in the Fuel Pressure Differential Feature table and then activating the sensor in the Fuel Pressure Activation table.

One benefit to the NexGen Flex Fuel software, is that the ethanol sensor and fuel pressure sensor calibrations are already set for our kit and the COBB 100 psi sensor included in the fuel pressure sensor kit. This includes DTC values. So now once it’s activated you can start tweaking the other base table values without needing to worry about additional setup.

For a list of the other main changes to the software for NexGen Flex Fuel compared to the original Flex Fuel kit, check out the article https://cobbtuning.atlassian.net/wiki/spaces/PRS/pages/2862153817

Other resources include: https://cobbtuning.atlassian.net/wiki/spaces/PRS/pages/2848587777

Blending Tables

The system is set up to use two identical sets of tables (Group A, and Group B), and blend between the two for varying ethanol contents. Using the table Flex Fuel A/B Table Ethanol Concentration Mode Table you can decide which table is for low ethanol, and which is for high ethanol content.

The system uses the ratio found in the Flex Fuel Tables (Blending Ratio) tables to blend between group A and B. It allows you to set the relationship as linear, or not depending on your desired behavior. There are a few cases where multiple tables share a single blending table for example there are 2 AVCS Exhaust Cam Retard Target maps but only one blending ratio.

Open

Example of Blending Ratio Table

The general math for how the blending works is
(High Ethanol Output Table X Blending Ratio) + (Low Ethanol Table Output X (1-Blending Ratio)) = Final Output

Here’s an example with the smallest table to make it the simplest (these are not valid values, do not use, this is an example only!)

Fuel Injector Scale High

Open

 

Fuel Injector Scale Low

 

 

Blending Table

 

You can see by the examples that at mixed percentages it’s using a portion of the high and low values to balance the output. The closer you get to a blend ratio of 1, the more closely it will follow the high content table, whereas as you get closer to 0 you’ll start to be influenced more heavily by the low content table.

The benefit of setting up a ratio of one vs the other rather than as a straight multiplier is that you can provide a non-linear response in proportion to an ethanol content change, or provide a specific blending within a smaller ethanol content range while ignoring others. In a nutshell, you can make the car have a larger range of performance and driveability based on the fueling choice made. You can also set it to completely ignore some ethanol levels if you so desire (though not recommended in most situations).

Fuel Based Flex Fuel Tables

Many of the Flex Fuel blending functions depend on the linear relationship that exists when looking at the change in ethanol content. For example, because we know that we need 63.2% more fuel when moving from 0% ethanol (E0) to 100% ethanol (E100) we can extrapolate for everything in between.

These tables should be set to a linear blend

EJ Engine

• Cranking Fuel Injector Pulse Width Base

• Fuel Economy Display Correction Factor

• Fuel Injector Scale

• Hot-Restart Enrichment Initial

• Post-Start Enrichment High/Low Speed (All)

• Tip-in Enrichment

• Warm-Up Enrichment Primary (All)
  
  

Having these tables properly scaled from the beginning shoudl allow for easily converting an existing tune to be Flex Fuel Capable. Additionally much of it is work you can do well in advance of even touching the car.

Typically the tables are set to be set up so that Flex Fuel Tables (Group A) are the group setup for low ethanol behavior (E0) and Group B are set up as the high ethanol tables. If you feel strongly about reversing this order, you can make that change in the Flex Fuel A/B Table Ethanol Concentration Mode table. However from here on out we’re going to assume that
Group A= Low Ethanol
Group B= High Ethanol

Determine Multiplier Value for High Ethanol Tables

In order to set these tables up appropriately you’ll need to first determine the fuel injector scaling.

For this example, we will assume:

• Group A and Group B are defined as default (A = Low Ethanol, B = High Ethanol)

We are converting an existing E10 gasoline tune to be truly E0-E100 compliant on COBB 1000cc injectors.

To start with, copy all of the existing tune data from the current Flex Fuel Group A tables to the Flex Fuel Group B tables.

In the absence of current accurate tuning data, the initial Fuel Injector Scale for COBB and Injector Dynamics brand injectors can be found within the Support section of the Injector Dynamics website.

To determine the two new Fuel Injector Scale values necessary for our Group A and Group B Tables, we will simply reference the stoich. ratio and multiplier table shown above. If our current injector scalar is 2900 for the current E10 fuel, in order to calculate our E0 (Group A) Scale, simply multiply 2900 by 96.1% ("M" Hotkey -> 0.961), which comes out to approximately 2788. To do the inverse for our E100 (Group B) Scale, multiply 2900 by 1.561% ("M" Hotkey -> 1.561), which comes out to approximately 4550. Once the vehicle is running, this conversion can be checked by logging "Fuel Injector Scale (Final)".

Assuming ethanol content has not changed, this value should be very close or identical to the previous reference value (in this case, 2900 at 10% ethanol).

Generate the other E100 (Group B) Fuel-Based Tables in the same fashion (increase existing values derived from the Group A tables by a factor of 1.561). While additional factors may come into play during final tuning, such as further adjusting Cranking Enrichment to compensate for the difficulties of cold-starting on high ethanol fuels, this initial mathematical scaling will lay the foundation for the completion of all tuning. Once this is done, fueling can seamlessly blend across any ethanol content. See below for this example:

Octane-Based FF Tables

For items such as Boost Targets and Ignition Timing, it is more common to use a linear blend that only spans certain ethanol content values. This is because things like the octane rating and effective knock threshold of the fuels do not change at a linear rate (more flat on the ends of the spectrum) as ethanol content changes. We cannot always calibrate the vehicle for true E0 and E100 to set the low and high ethanol tables for the ends of the ethanol content range, so the conservative approach suggests only increasing things like Ignition Timing up to the known thresholds established by the fuel and ethanol content range that is available for testing. We will refer to these as the "Octane-Based Tables".

In fact, blending between variable data for these types of tables is optional. While power output will only change based on the oxygen content change of the fuel, the vehicle will still seamlessly account for variable ethanol content as long as the Fuel-Based Tables have been configured correctly. 
For pragmatic purposes, the ends of these blend ratios are often set based upon the fuel types available during the calibration process. Generally, a vehicle will be tuned on pump gas, such as E10, then drained and refilled with higher ethanol fuel to complete the secondary FF tuning needs. If this new fuel is E85, the initial blend will often be between 60% and 80% (E70, for example). With this, you may choose E10-E70 as your blend rate for the Octane-Based tables. This means that the tune will always be no more aggressive than it was for the E70 fuel in use during calibration, even if ethanol content goes above this during subsequent fill-ups. Conversely, E0-E10 will be treated the same from an octane perspective, which is to be expected (ethanol content and octane rating are independent from one another).

Octane-Based Tables utilizing a non-linear blend generally include:

• AVCS Exhaust Cam Retard (If equipped)

• AVCS Intake Cam Advance

• Boost Limits

• Boost Targets

• Primary Ignition (All)

• Primary Open Loop Fueling

• Wastegate Duty Cycles (High/Low)

The easiest way to accomplish this is to adjust the Blending Table axis scaling. In this example case, Boost Targets are scaled upwards in a linear fashion as Ethanol Content increases from 20% up until ethanol content reaches 70%. For E0 to E20, the full low ethanol table values are targeted (0.00 blending value); for E70 to E100, the full high ethanol table values are targeted (1.00 blending value). See below for an example of how to configure this within the Blending Tables:

Ex: Octane-Based Tables: linear blending from E20-E70 for Boost Targets

Ethanol Content Measurement and Analog Input Considerations

• DTC functionality for Ethanol Sensor Low and High voltage errors is provided via "C0BB1" and "C0BB2" custom DTC codes.
  
  

• When the C0BB1 or C0BB2 DTC is present, ethanol content will be locked to the last known good value and the boost control system will be disabled (WGDC capped to 0.0%). This behavior will continue (DTC present, CEL illuminated, Boost Control disabled) until the hardware issue has been corrected and the ECU has been manually reset to clear the code.

  • If the hardware failure and triggering of the DTC occurs immediately upon key-on following a reset/reflash, and before the ECU has been able to store a known good ethanol content value, ethanol content will be locked to the value contained within the Ethanol Sensor Concentration Override table (default is 10.0%).
    
    

• Ethanol Sensor Calibration Deadband Range: the minimum size of the change in ethanol content necessary before "Ethanol Concentration FINAL" is updated. Usually does not require adjustment from the default value.
  
  

• Ethanol Sensor Calibration Sampling Rate: how frequently the analog sensor input for the FF kit is polled to check for an update to the current ethanol content value. The values are in a rough approximation of milliseconds. Usually does not require adjustment from the default value.
  
  

• Ethanol Sensor Calibration Smoothing Factor: a rolling average of the current ethanol content value. This helps smooth out an inconsistent signal while still providing a fast-enough response when ethanol content is legitimately changing. Usually does not require adjustment from the default value.
  
  

• Ethanol Sensor Concentration Failsafe: this threshold, which is in Load g/rev, allows for locking ethanol content above a certain Load value. Flex Fuel sensor accuracy can suffer from fuel pump cavitation, fuel boiling and other issues that can and will contribute to a temporarily inconsistent ethanol content reading from the FF kit. This normally happens when the fuel system is under high demand when fuel flow past the ethanol content sensor may be inadequate (or aerated).
  
  

• After polling the FF kit, the ECU stores the ethanol content value within the Ethanol Concentration RAW value. This is the raw ethanol content before smoothing or the deadband functions are applied. The final output is the Ethanol Concentration FINAL monitor; this is the value used by the ECU while operating on tables that are based on ethanol content.

  • You may observe minor "jitter" or wandering within the Ethanol Concentration RAW signal. The sampling rate, deadband, smoothing functions and load-based lock threshold are all designed to help mitigate this issue and produce a predictable and consistent Ethanol Concentration FINAL value.
    
    

Basic Outline to Enable and Tune for Flex Fuel

1. Set A/B Table functionality via Flex Fuel A/B Table Ethanol Concentration Mode table. This determines which set of tables (A or B) will represent low ethanol content and which will represent high. By default, the existing tuning data will be populated in the Group A tables, which would correlate to converting an existing low ethanol tune to add Flex Fuel functionality. If the vehicle is currently tuned on high ethanol, you may elect to keep this tuning data within the A tables and use the B tables to add low ethanol functionality; in this case, you will want to invert this toggle.
   
   

2. Enable the base Flex Fuel code and functionality via the "Flex Fuel Feature Activation" table.
   
   

3. Copy all current Flex Fuel Group A table data to Flex Fuel Group B tables. This can be accomplished via the Flex Fuel Copy Groups function in the Edit menu or is done automatically when initially opening an existing MAF or SD map in the COBB Custom Features (CCF) ECU. Axis values are shared between the two sets of tables. This will serve as your base starting point for tuning on the alternate ethanol content fuel. This will allow for the vehicle to run as-is no matter which set of tables are being referenced via the Blending Tables.
   
   

4. Configure Ethanol Sensor. Choose the input via "Ethanol Sensor Activation/Input Selection" table, input the calibration via the "Ethanol Sensor Calibration" table, and choose and input valid DTC voltage thresholds for the FF hardware in use via the "Ethanol Sensor DTC Limit (Voltage)(High)/(Low)" tables.

   1. It is advisable to set the DTC thresholds a few tenths of a volt between the ends of the valid calibration range and the ends of the voltage range; for example, if the kit uses 0.5v-4.5v for its E0-E100 range, setting the DTC limits to 0.30v and 4.70v should yield good results. A "C0BB1" DTC will be set for an Ethanol Sensor Low Voltage error and "C0BB2" DTC for an Ethanol Sensor High Voltage error. You may choose to adjust the Deadband Range, Sampling Rate and Smoothing Factor based on the hardware in use but the default values should work well for most FF kits.
      
      

   2. Depending on the analog input chosen, you may need to remove DTC's for the OEM hardware to prevent the CEL from illuminating.
      
      

   3. If the C0BB1 or C0BB2 code is set, Boost Control will be disabled and Ethanol Content will be locked to the last known good value. It is IMPERATIVE that the issue is corrected, and the DTC is cleared before the vehicle is refilled with fuel (and a potential change to current ethanol content is introduced). This functionality can be tested by quickly removing the Ethanol Content Sensor plug from the FF/DAC hardware while the vehicle is running.
      
      

5. Set Fuel-Based Group A and Group B Tables for E0-E100 compatibility. See above for a list of tables to include and how to calculate the multiplier value to use for calculating new Group B values.
   
   

6. Set Blending Ratio Tables. See the above descriptions for configuring Fuel-Based Tables and Octane-Based Tables for Blending Ratio. If you need to perform custom tuning on the Group A tune, be sure to set the Octane-Based Blending Tables so that the current ethanol content will be "locked" onto the Group A tables. If you choose to bypass this step, keep in mind you may experience blending between the Group A and Group B tables. For example, if current ethanol content is 9%, be sure to set all Octane-Based Blending Ratio tables to be 0.0 below E10. These can be revisited once alternate fuel has been added to the vehicle.
   
   

7. Perform calibration for Fuel A using Group A tables. Once this is complete, re-copy any newly changed Octane-Based Group A tables to Group B tables using the Flex Fuel Copy Groups function (in Edit menu). This ensures that things like Boost Target, Ignition Timing and Open Loop Fuel Targets are consistent and provides a starting point for tuning the Octane-Based Group B tables.
   
   

8. Change vehicle to Fuel B (high ethanol) to allow for calibration of Group B tables.
   
   

9. If you wish to tune Fuel B using Realtime tuning functionality as well, you will need to swap the current Group A and Group B tables using the Flex Fuel Copy Groups function (in Edit menu) with the Swap tables box checked, as well as invert the Flex Fuel A/B Table Ethanol Concentration Mode table. This will allow you to now tune the Group A tables for Fuel B (Fuel A table data now located in Group B).
   
   

10. Restart vehicle to cycle fuel supply and check newly updated ethanol content. Things like fuel trims should be very close thanks to the earlier configuration of the Fuel-Based Tables. If you find a significant issue, please stop and check all math and FF tune configuration settings.
    
    

11. Re-set Octane-Based Blending Ratio Tables so as to "lock" tuning to the B tables. For example, if current ethanol content is now observed to be 72%, be sure to set all Octane-Based Blending Ratios to be 1.0 above E70. If you choose to bypass this step, keep in mind you may experience blending between the Group A and Group B tables.
    
    

12. Perform calibration for Fuel B using Group B tables. Now that ethanol content has increased significantly, you may find the engine is able to tolerate more boost, more ignition timing, a leaner open-loop fuel target, etc. (or vise-versa if ethanol content has decreased). You can use the Blending Ratio tables to introduce non-linear blending or any variety of other tuning strategies based on the exact vehicle, modifications, tuning goals, etc.
    
    

13. Once Fuel B and Group B tuning has been completed, you effectively have a completely operational Flex Fuel tune! Double-check all Blending Ratio, Group A and Group B tables for sanity.
    
    

14. Keep in mind that if the auxiliary Ethanol Content hardware or associated wiring enters a failure state, it is possible to introduce either a direct short to the ECU's 5v power circuit or an overvoltage input to the ECU's 5v analog inputs. Either of these scenarios can potentially cause erroneous operation or ECU hardware damage.

Troubleshooting Flex Fuel Related Custom DTCs:

General:

• When any of the following DTCs are set, the check engine light illuminates and the ethanol failsafes are enabled. Ethanol failsafes are: WGDC becomes 0 under all conditions, and Ethanol Final locks to the last value before the DTC was set.

  • If your tune has Ethanol Sensor Concentration Override Always (Ignore Sensor and Use Fixed Value For Final Ethanol) enabled, the override value will continue being used.

• If the input voltage appears to be within the DTC limits, and sane, you may be experiencing momentary invalid readings due to electrical system fluctuations. Vehicles with aftermarket electronics i.e. stereo system, lighting etc. are fare more prone to electrical concerns. If the issue is minor, adjusting the associated DTC Delay values may prevent an unwanted response.

• Make sure the calibration flashed to the vehicle matches the mechanical configuration.

  • Input for each function matches the mechanical install i.e. Flex Fuel module is electrically connected to TGV L, and calibration Ethanol Sensor Activation/Input Selection set to TGV L.

  • Low and High voltage thresholds are appropriate for sensor/module output range, For Example:

    • 0-5V output doesn't have low and high error thresholds at 0.25V and 4.75V which would cause DTCs when there may not be an error.

    • 0.5-4.5 V outputs don't have error thresholds at 0V, 5V which would remove the error checking ability.

Ethanol sensor module voltage codes:

C0BB1 - Ethanol Sensor Voltage Low Input - Enabled if the input voltage is below Ethanol Sensor DTC Limit (Voltage)(Low)(C0BB1 DTC) continuously until Ethanol Sensor DTC Delay (Low)(C0BB1 DTC) expires.

C0BB2 - Ethanol Sensor Voltage High Input - Enabled if input voltage is above Ethanol Sensor DTC Limit (Voltage)(High)(C0BB2 DTC) continuously until Ethanol Sensor DTC Delay (High)(C0BB2 DTC) expires.

Verify voltage output from the Ethanol module to ECU using Accessport or Accesstuner live connect. Depending on which physical input you've connected the hardware to, check the corresponding Sensor Input Voltage monitor i.e. TGV L, TGV R, or Rear O2.

Is voltage in the appropriate range? COBB's flex fuel kit uses a 0.5-4.5 V range for normal operation. Other systems may use the full 0-5V range since they don't support error checking.

While you're at it, check to see if voltage is sane for the estimated ethanol content of the fuel in your vehicle. If not, you may have connected the flex fuel kit to a different input than your calibration expects i.e. hardware hooked up to TGV R and the calibration is set up for it to be hooked to TGV L.

If you're using a COBB flex fuel kit, here are some additional troubleshooting scenarios and tips to help take advantage of our custom module's error reporting capabilities:

0.0V - Power or ground supply fault, module fault
Suggestion: Check connections to rear o2 and TGV harnesses. BOTH must be plugged in (or wire in) for unit to function. Confirm supply voltage and ground continuity if the issue persists and harnesses are connected.

0.1V - Sensor fault, the module to sensor electrical connection fault, waiting for sensor data
Suggestion: Check connections for continuity, replace harness if necessary. If harness tests well, reset ECU and try again. If the issue returns, replace the sensor.

0.2V - Sensor error, internal fault
Suggestion: Replace the sensor.

4.8V - Sensor/Fuel error, compensation out of range
Suggestion: Remove fuel from the car if possible, flush fuel system with known good fuel and see if issues returns. If persists, replace the sensor.

4.9V - Fuel error (water, debris)
Suggestion: Remove fuel from the car if possible, flush fuel system with known good fuel. While water in your fuel is the most common cause, debris or some fuel additives may prevent ethanol sensors from getting a proper reading if in high enough concentration.

5.0V - Wiring short
Suggestion: Check to wire for damage, short.

Fuel pressure sensor voltage codes: (Non DIT Only)

C0BB3 - Fuel Pressure Sensor Voltage Low Input - Enabled if input voltage is below Fuel Pressure Sensor DTC Limit (Voltage)(Low)(C0BB3 DTC) continuously until Fuel Pressure Sensor DTC Delay (Low)(C0BB3 DTC) expires.
C0BB4 - Fuel Pressure Sensor Voltage High Input - Enabled if input voltage is below Fuel Pressure Sensor DTC Limit (Voltage)(High)(C0BB4 DTC) continuously until Fuel Pressure Sensor DTC Delay (High)(C0BB4 DTC) expires.

Verify voltage output from the Ethanol module to ECU using Accessport or Accesstuner live connect. Depending on which physical input you've connected the hardware to, check the corresponding Sensor Input Voltage monitor i.e. TGV L, TGV R, or Rear O2.

Is voltage in the appropriate range? COBB's fuel pressure sensor kit uses a 0.5-4.5 V range for normal operation. Other systems may use the full 0-5V range since they don't support error checking.

If you use a 0-5V system with an Accesstuner calibration that does not have the low and high limits set correctly for your hardware, you will throw a code when fuel pressure is in the low or high end of the sensor's range.

While you're at it, check to see if the voltage is sane for the estimated fuel pressure under current conditions. If not, you may have connected the fuel pressure sensor kit to a different input than your calibration expects i.e. hardware hooked up to TGV L and the calibration is set up for it to be hooked to TGV R.