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Flex Fuel Overview
"Flex Fuel " (FF) functionality is supported for select turbocharged Subarus utilizing the COBB Custom Features ECU. In short, COBB's Flex Fuel feature encompasses these custom code features:
Seamless switching of fuels between low ethanol fuel and high ethanol fuel.
Utilizes analog ECU inputs for ethanol content via tuner selection between the existing TGV (Left), TGV (Right) and Rear O2 Sensor analog 0-5v inputs.
All main tuning functions covered by Flex Fuel options: the full and variable blending of AVCS, Boost Control, Injector Scaling, Open Loop Fueling, Ignition Timing, Cranking Enrichment, Warm-up Enrichment, Fuel Mileage Display Correction (non-DIT), injection timing (DIT only), etc.
OEM-level functionality and finish: adjustable ethanol content signal sampling rate, smoothing factor, deadband, DTC detection, etc.
In general, there are two primary goals of a full-function Flex Fuel implementation:
Account Compensation for changes to ethanol content (stoichiometric ratio). This allows for the ECU to account for the variable .
The ECU needs to be able to adjust the amount of fuel
to achieve a correct air/fuel ratio as ethanol content changes. This is all based on a linear relationship and somewhat easy to calculate; scaling for these items generally requires a 1:1 relationship with the ethanol content delta. Tuning parameters like Injector Scalar and Cranking Enrichment will fall under this category. Correct tuning and blending of these tables is required for the vehicle to operate under variable ethanol content.
Account Adjustment for changes to fuel quality (octane, burn rate, EGT, etc.).
This allows the tuner to account for
the change in octane and burn rate as ethanol content changes to optimize efficiency and power in the vehicle. The need for this flexibility is based on the notion that things like effective octane rating will significantly increase as ethanol content increases.
While most of these items can be scaled with ethanol content
, they don’t typically require a 1:1 relationship with ethanol content. For example, you could leave boost and timing the same as on pump gas, make changes as content increases, or Target Boost and Ignition timing changes may be locked above E70 as diminishing returns to the knock threshold are usually observed above 70% ethanol content. The blending of these tables is not required for vehicle operation and is determined by the tuning strategy employed.
Additional Resources and Tools – Ethanol
If you have limited experience related to the math and science behind variable ethanol content and E85 tuning, we highly recommend some background education before utilizing the COBB Custom Features, especially the Flex Fuel feature and functionality. While easily understood once studied, ethanol and alcohol-based fuels introduce a variety of benefits and downsides; these must be accounted for both during the calibration process and during subsequent operation of the vehicle. While written with a humorous (and potentially NSFW!) approach, this article from Injector Dynamics helps cover many of those considerations: http://injectordynamics.com/articles/e85/
When adding custom auxiliary sensors and hardware, it is highly advised to have a copy of the applicable Subaru Factory Service Manual (FSM) available for reference. These are available via Subaru TechNet website, AllData shop service subscriptions, NASIOC and similar forums, etc. A digital multimeter (DMM) is a virtual necessity for verifying any custom wiring.
DSPORT Magazine published an informative article with contribution from COBB Tuning staff about Flex Fuel on the Nissan GTR platform, which is available here: http://dsportmag.com/the-tech/education/max-effort-stock-turbo-r35-gt-r-fuel-comparison/
If you have additional questions not covered by this help file, please contact COBB Tuning support via email at support@cobbtuning.com or by phone at (866) 922-3059.
Ethanol Content and Stoichiometric Ratio
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Ethanol Content and Stoichiometric Ratio
If the current fuel in use is 0% ethanol pump gasoline, a full ~63% more fuel mass will be required if transitioning to 100% ethanol. In reality, most will be switching from E10 pump gas to E85 ethanol, which requires a ~43% increase in fueling. This means taht As ethanol content rises, so does the demand on the primary components of the fuel system: the fuel injectors, low pressure fuel pump and the high pressure fuel pump.
Please reference the table below for the stoichiometric ratio of gasoline-ethanol blend fuels as well as the fuel mass change required when moving from one to the other. It is important to note that the stoichiometric ratios given are close estimates; the exact ratio will depend on the exact "mix" of hydrocarbons used to formulate the gasoline portion of the fuel. In order to manually calculate the change when moving from one ethanol content to another, simply divide the old stoichiometric ratio by the new stoichiometric ratio. Example: E10 to E70. 14.131 / 10.715 = ~1.319 (131.9%).***Keep in mind, air
Air/fuel ratio targets (AFR) in the software (and likely on your wideband) are displayed on the gas scale. In order to avoid confusion, we suggest working in lambda especially when fuel chemistry is variable. In ATP under the Edit menu → Configure Options → Display tab, if you un-click "Show Standard Units" air/fuel ratios will be displayed in lambda. Please note other units will be updated as well.
Ethanol Content | Stoich. Ratio | Multiplier (E0) | Percentage (E0) | Multiplier (E10) | Percentage (E10) |
0% | 14.700 | 1.000 | 100.0% | 0.961 | 96.1% |
10% | 14.131 | 1.040 | 104.0% | 1.000 | 100.0% |
20% | 13.562 | 1.084 | 108.4% | 1.042 | 104.2% |
30% | 12.992 | 1.131 | 113.1% | 1.088 | 108.8% |
40% | 12.423 | 1.183 | 118.3% | 1.137 | 113.7% |
50% | 11.854 | 1.240 | 124.0% | 1.192 | 119.2% |
60% | 11.285 | 1.303 | 130.3% | 1.252 | 125.2% |
70% | 10.715 | 1.372 | 137.2% | 1.319 | 131.9% |
80% | 10.146 | 1.449 | 144.9% | 1.393 | 139.3% |
85% | 9.862 | 1.491 | 149.1% | 1.433 | 143.3% |
90% | 9.577 | 1.535 | 153.5% | 1.475 | 147.5% |
98% | 9.122 | 1.612 | 161.2% | 1.549 | 154.9% |
100% | 9.008 | 1.632 | 163.2% | 1.569 | 156.9% |
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Initial Flex Fuel Setup
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Ethanol Content Analyzer with 0-5v analog output ("Flex Fuel Kit") wired to TGV-L, TGV-R or Rear O2 Sensor analog ECU input. Please see "TGV Duplication Mode" section of the Subaru Custom Sensor Logging document if planning to utilize TGV input option(s) while retaining TGV hardware.
Appropriately sized fuel system components (injectors and fuel pump) for maximum ethanol content desired. To estimate the change in fuel system demand, please see the table above. High ethanol will incur a roughly 1.5x increase in fuel system demand than zero/low ethanol fuels.
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Fuel Pressure Sensor with 0-5v analog output wired to TGV-L, TGV-R or Rear O2 Sensor analog ECU input. This will allow for on-the-fly fueling compensation based on fuel pressure, which facilitates more precise tuning and natively compensates against fuel pressure losses due to failing fuel pump, plugged fuel pump, etc. This will also allow the customer and the tuner to monitor fuel system performance. Please see the Subaru Differential Fuel Pressure Compensation document for further setup instructions.
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Blending Ratio Table Overview
The Blending Ratio Tables instruct the ECU on how to determine the final output value while "blending", or calculating a weighted average, between two tables. If the tables contain the same values, the blending ratio will be irrelevant.
The Blending Ratio Table defines how heavily to weight each of the low ethanol and high ethanol tables when determining a final output value, dependent on the current ethanol value. A blending ratio value of 0.00 means the equation is weighted to use the low ethanol table values exclusively; a blending ratio value of 1.00 means the equation is weighted to use the high ethanol table values exclusively. A Blending Ratio value of 0.500 means to use both tables in an equally-weighted average. This is accomplished by the following function:
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Blending Ratio Table Overview
The Blending Ratio Tables instruct the ECU on how to determine the final output value while "blending", or calculating a weighted average, between two tables. If the tables contain the same values, the blending ratio will be irrelevant.
The Blending Ratio Table defines how heavily to weight each of the low ethanol and high ethanol tables when determining a final output value, dependent on the current ethanol value. A blending ratio value of 0.00 means the equation is weighted to use the low ethanol table values exclusively; a blending ratio value of 1.00 means the equation is weighted to use the high ethanol table values exclusively. A Blending Ratio value of 0.500 means to use both tables in an equally-weighted average. This is accomplished by the following function:
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(High Ethanol Table Output * Blending Ratio) + (Low Ethanol Table Output * (1 – Blending Ratio)) = Final Blended Output |
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Beyond this, the Blending Ratio Tables can be customized to provide a non-linear response in relationship to an ethanol content change. They can also be specified to define blending only within a certain ethanol content ranges (while ignoring others). There are specific examples provided within the text below.
Fuel-Based FF Tables
As previously mentioned, many Flex Fuel blending functions will depend on a linear mathematical relationship that corresponds to changes in ethanol content. Because we know that we need 63.2% more fuel when moving from E0 to E100, it is easy to calculate any partial increase necessary anywhere between those two points. This allows for using any fuel between E0 and E100 seamlessly, even if the calibration was completed on less extreme blends (like E10 and E70, for example). We will refer to these as the "Fuel-Based Tables".
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The blending tables are set to accomplish this by default (E0 = Full Low Ethanol Tables, E100 = Full High Ethanol Tables): {EJ engine tables shown}
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Ex: Fuel-Based Tables: linear blending from E0-E100 for Fuel Injector Scale
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Determine Multiplier Value for High Ethanol Tables – Example
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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)
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Ethanol Content Measurement and Analog Input Considerations
Like most custom Flex Fuel implementations, COBB's Subaru Flex Fuel implementation requires a digital to analog converter that interprets the digital signal from an ethanol content sensor into a linear 0-5v analog output. The sensor, the digital to analog converter and associated power and signal wiring combine to comprise a "Flex Fuel Kit".
To power the external sensor hardware, assuming the device is a normal automotive sensor and draws low current, you may use the 5v circuit present at the TGV connector or the 12v circuit present at the Rear O2 sensor connectors. See applicable FSM for detailed wiring diagrams.
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).
DTC functionality is dependent on the hardware kit in use. Some Flex Fuel kits do not offer the ability to reports errors via their 0-5v output because they require the full 0-5v span to report the current ethanol content. Kits that have a narrower range, such as 0.5-4.5v, have the ability to trigger a DTC whenever a voltage is observed outside of the stated range, assuming this functionality has been included in the FF kit's design.
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
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.
Enable the base Flex Fuel code and functionality via the "Flex Fuel Feature Activation" table.
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.
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.
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.
Depending on the analog input chosen, you may need to remove DTC's for the OEM hardware to prevent the CEL from illuminating.
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.
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.
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.
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.
Change vehicle to Fuel B (high ethanol) to allow for calibration of Group B tables.
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).
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.
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.
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.
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.
WARNING! 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.
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Ex: Octane-Based Blending Ratio table while completing dyno tune on E9 and E81 fuels
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been added to the vehicle.
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.
Change vehicle to Fuel B (high ethanol) to allow for calibration of Group B tables.
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).
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.
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.
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.
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.
WARNING! 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.
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DIT Flex Fuel
Please read the above documentation first. While by and large, our flex fuel implementation for DIT engines works in the same fashion as the EJ engine implementation, there are differences based on the tables and logic present on each ECU.
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