This guide is for Australian Domestic Market (ADM) Vehicles ONLY. |
"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:
In general, there are two primary goals of a full-function Flex Fuel implementation:
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.
As ethanol content rises, so does the demand on the fuel system and two primary components: the fuel injectors and the fuel pump. 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.
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/fuel ratio targets 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% |
Ex: Stoich. Ratio and additional fuel mass required as ethanol content increases
Port injector selection suggestions based on max wheel HP and base fuel pressure. Consider this table a rule of thumb which requires several assumptions including, but not limited to: rev limit at or below 7000 RPM, FPR is manifold referenced, fuel system is maintaining target differential fuel pressure. Keep in mind some setups will make power more efficiently than others. More or less air and fuel flow will be required to achieve a given power level depending on ECU calibration and mechanical efficiency.
Minimum vehicle hardware requirements to utilize Flex Fuel feature:
Suggested accompanying vehicle hardware (non-DIT):
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:
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.
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".
It is advised to set these Fuel-Based Tables to use a linear blend between E0 and E100:
Properly scaling these tables from the beginning of the calibration process will allow for easily converting an existing tune to be Flex Fuel capable.
The blending tables are set to accomplish this by default (E0 = Full Low Ethanol Tables, E100 = Full High Ethanol Tables): {EJ engine tables shown}
Ex: Fuel-Based Tables: linear blending from E0-E100 for Fuel Injector Scale
In order to set the Fuel-Based Tables up appropriately to seamlessly account for a change in ethanol content, we need to define our low and high ethanol content Fuel Injection scaling (Flex Group A and B).
For this example, we will assume Group A and Group B are defined as default (A = Low Ethanol, B = High Ethanol) and that we are converting an existing E10 gasoline tune to be truly E0-E100 compliant on COBB 1000cc injectors. Copy all 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. In the future, these spreadsheets will be updated to include Fuel Injector Scale data for E0 and E100 automatically. Please check back regularly for updates.
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:
Ex: Fuel Injector Scale for Group A set to 2788 for E0 operation
Ex: Fuel Injector Scale for Group B set to 4550 for E100 operation
This works much like the EJ engine. Again we will assume FF B group is your high ethanol group. With our ethanol content table above including % mass change as a reference, multiply your FF B group Fuel Injector Trim (Fuel Pressure)(Multiplier) and Fuel Injector Trim (Fuel Pressure)(Offset) tables to account for the greater fuel mass injection required.
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:
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
Ex: Octane-Based Blending Ratio table while completing dyno tune on E9 and E81 fuels
Ex: Octane-Based Blending Ratio table to introduce binary WGDC duty cycle control. Below E50, Group A tables are used, above E50, Group B tables are used. A minor blend will occur if ethanol content equals exactly 50.0%. Using a sensor deadband of greater than 1.0% will help eliminate this possibility, as raw measured ethanol content must change more than 1.0% before Ethanol Content Final is updated.
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.
Ethanol Sensor DTC Max. Safe Ethanol Concentration for Tune (C0BB5 DTC) Delay - This delay allows you to avoid a momentary high ethanol reading from enabling the COBB5 DTC unnecessarily.
As with other DIT platforms i.e. VW, BMW, Mazda, the fuel pumps in Subaru DIT vehicles are not designed for ethanol use beyond 10-15%. That said, ethanol levels between 15-30% are generally well tolerated. Those comfortable with mild risk can run ~E30 and enjoy improved knock resistance, charge air cooling, and a mild improvement in engine power potential. While you can't get E30 at the pump, pumping a combination of E0/E10 gas and E85 in an appropriate ratio will allow the blending of a near E30 mixture in your tank.
On DIT platforms not set up for flex fuel from the factory, at greater than 30% concentration, ethanol has caused oscillating fuel pressure first, progressing to loss of fuel pressure, high-pressure fuel pump damage, even making the vehicle inoperable in some cases. Sudden loss of fuel pressure can cause a sudden loss of acceleration, stalling, which can reduce braking performance and steering control. Long story short, while the power potential is very attractive, the use of high ethanol content fuel is at your own risk.
COBB has performed internal testing in multiple geographic locations with different fuel sources in addition to compiling data from COBB retail shops, our Protuner network, and customers. The results varied significantly. Some users ran several tanks of E85 in a row without issue while some experienced serious loss of fuel pressure in as little as 1-2 tanks of fuel.
We suggest monitoring fuel pressure for oscillations. After having fuel pressure issues, some users were able to flush the HPFP (high-pressure fuel pump) out with low ethanol gasoline and get it to operate normally again, while others had to replace the whole pump assembly before they could safely drive the car again. After replacing the pump some users experienced repeat failures if they continued to run E85 fuel.
The ECDS system prevents Ethanol Final changes from the time it activates until the Exit Threshold based on fuel mass injected is achieved. This prevents large changes in engine operation based on ethanol sensor readings from occurring before fuel at the sensor reaches the combustion chamber.
While not a significant concern on return fuel systems because fuel is almost always circulating through the rails, this is critical on DIT's returnless fuel system when fuel ethanol content changes dramatically (ex. car had e10 in it, the customer fills up with E85 or vice versa).
The ECDS system has a few main components:
Ethanol Transition Activation (Ethanol Delta Threshold) - The ethanol delta between the current sample and the previous sample.
Key factors to note:
ECDS default values calibrated by COBB staff are only appropriate for COBB flex fuel hardware, installed in the exact manner we instruct. If you use alternate flex fuel hardware, you'll want to optimize ECDS settings for your needs. Even if you've chosen to use COBB flex fuel hardware, you may feel you have an alternate ECDS calibration strategy that better meets the needs of your customer. Here's an example workflow for that optimization:
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.
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 on very low or high ethanol content levels.
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.
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.
C0BB5 - Max. Safe Ethanol Concentration Exceeded - Enabled if Ethanol final exceeds the Ethanol Sensor DTC Max. Safe Ethanol Concentration for Tune (C0BB5 DTC) threshold continuously until Ethanol Sensor DTC Max. Safe Ethanol Concentration for Tune (C0BB5 DTC) Delay expires, while C0BB1 and C0BB2 are not set.
Each calibrator will need to decide what they deem to be the maximum safe ethanol content for the mechanical configuration and use.
Setting the Ethanol Sensor DTC Max. Safe Ethanol Concentration for Tune (C0BB5 DTC) threshold to 100.00% will disable this check.
COBBF - Map Programming Error - This error is caused by some type of corruption or error within the map that is reflashed to the ECU. Please try a different map or attempt to re-save the map in the latest version of the Accesstuner software. If you continue to have issues, please contact Cobb customer support with a memory snapshot (under Troubleshooting menu on Accessport) when the C0BBF code is present. DO NOT DRIVE OR OPERATE THE CAR IF THIS CODE IS PRESENT. Fatal engine damage can occur. Please have the vehicle towed to your home or preferred Protuner for diagnosis.