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 Soichiometric 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.
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If transitioning from 0% Ethanol to 100% ethanol ~63% more fuel mass will be required. (This is uncommon as 0% ethanol isn’t recommended for most vehicles and most available pump E85 will be 70-85%) If transitioning from 10% ethanol to 85% ethanol ~43% more fuel mass is required. |
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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Exact stoichiometric ratio will depend on the exact mix of chemicals used to formulate the gasoline portion of the fuel. |
Keep in mind air/fuel ratio targets in the software (and most external wideband gauges) are displayed on a gas scale. In order to avoid confusion it can help to work in lambda. This can be especially useful when fuel chemistry is variable.
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.
However the stock fuel system as already quite taxed to begin with, as a result you will not be able to make any increased power (and in fact will likely need to turn down the stock power level) if you want to run with 85% Ethanol on a stock fuel system. As a result we have set up our NexGen Stage2+FF maps to include the following.
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.
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NexGen Map Setup
Activation of the system is quite straightforward with only a few table changes required
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.
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 NEXGEN Flex Fuel Software Changes and Updates
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.
Table Value | Group A | Group B |
|---|---|---|
0 | Low Ethanol | High Ethanol |
1 | High Ethanol | Low Ethanol |
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.
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
Fuel Injector Scale Low
Blending Table
Assuming 50% ethanol content the math would go as follows:
(300x0.5)+(100x(1-0.5))= Final Output
(150)+(100x(0.5)=Final Output
150+50 = Final Output
200 = Final Output
Doing it again at 100% you’ll get:
(300x1)+(100x(1-1))= Final Output
300+(100x(0))=Final Output
300+0= FInal Output
300=Final Output
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 useful part of the blend ratio 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.