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.
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% |
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
Base Fuel Pressure | Injector Size For E85 | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|
Bar | PSI | 350 | 375 | 400 | 450 | 480 | 515 | 600 | 640 | 690 |
3* | 43.5* | 1050 | 1050 | 1300 | 1300 | 1700 | 1700 | 1700 | 2600 | 2600 |
3.5 | 50.8 | 1050 | 1050 | 1050 | 1300 | 1300 | 1700 | 1700 | 1700 | 2600 |
4 | 58 | 1050 | 1050 | 1050 | 1300 | 1300 | 1300 | 1700 | 1700 | 1700 |
*3 Bar/43.5 PSI is stock fuel pressure
These injector sizes are based on the Injector Dynamics (ID) series of injectors. Not all injectors that have the same or similar ratings can or will flow the same amount of fuel. If there is some question if the injectors you have or want to use will support the amount of horsepower you want to run, you should reach out to the injector’s manufacturer directly.