What the scalar does
Gasoline and ethanol need very different amounts of fuel to burn the same amount of air. Gasoline's stoichiometric ratio is about 14.7:1; ethanol's is about 9:1. With flex fuel, the ECU has to know where between those two the current fuel sits.
The Fuel Composition Scalar table decides exactly that. It sets the ratio of the fuel tuning's current stoichiometry between gasoline and ethanol. From that, the ECU works out how much fuel to inject to reach your target lambda.
When it's active
Only when the fuel type is set to Flex Fuel. With a single fuel type selected, the table isn't used.
How to set it
This table should follow what the fuel composition sensor reports. In practice, that means ethanol content on the axis and a matching blend value in the table, so a sensor reading of 50% ethanol puts the stoichiometry halfway between the two fuels. If the scalar doesn't track the sensor, the fueling will be off by a growing amount as ethanol content rises.
- Fit a composition sensor such as the Haltech flex fuel sensor (plug and pins sold separately).
- Confirm the sensor reads sensibly on a known fuel before relying on it.
- Check lambda on several blends to be sure fueling stays on target.
For timing and other changes with ethanol, see fuel composition correction and our flex fuel tuning guide.
