Fuel flow rate through an injector is affected by a number of different things. This page focuses on the effects of fuel type, primarily fluid viscosity.

Viscosity is a term you hear all the time, but probably haven't given it much thought in terms of injector flow rate. But imagine pouring honey through a straw compared to water! That's an extreme example, but the same concept applies to the difference in ethanol vs. gasoline.

Everybody knows (or should know!) that when you run E85 or any other ethanol blend, you have to adjust your ECU's fuel flow calculation to account for the difference in the stoichiometric ratio of gasoline (around 14.7:1) and ethanol (around 9:1). If you don't know that, read more on our E85 wiki page.

That's the most obvious change you have to make. But you may not realize that the viscosity of ethanol is also different from that of gasoline. In fact, it's different enough that it actually changes the volumetric flow rate through the injector. In fact, it changes it by enough to be measured and accounted for! This is a completely different and additional adjustment above and beyond the adjustment for stoichiometric ratio.

To illustrate this difference, we ran a number of tests with different injectors flowing gasoline and then again with E85. The same set of injectors was used in both cases. The gasoline measured out to be rough E6 (6% ethanol) and the E85 measured out to be exactly E85 (85% ethanol). Both cans were filled from the same station at the same time on the same day as the tests were run.

• First, we gathered data using stock DSM injectors. We noted a 13% drop in flow rate when flowing E85 vs. gasoline.
• Then we ran a set of FIC BlueMax 1450s. We noted a 11% drop in flow rate.
• Then we ran a set of FIC BlueMax 2150s. We noted a 6% drop in flow rate.

The results are summarized below.

STOCK 450s (click to enlarge)	FIC BM1450s (click to enlarge)	FIC BM2150s (click to enlarge)

So clearly viscosity needs to be taken into account when changing fuel types. This effect also appears to depend on the injector type/size too! Like everything else injector related, this just isn't as easy as it seems like it should be.