I have a customer that’s switching over to E85 fuel and a lot of new calculations have to be done to make sure the fuel system is capable of handling the required extra fuel. Here’s one tip for calculating fuel requirements.
ONE way to do calculations is to understand that E85 will require about 49% more fuel to achieve the same Lambda. You can’t use A/F when you calculate percentage fuel differences because the FUEL part is in the denominator (AIR is in the numerator). Switch it over to F/A by taking the inverse (1/x). That’s how the fueling is specified in the actual PCM strategy anyway. Diablosport tries to make it easier for CMR tuners by using A/F but they have to switch it back to F/A behind the scenes.
Here’s an example for ya…
| AIR | FUEL | RATIO |
| 14.7 | 1 | 14.7 |
| 14.7 | 1.5 | 9.8 |
| 14.7 | 2 | 7.35 |
As you can see, DOUBLING the fuel (2-parts instead of 1-part) results in an A/F of 7.35.
14.7 parts AIR with 1.49 parts FUEL gives you a ratio of 9.86 which is E85’s Stoichiometric ratio.
If you switch to F/A, it becomes easier to calculate %-differences in fuel..
Gasoline Stoich F/A: 0.0680
E85 Stoich F/A: 0.1015
Then, just do 0.1015/0.0680 and you get 1.49 = 149% = 49% MORE.