Hydrocarbon Chemistry
Jeff Pimental, via CarCraft.com: I was reading your column in the August issue on the air/fuel ratios of E85. Could you tell me what ratio to use if we mixed E85 with 50 percent 110-octane leaded race fuel? This would give us 2 grams of lead without hardened seats and octane to support 11:1 compression easily. By what percentage would we have to increase the volume of fuel through the carb?
The Innovate air/fuel ratio meter will read as either air/fuel ratio or in lambda. The beauty of lambda readings is that max power is always around 0.85 lambda, while the stoichiometric air/fuel ratio is always 1 for gasoline, ethanol, or methanol. Plus, if you talk in lambda, everybody will think you're an engineer. Just don't wear the pocket protector.>>>
Jeff Smith: I called on my friend Klaus Allmendinger, the resident engineer at Innovate Motorsports, the company that builds that trick air/fuel ratio meter we use for tuning. Here's his approach. The stoichiometric (abbreviated stoich) air/fuel (A/F) ratio for gasoline is 14.7:1, while the stoich ratio for 100 percent ethanol is 7.87:1. The lambda reading for stoich air/fuel ratios for both these fuels is 1.0. Keep in mind that stoich is a chemically correct A/F for least combined emissions, but not for best power. A best power A/F for gasoline is 12.5:1 while the 100 percent ethanol number is 9:1. As you can see from the different A/F ratios, ethanol requires a greater volume of fuel for max power. This is because of ethanol's reduced Btu heat content when combusted.
When looking at increasing jet size, what we're really concerned with is jet area. To use a simplified example, let's say you have a jet combination in a Holley carburetor of 85 jets in all four corners. Holley conveniently publishes the hole diameter for all its jets in its catalog. The drill diameter for a Holley 85 jet is 0.100 inch. According to Klaus, dividing the stoich ratios of gasoline by pure ethanol (14.7 / 9 = 1.63) is the jet-area factor. But because jet area changes with the square of the jet diameter, we have to take the square root of 1.63, which equals 1.278, or 27.8 percent. Hang on, we have to now go a little deeper.
If all we wanted to do was determine how much jet area to increase if we ran straight ethanol, we would take our 85 jet diameter of 0.100 inch and increase it by 27.8 percent, which would make it a 0.1278-inch-diameter jet. The Holley jet chart shows that the closest is a 100 jet. So far we're on track here, and it all makes sense because we've seen in previous testing that we need to increase fuel flow by roughly 25 to 30 percent when converting from gasoline to E85. This change to a 100 Holley jet will put you in the ballpar, but probably would require fine tuning to dial in the optimal jetting.