Some forced-induction guys swear by the stuff, and it’s of the few things that both hot rodders and tree huggers can embrace. Farmers love how it fattens up their piggybanks, and lobbyists are working hard to spread its popularity by bribing lawmakers. We’re talking about ethanol, E85 fuel to be exact, and few topics are as polarizing in both consumer and enthusiast circles. We’re all about performance, so we’ll focus on the benefits of E85 instead. There are lots of them, but E85 comes with plenty of drawbacks as well.

If there’s one universal truth when it comes to E85, it’s that there are a ton of misconceptions and urban mythology floating around out there. For example, E85 is said to have a much higher octane rating than regular pumps' gas, but did you know that the octane number for all ethanol fuel is pure bunk? Likewise, in instances where E85 does produce more power than gasoline, its higher octane number is rarely the reason why. What’s more, depending on the time of year when you fill up with E85, it might actually be E70. On the flip side of the equation, E85 and boost are often a match made in horsepower heaven, and it’s a heck of a lot cheaper than gasoline. To help us sort through all the mythology, we sought the expertise of Tim Wusz at Rockett Brand Racing Fuel. The company offers both high-octane gasoline and E85, so it has a good handle on the pros and cons of corn juice. Additionally, for advice on how to convert a gasoline fuel system and carb over for E85 use, we chatted with Marvin Benoit of Quick Fuel Technology. Here’s the scoop:

Octane Number

Tim Wusz: There is a lot of misinformation floating around regarding the virtues of ethanol and ethanol blends like E85 when compared to gasoline. Many hot rodders feel that E85 has a significant octane advantage over gasoline, but the truth isn’t so clear cut. In reality, it’s difficult to directly measure the octane rating of E85 because ethanol vaporizes at 172 degrees F. On the other hand, gasoline typically has a boiling range of 100 to 430 degrees F. The Motor Octane Number (MON) test method was originally designed for testing gasoline, and requires that the air/fuel mixture be at 300 degrees F when passing through an engine’s air intake system. Ethanol is completely vaporized at that temperature and cools the inlet charge significantly, which makes testing it very inconsistent. As a result, ethanol is normally tested in a 50/50 blend of a known-octane gasoline and ethanol. The octane number of that blend is then used in an algebraic calculation to determine the octane number of the ethanol. Results vary because of the known octane product used, and also because the test engine provides less consistent results when high volatility fuels like the aforementioned 50/50 mix are used in an engine not designed for this type of fuel. To make a long story short, ethanol octane numbers are calculated from results gathered in less than ideal operating conditions. That is why there are ranges for ethanol octane numbers posted in the E85 fuels that we offer.

Horsepower Advantage?

Tim Wusz: In instances where E85 does indeed produce more horsepower in an engine than gasoline, the common assumption is that it is due to E85’s higher octane number. However, when the air/fuel ratios are correct for both fuels, E85 and normal street gasoline will usually make about the same amount of power. Sometimes E85 will have a slight horsepower advantage, but that’s due to E85’s superior cooling effect over gasoline as the fuel evaporates, not its higher octane number. Since E85 absorbs more heat than gasoline as it vaporizes, this effect cools the intake system and improves volumetric efficiency, which improves cylinder pressure and also improves power.