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Why is There Even Ethanol in Our Gasoline?

by Classic Motorsports

Posted on 9/5/2024 at 9:00 AM

Ethanol sign on fuel pump



 Is that 10% ethanol found in most pump gasoline good or bad for your car, especially an older one? Like many things in life, it’s complicated.

Why Is Ethanol in Gasoline? Back in olden times–up until the ’70s–lead was added to gasoline to increase its resistance to knock. This tetraethyllead was found to cause smog and other issues, however, so it was eventually banned from pump gasoline. To further help reduce smog, in the ’80s an oxygenate called methyl tert-butyl ether, also known as MTBE, was added to gasoline.

Small problem with MTBE: As noted by Zachary J. Santner, senior specialist of quality at Sunoco, MTBE has a “low threshold to be detected by nose and taste.” In other words, a little goes a long way as far as making groundwater smell bad.

Today, ethanol is used to elevate the octane of pump fuel. The 87-and 93-octane grades regularly found at the pump, Santner explains, leave the pipelines with 83 and 90 octane ratings, respectively; it’s that 10% or so of ethanol added at the distribution terminals that increases the octane to its final values.

But don’t think of ethanol as a cheap fix: The science says that ethanol also increases horsepower. In fact, Santner notes, when NASCAR Cup teams moved from ethanol-free fuel to Sunoco Green E15, a product containing 15% ethanol, all of the teams were able to retune for more horsepower. In the most basic science terms, extra oxygen means extra horsepower.

Why Is Ethanol in Fuel an Issue During Storage? The presence of ethanol in gasoline tends to become a hot topic of discussion as winter approaches, a time when many cars are put away. Another feature of ethanol: It’s hygroscopic, meaning it can absorb moisture from the atmosphere. For most motorists, that’s not an issue because they use up the fuel soon after purchase. The problem arises when ethanol-enriched fuel is allowed to sit for a while.

The easiest fix for winter: Fill up with ethanol-free fuel. Sunoco offers Optima, an ethanol-free fuel designed for storage, while local options can be found at If you can’t obtain this type of fuel, Santner adds, a higher-octane gasoline will age better than a lower-octane product.