I came across your article on blending E85 with pump gasoline and I have a question. I have a C7 ZR1 that is un-tuned that needs 96 octane to prevent timing from being pulled.  I have e85, 93 e10, and limited 93 no E available. I am trying to stay close to 15% E, since my car has no modifications for using a higher ethanol percent. Do you think the 17-18% would be ok to run? I don’t run my car at the track, just some spirited driving. 

My car has an 18 gal. tank, assuming it is empty, are my figures below close to correct?

*** Mix #1 E10 93 ***

2 gal E85 + 16 gal E10 93
2 x 105=   210
16 x 93= 1488
1698/18= 94.3 Octane

2 x 85= 170
16 x10= 160
330/18= 18.3% E

*** Mix #2 No E 93 ***

3.75 gal E85 + 14.25 gal no E 93
3.75 x 105=  393.75
14.25 x 93= 1325.25
1719/18= 95.5 Octane

3.75 x 85= 318.75
14.25 x 0=0
318.75/18= 17.7% E


The story that B.H. refers to is an article I wrote for the EngineLabs website on mixing E85 with regular pump gas. You can read that article here on EngineLabs.com.

The car that this reader is using is a C7 Corvette with a ZR1 package, which is a supercharged 6.2L LT direct injection engine that Chevrolet rates at 755 hp. It’s also helpful to know that GM rated this engine using 93 octane fuel, a pump gasoline that is not prevalent across the country.

How Gasoline is Rated in the U.S.

Before we get into the details, let’s cover some basics. Gasoline in the U.S. is rated by combining the Research octane number (which is almost always higher) with the Motor octane number (which is lower) and averaging them—which is what you see on the gas pump as (R+M) / 2 = Anti-Knock Index (AKI).

Interestingly, European fuels are rated by their Research octane numbers only, which is a critical distinction that our friends who drive Porches should keep in mind.

Blending Octanes (& Ethanol)

The important thing to know is that when blending gasoline of different octane numbers, the curve is linear. This means if you blend 10 percent 100 octane race gas with 90 percent 91 pump gas, it will increase the octane of the 91 to 91.9—it’s a simple proportion problem. Or, if you mix it 50/50, then you’ll get 95.5 octane. 

Blending ethanol with pump gasoline operates much differently. Rather than a straight linear curve, the benefit of ethanol is that the first 10 to 30 percent blend of ethanol with a pump gasoline like 91 will radically increase the octane rating of the fuel. So creating an E30 fuel by blending 91 octane pump gas with E85 would generate a fuel with an octane rating of 94 AKI. Once you get to blend ratios over 60 percent, the octane benefit from ethanol is still there but the rate of benefit slows down—almost like a plateau. Think of this effect as like the Law of Diminishing Returns.

To illustrate this point, standard 87 octane pump gasoline with 10 percent ethanol actually starts as 84 octane without the ethanol. The additional three octane numbers are gained by the relatively minor 10 percent added ethanol.

This is why all the gasoline companies like ethanol—it’s an inexpensive way to increase octane. They just don’t want you to know how easy it is to mix ethanol with gasoline which will increase the octane rating of the fuel and also reduce the cost of the fuel at the same time, because E85 is generally less expensive than pump gasoline.

Other Things to Consider When Blending Fuel

It’s also important to note that one of the major advantages of ethanol is its cooling effect. When alcohol blended fuels are introduced into the engine, the alcohol evaporates much more quickly at lower temperatures. And when this occurs, the alcohol pulls heat out of the air.

An analogy would be when you go the doctor’s office and the nurse swabs your arm with alcohol. That spot feels cold because the alcohol evaporates and pulls heat off your arm.

The same thing happens when running an alcohol blend fuel. The alcohol evaporates and reduces the inlet air temperature. This is important because for every 25 degrees reduction in inlet air temperature, the engine requires one less octane to be detonation resistant. In other words, heating the inlet air also increases octane requirements.

We all know that superchargers heat the inlet air when as it is compressed. So using a fuel with 30 to 50 percent ethanol will radically reduce the inlet air temperature.

As an example, in a test we did years ago a supercharged small block running 9 psi of boost would generally generate around 150 degree F inlet air temperature after the supercharger on gasoline. With E85 fuel, our experience is that the discharge temperature was closer to 90 degrees F—or a reduction of more than 60 degrees F. That will reduce the engine’s octane requirement by roughly 2 points.

Now you begin to see why so many folks are converting to run on ethanol-blended fuel.

Checking Your Octane Blending Math

To address your math as far as blending mixtures go, your numbers are close but our numbers indicate that a blend of pump gas and E85 to produce a splash blend mixture of E30 would be worth 3 octane points. A blend ratio of E50 or 50 percent ethanol will deliver roughly 4.5 to perhaps 5 numbers. In other words, an E50 blend of E85 and pump 91 premium gasoline (that already has 10 percent ethanol in it) would produce a fuel with an AKI number of around 95 AKI.

Whether that overall AKI number is sufficient to allow your engine to not pull timing back is beyond our ability to predict—but in our humble opinion it would be very close to ideal.

Don’t Forget About Your Injectors

One final caveat is that this blend ratio assumes the engine’s current fuel injectors are large enough to supply the additional fuel volume. This is a real requirement because ethanol only produces about 75 percent of the heat of gasoline, so you have to run more fuel to make the same amount of heat.

If the injectors are too small, the duty cycle on the injector approaches or hits 100 percent. Generally, injectors do not operate consistently above 90 percent duty cycle and can actually deliver less fuel than at lower duty cycles so it’s best to avoid running the injector above an 80 percent duty cycle, especially with a supercharged engine where running lean can cause serious engine damage in a very short period of time. 

Fuel Octane Blending Guidelines

E85GasolineTotalBlend RatioOctane AKI
There are blender pumps beginning to make an appearance in the Midwest that will offer E30 right from the pump. Manually blending the fuel is not difficult but keep in mind that most pump gasoline is already mixed with 10 percent ethanol. (Image/Jeff Smith)
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Author: Jeff Smith

Jeff Smith has had a passion for cars since he began working at his grandfather's gas station at the age 10. After graduating from Iowa State University with a journalism degree in 1978, he combined his two passions: cars and writing. Smith began writing for Car Craft magazine in 1979 and became editor in 1984. In 1987, he assumed the role of editor for Hot Rod magazine before returning to his first love of writing technical stories. Since 2003, Jeff has held various positions at Car Craft (including editor), has written books on small block Chevy performance, and even cultivated an impressive collection of 1965 and 1966 Chevelles. Now he serves as a regular contributor to OnAllCylinders.