E85 Ethanol Fuel Test - The E85 Solution

We chose a Crane roller with a bit more duration than we normally would choose for a street motor since this was a 12.5:1 compression combination.

We chose a Crane roller with a bit more duration than we normally would choose for a stree

The Octane Game
The definition of octane is rather confusing. It's also called the antiknock index (AKI) and refers to a fuel's ability to suppress detonation. The higher the AKI number, the better the fuel's ability to prevent knock. Octane by itself does not add power to your engine. However, a low-octane fuel will not allow a performance engine to achieve its optimal ignition timing and power due to detonation. Detonation is defined as uncontrolled combustion where high cylinder pressure can auto-ignite end gases in the cylinder that tend to increase the opportunity for this process to be self-sustaining. This creates a massive pressure spike that literally rattles the pistons in their cylinders and kills power. Using a higher-octane fuel allows an engine to take advantage of an ideal timing curve without causing detonation. Once that timing has been achieved, there is no advantage to using higher-octane fuel.

You may have seen octane stickers on gas pumps that list the AKI using the equation R + M / 2 = Octane. The R stands for research octane number (RON) while M represents motor octane number (MON). RON is always much higher than MON, which is why the AKI uses an average of the two numbers to best represent the AKI for street-driven engines. The accompanying chart lists both RON and MON for ethanol and methanol plus a couple of popular race gasolines so you can get an idea of how ethanol and E85 compare.

Quick Fuel Technology supplied the annular discharge 850-cfm mechanical-secondary carb for our testing. The company also supplied the second set of billet metering blocks tuned for the higher fuel-flow rate required by E85.

Quick Fuel Technology supplied the annular discharge 850-cfm mechanical-secondary carb for

Straight ethanol has an AKI rating of 113-115. E85 is generally rated at between 100 and 105, depending upon the manufacturer. The E85 we tested, supplied by American Ethanol out of Chicago, is rated at 105. E85 is most often blended with 87-octane gasoline, since it is the least expensive. Another advantage to ethanol is that the greater volume of fuel required also tends to reduce inlet air temperatures, where evaporating fuel pulls heat out of the incoming air temperature.

Completed and on the dyno, our Ed Taylor-built 355 ran flawlessly once we got past too-hot spark plugs that cost a warped valve.

Completed and on the dyno, our Ed Taylor-built 355 ran flawlessly once we got past too-hot

Air/Fuel Ratio Chart
This accompanying chart offers comparisons of E85 and straight ethanol (or Jack Daniels, if you prefer), to gasoline. Stoichiometric is the term used to refer to the chemically correct air/fuel ratio that also creates the least emissions. This is also the air/fuel ratio most production engines are tuned to achieve at part-throttle. A leaner fuel mixture can generate better mileage, while a fuel-rich ratio generally will make more power by ensuring most of the air is used to create power.

By reading this chart, you can see that the air/fuel ratios for E85 are substantially richer than for gasoline from a percentage standpoint. What we found on the dyno was that we needed to increase E85 fuel flow through the carburetor by roughly 30 percent compared with gasoline. In a fuel-injection system, this could have been easily accomplished by merely increasing the volume of fuel flow (using the base fuel map) by one-third. This is a rough rule of thumb, but we found it to be pretty close for the purposes of our testing.

By Jeff Smith

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