E85 as a Racing Fuel

E85 is examined as a racing fuel.  E85 is a combination of ethanol and low octane gasoline.  Some racers report more power from E85 than from methanol.  On the other hand, when the Indy Car League converted from methanol to straight ethanol, dynamometer tests revealed a reduction in power after the conversion.  Regardless of those differing results, several racers are interested in the conversion because of good availability in some areas and low cost of E85.


The conversion from methanol to E85 requires leaning out the engine about 35-50% from the methanol mixture setup.  E85 contains less oxygen to support combustion than methanol.  That changes the air to fuel ratio.  In addition, converting to E85 from gasoline requires enrichment of about 20-30%.


The BTUs per pound of fuel indicate the heat energy from the fuel.  However, since various fuels are run at different air to fuel ratios, the BTUs per pound of the burn ratio mixture of air and fuel are a better indicator of heat energy.  BTUs per pound of mixture are a good measure of how much horsepower an engine will make.


According to engineering data presented in our methanol book, on p. 207, ethanol in the proper burn ratio with air has 1,150 BTUs per pound.  Gasoline with air has 1,274 BTUs per pound.  The E85 combination computes to 1,169 BTUs per pound.  Methanol has 1,146 BTUs per pound of mixture.  At the optimum burn ratios, the various values are close enough that it is almost a toss up between any of the fuels as far as horsepower is concerned.

These values are at the optimum or stoichiometric burn ratios.  Those are air to fuel ratios that produce complete burning.  No excess fuel or oxygen would be left over after combustion.  Unfortunately octane characteristics of E85 at the optimum burn ratio (economy mode) are not significantly different than high octane pump gasoline.  In the economy mode, that would limit its application in high compression or high boost racing engines.


However, E85 like methanol can be run rich for cooling to inhibit detonation.  Our methanol book describes how rich alcohol mixtures essentially raise the resistance to detonation in racing engines.  That is done with “intercooling” from the excess fuel.  Alcohol fuels including methanol and ethanol do not foul spark plugs from rich mixtures.  As a result, extra fuel from enrichment can be used.  Enough enrichment can provide enough cooling to keep the mixture below the self ignition temperature.  That effectively raises the octane characteristic.

Some sources quote E85 octane characteristics over 100.  That would be achievable only at rich mixtures that are seldom discussed.


Note that octane rating is a gasoline characteristic measurement.  It is a detonation resistance value compared to a gasoline standard called “octane”.  Combustion engineers report that rating alcohol fuels with the standard octane motor test is difficult.  At best, an approximate octane characteristic can be made.  However, since it is only an approximation, it is subject to interpretation or misinterpretation.  Racers should be careful at drawing comparisons between anti-knock properties of alcohol fuels or fuel mixtures and anti-knock properties of gasoline blends.  They may be valid. Or may not be valid.  It is possible for E85 to act like 114 octane gasoline in one test circumstance. It may detonate like a lower octane gasoline in another.  Octane rating may not be a valid rating for E85 in many circumstances.


While a little enrichment inhibits detonation, excessive enrichment reduces the cylinder temperature too much.  That reduces power.  In addition, excess liquid fuel will vaporize in the ignition spark of a plug.  That will absorb ignition energy, slowing down the preliminary ignition process. That reduces power as well.  How much enrichment for maximum power is complex.  Tracking of air to fuel ratio provides a valuable control number. Analysis will produce a number that simplifies tuning to achieve that value for maximum power.

Two identical E85 V-8 engines with different enrichment, engine temperature, & spark advance tune-ups can have a dramatic horsepower difference.  One HP value more than that engine on gasoline and the other less than the gasoline fueled engine.


The rich air to fuel ratio for best power for methanol is approximately 5 to one in a normally aspirated racing engine.  For E85, it is a value less than 10 to one.  The E85 value is much less fuel for the amount of air than the methanol value.  A reduced quantity of E85 is necessary to run a race engine previously set up for methanol.  However, fuel curves should be well developed on E85 before turning up the spark advance or compression; or the boost in a blown engine.


A common problem with commercial E85 is different mixtures of ethanol and gasoline.   The air to fuel ratio for “best power” would vary in a racing engine for different mixtures of ethanol and gasoline.  E85 should be a mixture of 85-15. Mixtures as different as 70-30 and anywhere in between that and 85-15 were reported in purchases labeled E85.


A hydrometer is recommended.  It can be used to measure specific gravity of the fuel to determine fuel mixtures of a new E85 purchase.  A fuel with a 70-30 mixture, for example, has more gasoline.  Most gasoline blends weigh less than ethanol.  So that specific gravity would be less than a fuel mixture of 85-15.

Once specific gravity data is acquired, the racer can use the average as a baseline for comparison for future tuning.

  • (a) If the specific gravity of a new fuel purchase is lower than the average, that would indicate a gasoline rich mixture.  That would indicate to lean the engine from the baseline tune-up.
  • (b) If the specific gravity of a new fuel purchase is higher than the average, that would indicate an ethanol rich mixture.  That would indicate to richen the engine from the baseline tune-up.


The standard for E85 is also affected by what gasoline is used in the mixture.  Gasoline is a blend of different weight hydrocarbons.  The specific gravity of gasoline varies with different blends.  Blends are varied by the manufacturer in response to location and seasons.  This would affect the specific gravity of an E85 fuel purchase as well.  Fortunately E85 purchases with various gasoline weights would require the same tuning trends as indicated for changes in the ethanol to gasoline mixture.

A lighter fuel from a light gasoline blend would need a new tune-up leaned from the baseline tune-up.  A heavier fuel from a heavy gasoline blend would need a new tune-up richened from the baseline tune-up.

Whether the specific gravity of a new fuel purchase varies from differing mixtures of gasoline or from differing blends of gasoline may be another issue that affects air to fuel ratios.  In that the case, it would be wise to run a bit rich on E85 to tolerate the differences.


I watched one racer convert to a “killer” brand of racing gasoline and go faster.  I saw another racer do the same conversion and go slower. Gasoline is a blend.  E85 is a blend as well as a mixture.  Blending & mixture variations can cause unpredicted outcomes in different race engine combinations.


Methanol is not a blend.  Methanol is methanol as long as it is not contaminated.  It is more predictable as a result.  We made over 400 drag race passes with our baseline air to fuel ratio for methanol.  We always adjusted fuel injection jetting for changes in air density and blower overdrive.  We saw a very stable tune-up.  Spark plug color remained virtually the same for all of the variations in air to the engine with a proportionate fuel adjustment.  That may not be the case with E85 due to variations in the mixture of gasoline dilution and variations in the blend of the gasoline dilution.  Adjustments of air to fuel ratios may be needed.


Optimum air to fuel ratios would vary in E85 labeled mixtures that vary from 85-15 to 70-30.  That variation would be from 2 to 4%.  For 70-30 fuel, mechanical fuel injection nozzle areas may be needed that flow 2 to 4% LESS fuel to the engine than nozzle areas for 85-15.

For electronic fuel injection, injector duty cycles may need to be reduced 2 to 4%.  If air to fuel ratios are mapped, then an up or down override would be appropriate.


Note that air to fuel ratio readings from air to fuel ratio meters are not necessarily calibrated the same from one manufacturer to another.  Nor are they necessarily numerically accurate.

An air to fuel ratio meter from one manufacture may read 14.7 to one for gasoline, E85, or methanol.  The actual air to fuel ratios by air to fuel weight would be:

  • gasoline: 14.7 to one
  • E85: 9.8 to one
  • methanol: 6.5 to one.


Care should be exercised in making tuning numerical decisions from air to fuel ratio meters.  While they may be inaccurate, they can be repeatable.  If they are repeatable, then they are a good indicator of tuning trends.

Ref. 1: Fuel Injection Racing Secrets, p. 175, has more information on air to fuel ratios and specific gravity values for various fuels.

Ref. 2: Several hundred pages of explanations for alcohol fuels throughout our methanol book, apply to ethanol and E85 as well.  They help to explain how to get the best power from a racing engine on alcohol.  Without that understanding, it is the cost of trial and error that can be lengthy and expensive; certainly a lot more than the cost of a couple of books.