E85 is a blend of ethanol and gasoline, with the ethanol content ranging from 51% to 83% by volume. Making it involves mixing fuel-grade ethanol with conventional gasoline at the right ratio, but the process comes with important technical, safety, and legal considerations that determine whether your blend will actually perform well in an engine.
What E85 Actually Contains
Despite its name, E85 doesn’t always contain exactly 85% ethanol. The ASTM D5798 standard, which governs E85 fuel blends for flex-fuel vehicles, allows ethanol content anywhere between 51% and 83%. The remaining volume is gasoline, which aids cold starting and provides additional energy density. The blend also has strict limits on contaminants: water content must stay below 1.0% by mass, sulfur below 80 mg/kg, and methanol below 0.5% by volume.
These aren’t arbitrary numbers. Water is ethanol’s biggest enemy in fuel applications. Ethanol absorbs moisture from the air far more readily than gasoline does. Even a 10% ethanol-gasoline blend can hold about 0.5% water by volume at 60°F before problems start. Once the water content exceeds the blend’s absorption capacity, the ethanol and water separate from the gasoline and sink to the bottom of the tank. This is called phase separation, and it can destroy an engine’s fuel system.
The Basic Blending Process
The concept is straightforward: combine fuel-grade denatured ethanol with unleaded gasoline in a clean, dry container. For a typical summer blend targeting around 80% ethanol, you’d mix roughly 4 parts ethanol to 1 part gasoline. For winter or cold-climate use, dropping the ethanol to around 51% to 60% improves cold starting, since gasoline vaporizes more easily at low temperatures. That means closer to a 1:1 ratio of ethanol to gasoline.
Start by ensuring your container is completely free of water. Even a small amount of residual moisture can trigger phase separation. Pour the gasoline in first, then add the ethanol and mix thoroughly. The two liquids blend easily since they’re fully miscible, but letting the mixture sit without agitation can allow minor stratification.
The ratio you choose matters for engine tuning. Higher ethanol percentages require more fuel volume per combustion cycle because ethanol contains about 30% less energy per gallon than gasoline. If your vehicle’s flex-fuel sensor reads the ethanol content and adjusts automatically, you have some flexibility. If you’re running a fixed tune calibrated for a specific ethanol percentage, precision in your blend ratio becomes critical.
Sourcing the Right Ethanol
The ethanol you use must be fuel-grade denatured ethanol, not food-grade spirits or industrial solvents. “Denatured” means a small amount of another substance has been added to make it undrinkable, which exempts it from beverage alcohol taxes. The federal government authorizes specific denaturing formulas for motor fuel use. The simplest and most common is Formula No. 28-A, which adds just 1 gallon of gasoline per 100 gallons of ethanol.
Other denaturing formulas use methanol (wood alcohol) at 5 gallons per 100 gallons of ethanol. These work for fuel but introduce a compound you want to keep minimal. The ASTM specification caps total methanol at 0.5% of the final blend. Some industrial denaturants use chemicals like methyl isobutyl ketone or acetaldol, which are designed to be nearly impossible to remove through distillation. These formulations are meant for non-fuel industrial purposes and can leave residues that corrode or clog fuel system passages. Avoid them entirely.
Race fuel suppliers sell ethanol specifically intended for blending, often at 200 proof (100% ethanol) already denatured with a small percentage of gasoline. This is the cleanest and most reliable source for DIY blending.
Why E85 Makes More Power
The performance appeal of E85 comes down to physics. Ethanol has a latent heat of vaporization of about 360 BTU per pound, compared to roughly 150 BTU per pound for gasoline. That means ethanol absorbs more than twice as much heat as it evaporates inside the intake and combustion chamber. This cooling effect lowers intake air temperatures significantly, which increases air density and reduces the likelihood of detonation (knock).
The knock resistance is what makes E85 so attractive for turbocharged and supercharged engines. With less risk of detonation, tuners can run higher boost pressure, more aggressive ignition timing, or both. The tradeoff is fuel economy. Because ethanol packs less energy per gallon, the engine needs to inject substantially more fuel to maintain the correct air-to-fuel ratio. Expect fuel consumption to increase by 25% to 30% compared to gasoline.
Material Compatibility
High-ethanol blends are more chemically aggressive than regular gasoline. Before running E85 through any fuel system, you need to confirm that every component in contact with the fuel can handle it. Fluorocarbon elastomers (commonly sold under the brand name Viton) and nitrile rubber are generally compatible with ethanol blends. Standard rubber hoses, cork gaskets, and some older-style fuel pump diaphragms will swell, crack, or dissolve over time.
Fuel lines should be rated for ethanol service. Many aftermarket fuel system components marketed for performance applications are already E85-compatible, but always verify. Aluminum fuel rails and fittings are typically fine, but certain zinc or brass components can corrode. Fuel injectors need adequate flow capacity since the engine demands significantly more volume per cycle on E85. Stock flex-fuel vehicle injectors are sized for this, but a gasoline-only car converted to E85 will almost certainly need larger injectors.
Storage and Handling
Ethanol is hygroscopic, meaning it actively pulls moisture from the air. This makes storage your biggest practical challenge. Keep ethanol and finished E85 blends in sealed containers with minimal air space above the fuel. Carbon steel and stainless steel containers both work well. Avoid containers with any standing water or condensation inside.
In warmer temperatures, storage tanks holding high-concentration ethanol blends develop visible condensation (“sweating”) on their exterior surfaces. Over time, this moisture environment promotes biological growth, an algae-like film, on the outside of the tank. Inside the tank, water absorption from humid air gradually degrades the fuel. For this reason, don’t store E85 blends for extended periods unless the container is truly airtight. Blend what you plan to use within a few weeks.
Ethanol is also a more aggressive solvent than gasoline. When first introduced to a fuel system that previously ran only gasoline, it can loosen deposits and varnish from tank walls and fuel lines, potentially sending debris into filters and injectors. Flushing the system and replacing the fuel filter after the first few tanks is good practice.
Legal Considerations
Federal law regulates fuel manufacturing and sale. Under 42 U.S. Code § 7545, any fuel or fuel additive sold or introduced into commerce must be registered with the EPA. The term “manufacturer” in this context includes anyone who produces fuel for distribution. Blending E85 for personal use in your own vehicle occupies a gray area that varies by state, but selling unregistered fuel blends is clearly prohibited.
Producing ethanol itself requires a federal fuel alcohol permit from the Alcohol and Tobacco Tax and Trade Bureau (TTB), even for personal fuel use. Purchasing already-denatured fuel ethanol from a licensed supplier sidesteps the distillation permit issue, but state regulations on fuel storage, blending, and emissions compliance still apply. Some states restrict the sale or use of non-certified fuel blends in on-road vehicles entirely. Check your state’s environmental and motor fuel regulations before blending at any scale.
Common Mistakes to Avoid
- Using wet containers or ethanol: Even small amounts of water cause phase separation. Once the ethanol-water mixture drops out of the gasoline, the blend is ruined and the separated layer at the bottom of the tank can hydrolock an engine or starve it of combustible fuel.
- Ignoring seasonal needs: Running a high-ethanol blend (above 70%) in cold weather makes cold starting extremely difficult. Ethanol doesn’t vaporize as readily as gasoline at low temperatures, so winter blends need more gasoline in the mix.
- Assuming any denatured alcohol works: Hardware store denatured alcohol often contains industrial denaturants like methyl ethyl ketone or other compounds that leave harmful combustion residues. Only use ethanol denatured specifically for motor fuel applications.
- Skipping fuel system upgrades: Running E85 through a fuel system designed for gasoline will result in a dangerously lean condition. The engine needs roughly 30% more fuel volume, which means larger injectors, a higher-capacity fuel pump, and often a recalibrated or aftermarket engine management system.