If you have spent any amount of time in the modern automotive tuning community, you have undoubtedly heard the legend of E85 fuel. Sometimes affectionately referred to as "corn juice," E85 has completely revolutionized the way enthusiasts extract power from their engines, particularly those equipped with forced induction. In the past, achieving massive horsepower numbers from a turbocharged engine required expensive race fuels that cost upwards of ten to fifteen dollars a gallon. Today, tuners are achieving those exact same, or even greater, power figures using E85—a fuel available straight from the pump at a fraction of the cost.
But why exactly do turbocharged cars love E85 so much? What makes this ethanol-based blend so superior to traditional premium unleaded gasoline when it comes to forced induction applications? The answer lies in the unique chemical and thermodynamic properties of ethanol, specifically its incredible knock resistance and its potent cooling effect on the engine's intake charge.
In this comprehensive guide, we are going to dive deep into the science of E85, the mechanics of turbocharging, and how the synergy between the two creates the ultimate recipe for cheap, reliable, and mind-bending horsepower.
Understanding the Basics of Turbocharging
To understand why E85 is the perfect fuel for turbocharged engines, we must first understand how a turbocharger works and the specific challenges it introduces into the internal combustion process.
A naturally aspirated engine relies on the vacuum created by the descending piston to draw air into the cylinder. The amount of air it can ingest is limited by atmospheric pressure. A turbocharger changes this equation entirely. A turbocharger is an air compressor driven by the engine's exhaust gases. As the exhaust gases exit the engine, they spin a turbine wheel, which is connected via a shaft to a compressor wheel in the intake tract. The compressor wheel sucks in ambient air, compresses it, and forces it into the engine's cylinders at pressures significantly higher than atmospheric pressure. This is what we call "boost."
By forcing more air into the engine, we can mix it with more fuel. More air and more fuel mean a larger explosion during the power stroke, resulting in more horsepower and torque. However, compressing air has a nasty side effect: it generates an immense amount of heat.
When you compress a gas, its temperature rises. By the time the boosted air leaves the turbocharger's compressor housing, it can be scorching hot. While intercoolers are used to reduce this temperature before the air enters the engine, the air entering the cylinders of a turbocharged car is still hotter and under vastly more pressure than it would be in a naturally aspirated state.
The Enemy of Forced Induction: Engine Knock
The combination of increased air pressure and increased heat creates a hostile environment inside the engine cylinder. This environment makes the engine highly susceptible to its ultimate nemesis: knock.
Also known as detonation or pre-ignition, engine knock is an uncontrolled explosion of the air-fuel mixture within the cylinder. In a perfectly operating engine, the spark plug ignites the air-fuel mixture, and the flame front propagates smoothly across the combustion chamber, pushing the piston down with a controlled, even force.
When an engine knocks, the intense heat and pressure cause pockets of the air-fuel mixture to spontaneously auto-ignite before* or *simultaneously with the spark plug firing, or before the primary flame front reaches them. This creates multiple flame fronts colliding with each other at supersonic speeds, producing massive, destructive spikes in cylinder pressure. This collision of flame fronts creates a distinct metallic "pinging" or "knocking" sound.
Engine knock is devastating. Mild knock will cause the Engine Control Unit (ECU) to pull ignition timing, robbing the car of power in an attempt to save itself. Severe knock can shatter pistons, bend connecting rods, blow head gaskets, and completely destroy an engine in a fraction of a second.
Because turbocharged engines operate with higher cylinder pressures and higher intake temperatures, they are operating much closer to the threshold of knock than naturally aspirated engines. To make more power, tuners want to run more boost and advance the ignition timing, but they are constantly limited by the octane rating of the fuel they are using. This is where E85 steps in to save the day.
What is E85 Fuel?
E85 is an alternative fuel blend consisting of up to 85% ethanol and 15% gasoline. The exact ratio can vary depending on the season and the region (sometimes dropping to E70 during winter months to aid in cold starting), but the primary component is always ethanol. Ethanol is an alcohol fuel, typically distilled from plant materials like corn, sugar cane, or switchgrass.
While gasoline is a complex mixture of hundreds of different hydrocarbons refined from crude oil, ethanol is a simple, single chemical compound (C2H5OH). This chemical simplicity, along with its specific properties as an alcohol, gives E85 its tuning superpowers.
Octane Rating and Knock Resistance: The Superpower of E85
The most prominent and highly publicized benefit of E85 is its incredibly high octane rating. But what exactly is an octane rating?
Octane is not a measure of how much energy is in a fuel; rather, it is a measure of a fuel's resistance to auto-ignition (knock). The higher the octane rating, the more heat and pressure the fuel can withstand before it spontaneously detonates.
Standard premium pump gas in the United States typically has an octane rating of 91 or 93 (using the Anti-Knock Index or AKI). In Europe and other parts of the world using the Research Octane Number (RON) scale, premium gas is usually around 98 RON.
E85, on the other hand, boasts an effective octane rating of approximately 100 to 105 AKI (around 108-110 RON). This massive leap in octane rating means that E85 is incredibly resistant to knock.
For a turbocharged car, this is nothing short of a miracle. The high knock threshold of E85 allows tuners to do two critical things to generate more horsepower:
1. Run Higher Boost Pressures
Because E85 resists detonation so well, tuners can safely increase the boost pressure coming from the turbocharger. More boost means more air entering the cylinder, which, when matched with the appropriate amount of fuel, results in a much larger combustion event and significantly more power. On pump gas, pushing boost too high would quickly result in engine-destroying knock. With E85, the engine can swallow massive amounts of boost safely.2. Advance Ignition Timing
Ignition timing refers to the exact moment the spark plug fires relative to the piston's position. For maximum power, you want the spark plug to fire slightly before the piston reaches Top Dead Center (TDC), so that the expanding gases reach peak pressure just as the piston begins its downward stroke. If you advance the timing too much on standard gasoline, the heat and pressure of the compression stroke will cause the fuel to detonate prematurely. E85's knock resistance allows tuners to advance the ignition timing significantly, optimizing the combustion event and extracting more mechanical work from every drop of fuel.The combination of higher boost and advanced ignition timing is the primary reason why turbocharged cars pick up massive amounts of power simply by switching to E85 and flashing a new tune.
The Secret Weapon: Latent Heat of Vaporization
While the high octane rating of E85 is fantastic, it is only half of the story. The other, often underappreciated, reason why turbocharged cars love E85 is its massive "latent heat of vaporization."
When a liquid turns into a gas (vaporizes), it absorbs heat from its surroundings. This is the exact same physical principle that makes sweating cool down your body. Different liquids absorb different amounts of heat when they vaporize. The amount of heat energy required to vaporize a specific quantity of a liquid is its latent heat of vaporization.
Gasoline has a relatively low latent heat of vaporization (around 150 BTU/lb). Ethanol, however, has a latent heat of vaporization that is more than double that of gasoline (around 396 BTU/lb).
When the fuel injector sprays E85 into the intake manifold or directly into the cylinder, the ethanol rapidly vaporizes. Because of its massive latent heat of vaporization, the E85 literally sucks an enormous amount of heat out of the incoming air charge.
This creates an intense cooling effect on the intake air. In some high-horsepower applications, E85 can drop intake temperatures by 50 to 100 degrees Fahrenheit simply by vaporizing in the intake tract.
Why is this Cooling Effect Important for Turbos?
As we discussed earlier, compressing air with a turbocharger generates a lot of heat. Hot air is less dense, meaning it contains fewer oxygen molecules per given volume. Engine power is dictated by how much oxygen you can pack into the cylinder.By drastically cooling the intake charge, E85 makes the air much denser. Denser air means more oxygen molecules packed into the cylinder, allowing you to burn more fuel and make more power. Furthermore, cooling the combustion chamber internals (valves, pistons, cylinder walls) provides an additional layer of protection against knock, compounding the benefits of E85's high octane rating.
In essence, E85 acts like a chemical intercooler, dropping intake temperatures and allowing the engine to run safer and make more power, even under grueling track conditions or repeated back-to-back acceleration pulls.
The Downside: Stoichiometric Ratio and Fuel Volume Needs
If E85 is so incredible, why doesn't every car run it from the factory? The primary challenge of utilizing E85 lies in its energy density and its stoichiometric ratio.
The stoichiometric ratio is the ideal ratio of air to fuel required for complete combustion. * For standard unleaded gasoline, the stoichiometric ratio is 14.7:1 (14.7 parts air to 1 part fuel). * For E85, the stoichiometric ratio is approximately 9.76:1.
This means that to burn the same amount of air, you need significantly more E85 than you would gasoline. Furthermore, ethanol has less energy density per gallon than gasoline.
To achieve the same air-fuel mixture and extract the power benefits, an engine running on E85 requires roughly 30% to 40% more fuel volume to be injected into the cylinders compared to running on standard gasoline.
Fuel System Upgrades are Mandatory
For a naturally aspirated commuter car, the factory fuel system might be able to stretch to accommodate E85, but for a turbocharged car making elevated power levels, the factory fuel system is almost never adequate.To safely run E85 in a turbocharged application, you typically must upgrade several key fuel system components: 1. Fuel Injectors: You will need significantly larger fuel injectors capable of flowing 30-40% more fuel without maxing out their duty cycle. 2. Fuel Pump: The factory fuel pump in the gas tank will usually be unable to supply the sheer volume of fuel required. High-flow aftermarket fuel pumps are mandatory. In very high-horsepower builds, dual or even triple fuel pump setups are required. 3. Fuel Lines and Filters: Ethanol is a solvent and can be corrosive to certain older rubber fuel lines and components. While most modern cars (post-2000) have fuel lines that can withstand ethanol, older vehicles may need complete fuel line replacements with PTFE hoses. High-flow ethanol-compatible fuel filters are also necessary.
The Impact on Fuel Economy
Because the engine must consume 30% more fuel to run correctly on E85, your fuel economy (Miles Per Gallon, or MPG) will drop significantly. If a car gets 20 MPG on premium gasoline, it will likely drop to 14 or 15 MPG on E85. However, because E85 is often heavily subsidized and cheaper per gallon than premium 93 octane gasoline, the actual cost-per-mile is often quite similar, depending on local fuel prices.Flex-Fuel Systems: The Best of Both Worlds
In the early days of E85 tuning, drivers had to run a dedicated E85 ECU tune. This meant you could only put E85 in the tank. If you took a road trip and couldn't find an E85 station, you were out of luck and stranded, or you had to carry a laptop to manually flash the ECU back to a gasoline tune.
Today, the aftermarket and OEM tuning worlds have embraced Flex-Fuel technology.
A Flex-Fuel system utilizes a specialized ethanol content sensor spliced into the vehicle's fuel return or feed line. This sensor constantly measures the exact percentage of ethanol in the fuel in real-time (from 0% to 100%) and sends this data to the engine control unit (ECU).
A competent tuner will program a Flex-Fuel tune that interpolates between a base gasoline map and a maximum ethanol map.
If the sensor reads 10% ethanol (standard E10 pump gas), the ECU runs the standard, conservative timing and boost parameters. If you fill up with E85 and the sensor reads 80% ethanol, the ECU instantaneously and automatically increases boost, advances ignition timing, and adds fuel volume to maximize power.
This allows drivers to mix 93 octane and E85 in the tank in any ratio they choose, without ever needing to plug in a laptop or switch maps. It offers total freedom and peace of mind.
E85 vs. The Alternatives: Race Gas and Methanol Injection
Before E85 became widely available at commercial pumps, the only way to get this level of knock resistance was to buy expensive race fuels (like VP MS109, Q16, or C16) in 5-gallon pails. These fuels cost a fortune, are highly toxic, and usually contain lead, which destroys oxygen sensors and catalytic converters. E85 provides race-gas performance for a fraction of the cost, is completely unleaded, and is much better for the environment.
Another alternative used by turbo owners is Water-Methanol Injection (WMI). This involves installing a separate tank of a 50/50 water and methanol mix in the trunk, along with a separate pump and a nozzle that sprays the mixture into the intake charge pipe under high boost. While WMI provides excellent cooling and octane boosting, it adds significant complexity to the car. You have to constantly refill a separate tank, ensure the pump doesn't fail, and ensure the lines don't leak. If a WMI pump fails during a wide-open-throttle pull, the engine will likely detonate and blow up.
E85 is elegantly simple. It comes straight out of the gas station pump, goes right into the main fuel tank, and utilizes the car's existing fuel delivery infrastructure.
Real-World Gains: What Can E85 Do For Your Turbo Car?
The power gains seen by simply upgrading the fuel system and tuning a turbocharged car for E85 are staggering. In many modern turbocharged platforms, E85 provides the highest horsepower-per-dollar ratio of any modification available.
Here are a few real-world examples of popular turbocharged platforms and the gains they typically see when switching from premium 93 octane gasoline to an E85 tune (with basic bolt-ons like an intake and exhaust):
* Subaru WRX STI (EJ25 Engine): A stock-turbo STI running a standard Stage 2 tune on 93 octane will make around 280-300 wheel horsepower (WHP). By switching to larger injectors, a better fuel pump, and an E85 tune, that exact same car will routinely jump to 330-360 WHP. That is a gain of 50-60 WHP purely from the fuel. * Nissan R35 GT-R (VR38DETT): The GT-R is a monster from the factory, but it absolutely loves ethanol. An FBO (Full Bolt-On) GT-R on 93 octane will make around 550 WHP. Switching to E85 often pushes these cars over the 600 or even 650 WHP mark on stock turbochargers. * BMW M-Cars (S55 / S58 Engines): Modern BMWs like the M3 and M4 have incredibly capable engines. A simple ECU flash tune on 93 octane yields great results, but putting E85 in these cars unlocks absurd potential. It is common for BMW tuners to see gains of 80 to 100+ horsepower at the wheels just by switching to full E85 or a heavy ethanol blend (like E50). * Volkswagen / Audi (EA888 Gen 3 Engine): The 2.0L turbo engine found in the VW Golf R, GTI, and Audi S3 is a highly tunable platform. Upgrading the high-pressure fuel pump and running an E85 tune on these cars transforms them entirely, often resulting in massive mid-range torque spikes and top-end horsepower gains that embarrass much more expensive sports cars.
Preparing Your Car for E85: A Quick Checklist
If you are convinced that E85 is the right move for your turbocharged project car, here is a quick overview of what you need to do to safely make the transition:
1. Consult with a Professional Tuner: Before buying any parts, talk to an experienced tuner who specializes in your specific vehicle platform. They will guide you on the exact fuel system requirements for your power goals. 2. Upgrade the Fuel Injectors: Purchase high-quality, flow-matched injectors capable of handling the 30-40% increase in fuel volume required by E85. 3. Upgrade the Fuel Pump(s): Ensure your low-pressure and/or high-pressure fuel pumps can flow enough liters per hour (LPH) to keep up with the injectors under maximum boost. 4. Install a Flex-Fuel Kit: While not strictly mandatory, a Flex-Fuel sensor kit is highly recommended for convenience and safety. 5. Check Fuel Lines and Filters: Ensure your vehicle's fuel lines are ethanol compatible (most modern cars are) and consider upgrading your fuel filter to an E85-compatible, high-flow unit. 6. Get a Proper Tune: Do not attempt to run E85 without a proper ECU calibration. Your tuner will need to adjust the stoichiometric tables, injector scaling, boost targets, and ignition timing. 7. Monitor Your Oil: Ethanol can sometimes dilute engine oil if the car runs overly rich or sees frequent short trips where the engine doesn't reach optimal operating temperature. It is recommended to reduce your oil change intervals when running E85.
The Environmental Aspect of E85
Beyond the sheer performance benefits, E85 also has significant environmental implications. Ethanol is a renewable resource, unlike finite petroleum products. Because it is largely derived from crops, the plants absorb carbon dioxide from the atmosphere as they grow, offsetting some of the carbon dioxide released when the fuel is burned in the engine.
Furthermore, ethanol burns much cleaner than gasoline. Vehicles running on E85 produce significantly fewer particulate emissions, lower carbon monoxide, and fewer toxic pollutants like benzene. For tuners who remove catalytic converters for track use (though this is illegal for street use), running E85 dramatically reduces the noxious odors and soot typically associated with catless exhaust systems.
Conclusion: The Ultimate Tuner's Elixir
In the eternal pursuit of horsepower, engineers and enthusiasts are constantly fighting the laws of physics, heat, and pressure. For turbocharged applications, engine knock is the absolute limiting factor for performance.
E85 fuel has leveled the playing field, offering everyday enthusiasts access to a fuel that possesses the knock resistance of expensive race gas and the cooling properties of chemical injection systems, all available at the corner gas station for a few dollars a gallon.
By drastically raising the knock threshold through its 105+ octane rating, and by acting as a powerful liquid intercooler via its massive latent heat of vaporization, E85 allows turbocharged engines to breathe deeper, push harder, and survive conditions that would instantly destroy an engine running on standard premium gasoline.
While it does require a commitment to upgrading the vehicle's fuel delivery system and accepting a reduction in miles per gallon, the benefits far outweigh the drawbacks. For anyone looking to maximize the potential, reliability, and sheer thrill of a turbocharged vehicle, E85 is, without a doubt, the ultimate elixir. It is no wonder that the automotive tuning world has fallen entirely in love with the corn.