Understanding E85 Fuel and Its Unique Properties

E85 is a blend of 85% denatured ethanol and 15% unleaded gasoline by volume. While the exact ratio can vary seasonally (to aid cold starts), the high ethanol content gives it fundamentally different combustion characteristics compared to standard pump gas. Ethanol's chemical structure – C₂H₅OH – contains oxygen, which changes how the fuel burns inside a combustion chamber. For turbocharged engines, these differences translate into measurable performance advantages.

The most impactful property of E85 is its high octane rating. Typical E85 has an octane rating between 100 and 105 (Research Octane Number, RON), whereas premium pump gas is usually 91-93 RON. Higher octane allows the fuel to resist detonation (knock) under high cylinder pressures and temperatures – exactly the conditions found in a boosted engine running at elevated boost levels. This knock resistance is the primary enabler of the power gains associated with E85.

Another critical property is ethanol's latent heat of vaporization. Ethanol absorbs a large amount of heat as it changes from liquid to vapor inside the intake port and cylinder. This charge cooling effect drops the intake air temperature, increasing air density and reducing the tendency to knock. For turbocharged engines already fighting heat soak and high intake temps, this cooling is a significant advantage.

Ethanol also contains oxygen by weight, meaning that a given volume of E85 carries its own oxidizer. This allows a richer air-fuel mixture (stoichiometric ratio for E85 is about 9.8:1 compared to 14.7:1 for gasoline) while still maintaining combustion stability. The combination of high octane, charge cooling, and oxygen content means a turbocharged engine can run significantly more boost and advanced ignition timing before reaching the knock limit.

  • Octane rating: 100-105 RON vs 91-93 RON for premium gasoline
  • Latent heat of vaporization: ~840 kJ/kg vs ~380 kJ/kg for gasoline
  • Stoichiometric AFR: ~9.8:1 vs ~14.7:1
  • Energy density: ~33.7 MJ/L vs ~46.5 MJ/L (lower energy per volume)

Why Turbocharged Engines Benefit from E85

Turbochargers force more air into the engine, raising cylinder pressures and temperatures. This environment pushes gasoline to its knock limit quickly – forcing tuners to dial back boost, retard timing, or enrich the mixture beyond optimal. E85 changes this calculus entirely.

Infinite Knock Resistance

On a properly sized fuel system, a turbocharged engine running E85 can often run as much boost as the turbo and mechanical components can physically tolerate without knocking. The effective octane of E85 increases as ethanol content rises, and the charge cooling further suppresses detonation. Many tuners report being able to run 25-35 psi of boost on a pump-gas-limited platform after switching to E85, with no knock whatsoever.

Charge Cooling and Air Density

The evaporative cooling of ethanol drops intake charge temperatures by 20-50°F compared to gasoline at the same ambient conditions. Cooler air is denser, containing more oxygen molecules per unit volume. This directly translates into increased power potential because the engine can burn more fuel with that extra oxygen. Additionally, lower intake temps reduce the load on intercoolers and help keep overall engine temperatures in check during sustained pulls.

Ability to Run Aggressive Ignition Timing

Because E85 is highly resistant to knock, tuners can advance ignition timing closer to the maximum brake torque (MBT) point. On many turbocharged engines, running gasoline requires retarding timing by 5-10 degrees from MBT to avoid detonation. With E85, timing can be set to optimal values, increasing cylinder pressure and extracting more work from every combustion event.

Oxygen Content and Enrichment

The oxygen in ethanol allows the engine to run richer mixtures (lower AFR) without misfiring. This enrichment helps cool the exhaust gas temperatures (EGTs) that would otherwise climb dangerously high under boost. Lower EGTs protect the turbocharger and reduce the risk of pre-ignition in the exhaust manifold. It also allows the engine to run higher boost levels without melting pistons or burning exhaust valves.

The AEM E85 FlexFuel Kit: How It Works

The AEM E85 FlexFuel Kit is designed to give modern turbocharged vehicles the ability to run any blend of gasoline and ethanol (E0 to E85) without needing to re-tune manually when fuel composition changes. The core of the system is an ethanol content sensor that measures the actual percentage of ethanol in the fuel line in real time.

The sensor works by measuring the dielectric constant of the fuel (which changes with ethanol content). It outputs a digital signal (typically a CAN bus or frequency-based signal) that is read by the engine management system (EMS). In conjunction with a fuel temperature sensor built into the unit, the ECU can then adjust injection timing, fuel pulse width, ignition timing, and boost targets seamlessly based on the detected ethanol concentration.

Key Components of the Kit

  • Ethanol content sensor: Typically a Continental (formerly Siemens) unit, capable of detecting 0-100% ethanol with ±2% accuracy
  • Wiring harness: Includes connectors and pins for integrating with various ECUs (standalone, factory reflash, piggyback)
  • Fuel temperature sensor: Integrated into the sensor for density compensation
  • Mounting bracket and hardware: For secure installation in the fuel line

Compatibility with Different ECUs

The AEM FlexFuel kit is not a standalone tuning device; it requires a compatible ECU that can interpret the sensor signal and adjust fuel and ignition maps accordingly. Many modern aftermarket ECUs (Haltech, MoTeC, AEM Infinity, Link, etc.) support ethanol content input natively. Additionally, many factory ECUs can be reflashed or used with a secondary controller (like a standalone fuel controller) to enable flex-fuel functionality. The kit outputs a 0-5V analog signal or CAN data depending on the specific model, making it broadly compatible.

For vehicles that retain a stock ECU but are tuned via reflash (e.g., Cobb Accessport, HP Tuners, Euro-spec flashing), the FlexFuel kit provides a signal that the aftermarket calibration can use to switch between gasoline and ethanol maps automatically. This flexibility means you can fill up with E85 when it's available, and run standard premium gasoline without any recalibration – ideal for daily drivers that travel between regions with varying ethanol availability.

Installation and Fuel System Considerations

Installing the AEM FlexFuel kit is straightforward, but it does require mechanical aptitude and careful planning of the fuel system. The sensor must be placed in the fuel line between the tank and the engine, typically after the fuel filter. It needs to be exposed to a steady flow of fuel – not in a dead-end or return line that might contain stagnant fuel.

Fuel System Upgrades for E85

While the FlexFuel kit makes it easy to switch fuels, the rest of the fuel system must be capable of delivering the significantly higher volume of fuel required by E85. Because E85 contains less energy per unit volume than gasoline (about 33% less), the engine needs roughly 30-40% more fuel volume to achieve the same air-fuel ratio for a given air mass. This means that even a stock turbocharged engine may require upgrades to support E85 at high boost levels.

  • Fuel pump: A high-flow, E85-compatible pump (such as a Walbro 450 LPH or AEM 320 LPH) is often needed. Stock pumps may not maintain pressure at higher flow rates and can fail due to ethanol's corrosive properties.
  • Fuel injectors: Larger injectors (typically 30-50% more flow than stock) are required. High-impedance injectors designed for ethanol are recommended to avoid stiction issues.
  • Fuel lines: Rubber hoses and seals degrade quickly with ethanol. Stainless steel braided lines (PTFE-lined) or ethanol-compatible nylon lines should be used.
  • Fuel filter: A finer mesh filter (10 micron) may be needed because E85 can carry contaminants from old fuel stations.

Real-World Performance Gains: The 60 HP Claim

One of the most compelling reasons to adopt E85 is the dramatic horsepower gains observed in turbocharged applications. The claim of 60 horsepower (crank or wheel) is realistic for many turbocharged cars with proper tuning and supporting modifications. To understand how this figure is achieved, it helps to look at a typical dyno comparison.

Example Scenario: A Modern Turbocharged 4-Cylinder Engine

Consider a 2.0L turbo engine in a 2015-2020 vehicle (e.g., Subaru WRX, Ford Focus ST, Volkswagen GTI) running 93-octane pump gas at 20 psi boost. A conservative tune might yield 280-300 whp. By switching to E85 (with the FlexFuel kit and larger injectors/fuel pump) and re-tuning to take advantage of the fuel's knock resistance, the same engine can run 25-26 psi boost and more aggressive timing. The result is typically an increase of 50-70 whp – exactly the 60 hp range. Some enthusiasts on forums like IWSTI.com or FocusST.org have documented gains as high as 90 whp on built engines with larger turbos.

Factors That Influence the Gain

  • Baseline tune: If the gasoline tune was already conservative (leaving power on the table for safety), the gain from E85 will be larger. If the gasoline tune was aggressive and knock-limited, the gain might be smaller but still significant.
  • Boost ceiling: The maximum boost the turbo can physically produce is a hard limit. On a small turbo, you might only pick up 40 hp because you can't push more airflow; on a larger turbo, 100+ hp gains are common.
  • Supporting modifications: Upgraded intercooler, downpipe, intake, and exhaust all compound the benefit. A car with a free-flowing exhaust and larger intercooler can take better advantage of the increased fueling capability.
  • Fuel system adequacy: If the injectors or pump are maxed out on E85, the gains plateau. The 60 hp figure assumes the fuel system is sufficient to deliver the required volume at the higher boost level.

Practical Considerations and Downsides

While the power potential is exciting, running E85 daily introduces some practical trade-offs that owners need to weigh.

Fuel Economy

Because ethanol has lower energy density, fuel economy drops by about 15-30% depending on driving style and blend. A car that normally gets 25 mpg on gasoline may return 18-20 mpg on E85. However, this is partially offset by the lower cost per gallon of E85 in many areas. The price per mile (not per gallon) is often comparable or slightly higher, but the extra power is usually worth it for enthusiasts.

Availability

E85 is common in the Midwest and parts of the South but can be scarce in the Northeast, Pacific Northwest, and mountain states. Apps like the E85 station finder or the Department of Energy's Alternative Fueling Station Locator help plan trips. The flex-fuel capability of the AEM kit ensures you can always fall back to gasoline when E85 isn't available.

Cold Start Issues

Ethanol is harder to vaporize at low temperatures. At ambient temperatures below about 40°F, engines running straight E85 can have difficulty starting. Most flex-fuel systems handle this by compensating with longer cranking times and richer mixtures. The AEM FlexFuel kit helps by allowing the ECU to adjust for ethanol content, so the cold-start strategy can be tuned accordingly. Many tuners reduce the acceptable ethanol content to E70 in winter blends to improve cold starts.

Oil Contamination

Direct-injected turbo engines that run E85 may experience fuel dilution of engine oil. Ethanol absorbs moisture from the air (it is hygroscopic) and can pass by piston rings into the crankcase, especially under high-load conditions. This can accelerate oil degradation and require more frequent oil changes (every 3,000-5,000 miles instead of 7,500-10,000). Using a high-quality oil designed for ethanol (such as a PAO or ester-based synthetic) helps mitigate this issue.

Material Compatibility

Older fuel systems with rubber or Viton seals, plastic fuel pumps with ethanol-sensitive cores, and non-coated fuel tanks can be damaged by ethanol's solvency properties. Modern vehicles (post-2000) generally have ethanol-compatible components, but some fuel pumps and fuel sending units may still fail prematurely. The AEM kit itself uses ethanol-resistant materials, but the rest of the system should be inspected.

Conclusion: Is E85 Right for Your Turbocharged Vehicle?

The impact of E85 on turbocharged engines is clear: it offers a cost-effective path to significant power gains – often 60 horsepower or more – by leveraging the fuel's high octane, cooling properties, and oxygen content. The AEM E85 FlexFuel Kit makes the transition practical for daily-driven vehicles by automating fuel blend detection and allowing seamless switching between gasoline and ethanol.

However, the 60 hp gains quoted by many users are only achievable with a solid foundation: adequate fuel system capacity, a competent tuner, and supporting hardware like a larger intercooler and exhaust. The trade-offs in fuel economy, availability, and maintenance must be considered. For enthusiasts who tune their cars and are willing to manage these factors, the AEM FlexFuel kit is an excellent investment that unlocks the true potential of a turbocharged engine without sacrificing daily drivability.

As emissions regulations tighten and alternative fuels become more common, understanding how E85 interacts with forced induction will become increasingly valuable. Whether you are building a track-focused rocket or a weekend street toy, the combination of a turbocharger and E85 remains one of the most effective ways to extract horsepower from a four- or six-cylinder engine.