The FA20 Engine and E85: Unlocking Significant Horsepower

The FA20 engine, found in the Subaru BRZ, Toyota 86, and Scion FR-S, is a 2.0-liter naturally aspirated boxer that has earned a reputation for its high-revving character and efficiency. While it provides a solid foundation for sports car driving, enthusiasts quickly seek more power. Among the most effective modifications is converting the FA20 to run on E85 fuel, a blend of 85% ethanol and 15% gasoline. This conversion can yield up to 40 additional horsepower with proper tuning, transforming the driving experience. But achieving these gains requires understanding the fuel, the necessary hardware, and the tuning strategy. This article expands on the original overview to provide a comprehensive guide for anyone considering this popular upgrade.

What Makes E85 Different?

E85 fuel offers two primary advantages over standard pump gasoline: a significantly higher octane rating and the cooling effect of ethanol. The octane rating of E85 typically ranges from 100 to 105 (R+M/2 method), compared to 91 or 93 for premium unleaded. This higher octane rating allows the engine to run more aggressive ignition timing and, in forced-induction setups, higher boost pressure without encountering knock (detonation). Additionally, ethanol's latent heat of vaporization is much higher than gasoline's. As the fuel vaporizes in the intake port and cylinder, it cools the air-fuel mixture, reducing intake air temperatures and further suppressing knock. This cooling effect also increases charge density, contributing to power gains.

Octane Rating Explained

Octane rating measures a fuel's resistance to auto-ignition. A higher octane fuel can withstand higher compression ratios and advanced ignition timing without detonating. The FA20 stock compression ratio is 12.5:1, which is relatively high for a naturally aspirated engine. With E85, tuners can advance ignition timing significantly beyond what is safe with pump gasoline, extracting more work from each combustion cycle. This is the primary source of the 30-40 horsepower gain reported by many owners.

Performance Gains: Up to 40 HP at the Wheels

Real-world dyno results from reputable tuners consistently show gains of 30-40 horsepower at the wheels on a naturally aspirated FA20 when switching from 93-octane pump gas to E85. These gains are achieved without any changes to the engine hardware other than fuel system upgrades. The torque curve also improves, with a noticeable flattening and increase in the mid-range. A typical before-and-after dyno run on a stock FA20 might show 170-175 whp on 93 octane and 205-210 whp on E85, with torque climbing from 140 lb-ft to 165 lb-ft. The gains are even more pronounced on turbocharged FA20 builds, where the high octane allows for substantial boost increases.

Typical Dyno Results (Naturally Aspirated FA20)

  • Stock on 93 Octane: ~170-175 whp, ~140 lb-ft torque
  • E85 with Tune (stock injectors/fuel pump): ~195-200 whp, ~155 lb-ft torque
  • E85 with Upgraded Injectors & Fuel Pump: ~205-210 whp, ~165 lb-ft torque

Note: Peak gains depend on the specific tune, ambient conditions, and the quality of the E85. Many owners report a 15-20% increase in power.

Required Hardware for a Reliable E85 Conversion

Simply pouring E85 into the tank and uploading a new tune is not sufficient. The FA20's fuel system must be upgraded to deliver the extra volume of ethanol required. E85 has a lower energy density than gasoline (about 30% less), so the engine needs to inject roughly 30-40% more fuel by volume to achieve the correct air-fuel ratio. This can overwhelm the stock fuel system, leading to lean conditions and potential engine damage.

Fuel Injectors

Stock FA20 injectors flow approximately 260 cc/min at the factory fuel pressure. For E85, most tuners recommend upgrading to injectors that flow at least 400-500 cc/min. Popular options include injectors from the Subaru WRX/STI (which are a direct fit) or aftermarket injectors from brands like Injector Dynamics or DeatschWerks. Upgraded injectors ensure adequate fuel delivery at high RPM and high load.

Fuel Pump

The stock in-tank fuel pump can also struggle with E85, especially if the ethanol content varies. Ethanol is more corrosive to fuel system components designed for gasoline, and the pump may wear faster. A drop-in replacement high-flow fuel pump, such as a Walbro 255 LPH or AEM 340 LPH, is recommended. These pumps provide the flow margin needed for E85 and are more resistant to ethanol's effects.

A flex fuel sensor allows the ECU to automatically detect the ethanol content in the tank and adjust the fuel and ignition maps accordingly. This is especially useful when E85 availability is inconsistent, and the driver may sometimes fill with regular gasoline. A flex fuel kit (sensor, wiring harness, and tuning support) provides the convenience of running any blend from 0% to 85% ethanol. Without it, the car must be tuned strictly for E85, and running gasoline on that tune would cause a dangerously rich condition.

Tuning Considerations

The tune is the most critical element of an E85 conversion. The factory ECU must be reprogrammed via a standalone ECU (like a Motec or Haltech) or through an Accessport with custom calibration from a qualified tuner. The tuner will adjust the following parameters:

  • Fuel tables: Increased injector pulse width and adjustments to the fuel pressure regulator target (if applicable) to match the required lambda.
  • Ignition timing: Up to 6-10 degrees of additional advance compared to gasoline, dialed in to find the maximum torque without knock.
  • Cold start enrichment: Ethanol is harder to vaporize when the engine is cold, so extra fuel and timing adjustments are needed for reliable starts in cold weather.
  • Warm-up strategies: The ECU must account for ethanol's different combustion characteristics during warm-up.

A good tuner will perform multiple dyno pulls and road logs to ensure the air-fuel ratios are safe (typically lambda 0.85-0.88 for maximum power on E85) and that no knock is present.

Supporting Modifications for Maximum Gains

To fully exploit the potential of E85, consider these complementary modifications:

  • Exhaust system: A full cat-back or header-back exhaust reduces backpressure, allowing the engine to breathe more freely. Gains of 5-10 hp on top of the E85 tune are common.
  • Cold air intake: A high-flow intake can improve throttle response and add a few horsepower, especially combined with the denser E85 charge.
  • Lightweight flywheel: Reduces rotational inertia, improving acceleration and throttle response, complementing the power gains.

Real-World User Experiences

Online forums and Facebook groups for the 86/BRZ/FR-S community are filled with positive reports about E85 conversions. Common comments include:

  • "The car feels alive; the throttle response is instant, and it pulls hard past 7000 rpm."
  • "I was skeptical about spending the money, but the seat-of-the-pants difference is dramatic. It's how the car should have come from the factory."
  • "Even with a mild tune (just injectors and pump), the gains are very noticeable. It transforms the car on track."

However, some users note the inconvenience of reduced fuel economy and limited availability of E85 in rural areas.

Potential Drawbacks and Considerations

Despite the impressive gains, E85 conversion comes with trade-offs:

  • Fuel economy: Expect a 20-30% decrease in miles per gallon. The engine consumes more fuel volume to produce the same energy output.
  • Availability: E85 is common in the Midwest and parts of the South, but scarce in the Northeast, Pacific Northwest, and many rural areas. Check E85prices.com for station locators.
  • Cold start issues: In temperatures below freezing, pure E85 can be very difficult to start. Many flex-fuel tunes allow the car to run on a gasoline blend or require a starting strategy that cycles the fuel pump multiple times.
  • Fuel system corrosion: Ethanol attracts water and can degrade rubber components, fuel lines, and O-rings designed for gasoline. Upgraded components should be ethanol-rated.
  • Oil dilution: In some driving conditions, unburned ethanol can seep past the piston rings into the oil, reducing its lubricity. More frequent oil changes (every 3,000-4,000 miles) are recommended.

Cold Start Solutions

For owners in colder climates, installing a flex fuel sensor and tuning for a minimum ethanol blend of 50% (E50) during winter can improve starting. Some tuners also increase the ignition timing during cranking and extend the cranking time on the ECU. Another option is to keep a small amount of gasoline in the tank to lower the ethanol content.

Cost Breakdown

The cost of an E85 conversion for a naturally aspirated FA20 varies:

  • ECU tuning (custom dyno tune or Accessport + e-tune): $700 – $1,500
  • Fuel injectors (upgraded): $300 – $600
  • Fuel pump (high-flow): $100 – $200
  • Flex fuel kit (optional): $200 – $400
  • Labor (if not DIY): $300 – $600

Total estimated cost: $1,400 – $2,800. While not trivial, the gains of 30-40 hp make it one of the best cost-per-horsepower modifications for the FA20. For comparison, a supercharger kit costs $4,000-$6,000 and yields 60-80 hp, but E85 provides a natural, reliable increase without adding forced induction complexity.

Comparison with Other Fuels

How does E85 compare to other high-octane fuels or additives?

Fuel Type Octane (R+M/2) Typical WHP Gain (FA20) Cost per Tank (approx)
93 Octane 93 Baseline $45
100 Octane (Race Gas) 100 +10-15 hp $120
E85 100-105 +30-40 hp $30
Water/Methanol Injection N/A (additive) +15-25 hp (with tune) $15-20 (per fill of meth tank)

E85 offers the best blend of performance and cost among fuels, though race gas provides convenience in areas without E85. Methanol injection can be used to supplement pump gas but adds complexity and maintenance.

Conclusion

The FA20 engine responds exceptionally well to an E85 conversion, delivering up to 40 additional horsepower and a much more responsive driving experience. By upgrading the fuel system (injectors and pump) and investing in a custom tune, enthusiasts can unlock the engine's latent potential without resorting to forced induction. The drawbacks—reduced fuel economy, cold start issues, and limited availability—should be weighed carefully against the performance gains. For those who have access to E85 and are willing to manage the trade-offs, this conversion is one of the most satisfying and cost-effective modifications for the Subaru BRZ, Toyota 86, or Scion FR-S. For further reading, consult tuning resources on FT86 Club or professional tuning shops like Delicious Tuning and SubiSpeed. Always work with an experienced tuner to ensure engine reliability and safety.