Why E85 Requires More Fuel
E85 (85% ethanol, 15% gasoline) has become increasingly popular in performance applications due to its high octane rating and cooling properties. However, it comes with a significant trade-off: E85 contains less energy per gallon than gasoline.
The stoichiometric air-fuel ratio (AFR) tells the story:
Gasoline
14.7:1 AFR14.7 parts air to 1 part fuel
E85
9.8:1 AFR9.8 parts air to 1 part fuel
This means E85 requires approximately 30% more fuel by volume to achieve the same power output as gasoline. Your fuel system must be capable of delivering this increased flow.
The 30% Rule
The simplest way to estimate E85 injector requirements is the 30% rule:
E85 Injector Size = Gasoline Injector Size × 1.3 Add 30% to your gasoline injector requirement
Example Calculations
| Gasoline Requirement | E85 Requirement | Common Upgrade |
|---|---|---|
| 440cc | 572cc | 600cc or 650cc |
| 550cc | 715cc | 750cc or 800cc |
| 750cc | 975cc | 1000cc |
| 1000cc | 1300cc | 1300cc or 1500cc |
When your calculated E85 requirement falls between common injector sizes, always choose the larger option. The small cost difference is worth the added safety margin.
BSFC Values for E85
If you're using the full injector sizing formula, you'll need to adjust your BSFC (Brake Specific Fuel Consumption) value for E85:
| Engine Type | Gasoline BSFC | E85 BSFC |
|---|---|---|
| Naturally Aspirated | 0.45 - 0.50 | 0.60 - 0.68 |
| Turbocharged | 0.55 - 0.65 | 0.75 - 0.85 |
| Supercharged | 0.55 - 0.65 | 0.75 - 0.85 |
Using the correct BSFC in your calculations automatically accounts for E85's higher fuel consumption.
Flex Fuel Considerations
Many enthusiasts want the flexibility to run both gasoline and E85. This "flex fuel" capability requires careful planning:
Size for E85, Tune for Both
Your injectors must be large enough to support full power on E85. When running gasoline, the ECU simply uses shorter pulse widths. Modern ECUs handle this transition smoothly.
Flex Fuel Sensor
A flex fuel sensor (ethanol content sensor) measures the actual ethanol percentage in your fuel and reports it to the ECU. This allows automatic adjustment of:
- Fuel injector pulse width
- Ignition timing (E85 tolerates more advance)
- Boost levels (if applicable)
- Target AFR values
Idle Quality Concerns
Injectors sized for E85 will be oversized when running gasoline. This can cause:
- Slightly rougher idle on gasoline
- Reduced low-load fuel economy
- More demanding tune requirements
Quality injectors with good low-flow linearity minimize these issues. Cheap injectors struggle with the dynamic range required for flex fuel.
Real E85 vs Pump E85
Here's a critical consideration many overlook: pump "E85" isn't always 85% ethanol.
Pump E85 can range from 51% to 85% ethanol depending on season, region, and supplier. Winter blends often contain more gasoline to aid cold starting.
This variability means:
- A flex fuel sensor is highly recommended, not optional
- Your tune must accommodate the full range of ethanol content
- Size injectors assuming true E85 (85% ethanol) for safety
Fuel System Upgrades Beyond Injectors
Injectors are just one part of the equation. E85's higher flow requirements affect the entire fuel system:
⛽ Fuel Pump
Must flow 30% more volume. A 255 lph pump on gasoline needs to be upgraded to 340+ lph for E85 at similar power levels.
🔧 Fuel Lines
E85 is more corrosive than gasoline. Ensure all fuel lines, fittings, and seals are ethanol-compatible (check for E85/flex fuel rating).
⚙️ Fuel Pressure Regulator
Must maintain consistent pressure at higher flow rates. Adjustable regulators allow fine-tuning for E85's requirements.
🛢️ Fuel Tank
Some older plastic tanks degrade with E85. Most modern tanks are compatible, but verify before switching.
Benefits of E85
Despite the fuel system requirements, E85 offers significant advantages:
High Octane Rating
E85 has an effective octane rating of 100-105, allowing more aggressive timing and higher boost without knock.
Charge Cooling
Ethanol's high latent heat of vaporization cools the intake charge significantly, reducing intake temps and increasing density.
More Power Potential
The combination of timing, boost, and cooling advantages typically yields 5-15% more power than gasoline on the same setup.
Often Cheaper
In many areas, E85 costs significantly less per gallon than premium gasoline, partially offsetting the higher consumption.
Quick Sizing Reference
Approximate injector sizes for E85 applications (turbocharged, 80% duty cycle):
| Target HP | 4-Cylinder | 6-Cylinder | 8-Cylinder |
|---|---|---|---|
| 300 HP | 750cc | 500cc | 375cc |
| 400 HP | 1000cc | 650cc | 500cc |
| 500 HP | 1300cc | 850cc | 625cc |
| 600 HP | 1500cc | 1000cc | 750cc |
| 800 HP | 2000cc | 1300cc | 1000cc |
Calculate Your E85 Requirements
Use our injector calculator with E85-specific BSFC values to find the right size for your build.
⛽ Open Injector Calculator