E85 & Flex Fuel on Mercedes-Benz — Pros, Cons & What You Need to Know

Running E85 or flex fuel in your Mercedes-Benz can unlock serious power potential — but it’s not as simple as pumping and going. There are trade-offs, requirements, and platform-specific nuances you need to understand before jumping in.

⚙️ What Is E85?

E85 is a blend of up to 85% ethanol and 15% gasoline. It's a high-octane fuel (typically 105+) that burns cooler and allows for more aggressive ignition timing and boost targets — making it ideal for performance builds.

✅ Pros of E85

• Higher Octane: Reduces knock and allows for more boost and timing in tuned setups.
• Cooler Combustion: Lowers intake temps and helps prevent heat soak under hard driving.
• Power Gains: On turbocharged Mercedes models, E85 can yield 30–70+ horsepower with proper tuning.

❌ Cons of E85

• Lower Fuel Economy: Ethanol contains less energy per gallon than gasoline. Expect 20–30% worse MPG.
• Requires More Fuel: Larger injectors and a higher-capacity fuel pump may be needed to maintain safe AFRs.
• Cold Start Issues: Ethanol is harder to ignite in cold weather. Some ECUs struggle unless tuned correctly.
• Corrosive Properties: Ethanol attracts water and can degrade rubber lines and seals if the system isn’t compatible.
• Inconsistent Ethanol Content: Pump E85 can vary from E60 to E85 depending on the station and region. Without a flex fuel sensor, this can lead to:
• Lean Conditions: Too little ethanol for a full E85 tune can cause detonation and engine damage.
• Over-Enrichment: Too much ethanol when running on a lower-ethanol tune can foul plugs and cause rough operation.
• Fuel Trim Instability: Inconsistency leads to surging, hard starts, and poor drivability without adaptive tuning.
• Unstable Tuning: Fixed ethanol tunes won’t adapt — fluctuations in blend make performance unpredictable and unsafe.

🔧 What You Need to Run E85 Safely

• Flex Fuel Sensor: Detects ethanol content so the ECU can adjust fuel and timing.
• Compatible Fuel Pump: A higher-flow pump like a Walbro 450/525 or equivalent is often required.
• Larger Injectors: E85 needs ~30% more fuel volume. Factory injectors may not keep up at higher power levels.
• Proper ECU Tuning: You need a tune designed for E85 or flex fuel blending. Off-the-shelf tunes won’t cut it.

💡 Not All Mercedes Platforms Are Equal

Some platforms, like the M276 and M177, handle E85 conversions easily. Others, like the M156 or older NA motors, either don’t benefit much or require more extensive upgrades. Always consult with a platform-experienced tuner.

🛠️ E85 Conversion Kits Available

BenzUnited Parts offers full E85 conversion kits designed specifically for modern Mercedes platforms. These kits include injectors, pumps, ethanol sensors, and plug-and-play harnesses where available.

🚀 Final Thoughts

E85 can unlock significant power gains and reduce heat soak on forced induction setups — but it’s not a one-size-fits-all solution. Know your platform, install the right supporting mods, and get a reliable flex-fuel tune to keep things safe.

BenzUnited Parts is here to help you build your Mercedes the right way — with performance, reliability, and efficiency in mind.