Can I run 100LL AvGas in my 2-Stroke Outboard?

Optimizing High-Performance 2-Stroke Outboard Engines with Aviation Fuel 100LL (AvGas)

For high-performance 2-stroke outboard engines used in race boats, bass boats, and offshore applications, achieving the right fuel octane rating is critical for maximizing performance, reliability, and combustion efficiency.

While 100LL AvGas (100 Low Lead Aviation Fuel) is primarily designed for turbocharged aircraft piston engines, its properties make it an effective and affordable alternative for boosting octane levels in 2-stroke marine engines.

100LL AvGas delivers high-octane performance comparable to many premium racing fuels. It has key characteristics that make it well-suited for 2-stroke outboard applications, including a high AKI rating of 105, a consistent distillation curve, and a lead content similar to traditional racing fuels.

Mercury Hi Performance recommended this fuel for the Mod VP 2.4 Liters and others back in the late 1980s and early 1990s.

This allows for better lubricity and clean combustion. Its specific gravity, comparable to high-octane racing fuels, ensures proper air-fuel ratios and optimized performance.

100LL AvGas can be run directly in your engine or blended with lower-octane pump gas to achieve the desired octane level, making it a cost-effective solution for performance applications.

To use AvGas effectively, it’s important to understand octane ratings and how they are measured in the U.S.A. versus Europe. In the U.S.A., octane is measured as AKI (Anti-Knock Index), which is the average of RON (Research Octane Number) and MON (Motor Octane Number).

The formula is (RON + MON) / 2 = AKI, and the number displayed at gas pumps reflects this value (e.g., 87, 91, 93). In Europe, however, the octane rating at the pump represents only the RON value. For reference, EU 95 RON equals U.S. 91 AKI, and EU 98 RON equals U.S. 93 AKI.

Blending 100LL AvGas with pump gas is a common practice to reach the ideal octane level for 2-stroke engines. For example, 100LL AvGas has an octane rating of 105 AKI, while premium pump gas in the U.S. is typically 91 AKI.

Using a 50/50 blend of AvGas and pump gas results in (105 + 91) / 2 = 98 AKI. This blended fuel provides a safe, high-octane solution for performance outboards, reducing the risk of detonation (knock) at high RPMs and under heavy loads.

Integrating AvGas into your fuel system delivers significant performance benefits. The higher octane rating prevents pre-ignition and detonation, ensuring reliable operation under higher compression and more advanced timing. AvGas burns consistently, which reduces carbon buildup and promotes cleaner combustion.

The lead content enhances lubrication for critical engine components such as rings, pistons, and cylinder walls, extending engine life. Compared to specialized racing fuels, 100LL AvGas is more affordable and readily available, making it a practical alternative for race boats, bass boats, and offshore applications.

Here are the key specifications of 100LL AvGas for 2-stroke outboard engines:

• Octane Rating (AKI): 105 AKI

• Lead Content: 0.56 g/L (Low Lead)

• Specific Gravity: 0.72–0.74 at 60°F (15.6°C)

• Distillation Range: 158°F to 338°F

• Fuel Compatibility: Blends with pump gas, ethanol-free preferred

• Storage Stability: Excellent, with a longer shelf life than pump gas

While 100LL AvGas offers excellent benefits, there are a few considerations. The lead content, while beneficial for older engines, can cause spark plug fouling over time, so regular maintenance and plug checks are necessary.

AvGas also has excellent shelf life compared to pump gas but requires proper storage in sealed containers to avoid contamination. Avoid blending AvGas with ethanol-containing fuels (like E10 or E15), as ethanol can cause phase separation and fuel system damage if not burned immediately.

In conclusion, I do prefer true race fuels for particular racing applications but for most performance boaters, tournament anglers, and racing enthusiasts, 100LL AvGas is an good solution for achieving high octane and consistent fuel delivery in 2-stroke outboard engines.

Its 105 AKI rating, stable burn properties, and lubrication benefits make it a reliable alternative to traditional racing fuels. Blending 100LL with pump gas provides a practical way to fine-tune octane levels and reduce the risk of detonation, ensuring peak performance at high RPMs and under heavy loads.

By understanding octane ratings, proper blending methods, and fuel compatibility, you can maximize your engine’s power, durability, and reliability on the water. Upgrade your fuel game with 100LL AvGas and keep your high-performance 2-stroke engine running clean, powerful, and race-ready.

Finally, running AvGas may require minor carburetor, and timing adjustments to optimize air-fuel ratios for clean, efficient combustion.

General minimum octane guidance for the Mercury 2.5 Liter at 25 degrees timing advance is estimated at 93 AKI octane for a motor under 140psi compression, 95 AKI octane at 140-155 psi, 96 AKI at 160psi, 97 AKI octane at 170psi. Talk with your motor builder and share your magic numbers! If you are not sure, it's best/safest to run a higher AKI octane rating vs sorry.

You might be able to find a local airport to buy 100LL fuel. Be sure to bring your 5-gallon jugs or some may allow you to pull in your boat. https://www.airnav.com/fuel/local.html

Note:

The transition to blue 100LL (100 low-lead) aviation gasoline (avgas) in the United States happened around 1975. This change was prompted by environmental and operational concerns with the older green 100/130 avgas, which had higher levels of tetraethyl lead (TEL).

The blue dye in 100LL was introduced to differentiate it from other fuels and to indicate its reduced lead content compared to the older green avgas.