Katerina Swanson
Flex fuel vehicles are designed to run on a range of fuel types, including regular unleaded gasoline and ethanol-enriched fuels. The term flex fuel refers to the vehicle's ability to use multiple fuel types rather than a specific type of fuel. Octane ratings, which indicate a fuel's resistance to detonation, vary across fuel types. For example, E85 has an octane rating above 93, while regular unleaded gasoline is typically rated at 87 or 91. When choosing the right fuel for a flex-fuel vehicle, it's important to consider factors such as fuel economy, engine design, and cost. While premium fuels may offer higher octane ratings, they may not provide significant benefits for all vehicles, and in some cases, mixing fuel types may be necessary to achieve the desired octane rating and fuel efficiency.
Can I use 93 octane in a flex fuel car?
| Characteristics | Values |
|---|---|
| Octane rating of E85 | Above 93 |
| Octane rating of E15 | 88 |
| Octane rating of regular fuel | 87 or 89 |
| Octane rating of premium fuel | 91 or 93 |
| Flex fuel | Contains more than 10% ethanol |
| Flex fuel | Can take ethanol-enriched fuels and regular fuel |
| E85 | Burns cleaner than pure gasoline |
| E85 | 30% less efficient than regular gas |
| E85 | More expensive per mile |
| Premium fuel | More expensive |
| Premium fuel | May have higher levels of detergents |
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What You'll Learn
Mixing E85 and 93 octane
It is important to note that the heat generated by combusting a gallon of E10 gasoline is higher than that of a gallon of pure ethanol, so more fuel would need to be delivered to the engine when using a blend. Additionally, moving to a higher concentration of ethanol, such as E85, does not always generate a significant jump in octane.
Some car owners choose to mix E85 and 93 octane fuel to gain significant horsepower without the need for a tune. For example, a 2024 Mustang GT owner mixes 6 gallons of E85 with 10 gallons of 93 octane per tankful, resulting in a blend that is under E50. This blend is said to provide more horsepower while remaining within the limits of what the ECU can accommodate without generating an error code.
It is worth noting that some car owners have expressed concerns about the potential negative consequences of mixing E85 and 93 octane. One user mentions that adding too much E85 can cause AFRs to lean out and potentially damage the fuel pump. However, another user reports adding 1-1.5 gallons of E85 to their 2019 WRX with no issues, as long as the amount of E85 does not exceed 3 gallons.
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Octane ratings and engine performance
The octane number is the average of two different octane rating methods: Motor Octane Rating (MOR) and Research Octane Rating (RON). Octane ratings are measures of fuel stability, specifically the pressure at which a fuel will spontaneously combust (auto-ignite) in a testing engine. The higher the octane number, the more stable the fuel.
Higher-compression gasoline engines use fuels with higher octane ratings, which may yield higher power. The added power comes from the way the engine is designed to compress the air/fuel mixture, and not directly from the rating of the gasoline. Gasoline engines rely on the ignition of a compressed air and fuel mixture, which is ignited only near the end of the compression stroke by electric spark plugs. Being able to compress the air/fuel mixture without causing detonation is important mainly for gasoline engines.
Using gasoline with lower octane than an engine is built for may cause engine knocking and/or pre-ignition. Knock occurs when a secondary flame wave forms from instability and then travels against the path of the primary flame wave, thus depriving the power stroke of its uniformity and causing issues including power loss and heat buildup. The higher the octane, the higher the activation energy, or the amount of applied energy required to initiate combustion.
In recent years, car manufacturers have been requiring or recommending premium gasoline (a high-octane grade of fuel) for use in more of their vehicle models. Higher octane fuels are often required or recommended for engines that use a higher compression ratio and/or use supercharging or turbocharging to force more air into the engine. Increasing pressure in the cylinder allows an engine to extract more mechanical energy from a given air/fuel mixture but requires higher octane fuel to keep the mixture from pre-detonating. In these engines, high octane fuel will improve performance and fuel economy. Using a lower octane fuel than required can cause the engine to run poorly and can damage the engine and emissions control system over time. It may also void your warranty.
In the Rocky Mountain (high elevation) states, 85 AKI (90 RON) is the minimum octane, and 91 AKI (95 RON) is the maximum octane available in fuel. The reason for this is that in higher-elevation areas, a typical naturally aspirated engine draws in less air mass per cycle because of the reduced density of the atmosphere. This directly translates to less fuel and reduced absolute compression in the cylinder, therefore deterring knock. It is safe to fill a carbureted car that normally takes 87 AKI fuel at sea level with 85 AKI fuel in the mountains, but at sea level, the fuel may cause damage to the engine.
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Flex fuel vs regular fuel
Flex-fuel vehicles are capable of running on more than one type of fuel. They can use anything from 100% gasoline to blends containing up to 85% ethanol. The percentage of ethanol in flex fuel depends on the geographical location and time of the year. Producers add more ethanol in the summer than in the winter.
E85 gas, also known as flex fuel, is a combination of 51% to 83% ethanol blended with gasoline. Ethanol is derived from several plant materials, including corn, sugar cane, or various grasses. It is a good alternative to purchasing foreign oil. Regular gasoline sold in the U.S. usually contains up to 10% ethanol.
Flex-fuel vehicles have a few modifications to the engine and fuel system but are otherwise identical to gasoline-only models. Modern flex-fuel cars have electronic sensors that gauge the blend of fuel and adjust the fuel injection and timing accordingly.
One of the main concerns about driving a flex-fuel car is its gas mileage. Ethanol has about one-third less energy than pure gasoline, so drivers will notice a decrease in miles per gallon when using E85 instead of regular gasoline. Despite the lower miles per gallon, flex-fuel vehicles often put out more torque and more horsepower when running on E85.
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Cost savings of different fuel types
The cost savings of different fuel types depend on a variety of factors, including the type of vehicle, annual mileage, and fuel prices. While there isn't a specific type of fuel designed to save the driver money, choosing the most suitable fuel type for your vehicle can lead to cost savings.
For example, diesel vehicles often have higher fuel efficiency than standard gasoline vehicles, but diesel fuel is typically more expensive. On the other hand, plug-in electric vehicles can achieve high fuel efficiency and benefit from relatively inexpensive electricity, resulting in significant fuel cost savings over time, despite the higher upfront vehicle cost.
The octane rating of fuel, such as 91, 93, 95, or 98, also plays a role in cost savings. Higher octane fuels are designed to maximise the efficiency and power of different engine types. For instance, a turbo vehicle will benefit from higher octane fuel due to better combustion. However, using a higher octane fuel than recommended may not provide noticeable performance gains or cost savings for everyday driving.
Additionally, ethanol-based fuels like E10 and E85 offer more environmentally friendly options. E10, for example, might be slightly cheaper, but using it in a car recommended for unleaded 91 could lead to lower fuel economy, offsetting any potential cost savings.
To determine the most cost-effective fuel type for your specific situation, it is essential to consider factors such as your car's engine, your driving habits, and the availability and prices of different fuel types in your area. Online calculators can help compare the costs of different vehicles and fuel types, taking into account factors like EPA's MPG data, driving conditions, and vehicle prices.
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Ethanol content in flex fuel
Flex fuel, also known as E85, is a blend of ethanol and gasoline. The ethanol content in E85 can vary between 51% and 85%, with the remaining percentage consisting of gasoline. The specific percentage of ethanol in E85 depends on factors such as the season, geographical location, and distributor. For example, during colder months, E85 often contains lower levels of ethanol to facilitate easier engine starting in cold temperatures. The higher ethanol content in E85 results in a decrease in miles per gallon compared to regular gasoline. However, E85 has higher octane content, which can provide more engine power.
Flex-fuel vehicles (FFVs) are designed to use both regular gasoline and ethanol-gasoline blends like E85. These vehicles have sensors that detect the gasoline-ethanol ratio in the fuel and adjust for optimal performance. FFVs are marked with a "Flex-Fuel," "FFV," or "E85" badge or a yellow gas cap to indicate their compatibility with E85 fuel.
While flex-fuel vehicles can use any octane level of gasoline, it is important to note that E85 fuel is not suitable for traditional gas engines. Putting E85 in a car not designed for flexible fuel can cause damage. Therefore, it is essential to refer to the owner's manual to determine the appropriate fuel for a specific vehicle.
The use of E85 in flex-fuel vehicles offers benefits such as reduced greenhouse gas emissions and lower reliance on foreign oil. Additionally, E85 is often priced lower than regular unleaded gasoline, making it a cost-effective option. However, the availability of E85 may vary across regions, with a higher concentration of E85 stations in the Midwest due to the abundance of corn, a primary ingredient in ethanol production.
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Frequently asked questions
Yes, you can use 93 octane in a flex-fuel car. Flex-fuel cars can take ethanol-enriched fuels as well as regular fuel.
Examples of ethanol-enriched fuels include E85, E15, and regular 87.
There is no benefit to putting premium fuel in a flex-fuel car unless it is designed to take what is commonly regarded as premium (91 to 93 octane).
