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Technical Brief on E85 Options for Mustang Owners

The Mustang community has long been a supporter of burning E85 ethanol-based fuel and was an early adopter of this potent “race gas at the pump”. When managed properly running E85 in the Mustang produces more horsepower and torque while reducing emissions and cooling operating temperatures. However, Mustangs are not E85 ready from the factory and, as such, a Mustang owner who wishes to run E85 or achieve flex fuel capability must make modifications to the car before using this powerful fuel.

To be clear, E85 is different from flex fuel. E85 is the name given to fuel blends with high ethanol content, typically defined as over 50% ethanol. Flex fuel, often used incorrectly as an alternative name for E85 fuel, is defined as the ability to switch fuels from gasoline to ethanol-based fuels on demand.

Running E85 produces some challenges for Mustang owners. The first is that E85 at the pump is inconsistent in content and can be anywhere from 51-83% ethanol. The second is that E85 is not always readily available which means drivers need to be able to use gasoline when E85 is not convenient. Third, running a tune for E85 that does not match the fuel blend can be harmful to the engine and reduce performance. Because of these hurdles some confusion exists in the Mustang market concerning the safest and most effective way to run E85 ethanol-based fuels.

Mustang owners have several options for running E85:

E85 Fixed Map Tune (E85 Tune)

Early adopters of E85 turned to a method of tuning called running an “E85 tune”. An E85 tune simply fixes fuel delivery at a higher percentage than gasoline, usually between 28 and 30%. This is the least sophisticated and least desirable method for running E85 and is antiquated.

Because pump E85 is never actually 85% ethanol, running an E85 tune means that the only fuel the engine is compatible with is race grade E85 which is guaranteed to contain 85% ethanol. An E85 tune does not allow the owner to run gasoline and marries the car to a barrel of race fuel. The E85 tune is only appropriate for dedicated race cars that always run 85% ethanol fuels.

Switch Map Tuning

To solve the problems of a fixed map E85 tune the tuning industry developed switch map tuning. This requires the car owner to use a handheld tuning tool to switch from an E85 fixed map tune to a gas tune or vice versa when switching fuels. The problem is, when switching fuels, the car will never have either pure E85 or pure gasoline in the tank which means neither tune ever matches the fuel content and consequently, never calls for the right amount of fuel. Combine this with variations of fuel content at the pump and the lack of an accurate way to measure content in real time and a switch map quickly becomes a problem.

For instance, if the car pulls up to the pump with a couple of gallons of gas still in the tank and tops off with E85 the resulting blend may be something like 62% ethanol. Neither the gas tune or the E85 tune options are appropriate for this resulting fuel blend.

When running the E85 tune with less than 85% ethanol content the fuel system is over injecting fuel, meaning wet or unburned fuel will be present in the cylinder. Wet fuel in the cylinder washes the cylinder walls, removing vital lubrication and causing significant wear on the sealing rings and possibly scoring the cylinder walls. This expensive damage happens over time and is hard to detect until it is too late. Because this condition happens over time the cause is often overlooked or misdiagnosed.

Alternatively, running the gas tune with over 20% ethanol content will cause a lean burn condition that can quickly burn holes in the pistons.

Barring engine damage, when the tune does not match the fuel blend the car will lose horsepower and will often start and run poorly.

For street cars and cars that run pump E85 a switch map tune is damaging to the engine, reduces performance and should not be considered.

Ford Flex Tune

A slightly more sophisticated way of running E85 in your Mustang is the Ford flex tune, or wide band system. The flex tune uses historical data from the oxygen sensors to adjust for rich and lean conditions and slowly adjusts the fuel delivery to attempt to correct these conditions. This system does provide the convenience of being able to run either E85 or gasoline when desired but there are significant drawbacks.

Primarily a flex tune is slow. A flex tune does not actually measure fuel content and suffers from a condition called latency. Latency is defined as the delay between when you switch fuels and when you achieve proper fuel delivery. The ECU can only make incremental changes to correct a rich or lean condition, which means sampling oxygen sensor data over many miles of driving. As described previously, running rich or lean during the latency gap can do catastrophic damage to the engine, especially under load.

Additionally, because a wide band system uses historical data to make its adjustments the system is always looking at what happened a few minutes ago, not what is happening now, so it is likely to be inaccurate most of the time.

Mustang owners running the flex tune have switched from gas to E85 on the way to the track and burned their pistons on race day because the ECU has not been given the proper amount of run time to cross the latency gap. Unfortunately, there is no indication that the computer has made the proper adjustments when driving the car, so the owner is always flying blind and cannot reliably determine if it is safe to run at wide open throttle (WOT) when switching fuels.

Over time, switching fuels with the flex tune wide band system will invariably cause engine damage because of the latency gap and lack of a feedback loop.

Real Time Adaptive Flex Fuel

The safest and most effective method for running E85 is called a real time adaptive flex fuel system. In a real time adaptive system, the fuel content is accurately measured by a sensor that feeds real time content data to a computer that adjusts fuel delivery instantaneously.

Real time adaptive flex fuel has no latency gap and always delivers the right amount of fuel for the exact blend at the injector. This allows the car owner to switch fuels as often as desired with no delay in using the car for ultimate performance.

Adaptive flex fuel provides the best possible performance results for any given fuel blend because it maximizes horsepower at all percentages of ethanol from 0% (pure gasoline) to 100% ethanol.

Real time adaptive flex fuel is also the safest method for running E85. Unlike an E85 tune, switch map tune or wide band flex tune, fuel delivery is always accurate which eliminates the possibility of a damaging rich or lean condition.

Another advantage of real time adaptive flex fuel systems is compatibility with gas tunes and bolt-ons. Because these systems operate after the ECU they are compatible with all gas tuning and performance upgrades and instantly adapt to changes in performance parameters.

Advanced Fuel Dynamics developed and patented real time adaptive flex fuel and produces systems for most Mustang, GT350 and GT500 models. These plug and play ProFlex Commander systems can be installed in about 45 minutes and give the car full flex fuel capability on all stock, performance upgraded and tuned cars.


The Mustang marketplace has come to rely on the power and performance of running E85 ethanol-based fuels. While E85 tunes, switch map tunes and the Ford flex tune are available to provide compatibility they all have significant limitations and the likelihood of short-term or long-term internal engine damage.

The most technologically sound way to run E85 in your Mustang is real time adaptive flex fuel system from Advanced Fuel Dynamics, the leader in flex fuel technology for Mustangs. The ProFlex Commander real time adaptive flex fuel system from AFD provides the safest option for your engine and maximizes performance for all blends of gasoline and ethanol. With the freedom to tune and bolt-on power adders and easy installation it is clear that Advanced Fuel Dynamics has finally made running E85 a safe, reliable high-performance option for Mustang owners looking for the right E85 solution.


To order Advanced Fuel Dynamic’s ProFlex Commander for your Mustang, click here. Use Discount Code MUSTANG20 at checkout to save 20% on your order!