Simply put, flex fuel is a term given to an add-on to the fuel system that allows your car to properly and safely run gasoline and ethanol mixes and automatically adjust between the two. Ethanol is a great way to safely add power to the car. You can find a massive increase in performance by simply adjusting the ignition timing, target air fuel, and boost levels on the car. Ethanol will also burn cooler and cleaner than regular gas which is an added benefit...especially for boosted cars. When you hear terms like e30, e50, and e85, that’s telling you a percentage of ethanol in your gas. E30 means 30% ethanol/70% gas and E85 is 85% ethanol/15% gas. Without a flex fuel kit, you are guessing how much ethanol your car is getting. Being that ethanol has a different energy density than regular gas, your engine will consume around 30% more fuel at any given horsepower over gasoline on a full mix of e85. Not knowing exactly how much ethanol is in your car can be disastrous and can cause an engine failure.
The flex fuel kit has the ability (once tuned) to read the ethanol content percentage in your fuel and automatically adjust the tune. As the ethanol content in the tank changes, the flex fuel sensor will see this and start adjusting fuel, timing, and boost levels all independently of each other however your tuner finds best. The tune will change almost linearly anywhere in between the regular gas up to your full ethanol content. Your tuner will first tune the car on regular gas (which is anywhere from 0-12% ethanol) and then finish on the full amount of ethanol your car will run. The amount of ethanol your car can use will depend on your fuel system. A stock fuel system cannot support full e85 as it will not have the ability to flow enough to keep the engine from running too lean. This is why having the proper fuel mods are an absolute must.
The only downside for ethanol is the potential for direct injector failures. It doesn’t matter which brand of direct injector(DI) you run. They are all prone to the exact same failures for the exact same reason. This is why I always over stress making sure your engine and working area are beyond extremely clean when changing any part of the fuel system on these cars. There are many theories as to why injectors seem more prone to failure. In my opinion, based on my tuning experience with these cars, these injectors fail because of the dirt and debris getting stuck in them. I think AMS did an extensive study where they said it only takes a piece of dirt 30 microns in size (correct me if I’m wrong) to lock up an injector. That is the size of a piece of dust. This is also why I always recommend a fuel filter as a must when running a flex fuel kit. If your car has some miles on it and the fuel quality the car got most of its life was average or poor, you will get some build up in the tank and lines over time. The ethanol will not only act like a detergent, being that it is mildly corrosive, but it is also hygroscopic. That means it can break up any of the junk built up in the tank and lines and send them right to the injectors as well as having a propensity to absorb moisture. This is why you shouldn’t leave the same tank of mixed fuel in the car for long periods of time and it’s usually a good idea to run a tank of regular gas through every once in a while.