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Flex Fuel


What is flex fuel?


Flex fuel has been around for a while now.  It is basically the concept of a vehicle being able to run on blend of gasoline to blends and ethanol (E85).  If you don't know what E85 is, click over to the E85 page and read up so that flex fuel will make more sense.  Car manufacturers wanted to support E85 but since E85 isn't readily available everywhere, they had to setup the vehicles to be able to detect the amount of ethanol and adjust the tuning correctly.  This is known as being a flex fuel vehicle (FFV).  Initially, most companies used a physical ethanol content sensor.  As fuel passes through this sensor, it actually measures the amount of gasoline, despite being called an "ethanol" content sensor.  It assumes the rest to be ethanol, which will generally be true unless water has found its way into the blend somehow.  This information about ethanol content is passed to the ECU and it adjusts the tuning accordingly to deliver fuel correctly and adjust any other engine operating parameters the manufacturer has deemed necessary.  Some manufacturers have switched to a model of "virtual" content sensors.  All modern cars have oxygen (o2) sensors and the capability to adjust fueling in both a short term and long term manner.  The virtual content sensor model leans on this concept heavily.  Ethanol requires more volume than gasoline to get the same burn at a given load.  If the ECU is having to do heavy short term fuel correction, this becomes heavy long term correction and if the trend continues for a long enough period of time, the ECU uses this to indicate the ethanol content of a given blend and adjusts tuning accordingly.


Flex fuel lets your car safely run any ratio of gasoline and ethanol safely.  The tune adjusts on the fly.



How does this apply to a non Flex Fuel car?


Many non-flex vehicles have the ability to run E85.  This is accomplished through a combination of upgraded parts and tuning changes.  Ethanol requires more volume of fuel, so at the least, upgraded fuel injectors and fuel pump are needed on any Subaru with port fuel injection.  The tuning accounts for E85 by telling the ECU to inject the extra fuel needed and by adjusting various tables to optimize things for E85.  The problem is that the ECU has no concept of ethanol content.  It doesn't even know its running E85; the tuning has tricked it in a way.  Monitoring the air to fuel ratio (AFR) at high loads with a wideband gauge can give a clue as to the ethanol content and at least keep a user informed as to whether or not the AFR is in a good, intended range.  Beyond that, a user has to check ethanol blends manually, using an ethanol content sensor and gauge or just trust the pump and watch the wideband.  Ethanol blends change with the seasons in most areas.  The reason is simple, ethanol itself doesn't ignite well in cold weather with cold cylinder temps.  It relies on the gasoline mixed in to get things going.  15% gas doesn't start very well and manufacturers found 30% works much better so most regions blend down to 70% ethanol for Winter.  Warmer regions can be an exception to this.  These varying blends can have an affect on the air to fuel ratio depending how the car has been tuned.  Ethanol has a more forgiving AFR range but it is still possible to slip outside this optimal range from the lowest blend to the highest blend available.  Along with the issue of changing blends, there's the inherent issue of not being able to simply pump in some gas if needed like a normal FFV. If a person wants or needs to run gasoline, they have to run the tank down very low on E85, fill up all the way on gasoline and then switch maps, hoping the tune works well enough with the gas blended with the E85 that is left in the tank. Most people choose to drive easy on that first tank and stay out of boost and do a 2nd fill up on gas before running the car hard. The same can apply going from gas back to E85.


We can now upgrade the fuel system to take advantage of E85 but do it more effectively using flex fuel.



How can we have Flex Fuel on a Subaru?


Adding flex fuel support to a non FFV is nothing new and now it is finally going to be possible on many turbo Subarus thanks to a combination of CarBerry flex fuel (for 02-05 WRX models using Open Source tuning) and Cobb AccessPort (AP) Flex fuel (for 06+ WRX / 04+ STi using a Cobb AccessPort V3).  Adding flex fuel support to a non-FFV is a lot of work.  It requires completely reverse engineering the OEM logic inside an ECU and hi-jacking into it.  You have to introduce the concept of ethanol content and manage it throughout the tune.  That is no small task.  A lot of new logic and supporting tables have to be added to support this.  With all this in place, the tune can be setup to handle ethanol content.  From there it can adjust fueling, timing, boost and all the other important tables that are needed to handle different blends.  On a gasoline blend, the car can make safe power and as the ethanol blend goes higher towards 70-85% ethanol, the car will make more power until its making it is peak power on the higher ethanol blends.  When tuned correctly, it will be seamless to user.  There will be no need to switch maps or check ethanol blends. The Cobb AccessPort will show the blend of ethanol that is in the tank and the flex fuel tune will run the show.



What kind of hardware is needed to support Flex Fuel?


The ECU needs a linear 0-5 volt input that represents ethanol content.  An ethanol content sensor itself doesn't actually output a 0-5v signal so it is not quite as easy as simply plugging that into the ECU.  Something is needed to go between the content sensor and ECU to convert the content sensor output into the needed 0-5v input for the ECU.  Some people call this an Ethanol Content Analyizer (ECA).  There are several options on the market and some are even plug and play.  It is best to consider an ECA that outputs either a custom linear voltage range or a specific linear range that is inside of 0 to 5 volts, meaning something such as .5 volts to 4.5 volts where .5 represents 0% ethanol and 4.5 represents 100% ethanol.  The important reason behind this is simple; fault detection.  You can't have a safe flex fuel system if you don't account for situations where the ever-so-important ethanol content signal is lost.  The flex fuel tune can react to a situation of voltage going outside the normal range.  Cobb will even throw a custom check engine light to alert the user.  If the ECA operates on a fixed scale where 0 volts means 0% ethanol and 5 volts means 100% ethanol, it doesn't lend itself to as accurate of fault detection.  In reality, unless a person wishes to frequently run no-ethanol fuel or 100% ethanol fuel, a 0-5 volt ECA can have its fault detection limits set very close to 0 and 5 volts and still work fine.  Let's be honest, most people with flex fuel will want to keep as much ethanol in the tank as they can at most times and rarely does anyone run 100% ethanol since it is not something found in any pump out there.  This is good news for early adopters with a Zeitronix ECA installed already, since it outputs on a fixed 0-5v scale.  



Questions and answers:



Q: What about the older Cobb V2 AccesPort?


A: Unfortunately due to hardware limiations, these older AccessPorts are not supported.  Cobb offers a great trade in program to trade in your V2 for a half off a new V3.  Take adnvantage of that or sell your V2 and pick-up a V3 tuning package on the Shop page.  You can get more information on Cobb's trade it program here:

Q: What about Cobb AP and the 02-05 WRX?


A: Flex fuel requires custom code and tables to be added to the rom file that flashes to the ECU.  The 02-05 WRX has a very small rom file compared to newer ECUs and there simply isn't the room to fix everything in there.  The CarBerry rom available via open source tuning is the answer for flex fuel on the 02-05 WRX.  You will just need to supply the ECU with some kind of ethanol input from something such as an Innovate or AEM Ethanol gauge.


Q: What about the other turbo Subaru models such as the Legacy GT and Forester XT?


A: There just aren't as many non-Impreza turbo Subarus in the market that are going to be interested in Flex fuel.  Don't get me wrong, I LOVE all turbo Subarus and want to support them.  On Cobb's part, it is a massive amount of effort to create the custom flex fuel logic and tables.  Subaru ECUs vary from model to model and year to year.  Factor in Auto & Manual transmissions ECUs and it is a lot of work.  The good news is, Cobb did not say "no" to extending support.  They just want to hear from people who are interested so they can gauge the level of overall interest.  Send them an email, give them a call.  Speak up!


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