Posted: July 3, 2015


E85 is the shorthand way of saying 85% ethanol fuel and 15% gasoline or other hydrocarbon fuel. This fuel is typically made by distilling corn sugar into alcohol then blending it with gasoline to the desired ratio which can actually be anywhere from 53% to 83% ethanol according to government regulations.

There is some chemistry there that we won’t go far into but basically, gasoline burns at a stoichiometric (complete combustion reaction) ratio of 14.7:1 with oxygen (it takes 14.7 oxygen molecules to completely burn one gasoline molecule) whereas e85 burns at a rate of 9.85:1 with oxygen.

The difference would result in a 67% ratio of gasoline to e85 except our gasoline is now actually e10 which has a stoich ratio of 14.08:1 for a ratio of 70%. This basically means it takes 30% more e85 than premium gas to burn the same amount of oxygen which should result in 30% less fuel economy. The simple math, if mileage is 30mpg on gasoline, after switching to e85 mileage should decrease to 20mpg. We’ll have more on that later.

In order to convert the 2015 WRX to ethanol-based fuels one must do the following steps:

  • Drain the tank of all gasoline; do this by driving or using a pump. Unfortunately there is no drain on the bottom of the tank to make this super easy like older Subaru vehicles. We can’t all be perfect!

  • Fill tank with ethanol; through the fill neck, just like pump gas.

  • Tune vehicle for e85; this can be as simple as changing one table in the tune file although many tables must be changed for optimal results and maximum horsepower.

  • Drive!

Yes, it’s that simple. No need for injector changes, fuel pump changes, expensive flex kits, and the like. The reason for this is the vehicle's ability to step up pressure in the direct injection system to compensate for the additional fuel demand. By raising pressure the volume flow rate is increased.


When discussing the switch to ethanol fuels the first thing out of most people’s mouths is “but that fuel economy though.” Well, this is the first time I think I’ve ever been able to proudly answer that question with “who cares.” First off, the fuel economy gauge in the 2015 WRX is woefully inaccurate.

Hopefully soon COBB will release scalar factors for it so we can rein it back in but at the moment it’s extremely liberal. I guess if the gauge says 34mpg it must be true! (but it’s not) Actually, at one point we observed 34.5mpg sustained on the gauge while running e85, clearly a miscalculation.

Even on gasoline and a completely stock tune, it likes to show 32-33mpg when if the math is done results show closer to 28mpg.

E85 Ethanol Fuel: 23.33 mpg measured (not observed) on the 349 mile trip to Spokane, WA from Portland, OR. Please note, this was done on ONE tank of fuel and while we actually ran out at our destination of Spokane County Raceway, we did make it. The actual measured ethanol content in this test was 83% ethanol which seems to be pretty average for Jay’s Garage in Portland where we source our ethanol.

92 Octane (E10 Unleaded Gasoline): 28mpg highway rated, 28.5 measured (not observed) on the 349-mile trip home from Spokane, WA to Portland, OR.

The surprising thing is that we only saw a decrease of 18.3% in fuel economy while running on e85. Mathematically the fuel consumption for e85 fuel should be much higher, upwards of 30% more in fact. There are two primary factors that caused this variance in expected vs. measured fuel economy, direct injection, and high compression. These factors allow the engine to run extremely efficiently at low rpm making more torque for the same given air mass coming into the engine.

This extra torque means the engine doesn’t have to work as hard to maintain engine RPM, so at 75mph in 6th gear, the actual engine load is significantly lower on the ethanol fuel than the gasoline fuel. Given the cost of e85 being typically 15-20% lower than premium unleaded the cost to run e85 in this car is basically a wash and with the added benefits, it seems like e85 is definitely the way to go.

Too bad the bio-fuel infrastructure in the Portland area (really the whole Pacific Northwest) is basically non-existent making availability the only real hurdle here to overcome.

Ethanol isn’t the answer for all our nations’ fuel problems but it’s a no-brainer that we need to reduce our dependency on foreign oils. With more and more vehicles coming to market with direct injection, most having significantly higher compression, I really hope support for ethanol grows and new engineered bio-fuels show their face in the years to come.


Now for the section everyone actually cares about, the POOOOOOWWWWWAAAAA! Ethanol fuels contain a few properties that make them absolutely insane for making big power numbers.

Knock Resistance – The first of these properties is octane also known as knock resistance. This property is basically the fuel’s resistance to combustion which actually makes it burn slower. A slower burn means we get to start the burn sooner before the combustion becomes uncontrolled. This advance in ignition timing allows for serious power gains even at the same boost levels as gasoline.

Cool Combustion – Along with this slower burn we have really cool (literally) thermal characteristics of all alcohols, they burn very cold. Colder burn means lower engine temperatures and lowers exhaust gas temperatures. This cooling effect is very important on the 2015 WRX due to the high intake temperatures it tends to see due to its poor intercooler design. This is another reason the engine became so much more efficient on the switch to e85, even sustained highway driving saw lower manifold inlet temperatures by about 8 degrees on two days with the same ambient temperature.

Evaporative Cooling – While in boost this cooling effect is amplified because the ethanol cools the charge similar to an intercooler through an evaporative cooling effect. The basic laws of thermodynamics state that colder air is a denser air (or charge as we call it), denser charge means more oxygen molecules to burn at the same boost level which means more horsepower! See where we’re going here?

These cooling effects along with the increased octane allow us to run more boost than would be safe on gasoline. Turning the boost up results in more oxygen molecules in the air charge, the more air forced into the engine to combust, the more fuel we can add, the more power we can put to the wheels. After turning up the boost, remapping the ignition timing, and adjusting the air-fuel ratios we ended up with a grand total of 274.1 wheel horsepower and 282 ft-lb of torque!

That’s a peak gain of 43.5 horsepower and 25ftlb of torque by changing nothing but the fuel and adjusting the tune. More important is the overall gains under the graph which can be seen in the picture below. Torque is considerably higher throughout the entire pull which equates to seriously faster acceleration. How much faster? We dropped almost an entire second in the quarter-mile!


This graph shows the 2015 WRX in stock trim with the factory tune on pump gas in red and a PREracing custom COBB Accessport tune on e85 in blue. Substantial horsepower and torque gains can be seen throughout the entire powerband and power delivery is much more smooth.

Fastest quarter mile pass on e85 : 12.989 @ 106.25 mph.

The only change was the addition of e85 fuel and a custom tune. Interestingly other passes “soft launching” had mph up as high as 107.28. So far on e85, we haven’t been able to run a true “clean” pass with a good hard launch because of the next weakest link in the vehicle, the rear end.

The fastest time we have run bogged pretty hard off the line. With a good hard launch and ethanol, we’re confident this vehicle has a mid-12-second pass in it. Stay tuned for that!

Overall this 2015 WRX is a completely different vehicle when fed with e85. Throttle response is crisper, acceleration is significantly improved, and the whole feel of the car is improved. We really like the way everything came together and hope to really show off the capabilities of the platform in the near future.


With all racing comes the risk of broken parts and as we have turned up the power we’ve already started finding weak links. The first serious issue we’ve run into is a result of the hard launches.

Both times we’ve been to the dragstrip and launched the car with ethanol in the tank we came home with a broken rear end. Both times the spider gears in the differential shattered during a launch causing catastrophic differential failure.


The car is just too heavy with too much torque for the tiny Subaru R160 rear differential. The STI models have an upgraded R180 differential and beefier axles to help with this issue but unfortunately the WRX was not so lucky. We have looked into different options and think we will probably go with the Quaife differential out of England. This will remove the vehicle from its “physically stock” status but it’s getting tiresome replacing these rear ends. Hopefully the Quaife will hold together and we can get a few hard launches on the all stock engine components to really show what this new direct injected corn fed beast can do.


These rear-end issues seem to be isolated to drag launches as to be expected and fortunately there is a new form of racing that’s gaining momentum throughout the country, roll racing. This is where the race is started while already moving at a set mph, usually 30-40.

This reduces drivetrain stress but still provides the exhilaration of racing head to head down a drag strip or other track. We had the great opportunity to sponsor such an event at the Spokane County Raceway in Spokane Washington.

From a roll this little WRX did not disappoint and while usually outgunned it held its own against quite a few opponents with a whole lot more horsepower. Trapping as high as 121.43 on the roll the WRX performed admirably.

Few people could believe it was a completely stock powertrain with just a tune and it definitely earned some respect. We can’t wait to be part of future roll events as we watch this new aspect of the sport grow.

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