Driving Sports

All across the United States, at the time of this writing, the cost of premium unleaded gasoline, the lifeblood of our high-performance engines, is approaching the $5 mark. Even the most resolved performance enthusiasts now at least consider for a moment the costs of fuel when the total tab for a fill-up approaches $60.

Not everyone gets the same performance for their money. Despite uniformly high fuel prices across the board, some regions get 93 or 94 octane fuel while other, less fortunate enthusiasts in California and the desert Southwest are bridled by poor-performing, lower-octane premium fuels. Higher octane means the engine can be tuned to a greater efficiency and power production because higher octane fuels are, by definition, more detonation resistant. Greater efficiency not only translates into better performance but also better fuel economy, as the engine is able to generate more mechanical force with optimized ignition timing and fueling strategies.

Turbocharged engines are particularly sensitive to overall fuel quality and detonation resistance. For example, an optimally-tuned Subaru STI running 94 octane fuel from Ohio will be able to generate 20 more wheel horsepower and torque than its mechanical twin located in the Los Angeles area, where 91 octane is the highest octane pump fuel available. Furthermore, the STI tuned to run on 91 octane will run a richer fuel mixture than the car fueled with Midwest 94 octane. The difference in "on boost" air-fuel ratio (AFR) can require as much as seven percent additional fuel to run safely on 91 octane.

High-Octane Alternatives: E85 and Methanol Injection
The most common strategy used to offset lower octane fuels is a combination of methanol and water injection. In a previous issue of Subiesport ("Tuning for Water and Methanol Injection", September 2007, p. 86) we reviewed in detail the tuning techniques, advantages and risks associated with water or water and methanol injection. However, there is a new fuel on the market in some parts of the country that now provides a very good alternative to methanol injection and has the advantage of actually lowering overall fuel costs. Properly calibrating a car to run on either methanol injection or E85 requires some form of aftermarket engine management as well as custom tuning on a dyno, so either approach requires a similar initial investment.

E85: Low-Cost, High-Octane
This new fuel is called E85. E85 is a blend of 15 percent gasoline and 85 percent ethanol. Yes, the same ethanol that you enjoy in alcoholic beverages is a viable fuel. Just like a gin and tonic, ethanol packs a punch in the form of a comparatively high octane rating of about 105. To top it all off, E85 is less expensive compared to premium unleaded fuels. According to the California Energy Commission, the average price of E85 in June of 2008 was $3.29, while the average price of premium unleaded was $4.29 per gallon. This represents nearly a 25 percent difference in price. Based upon this information alone you may be asking yourself, "Where can I get E85?" There are, however, many factors that mitigate the pure economic value of this fuel alternative.

Despite having a higher octane rating, E85 has a lower overall energy density than pure pump fuel. Pure gasoline contains approximately 125,000 BTUs per gallon, while E85 contains approximately 84,000. This means that a greater volume of E85 must be utilized to realize the same energy content. This lower hydrocarbon content is mirrored by the stoichiometry of E85 and pure unleaded gasoline. Complete combustion of pure gasoline requires 14.7 parts air (mixed gas air with 23 percent oxygen) with one part fuel. In contrast, complete combustion of E85 requires a much higher relative content of fuel with only 9.7 parts air required. The distillation of this information means that larger volumes of E85 are required for equivalent combustion when compared to pure pump fuel. Thus, the cost savings associated with lower E85 prices is offset by an overall reduction in fuel economy.

Calibrations Compared for Premium Unleaded, Methanol/Water Injection, and E85 in a 2006 Subaru STI.
You can read about E85 anywhere. However, direct experience on a Subaru platform not only is the better story, but it provides more valuable information. We convinced the owner of the same 2006 Subaru STI featured in our earlier tuning exercise with water and methanol injection to volunteer his car once again to test with E85. For the best comparisons, we tuned the car on pump fuel, methanol and water plus pump fuel, and E85 during the same evening. This particular car is equipped with the stock Vf39 turbocharger, upgraded Walbro 255 fuel pump, Perrin Performance front mount intercooler, and larger 850cc Deatschwerks fuel injectors.

Critical to properly tuning this car for different fuels is the addition of an AccessPORT from Cobb Tuning. This device allows the owner to switch from one calibration for pump fuel to a totally different calibration with ignition timing, boost control and fuel parameters optimized for a specific fuel. AccessPORTs are preloaded with calibrations specifically designed for regional differences in pump fuel and accommodate popular bolt-on part configurations. If these calibrations don't fit the parts configuration or fuel, a nationwide network of authorized Cobb AccessECU professional tuners can create custom AccessPORT mapping (www.cobbtuning.com).

Methanol Injection
We started our tests and comparisons by first creating an optimized calibration for 92 octane pump fuel. This typical pump fuel tune yielded a conservative 320 wheel torque and 290 wheel horsepower (Figure 1). In this case, the torque and power output were maximized but limited to a reliable calibration for 92 octane fuel. We saved this map and moved on to create a calibration for use with a mixture of 80 percent methanol and 20 percent water. As was discussed in detail in the earlier article, we utilized more aggressive boost, timing and fuel strategies to take advantage of the added octane and heat reduction offered by the water and methanol injection system. These differences produced reliable power with realized gains of 35 ft/lbs of wheel torque and 10 additional peak wheel horsepower (Figure 1). The focus of this article is E85.

E85 Power Production
Maps specifically tuned for pure pump fuel, as well as pump fuel plus water and methanol injection, were saved to our AccessPORT. We drained the fuel tank by disconnecting the return line and attaching an extension that drained into an external fuel can. We let the car run for a few minutes, and the returning fuel now filling the gas can quickly depleted the remaining pump fuel from the tank. We poured in 10 gallons of E85 and did some quick changes to the fueling parameters to create a starting point map. E85 requires about 25-30 percent more volume in order to achieve a stoichiometric air-to-fuel ratio. After adding an average of 24 percent more fuel volume to our calibration, we were able to start, idle and drive the car under power with little to no drama.

Utilizing similar boost pressures as those for methanol injection, we now started to change fuel and timing parameters to best match the high-octane character of the E85. Surprisingly, additional ignition timing over that used for methanol injection did not heavily impact torque and power output (Figure 1, lower panels). However, the timing was higher than that used for pump fuel as we ran much higher boost (22 compared to 19 PSI), which normally requires a substantial drop in ignition timing. Added ignition timing that does not produce substantially more torque is dangerous. Even without the presence of detonation it is never a good idea to run ignition timing that is beyond the minimum required to produce maximum torque (MBT, or Minimum timing for Best Torque).

The lambda used for E85 overall was very similar to that used for pump fuel and pump-plus-methanol (Figure 2, top). E85 can be tuned as lean as 0.86 and as rich as 0.74 when on power. Over a period of several runs we found that lambda leaner than about 0.82 produced no extra power on this specific vehicle. Because richer fuel mixtures are always more conservative, we tuned for an average on power lambda of about 0.80 (Figure 2). Lambda is a reflection of stoichiometry. When comparing air-to-fuel ratios from the same data, it is clear that E85 requires much more fuel mass than does pump fuel (Figure 2, bottom).

E85: more fuel means larger injectors
Because of the greater mass of E85 needed to accomplish proper stoichiometric air-to-fuel ratios, the fuel system has to deliver a greater volume of fuel compared to pump fuel. We predicted that we would need at least 25 percent more fuel, that our 560cc stock fuel injectors would not be sufficient for E85. In fact, the stock fuel system on an otherwise stock STI is nearly insufficient for stock pump fuel calibrations. To allow some headroom in our fuel system, we changed our stock 560cc injectors for some high-flowing 850cc side feed injectors from Deatschwerks (www.deatschwerks.com). Using these injectors, our peak injector duty cycle was 80 percent when using E85 and approximately 67 percent for pump fuels (Figure 3, top). These injectors provided a comfortable margin, given the fact that our stock turbo is pumping its poor guts out and will not make more boost at higher engine speeds.

To illustrate the need for larger injectors when using E85, I transformed that injector duty cycle data to match those that we would have observed IF we had attempted these tunes with stock 560cc injectors. As predicted, 560cc injectors would have been statically open just above 4000rpm (Figure 3, bottom). When injectors are above 100 percent duty cycle (IDC) there is no more fuel. In other words, our E85 tune if attempted with stock injectors may have resulted in dangerously lean lambda and possible engine damage. This calculation clearly shows that larger injectors are required for E85 unless boost is lowered substantially.

E85, the Golden Egg?
Given the cost of fuel, the move toward E85 is motivated in part by its lower cost-per-unit volume. However, the energy density of E85 and its stoichiometry indicate a reduced fuel mileage. The national average difference in E85 cost vs. premium pump fuels is a 25 percent savings per unit volume. Our very short evaluation of E85 produced about a 15 percent reduction in fuel mileage compared to pump fuels. This negative impact on fuel economy makes E85 a small savings compared to premium pump fuels. The pure economic value may be different in your region, given variances in fuel costs. Furthermore, just like methanol injection, there is an investment associated with running E85. Larger injectors, engine management and a custom tune are a few of the major costs. However, for most Subaru enthusiasts willing to step up to E85, there is a rationale that transcends simple economics. High-octane fuels are the stuff of dreams for turbocharged internal combustion engines. E85 allowed our test Subaru to create a reliable, additional 55ft/lbs of torque and 18hp at the wheels when compared to pump fuel. Is E85 a value? If you value power, E85 is the deal of the century.

Please direct your tech questions to Timothy Bailey, Ph.D, at tbailey@subiesport.com

Key Product Suppliers

Cobb Tuning - AccessPORT Engine management
www.cobbtuning.com

Deatschwerks - Upgraded Fuel injectors
www.deatschwerks.com

Perrin Performance - exhaust, intercooler, methanol injection, dyno testing facility
www.perrinperformance.com

SurgeLine Tuning - AccessECU qualified Professional Calibrator
www.surgelinetuning.com