What's your estimate for cost of materials and time it takes to do this? Sounds like you believe the OEM set up is not able to handle the E85 at increased HP levels or boost. Is that correct? Want to understand motive of this mod. And the benefit.
I have '14 RS that should already take flex fuel/E85. I have the braided line that others pointed out indicates it can take it. Although I did get a CEL when I ran it... Blamed it on the tune file but never followed up. Just went back to 91
That's a very good question. The answer to it is worth getting out in the open for anyone to read, but I couldn't find a good place to put it within the write-up as it's a bit of a tangent. I'm going to put a little glossary of terms at the end of the write-up in case things get confusing at any point.
Cost of materials can vary: Cheapest possible- I could see doing it for $110 or less assuming you already have the tools and a few supplies. Most expensive- Probably around $225. The expensive route assumes you use the Russell fittings at $20 a piece and you need 4 of them. This assumes you get the sensor for around $60 on the cheap route and $80 on the more expensive (mounting tabs vs no tabs); they're $75 for the mounting tab version right now on GMPartsDirect.com, and around $80 most anywhere else.
Time to install, assuming I complete this write-up as well as I'd like to and all the steps are reasonably clear going in to it, probably 2-3 hours for a competent installer.
How your car operates from the factory
From the factory the Sonic 1.4T cars (not sure on 1.8) have no means of handling ethanol contents greater than 10% reliably. "Stoich" air/fuel ratio for 100% gasoline is 14.7:1. That is, 14.7 parts of air to 1 part of gas. All fueling logic within the factory tune is based off this number. Cruise and idle (closed loop operating) the car is going to provide the amount of fuel that
should correspond to this air/fuel ratio. As the name "closed loop" should imply, there is a loop logic. Your oxygen sensor detects actual air/fuel ratio and tells the car to add or take away fuel from the next cycle based on its feedback. The factory sensor is "narrow-band" style. This means it can only accurately sense values near "lambda". A lambda value of 1.00 corresponds to "stoich", no matter what fuel is being used. If you have 100% gasoline, then your oxygen sensor is going to tell the computer that the car is running damn near perfectly, resulting in very little fueling change in closed loop operation.
Above certain RPM, airflow, and throttle percents the ECM goes in to open loop operation, during this time is uses a multiplier to increase fuel delivery to richer targeted air/fuel ratios. Again, this is all based on the stoich value I mentioned above. The oxygen sensor may produce readings, but they no longer effect immediate fueling strategy.
As you've probably seen at the gas pump, 99.9% of stations in the USA have "up to" 10% ethanol content in any octane of fuel. Why is this a problem? The energy content of ethanol is a bit different from gasoline. The "stoich" air/fuel ratio of 100% ethanol is 9:1, 39% more fuel required to reach a stoich level. For reference: E0- 14.7:1, E10- 14.04:1, E85- 9.75:1, E100- 9:1
As I mentioned before, your oxygen sensor (some call the lambda sensor) sees 1.00 lambda as being stoich, it doesn't matter the fuel; if the sensor is reading 1.00 then you're right where you need to be for idle and cruising. So if you run E10 (all US stations sell this) in your car which expects E0 (as the Sonic ECU expect), you can see that the amount of fuel required is a bit different. E10 requires a higher volume of fuel to be stoich when compared to E0. This is NOT a problem. Closed loop feedback from the sensor tell the ECU to add a few percent of fuel, and everything is happy.
Now let's imagine you put E85 in a car designed for E0. You start the car, it idles, and you drive away. Suddenly during closed loop operation, the oxygen sensors are reporting that you're not burning nearly enough fuel. The ECU adds more fuel, but it's still too lean. It finally has enough fuel when you've increased fuel delivery by 34%. The car sees this as an error because it was never expecting to need this much fuel to run. When the car sees an error, it sends a check engine light.
Even worse is if you go full throttle with E85 on a car that is not prepared for it. The car will deliver enough fuel for what would be probably 12.5:1 air/fuel ratio on gasoline, when in reality it needs closer to 7.8:1. The engine is starving for fuel.
How the "Virtual" Flex Fuel Sensor works
GM sells many "Flex Fuel" vehicles, but only a small percentage have the sensor installed that inspired this whole thread. So how exactly do they work, and why do I not think it's an adequate method for our purposes?
Let's just ump straight to the E85 example I gave above. During closed loop driving the car sees the oxygen sensor telling it that 34% fuel is needed. With a virtual sensor, the ECU is programmed to understand that error number to mean there is alcohol in the fuel, and it will estimate the alcohol content until it starts seeing a proper signal from the oxygen sensor again. Although this is more or less accurate (I'm not sure what the % error is, but I'd imagine it's within 5%), it has a few problems. The first problem is that it's pretty slow to act. The car is triggered to estimate alcohol content any time it detects an increase in fuel level, as it assumes you just filled up the car with a possibly new ethanol content. After it starts its relearn process, the car must go in to closed loop operation for a number of cycles before it is confident it has correctly estimated alcohol content. During this time your open loop fuel tables, and all aspects of timing are in a state of flux where anything can happen. This brings me to my second point: precision. Sure, for a Tahoe it is fine if fueling and timing are within 5% of correct... it's a Tahoe. For a performance enthusiast, boost levels, timing, and air fuel ratios need to be as dialed in as possible. Having correct ethanol content information down to 0.1% is very valuable information to have in this case. There are also other reasons your oxygen sensor could report a bad value: bad MAF sensor, bad oxygen sensor, boost leaks, etc. These are all problems for performance, but the virtual sensor will cover up the problem.
The E78 (Sonic) ECU is equipped to "sense" ethanol in this way, but no tuner would recommend it.
How the "Real" sensor works
Alright, at this point you now understand far more about how your car determines fueling than the average person. You also have a pretty good idea of ethanol's effect on fuel delivery needs.
As mentioned above, correct ethanol content is incredibly valuable for tuning. The GM Flex Fuel sensor is perfect for this. Using its signal pin, it pulses a square wave to the 5V signal wire of the ECU to deliver ethanol content. The frequency of this square wave corresponds to ethanol content + 50 (0 ethanol is 50Hz), and the wavelength corresponds to fuel temperature (1ms to 5ms wavelength). With this information, fueling and timing can be dynamically adjusted on the fly, as the fuel reaches the engine.
This is ability is awesome to have. Your fuel delivery is always going to be reasonably close no matter what your ethanol content, whether you can find e85 or not, whether you run 100% gasoline... always. Further, e85 varies widely in ethanol content. In the summer it is close to 85% (normally about 80%) but in the winter it can drop as low as 51% to combat the cold (ethanol doesn't burn as well in a cold engine, hard cold starts). If you're running a static tune with no sensor, again you are at the mercy of the estimated ethanol content. A real sensor fixes this problem.
Terms:
Stoich- An ideal air/fuel ratio for the fuel that is being burned.
Closed Loop- Typically idle and steady state cruising in your car.
Open Loop- Times of varied throttle, load, and rpm in your car.
Rich Air/Fuel- More fuel than is needed
Lean Air/Fuel- Less fuel than is needed
Lambda- Oxygen sensor value corresponding to "Stoich" for your fuel type
EXXX- E prefix means ethanol, number value behind indicates percent of ethanol in the fuel