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Originally Posted by Big Dave
Let’s take my trusty old B-body Impala.
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The EPA won’t let me jack up the compression ratio or advance the spark timing. That increases NOx emissions and the EPA views that as “tampering.” So the car still gets 172.4 miles per MMBTU.
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The thing is, the EPA does let you change spark advance (nobody is using variable compression ratio as far as I know). That is why flex fuel vehicles have ethanol sensors, to detect what mix their fuel is and optimize spark ignition for it (and adjust injector duty cycle).
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Flexible fuel vehicles use a sensor to determine the percentage of ethanol in the fuel mix (which varies not only with the season and location, but also with the relative amounts of E85 and gasoline in the tank at fill-up) and adjust the fuel injection and timing to suit the fuel.
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Reference:
Flex Fuel with MegaSquirt
I'm going to have to say Frank is right in that just because it has a 30% lower BTU content doesn't mean it gets 30% lower fuel economy. There is some optimization for ethanol built into flex fuel vehicles (which obviously isn't all vehicles on the road).