Let me explain why the extra alcohol, since I missed it: you are paying to get a fuel of a specific octane rating from the gas station; the fuel MUST meet that rating at a minimum. Alcohol is a hell of an octane booster, but has less total energy available than gasoline. Your 91 gasoline will be 91 octane for most of its shelf life. Your 91 octane e-10 will likely be higher in the beginning.

Alcohol will deteriorate much more quickly than the gasoline, and as it does, it will have less and less effect on the base octane of the gasoline. Any gas sold within the tank life of that delivery (roughly 60 to 90 days max for e-10) must still have the minimum octane rating listed. Really fresh stuff will actually have a higher octane rating, but the state is OK with that. ETOH is actually 99.5 AKI, so it is actually a superb fuel, except for needing a bit more of it.

 

Which is a crime in most, if not all places. I presume you've reported this?

Actual testing here (Canada) shows that most stations actually deliver less than the maximum 10% ethanol permitted. That's why the sings say up to 10%. Since ethanol is more expensive to produce than gasoline, why would a refiner lose out on profit by adding more than required or permitted by law?

 

So now please answer why/how 'most computers will crank the timing forward a bit when they sense the e-10 (actually higher octane than straight gas'.

Cars do not have any sort of ethanol detectors nor does it directly know the octane rating of a fuel. There is a simple knock sensor. If there's no knock it does not retard the timing. These engines do not knock on 91 so higher octane is a complete waste.

 

Certainly not a crime, when I had to do it to put that little certification sticker from the commonwealth on that pump for each and every one of 'em! The counties actually did the volume certification, but the state does the octane.


The knock sensor will advance the timing within predefined limits up to the point where the engine starts to knock, then roll it back.


Your "91 octane" e10 from a fresh batch may be 93 octane or slightly better, so the timing can go further up before the sensor hears the first chuckle of preignition.


Remember that the higher the octane, the less likely the fuel is to fire off before it sees that big, fat, juicy spark from the plug. Lower grade fuels burn easier.


Oh, we tested for acetic acid, too...acetobacter eats ethanol, and pees vinegar, which is why the mix sours (and why new gas tanks are plastic!)


In normal practice, this will never be an issue for most drivers.

 

The above is so full of myths and misconceptions I don't know where to begin.

You use the terms detonation (knock) and pre-ignition as though they are interchangeable and mean the same thing. They most certainly do not.

 

Preignition is when the fuel/air mix fires off BEFORE the spark.


Detonation is when the end-gas ignites after the spark, and out of time with the main wave front.


Detonation ring at 6.4 kHz is what your knock sensor listens for. It is NOT the noise of the flame fronts themselves meeting, but rather engine vibration caused by the resulting pressure spike. In and of itself, it is not harmful at the levels detected by the knock sensor.


Yes, I did use the tem preignition colloquially, and therefore incorrectly, but that does not change the nature of how the knock sensor works...the timing is advanced to the threshold of detonation, then dialed back.

 

And again using terms colloquially changes the meaning of everything.

Pre-ignition is when the fuel/air mix ignites and burns normally (vs. 'fires off') BEFORE the spark.

Detonation is when the end-gas explodes/detonates (vs. 'ignites') after the spark, and out of time with the main wave front.

The 6.4 Khz signature 'ring' detected by the knock sensor is produced as a direct result of the detonation event.

No Jag engine I know of operates in the manner you describe where timing advance is dialed in until a detonation event is detected, timing is dialed back and then followed by the advance/retard process repeating.

The OEM limits the timing advance to suit the recommend fuel and the car is certified accordingly. In N. America that's 91 AKI. Anything higher is a waste.

 

Look at the module. It likely says "FoMoCo", "Bosch", "NGK", "Delphi", or "SMPE". It is a simple acoustical (piezo) device invented by Bosch. No knock, Logic Low. Knock, Logic High.


No car that can be afforded by mere mortals is using an ion sensor system.


Computers listening for these sensors are programmed exactly the same way.


That is how they work. You have a "premium gas recommended" sticker, that is how they are programmed. Actually, non-premium cars are set up the same way, but they allow for lower maximum timing (based on the engine specs).


The OEM set the baseline, and the maximum timing. The engine shoots for maximum, and retards itself until there is no ringing. Super simple, no remapping fuel curves, and keeps the emissions in spec.


Go measure your timing advance on premium, see where it maxes out. Now, cook off the hi-test, and put some 87 R+M/2 in it. Measure the advance again.


How does it know? By retarding the timing until the 6400 is gone.

 

Agreed on all almost points except that Jag sets the max advance timing to operate detonation free with the recommended 91AKI octane. Using higher octane doesn't get you more advance, the software has already set the maximum permissible.

Of course using lower octane runs the risk of inducing some level of detonation which would then be detected and corrected by the knock sensor and ECU.