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I took two road trips recently and tracked the gas mileage on my 06 A8 with the 4.2 engine. The difference between 10% ethanol gas and pure gas was surprising. One way to tell the difference is to look at the octane rating on the pump. E10 premium gas is usually rated at 93 octane. Pure gas is at 91.
With E-10 gas I got 22 MPG at 82 MPH
Gas without ethanol yielded about 26 MPG at 82 MPH.
Has anyone else experienced this?
The cost of the E10 and pure gas was about the same. Gas prices varied more by location than type of gas. Downtown Chicago was pricey for E10. Almost $4 per gallon. Rural Oklahoma was cheaper for pure gas.
That is too big difference. 10% ethanol gas has just about 2.5% less energy. Higher octane value compensates that a bit. You've got 15% difference. It comes from other influences, not the fuel. Could be road configuration, traffic, wind, temperature and so on.
Since the Audi engines have a relatively high compression ratio (11:1 if I recall correctly), that means that the engine will knock if run on regular gas. Knocking isn't just an annoyance, it is damaging the engine.
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Audi motors have had knock sensors for up to 30 years, and most all for 20+ years. Knock sensor pulls back timing as needed, and with triangulation and coil packs/electronic ignition can do it on a per cylinder basis. Not comparable to old days, even at 11:1 (or more) now vs. old 10:1 "high compression." There you really did need premium to avoid knocking in pre-knock sensor days, and more so with distributor set ups that couldn't vary by cylinder either.
Watch your ignition timing when driving when filled with premium and with regular. With regular it will be severely retarded. In addition, I don't think there is enough timing adjust range there to compensate for regular gasoline. That notion is supported by the fact that the engine is pinging and by the fact that Audi says to use 91 or higher octane fuel right on the fuel door.
E10 gas has less energy per gallon than pure gas. That is part of the reason why 10-30% better mileage can be attained with pure gas. Most of the propaganda says 3% but real world data says something different.
I am not sure why I am even bothering with the octane/knock/pinging issue. This topic is so well documented that repeating it here is redundant. A google search for "engine detonation" or "engine pinging" will yield all manner of hits showing what happens to an engine that pings or knocks due to low octane fuel.
For those that want some pictures anyway, here are but a few.
"Your vehicle may also be operated using regular gasoline with a minimum octane rating of 87 AKI. However, using 87 AKI fuel will slightly reduce engine performance."
The manual does recommend 91 AKI but mentions nothing about 87 AKI potentially damaging the engine. One could correctly presume that Audi is stating regular fuel is fine to use without concern other than a small reduction in performance. (They make a bigger deal of saying to use the correct fuel with the necessary additives).
That said, I purchased a high performance, luxury sedan and wish to achieve its top performance. Therefore, I use 93 AKI Shell Premium (I'm in MA), a listed Top Tier fuel, rather than save a couple of hundred dollars a year using regular. However, not because it would damage my engine.
E10 gas has less energy per gallon than pure gas. That is part of the reason why 10-30% better mileage can be attained with pure gas. Most of the propaganda says 3% but real world data says something different.
Pure ethanol has ~25% less energy than pure gasoline. Math is simple and the result is 10% ethanol has 2.5% less energy than pure gas. Real world is too complicated for simple road experiments, so they can and will always prove what the one performing them wants.
Actually, I am not sure why I am even bothering with the octane/knock/pinging issue either. ...This topic is so well documented that repeating it here is redundant... Funny how that also fits the counter. Well documented that for many motors (esp. w/ sensors...) premium is often an unnecessary expense brought to you by oil co's eager to upgrade your purchase price and lighten your wallet. Car companies may be incented too if the higher octane allows a bit of mileage boost, which then presumably they need to spec for the car so they can't game the test. Missing fuel economy targets even slightly costs a manufacturer big bucks.
Knock sensors. The key as mentioned before. As another poster noted, contrary to the prior reference about Audi's supposed requirements, the manuals generally talk about performance not engine damage. Care to revise that? Yes, I generally use premium, but no the world won't end and the motor won't blow up in general otherwise. I am comfortable using other grades in certain scenarios as described before. I don't think Mister Bally's motor at 240+K miles or 165 mph has melted down yet either. But then hard to prove a negative as they say, and pictures of many good motors I have been inside of (for other reasons) wouldn't show any relevant issues.
Concepts from pre-knock sensor motors are not current. I know what knock is from 40 years of driving experience and personal internal motor work and rebuilds, from later 60's 10 to 1 motors to smog choked 70's motors, to Audi motors I bumped the compression on and others I advanced the timing. I also know the pinging I do or don't hear from the motors themselves in on road driving. Don't believe everything you read on the internet, especially if not apples to apples in comparable technical contexts. Also think about it; turbos now run way higher effective compression when in boost.
In MI,everything has 10% ethyl. I use mid grade (89). On the highway, I can get 23-24. Non highwayh (a mix of semi rural and some stop and go), I can only get 18-19. I drive like an old fart, which I am. I'm gentle on the gas at a stopsign or light to the point where some smart *** always roars around me. However, by gently accelerating I general pass the smart *** (gently). I also watch lights, traffic on local roads and the highway. It makes no sense to come right up behind a big truck which is going to take a mile to pass a slightly slower truck.
The only place where I can get non ethynol gas is at a little airport near me but the price scares me away.
08-30-2015, 07:09 PM
