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Old 09-06-2011, 07:00 PM   #11 (permalink)
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Another thing to note when testing is that it may take a tank or pulling a fuse or disconect the battery to reset the Long Term Fuel Trims that the computer stores depending on the fuel you run. Depends on the car as to how to reset them, but the car is going to react different with differtn fuel trims when gas is changed.

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Old 09-08-2011, 07:10 PM   #12 (permalink)
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I just ran across this reply in an old popular mechanics:
[quote]Knock Three Times
My father has a 2003 VW Passat with the W8 engine, and he uses 89 octane fuel instead of the 91 recommended. He says the knock sensor will adjust the engine timing to prevent knocking and engine damage. I contend that even if the driver doesn't hear pinging, that doesn't mean that it's not occurring, and can lead to engine damage. Can this lower-octane fuel damage his engine?

Your father will not damage his engine running any gasoline he can get in the United States, which might be as low as 87 in some areas. The knock sensors (many engines have more than one because they are bolted to the head, not the block) will sense the rapidly building pressure waves caused by gasoline igniting prematurely inside the combustion chamber. The engine-management computer will retard this spark and prevent this condition. The system is good enough at this to preclude any damage. The downside is that fuel economy will suffer from the retarded timing, making his bargain low-grade fuel less of a bargain.

found here
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Old 09-08-2011, 08:05 PM   #13 (permalink)
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Originally Posted by Rick Rae View Post
I suspect the vehicle (and driver, and environment) of interest would need to be tested. There are probably way too many variables to make a useful blanket statement.

Before the Versa I had a Cabrio; its 2L was the only VW engine rated for regular gas. Back in 2007-2008 I ran a test on Shell Regular, Plus, and "V-Power." It was real-world, not tightly controlled, and not blinded, so it hardly qualified as an "experiment." But I ran the test for a LONG time to try to get reasonable data.

In my specific car (with me driving, etc.) I saw a slight FE improvement with premium, but not enough to offset the additional cost.

The data is available if you're curious, but it's on another forum and I don't know if I'm allowed to mention it here. Hardly worth looking at anyway; the improvement in my case was very subtle.

you left uot the most important fact: what was the recommended fuel.

That's really the issue.
And it can't be 'retrained' by pulling the ecu.
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Old 09-08-2011, 08:16 PM   #14 (permalink)
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Originally Posted by mcrews View Post
you left uot the most important fact: what was the recommended fuel.
Sorry, I thought the implication was clear. As I stated, the 2.0L is the only VW engine rated for 87 octane, and that's what was specified in the owner's manual.

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Old 09-09-2011, 05:56 PM   #15 (permalink)
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Not sure if this is "premium gas" or not but I tend to fill up at supermarkets over here in the UK as they are cheaper.

Some on forums related to the Aygo suggest fuel from branded garages (e.g. Shell, Esso etc.) results in better MPG and I intend to try a couple of tanks to see if there is a big difference.

Most Petrol in the UK comes from the same refineries, and the differ in terms of added chemicals for cleaning, octane boosting and so on. Supermarket fuel is the basic blend, branded fuel has more additions - maybe.
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Old 09-09-2011, 07:16 PM   #16 (permalink)
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Originally Posted by RRC View Post
In my case (no turbo) and old engine difference between lower and higher octane is about 0.5L/100km when we are talking about FE.
So yes the permium gas gives me a little bit better FE.
That's a nice improvement.
3 times what I'm getting with premium diesel.
At that kind of difference, it starts paying off to use the premium fuel.

The best what you can do is check it by self with your car.
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Old 09-09-2011, 08:03 PM   #17 (permalink)
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Thanks everyone for the reply's...I'm impressed, not many places you can ask a question and get responses from both sides of the Atlantic!
I think in my case I'll just stick to the 87 octane gas.
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Old 09-09-2011, 10:26 PM   #18 (permalink)
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You've pretty much got your answer, but I'll add that you will get the most power from a fuel with the lowest octane level that just barely prevents detonation. Additional octane, if not required to control detonation, will cost you power. I'm not certain if that translates directly to MPG numbers also, but I expect it might.

The only time a high octane fuel may give you more MPG, when not required to control detonation, is when it doesn't contain Ethanol like the lower grade. I've found about a 4% increase in mileage with my cycle when using 91 octane (87 is required) without Ethanol.

I don't think there is any definite answer to the exact penalty of 10% Ethanol - every car/cycle reacts differently.
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Old 09-09-2011, 10:52 PM   #19 (permalink)
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Originally Posted by Arragonis View Post
Some on forums related to the Aygo suggest fuel from branded garages (e.g. Shell, Esso etc.) results in better MPG and I intend to try a couple of tanks to see if there is a big difference.
IIRC there's an old thread on EM or gassavers with extensive debating/testing of "top tier" fuels vs the others. I didn't pay too much attention to it though because I'm always going to fill with the cheapest swill I can find.

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Old 09-09-2011, 11:21 PM   #20 (permalink)
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I tried a few tanks of 91 octane premium in my Mustang last summer. I have a Hypertech power programmer that can re-program my PCM for best performance with 87 octane regular or 91 octane premium. I had it programmed with the 91 octane performance tune. I got lower MPG with premium and the premium performance tune than I get with regular and the regular performance tune. There was a noticeable difference in performance, but the MPG loss "inverse squared" the price difference. I'll feed my pony the less expensive "oats". Unless I'm racing..........

Fuel economy is nice, but sometimes I just gotta put the spurs to my pony!

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