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Discussion Starter #1
I have a 2004 VW Passat GL. Do I really need minimum 91 fuel octane rating as recommended in the manual?
 

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No... but you will notice reduced performance without it. It has been discussed many times on this forum, searching will turn up many different opinions. In my opinion, the extra .60 I save at each fill up isn't worth the lost performance. (Lower octane means lower HP, which means an engine working harder, which means lower gas milage in the long run)
 

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I think the car only needs the higher octane when you really have your foot in it. I run the 90 octane most of the time - especially in winter.

I have a VAG-Com and am watching if the ignition is being pulled back. It rarely is, so I am getting enough octane for my driving.
 

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Welcome to the club. Pull up a chair and pay attention....

Your VW Passat is programmed to run on 91 octane. It will accept a lower premium however due to the nature of running a lower octane fuel it will combust differently and the likely result is referred to as "knock". German engineers being the somewhat bright individuals we like to think they are included sensors to detect this condition because if left unchecked it will damage your engine over time. These sensors detect knock for a period of time (or knocks) and retard the timing of the engine gradually until the knocks are no longer detected. In the meantime your engine running at 1500rpm in 5th gear at 45mph or so has knocked 5, 50, 100, 1000 times. Retardation of timing equals less performance from your engine.

What I do not know is if the engine resets the timing back to normal everytime you turn off the engine. Or does it try to advance the timing until it detects knocking and the frequency or timing of when it attempts to advance the timing.

In my humble opinion at the savings of $3-4 a tank of gas it is unneccesary wear and tear on your engine. If you plan on chipping your engine you will be required to run premium fuel anyway.
 

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There is a thread somewhere that the math was done. At 15000 a year in mixed driving 27mpg you save like $80 or $1.64 a week.
 

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http://caranddriver.com/article.asp?section_id=4&article_id=3604&page_number=1

Here's the executuve summary:
"Our tests confirm that for most cars there is no compelling reason to buy more expensive fuel than the factory recommends, as any performance gain realized will surely be far less than the percentage hike in price. Cheapskates burning regular in cars designed to run on premium fuel can expect to trim performance by about the same percent they save at the pump. If the car is sufficiently new and sophisticated, it may not suffer any ill effects, but all such skinflints should be ready to switch back to premium at the first sign of knock or other drivability woes. "

My emphasis added.
 

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Altair 4 said:
http://caranddriver.com/article.asp?section_id=4&article_id=3604&page_number=1

Here's the executuve summary:
"Our tests confirm that for most cars there is no compelling reason to buy more expensive fuel than the factory recommends, as any performance gain realized will surely be far less than the percentage hike in price. Cheapskates burning regular in cars designed to run on premium fuel can expect to trim performance by about the same percent they save at the pump. If the car is sufficiently new and sophisticated, it may not suffer any ill effects, but all such skinflints should be ready to switch back to premium at the first sign of knock or other drivability woes. "

My emphasis added.
Agreed for MOST cars, but the difference is you are driving a beautiful german lady... And she has a turbo! :)
 

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rpaller said:
Welcome to the club. Pull up a chair and pay attention....

Your VW Passat is programmed to run on 91 octane. It will accept a lower premium however due to the nature of running a lower octane fuel it will combust differently and the likely result is referred to as "knock". German engineers being the somewhat bright individuals we like to think they are included sensors to detect this condition because if left unchecked it will damage your engine over time. These sensors detect knock for a period of time (or knocks) and retard the timing of the engine gradually until the knocks are no longer detected. In the meantime your engine running at 1500rpm in 5th gear at 45mph or so has knocked 5, 50, 100, 1000 times. Retardation of timing equals less performance from your engine.

What I do not know is if the engine resets the timing back to normal everytime you turn off the engine. Or does it try to advance the timing until it detects knocking and the frequency or timing of when it attempts to advance the timing.

In my humble opinion at the savings of $3-4 a tank of gas it is unneccesary wear and tear on your engine. If you plan on chipping your engine you will be required to run premium fuel anyway.
Very well put!
 

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Knocking more clearly defined:

"When gas ignites by compression rather than because of the spark from the spark plug, it causes knocking in the engine. Knocking can damage an engine, so it is not something you want to have happening. Lower-octane gas (like "regular" 87-octane gasoline) can handle the least amount of compression before igniting."
 

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Okay, let's do the premium fuel economy math again:

Let's say you get 22 mpg (on average) and drive 12,000 miles a year. That's 545 gallons of fuel a year.

Using the average prices posted on the AAA website (http://www.fuelgaugereport.com/) for my area for today, I would pay:
87 AKI: $1.692 x 545 = $922.
89 AKI: $1.784 x 545 = $972.
93 AKI: $1.863 x 545 = $1,015.

So the difference over a year is $93. That's about 25 cents a day, $1.79 a week, or $7.75 a month. Or if you want to look at based on a per mile difference, it would be $0.00775 per mile, not even one cent.

And these values don't reflect any difference in economy or performance. The spread narrows if the Car and Driver article is correct in that both suffer on cars that were designed for premium but use regular.

$1.79 a week...that's a cuppa coffee at Panera's. Seems worthwhile to me to maximize my performance, economy, and peace of mind.
 

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Yea... as mentioned above, you will not save any money. Remember, your car is DESIGNED to run on premium fuel. If you run 87 octane, you will get less milage, if but it costs less, so thats ok... if you run 91 octane, you will gain milage, but it costs more. Looked upon from a cost aspect, it makes no sense, because you will spend almost exactly the same on gas over the course of ownership. This is not even factoring in the damage to your engine, and the premature wear of components due to overuse (the knock sensor was not designed to be in operation at all times, it was designed with the occational tank of bad gas in mind)
 

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Altair 4 said:
... $1.79 a week...that's a cuppa coffee at Panera's. Seems worthwhile to me to maximize my performance, economy, and peace of mind.
Yeah ... but that $1.79 is milk money for my babies. Also been putting my pennies aside to save up & buy a blanket for my poor old mother :cry: :cry:
 

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vuvision said:
Altair 4 said:
... $1.79 a week...that's a cuppa coffee at Panera's. Seems worthwhile to me to maximize my performance, economy, and peace of mind.
Yeah ... but that $1.79 is milk money for my babies. Also been putting my pennies aside to save up & buy a blanket for my poor old mother :cry: :cry:
I think Geo metros run on 87 octane... :p :cry: :lol:
 

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For what it's worth, when I first got my car i was filling with mid grade and got around 305 or so before fililng up...on the same conditions I filled with premium and went 325-330 miles...so I figure it's a wash. I get an extra day or two with prem. :) I've been running prem ever since.

Though, premium prices SUCK ASS now. :mad:
 

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Detonation or engine "knock" is caused when the the compessed fuel mixture ignites before the piston reaches top-dead-center on the compression stroke (bad for piston, rings and rod bearings). The pre-ignition is caused by heat (not spark) in the combustion chamber. The heat is created by the added compression of the turbo boost. Pre-ignition can also be caused by carbon build-up in the combustion chamber (but thats another story). The addition of octane in gasoline makes the fuel less likely to be ignited by heat caused by the compressed fuel/air mixture (pre-ignition, detonation, knock) and more likely to be ignited by the spark at top-dead-center. The addition of octane to the gasoline doesn't add horsepower, it only lets the engine operate without deceasing horsepower (decreasing timing) to stop detonation.

The question we should ask is: Does my engine detonate using lower octane fuel? If it does, then higher octane fuel will help the engine perform to it's true potential. If it doesn't, then using higher octane fuel doesn't do anything but lighten your wallet. :thumbup:
 

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Discussion Starter #17
Thank you for your replies.
I'm not worried about money, it's just the information that there's no difference between regular and premium gasoline. Also, I've been driving the Passat for 2 weeks and I don't go, as per trip computer, more than 11 miles per gallon (NYC driving). I use the Tiptronic most of the time. :???:
 
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