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  #1  
Old 04-30-2007, 02:07 PM
Figaro Figaro is offline
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Do I really need premium gas? (VW Passat car question)

The manual says I do. Click and Clack on Car Talk say I don't. Googling reveals a consensus that regularly-aspirated engines that are designed for premium gas will suffer no harm from using regular unleaded -- the loss of a few maximum horsepower is the only result.

But there is less consensus (as far as I can tell) about turbo-charged engines, apparently because the rapid pressure build-up can prevent the anti-knock sensors from keeping up and result in detonation.

I've got a 2003 VW with a 1.8 liter turbo engine.

So...dopers in the know...do I need to pay extra for my gas?

p.s. I couldn't find a previous thread on the subject. Sorry if I missed something in my search.
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  #2  
Old 04-30-2007, 02:29 PM
yabob yabob is offline
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I'd follow the manual, at least as long as you're on warranty. Don't give the dealer an excuse to void it, valid or not.

Click and Clack's point is that most modern cars have anti-knock sensors which retard the timing if knocking is detected, hence the lost power. Before this was the case, using lower octane than designed for caused knocking, which was not good.

I'd still follow the manual. For one thing, with high gas prices, the difference between premium and regular has remained more or less constant, and now makes very little percentage difference. If the manufacturer says it's supposed to use 91 octane, buying 89 is probably false economy. Consider it cheap insurance, if nothing else. For another, if you bought a high performance engine, you presumably wanted the power. Why defeat your purpose?
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  #3  
Old 04-30-2007, 02:29 PM
Rick Rick is offline
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Click and Clack are an entertainment show. Any useful information coming from that show is strictly a lucky coincidence.
Your car maker knows more about your engine than Click and Clack do. Hell for that matter they know more about it than I do.
I seriously doubt that the knock sensors "can't keep up" Volvo used the same knock sensors, on the same type of injection system (Motronic) on turbo motors, and this was never an issue.
What could happen is regular fuel could knock so badly that even at max retard the engine is still knocking. Engine knock can cause severe damage to both the engine and the catalytic converter.
It's your car, and your decision, but if it were mine, I would run the recommended fuel.
ETA: If C&C know that your car has a knock sensor, the car maker for sure knows it has knock sensors. With that knowledge they still recommend premium.

Last edited by Rick; 04-30-2007 at 02:32 PM.. Reason: one more thought.
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  #4  
Old 04-30-2007, 02:30 PM
Iggins Iggins is offline
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I have the same year/style Passat. I do buy the premium fuel, along with the synthetic oil and oversized oil filter. I don't think premium fuel (really, the octane rating) is critical, unless you have done the chip upgrade to the car. The extra octane really gives it a pop!

There has been a recall of the fuel pump:

http://www.consumeraffairs.com/recal...6/audi_vw.html

(on preview, I do agree with Rick and yabob.

Last edited by Iggins; 04-30-2007 at 02:33 PM..
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  #5  
Old 04-30-2007, 02:38 PM
crazyjoe crazyjoe is offline
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Does your manual say "recommended" or "required"? I had a PT Cruiser with a 2.4 Turbo engine. The manual said that premium was recommended, but the engine would run fine (albeit with lower performance) if I used regular. For a good long time when gas prices were at 3 bucks per gallon, I used regular.

In fact, the "touring" version of the PT intrduced in 2004 had a turbo engine option. The engine was the same, it was just tuned to run on regular fuel (and premium produced no increase in power).

However, all of this is irrelevant if the mfr of your car says "Premium unleaded fuel only" in the manual.
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  #6  
Old 04-30-2007, 02:46 PM
Figaro Figaro is offline
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Quote:
Originally Posted by crazyjoe
Does your manual say "recommended" or "required"?
It says required, and I've always used it up until now. (and the synth oil, too, Iggins)
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  #7  
Old 04-30-2007, 02:51 PM
GorillaMan GorillaMan is offline
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Is the engine the same as the European model, but the manual an American version? If so, I suspect they're getting you to use the closest to the 95 octane of regular European fuel.
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  #8  
Old 04-30-2007, 02:58 PM
Figaro Figaro is offline
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Quote:
Originally Posted by GorillaMan
Is the engine the same as the European model, but the manual an American version? If so, I suspect they're getting you to use the closest to the 95 octane of regular European fuel.
Eurogas is 95 octane? Criminy. No wonder it costs so much...

In answer to your question, I'm assuming the engine is the same as the euro model, since it's (I believe) a 100% import model built in Deutschland. But I could be wrong about that.
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  #9  
Old 04-30-2007, 03:01 PM
guppy guppy is offline
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The master speaks.


Hah! I finally get to post that!
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  #10  
Old 04-30-2007, 03:06 PM
Thudlow Boink Thudlow Boink is offline
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Here's a recent thread, with a link to a Straight Dope column, but it doesn't mention the Passat specifically. (On preview—the thread was started by guppy, and the linked column is the same one guppy has just now linked to here. Ah, the circle of life; or something.)
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  #11  
Old 04-30-2007, 03:21 PM
Figaro Figaro is offline
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Thanks for the links and responses. (odd that my forum search for "premium gas" didn't turn up a thread from last month called "premium gas" ).

I'm not willing to risk engine damage to save a few bucks, and it sounds to me as if it's a legitimate concern with my engine. (I'm assuming that 1.8 liters pumping out 170 peak horsepower means there's some high compression going on).

Thanks, all.

~fig
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  #12  
Old 04-30-2007, 03:28 PM
Stranger On A Train Stranger On A Train is offline
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Quote:
Originally Posted by Iggins
The extra octane really gives it a pop!
Octane does not increase the amount of energy in the fuel; in fact, it's purpose is to decrease volatility under the higher combustion pressures (and thus increase engine efficiency and/or output under load) and in doing so slightly reduces the energy density (though by a barely discernable fraction). The extra "pop!" you get is the engine running at the correct timing and combustion ratio. The difference in cost between regular 87 octane and premium 91 is currently about $0.20-$0.25. With regular unleaded running about $3.40 a gallon these days, that's about 6-7% difference in price, not even accounting for how inefficient it is for the anti-knock sensors to adjust for running rich on non-premium fuel. Compared to what it's going to cost you to rebuild the engine that's a bargin, and despite the decreased volatility you're still likely to get better economy.

And as Rick has said, Car Talk is entertainment only (and the "Tappet Brothers" seem to entertain themselves more than they do anyone else). I'm an engineer whose practical experience with IC engines is limited to being a shade tree mechanic, and yet I've caught them pulling out verifiably spurious reasoning on many of the few occasions that I've listened to their show. In any case, I'd always stick with what the manual says unless you have some very good reason to do otherwise. The manufacturer has to live by their recommendations, but the Magliozzis are indemnified from the results of their advice.

Stranger
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  #13  
Old 04-30-2007, 04:14 PM
Kevbo Kevbo is offline
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Something to consider when fueling EuroCars:

There are two ways to determine octane ratings, known as the research method, and the motor method. They come up with different answers, to the tune of about 4-6 points.

In the US, the pumps are marked with (r+m)/2 octane numbers...an average of the two methods mentioned above.

As I understand, the European convention is to only use the research number, which is typically about 2-3 points higher than the (r+m)/2 number for the same fuel. Thus European 95 octane is equivalent to US 92 octane.

I don't know about car manuals, but I do know that BMW motorcycles do not seem to account for this in their manuals.

If you drive at high altitude, there are few vehicles that need anything but regular, and the refineries normally supply fuel a couple of octane points lower in each grade in these areas.
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  #14  
Old 04-30-2007, 05:14 PM
Doug Bowe Doug Bowe is offline
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Consumer Reports has liked the Passat and recommended it for a number of years. A lot of good things in the Plus column. In the minus column they've always listed requires premium fuel.
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  #15  
Old 04-30-2007, 05:39 PM
GorillaMan GorillaMan is offline
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Quote:
Originally Posted by Kevbo
As I understand, the European convention is to only use the research number, which is typically about 2-3 points higher than the (r+m)/2 number for the same fuel. Thus European 95 octane is equivalent to US 92 octane.
That explains the huge discrepancy! Thanks.
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  #16  
Old 04-30-2007, 07:20 PM
Santo Rugger Santo Rugger is offline
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Quote:
Originally Posted by Kevbo
<snip>
If you drive at high altitude, there are few vehicles that need anything but regular, and the refineries normally supply fuel a couple of octane points lower in each grade in these areas.
Why's that? Because there's less oxygen, so it's harder to get pre-ignition? Does the fact that the lower octane has more energy make up in any way for said lack of oxygen in terms of horsepower?
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  #17  
Old 04-30-2007, 08:13 PM
Rick Rick is offline
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Quote:
Originally Posted by Pygmy Rugger
Why's that? Because there's less oxygen, so it's harder to get pre-ignition? Does the fact that the lower octane has more energy make up in any way for said lack of oxygen in terms of horsepower?
Lower octane does not have more energy.
Higher altitudes = lower cylinder pressures = less chance for pre-ignition = lower octane requirements.
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  #18  
Old 04-30-2007, 10:14 PM
Stranger On A Train Stranger On A Train is offline
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Quote:
Originally Posted by Rick
Lower octane does not have more energy.
Higher altitudes = lower cylinder pressures = less chance for pre-ignition = lower octane requirements.
Pedantic nitpick: fuels that have a higher octane rating have a (very slightly) lower mass energy density. While the molar enthalpy of combusion is substantially higher for octane then hexane (1307 kcal/mol vs. 997 kcal/mol) the density of hexane at standard temperature and pressure is greater, giving a density of 11.6 kcal/g for hexane versus 11.5 kcal/g for octane. (Heptane falls somewhere in between but my Marks' is at the office, my useless thermo book doesn't have values for organic chains higher than butane, and I'm too lazy to walk over to the library and look up the numbers in the CRC Handbook of Chemistry and Physics.) A higher octane fuel will have a slightly lower energy density, although it's clear from the numbers that we're talking a fraction of a percent, so for all intents and purposes Rick is correct that high octane fuels have an essentially identical energy content. (I'm surprised it's that close--I would have anticipated a larger difference--but clearly it's not much.)

The benefit of using a high octane fuel is that you get greater compression, thus more complete combustion, more power per the same volume of fuel, and (generally, under most engine speeds) higher efficiency, both theoretically (see Carnot cycle) and practically. At higher altitudes the air is thinner, thus less resultant compression. Modern electronic fuel management systems (please correct me if I'm wrong, Rick) will compensate by thinning the fuel flow and retarding ignition to achieve optimum combustion efficiency, but the result (without some kind of precompression) is lower pressure and lower power output per cycle.

Stranger

Last edited by Stranger On A Train; 04-30-2007 at 10:15 PM..
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  #19  
Old 05-01-2007, 02:01 AM
DrDeth DrDeth is offline
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There is the "pump in the middle" which might be worth a try.
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  #20  
Old 05-01-2007, 12:36 PM
control-z control-z is online now
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I would try the low octane gas and see how it runs. My understanding is that the computer will adapt to the lower octane, making your vehicle run less efficiently. But the loss may not be noticable, and you're saving 20 cents a gallon.

My theory is that all 3 grades of gasoline are pumped from the same storage tank anyway!
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  #21  
Old 05-01-2007, 12:39 PM
Thudlow Boink Thudlow Boink is offline
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Quote:
Originally Posted by control-z
I would try the low octane gas and see how it runs. My understanding is that the computer will adapt to the lower octane, making your vehicle run less efficiently. But the loss may not be noticable, and you're saving 20 cents a gallon.
Is there really a 20 cent difference where you are??
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  #22  
Old 05-01-2007, 12:41 PM
control-z control-z is online now
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Quote:
Originally Posted by Thudlow Boink
Is there really a 20 cent difference where you are??
Yeah it's always like that, right now regular is about $2.89, mid-grade is $2.99, and premium is $3.09.
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  #23  
Old 05-01-2007, 12:55 PM
Velma Velma is offline
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I have a Passat (1997, supposedly the worst year for them) and I use the mid-grade. My manual says something like "for best performance, this fuel is recommended" but I don't think it says "required." Everything is so freaking expensive to fix on this car that I am not going to risk it. My husband, an over-all car guy but not a licensed mechanic, says it makes a difference in the way the car runs, and the price difference is not big enough to make a big deal about it. Either I spend $30 a week on gas or $33.
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  #24  
Old 05-01-2007, 01:12 PM
Maus Magill Maus Magill is offline
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It so happens I drive a 2004 Passat, and wondered the same thing, so last year I cooked up a little test. I drove for three weeks on 87 octane, three weeks on 89 octane, three weeks on 93 octane, and three weeks on a blend of 89 and 93 octane.

I drove my regular route from home to my son's daycare to work, then directly home. All city driving about 10 miles each way, but with lots of hill, so I do a lot of coasting. I tried to avoid using they car on weekends of errands, but was not totally successful. Using the car's trip computer I recorded the following results.

87 Octane:
Miles: 338.4
Average speed: 24.8
MPG: 30.4

89 Octane:
Miles: 334.8
Average speed: 25.2
MPG: 31.8

93 Octane
Miles: 342.1
Average speed: 25.0
MPG: 32.2

Blend:
Miles: 339.4
Average speed: 24.9
MPG: 33.4

Like I said, I tend to coast when I can. I also turn off the engine at traffic light that I know last longer than a minute I'm sure the differences would be more pronounced if I engaged the engine more often.

My conclusion is that while I didn't notice much difference in performance, my car's computer noticed a difference in fuel consumption.
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  #25  
Old 05-01-2007, 01:34 PM
Stranger On A Train Stranger On A Train is offline
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Quote:
Originally Posted by control-z
I would try the low octane gas and see how it runs. My understanding is that the computer will adapt to the lower octane, making your vehicle run less efficiently. But the loss may not be noticable, and you're saving 20 cents a gallon.

My theory is that all 3 grades of gasoline are pumped from the same storage tank anyway!
While you're at it, why not just use the same fluid for power steering, power brakes, and engine lubrication?

Quote:
Originally Posted by Maus Magill
Like I said, I tend to coast when I can. I also turn off the engine at traffic light that I know last longer than a minute I'm sure the differences would be more pronounced if I engaged the engine more often.
This is a poor idea; what you're saving in fuel you're losing in bearing wear (not to mention the loads on your starter, alternator, and battery which will reduce their lives). The lubrication of the engine bearings depends upon a thin film of pressurized oil, which essentially isolates the crankshaft from the bearings under all but heavy loads. The pressurization is due to the movement of the shaft itself, due to the viscosity of the fluid (i.e. when it spins it drags along a layer of oil with it). Every time you stop the engine, this layer is squeezed out, and when you restart the engine, the bearings are dry, causing a brief moment of direct contact friction and accompanying wear. Unless you're going to be sitting around idling for more than a few minutes, it's really better just to leave the engine running; most car engines running at idle only sip a very small amount of fuel, so the real savings you get from shutting the engine off for a minute isn't much.

The fact that your blend of 90 and 93 got the highest fuel efficiency (and all other numbers differ only by a percent or two) suggest that it doesn't really make much difference in your car. I would still go by what the owner's manual says to use, and would tend toward what is recommended. It's true that modern fuel systems will detect detonation and adjust compression accordingly, but this doesn't make it a healthy state to operate in; as Rick states, even after adjustment detonation is possible, as is damage to the cat. Both of those are expensive things to replace. It's your car, but I tend to place more confidence in the engineers who designed it than casual experiments which don't control or observe all conceivable consequences.

Stranger
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  #26  
Old 05-01-2007, 01:36 PM
control-z control-z is online now
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Very interesting, Maus Magill. Of course you're not done yet, we hope you also have the dollar figures to calculate whether lower octane gas saved you any money.
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  #27  
Old 05-01-2007, 01:36 PM
pulykamell pulykamell is offline
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Quote:
Originally Posted by Maus Magill
I also turn off the engine at traffic light that I know last longer than a minute
Just out of curiosity, how much gas would this save? I assume not a heck of a whole lot. Plus wouldn't the savings be outweighed by the safety of not having a car turned off in the middle of a street?
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  #28  
Old 05-01-2007, 01:47 PM
Rick Rick is offline
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Perhaps several of you missed the posts where Volkswagen (the people that built the engine, and who know more about it than you do) say that premium fuel is required.
Here I found a copy of a Passat owner's manual allow me to qoute:

"You vill use premium fuel, and you vill like it. Ve have tested different fuels, and ve know what vorks, and vhat does not vork. Only a schienhunt vould try to get by on regular. If you try and use regular in shis fine Sherman automobile, you vill have to speak with the Gestapo our consoomer affairs department. Ve will also need to see your papers. Papers, ve must see your papers!"

seems pretty clear to me.
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  #29  
Old 05-01-2007, 01:53 PM
control-z control-z is online now
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Quote:
Originally Posted by Stranger On A Train
While you're at it, why not just use the same fluid for power steering, power brakes, and engine lubrication?

Stranger
I don't know if I'm feeding a troll, but of course I wouldn't use the same fluid for power steering, brakes, and engine lubrication. Let's compare apples to apples.

I would try a cheaper grade of fuel and see how things went. There are many variables involved; the car, the fuel, the driver, the environment the car is driven in. Just because someone along the line at VW says use premium fuel doesn't mean the car couldn't possibly be driven efficiently with regular fuel.
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  #30  
Old 05-01-2007, 02:09 PM
Stranger On A Train Stranger On A Train is offline
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Quote:
Originally Posted by control-z
I don't know if I'm feeding a troll, but of course I wouldn't use the same fluid for power steering, brakes, and engine lubrication. Let's compare apples to apples.

I would try a cheaper grade of fuel and see how things went. There are many variables involved; the car, the fuel, the driver, the environment the car is driven in. Just because someone along the line at VW says use premium fuel doesn't mean the car couldn't possibly be driven efficiently with regular fuel.
This isn't "apples to apples", and it's not just a matter of efficiency; by the time you "see how it went" you may have done irreparable (or at least very expensive) damage to your engine and exhaust system. If the manual says "Use premium fuel" then you should use premium fuel. To reinforce this, they don't just stick it on an obscure page in the manual, but also print it on the instrument console adjacent to the fuel guage and typically on a placard inside the fuel access door. Do you think that they're going to all this trouble because it is optional?

Quote:
Originally Posted by Rick
"You vill use premium fuel, and you vill like it. Ve have tested different fuels, and ve know what vorks, and vhat does not vork. Only a schienhunt vould try to get by on regular. If you try and use regular in shis fine Sherman automobile, you vill have to speak with the Gestapo our consoomer affairs department. Ve will also need to see your papers. Papers, ve must see your papers!"
Will they put me in "The Cooler" if I don't cooperate? 'Cause I've always wanted to be a Cooler King.

Stranger

Last edited by Stranger On A Train; 05-01-2007 at 02:11 PM..
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  #31  
Old 05-01-2007, 02:22 PM
Figaro Figaro is offline
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Quote:
Originally Posted by Rick
Perhaps several of you missed the posts where Volkswagen (the people that built the engine, and who know more about it than you do) say that premium fuel is required.
Here I found a copy of a Passat owner's manual allow me to qoute:

"You vill use premium fuel, and you vill like it. Ve have tested different fuels, and ve know what vorks, and vhat does not vork. Only a schienhunt vould try to get by on regular. If you try and use regular in shis fine Sherman automobile, you vill have to speak with the Gestapo our consoomer affairs department. Ve will also need to see your papers. Papers, ve must see your papers!"

seems pretty clear to me.
You must have the "bad cop" version of the manual...mine just says that if I'm a good boy and follow the fuel recommendations in the manual for the life of the car VW will send me a free case of frozen Schnitzel

And, just to reiterate, I am fully convinced by this thread (and googling) that there are two kinds of premium fuel cars: those that suggest it, and those that require it -- and that I am in group two.

I'm not going to experiment with my baby.

Last edited by Figaro; 05-01-2007 at 02:25 PM..
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  #32  
Old 05-01-2007, 02:27 PM
control-z control-z is online now
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Quote:
Originally Posted by Stranger On A Train
This isn't "apples to apples", and it's not just a matter of efficiency; by the time you "see how it went" you may have done irreparable (or at least very expensive) damage to your engine and exhaust system. If the manual says "Use premium fuel" then you should use premium fuel. To reinforce this, they don't just stick it on an obscure page in the manual, but also print it on the instrument console adjacent to the fuel guage and typically on a placard inside the fuel access door. Do you think that they're going to all this trouble because it is optional?
Obviously it is optional because I can put what I damn well please in my cars.

High octane is not optional if you want to get the horsepower figures the manufacturer quotes you. They require the high octane to get the maximum performance out of the engine.

If you're willing to accept less performance a modern car's engine can cope with lower octane. As long as your engine's anti-knock system does it's job, you're fine.

Let's go to Wikipedia: http://en.wikipedia.org/wiki/Octane_rating

Quote:
Using high octane fuel for an engine makes a difference when the engine is producing its maximum power. This will occur when the intake manifold has no air restriction and is running at minimum vacuum. Depending on the engine design, this particular circumstance can be anywhere along the RPM range, but is usually easy to pin-point if you can examine a print-out of the power-output (torque values) of an engine.
Furthermore:
Quote:
When this occurs, if a fuel with below recommended octane is used, then the engine will knock. Modern engines have anti-knock provisions built into the control systems and this is usually achieved by dynamically de-tuning the engine while under load by increasing the fuel-air mixture and retarding the spark. Here is a white paper that gives an example: [1] . In this example the engine maximum power is reduced by about 4% with a fuel switch from 93 to 91 octane (11 hp, from 291 to 280 hp). If the engine is being run below maximum load then the difference in octane will have even less effect. The example cited does not indicate at what elevation the test is being conducted or what the barometric pressure is. For each 1000 feet of altitude the atmospheric pressure will drop by a little less than 1 inHg (11 kPa/km). An engine that might require 93 octane at sea level may perform at maximum on a fuel rated at 91 octane if the elevation is over, say, 1000 feet.
So yes, premium gas is necessary to get all 300 horsepower out of your Mustang GT, but if you drive conservatively I submit that you may very well save money and do no damage by using lower octane fuel.
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  #33  
Old 05-01-2007, 02:40 PM
Cheez_Whia Cheez_Whia is offline
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Quote:
Originally Posted by Rick
Lower octane does not have more energy.
Higher altitudes = lower cylinder pressures = less chance for pre-ignition = lower octane requirements.
I live at over 4500', and have to use premium gas in my 2003 Vibe GT. The octane rating at this altitude for premium gas is 91, which is, IIRC from trips "downhill", lower for the same type of gas at a lower altitude. So if you drive up in the mountains, the octane is adjusted already by the gasoline companies.
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  #34  
Old 05-01-2007, 02:43 PM
Cheesesteak Cheesesteak is offline
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Quote:
Originally Posted by control-z
Obviously it is optional because I can put what I damn well please in my cars.
control-z, you seem to have this forum confused with IMHO. Your willingness to risk engine damage doesn't actually mean that the VW Passat doesn't require high octane fuel.

The people who built the engine, VW, are the ONLY people who have the detailed information necessary to determine if that particular engine requires a particular octane rating, and have issued a statement on the topic. They have said high octane is required, the requirement has a legitimate basis in IC engine theory, I question how anyone outside of a disgruntled VW engineer can suggest they are wrong.
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  #35  
Old 05-01-2007, 02:46 PM
Stranger On A Train Stranger On A Train is offline
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Quote:
Originally Posted by control-z
Obviously it is optional because I can put what I damn well please in my cars.

High octane is not optional if you want to get the horsepower figures the manufacturer quotes you. They require the high octane to get the maximum performance out of the engine.

If you're willing to accept less performance a modern car's engine can cope with lower octane. As long as your engine's anti-knock system does it's job, you're fine.

Let's go to Wikipedia: http://en.wikipedia.org/wiki/Octane_rating
And you're also free to use mineral oil in your engine lubrication. But don't go crying when you have to do a rebuild after a few thousand miles. Wikipedia is not a credible source for authoritative information; your car manual is. If you decide to run nonpremium fuel in an engine where the manufacturer mandates the use thereof, it's your choice, of course, but to claim that the only difference is that the engine will run at lower efficiency ignores both the experience offered by an automotive technician trainer (who stated that anti-knock provisions may not be enough under load to prevent detonation) and the long history of emprical knowledge from the manufacturer of the engine.

Stranger
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  #36  
Old 05-01-2007, 02:59 PM
Mr. Slant Mr.  Slant is offline
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On a side note, follow all OEM oil reccomendations super-close.
That hot little engine loves to sludge motor oil.
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  #37  
Old 05-01-2007, 04:03 PM
Rick Rick is offline
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Let's do a little math here shall we?
Using Maus Magill's numbers from upthread on the same model car there is about a 2 mpg improvement using premium. On a gas tank of just over 15 gallons that works out to saving about one gallon per tank.
Regular gas is right at $3.50 / gallon here in LA. The extra cost of premium is 0.20/ gallon. Looks to me like it is about 0.50 cents per tank more expensive to burn regular.
When you add to that the people that know the most about the engine say that premium is required, it seems to me that anyone that is arguing for running regular in this particular model car is stepping over a crumpled $10 bill to pick up a shinny penny.
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  #38  
Old 05-01-2007, 06:31 PM
Kevbo Kevbo is offline
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Quote:
Originally Posted by Stranger On A Train

The benefit of using a high octane fuel is that you get greater compression, thus more complete combustion ... At higher altitudes the air is thinner, thus less resultant compression. Modern electronic fuel management systems (please correct me if I'm wrong, Rick) will compensate by thinning the fuel flow and retarding ignition to achieve optimum combustion efficiency, but the result (without some kind of precompression) is lower pressure and lower power output per cycle.

Stranger
My pedantic nitpicks:

-Compression is determined by engine design and condition. A higher compression engine may require a higher octane rating fuel, but just putting high octane fuel in the tank does not change compression.

-At lower air density, ignition needs to be advanced, not retarded, for best efficiency, and cars have been doing that since at least the 1960's (Vacuum advance we called it, and we LIKED it!)
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  #39  
Old 05-01-2007, 07:00 PM
Stranger On A Train Stranger On A Train is offline
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Quote:
Originally Posted by Kevbo
-Compression is determined by engine design and condition. A higher compression engine may require a higher octane rating fuel, but just putting high octane fuel in the tank does not change compression.
I didn't intend to imply otherwise; using high octane fuel in a naturally aspirated low compression engine gets you nothing. In a high compression (turbocharged or supercharged) engine, however, it's a necessity. Modern fuel management systems have a good deal of intelligence built into them to optimize the fuel mix for efficient combusion. Trying to second guess this by using something other than the recommended fuel composition is like buying an HP RPN calculator and then trying to write a program to make work like a normal algebraic calculator.

Stranger
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