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.
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?
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.
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!
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.
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.
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.)
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” :dubious: ).
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).
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.
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.
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.
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?
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.
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!
