When gas goes above 4 bucks a gallon, it's the premium cars who hurt the most...

August 22, 2008 3:39 PM

I want to use cheap gas in my 2006 Audi A4. Will this void my warranty?

Basically with gas so high, filling up on premium is more of a pain than it used to be. I'm looking for any savings I can 30 cents a gallon difference in California wouldn't be bad. That said, I OWN the car and don't lease it so I don't want to ruin it. I plan on giving the car to my nephew when the warranty runs out in 2010 to be a cool uncle.

That said I would need to know

1) if it really hurts the car in any real way

2) If I DO use cheap gas, is there a computer or sensor doohickey that will tell the Audi people when I bring it in for scheduled maintenance

3) If they CAN tell somehow, will this void my warranty?

Any automotive or Audi experience will be appreciated.

posted by rileyray3000 to travel & transportation (14 comments total)

A USA Today article on the topic from, heh, 2003 when premium gas was... $1.99!!!!!! SHOCK!

Most cars have a way to modify their sparkplug timing to avoid engine knock so you shouldn't hurt the car. That said, if the engine does begin to knock, fill up with premium.

From the article: "The only modern engines that should really need premium are those with superchargers, which force-feed fuel into the cylinders. "You're driving along and just tramp the gas and the knock sensor cannot sense the knock fast enough in some cases," because the supercharger boosts pressure so fast, says Bob Furey, chemist and fuels specialist at General Motors."
posted by GuyZero at 3:47 PM on August 22, 2008

Check your manual.

It doesn't void the warranty on my '08 BMW. The manual says that the engine will detune to compensate, performance may be affected, and scheduled maintenance may be required sooner.
posted by ikkyu2 at 3:59 PM on August 22, 2008

Try it for a couple of weeks and see if you like it. On that (high-compression, turbocharged, if it's the 1.8T) engine, you'll likely experience considerably worse performance, poor fuel economy, and dirtier emissions. Is it worth the $5 a week?

But yeah, it probably won't void the warranty. My car wants 93, and I use it, but the manual says it'll take 89 in a pinch, and I'm sure it'd survive on 87.
posted by uncleozzy at 4:10 PM on August 22, 2008

Putting cheaper gas than what the car is rated may decrease your MPG. Your car is rated for 91, so you might be able to get away with 89.

That said, I think saving 3$ (on a 10gal fillup) can and should be applied elsewhere.
posted by wongcorgi at 4:13 PM on August 22, 2008

People are weird. They'll pay $10 for a cocktail or two at a bar, pay $100/month for hundreds of cable channels they never watch, and spend hundreds of dollars eating out when it takes 30 minutes to cook at home, but when it comes to filling up a tank of gas, they start pinching pennies.

This was just in the NYTimes. Ultimately, you're driving a car you paid anywhere from $25-$35K, and what concerns you is a couple of bucks. People used the same logic to extrapolate that mansion dwellers who live out in the country would trade in their fuel inefficient cars, and maybe even move closer to where they work. Turns out they were wrong. Why? Because when you make "x" amount of dollars, a doubling of fuel costs is a meaningless added expense. Drink one or two less lattes a month and suddenly you've got your money back.

My take (given all the evidence)? It won't kill your car, or even damage it, but you bought a premium automobile, so you clearly have a different value for money. (So did I, so I use the same logic for myself.) Why pinch pennies when it comes to fuel, or oil, or transmission fluid? You bought an expensive car, which means that you took into account the significant increase in maintenance costs (or should have). If those $2-3 dollars per month mean something to you, you probably own too much car.
posted by SeizeTheDay at 4:23 PM on August 22, 2008 [7 favorites]

Upon further review, this document (PDF) from 2003 suggests that the warranty doesn't cover any damage caused from the use of improper fuel. However, since most (if not all) auto experts agree that non-premium fuel doesn't damage your car, you should be fine.

I was googling around until I realized that these manuals are typically not available online because there's usually a cost associated with them.
posted by SeizeTheDay at 4:51 PM on August 22, 2008

The 2006 A4 has a turbocharged inline-4. A bit of googling found this very thorough explanation:

Why do these cars require premium fuel? Is it worth it?

Strictly speaking, DSMs do not require premium fuel. However, the car was designed to accept higher-octane gasoline for a reason.

Octane is not a measure of the amount of energy in gasoline - rather, it is a measure of how well the fuel resists preignition, also known as detonation or 'knock'. Detonation is very hard on the engine, and must be avoided. The DSM cars incorporate a knock sensor specifically for the purpose of detecting knock. Higher-octane gasoline resists knock better than lower octane gasoline.

During normal operation, the engine control unit (ECU) adjusts the engine timing as far forward as it will go. This provides maximum power, but also increases the possibility of knock. This is especially true for turbocharged or supercharged engines, such as the 2.0L turbo 4-cylinder found in the upper model DSMs. Note that the ECU hears knock all the time - the problem has to be persistent to generate a reaction from the computer.

If the ECU detects excessive, continuous knock via the knock sensor, the engine timing is retarded until the knock goes away. This results in a power loss, but saves the engine from damage. So, if you fill up with 87 octane gas, your ECU will be forced to retard the timing quite a bit, and you will not get the rated power from the engine. Not only that, but the ECU may also be forced to limit turbo boost levels to accommodate the lesser quality gasoline, which again means a significant power loss. And yes, the ECU does have the power to do that - see "My boost is not strong enough. Is there a fix?".

Once you switch back to premium, the ECU will eventually advance the timing back to original levels - after a while.

Source.
posted by randomstriker at 5:36 PM on August 22, 2008

audiforums.com

When your nephew decides he wants to mod the A4 you give him, show him this.

Both of my cars require premium (91 here in CA); both are high compression engines (one's a turbo WRX wagon, the other's a naturally aspirated S2000). I've never been willing to run either on regular, because I'm not willing to give up the performance gains, and I'm not willing to risk engine damage. And besides, the difference between regular and premium, tank to tank, is only a few dimes.

As I understand it, the higher octane rating means the fuel can take more compression in the cylinder before exploding (with more vigor), thereby delivering more power. A lower octane fuel will ignite at a lower level of compression. Under high compression, it will ignite without a spark ("detonation").

Modern high compression engines with computer controlled fuel mixture and ignition systems can adjust fuel/air mixture and spark timing to accommodate lower octane fuel and reduce detonation, but at the cost of reduced performance and efficiency.
posted by notyou at 5:44 PM on August 22, 2008

You will almost certainly not void your warrantee or hurt your engine. As others have said, your engine computer should adjust for the lower octane gas and prevent any knocking.

Go for it - if you hear knocking (listen while accelerating), then switch back. But otherwise, you're all set.

Assuming you drive 10k miles/year, your car gets 30mpg, and the price difference is 10c / gal between your usual gas and the cheap stuff, you will save about $33 dollars/yr over using premium gas.

If we go with your 30c / gal price difference (which implies you're dropping from 91 octane to 87 octane), then you'll save $100/yr.
posted by zippy at 7:08 PM on August 22, 2008

It's really kind of a waste of a high compression turbocharged engine (if you have that engine and not the V6) to put low-octane gas in it. I wouldn't be surprised if you see significant performance losses, and you may end up getting poorer mileage as well. It's like using your Audi to haul around firewood.
posted by iknowizbirfmark at 7:44 AM on August 23, 2008

Anecdotal: I am not a mechanic or an engineer. I drive a 2003 325i. I periodically put premium in when I can get over my disgust at the price. Otherwise I use regular. I do see an average of 2-5 mpg difference. I can't tell if the engine is actually running better (I'm hearing impaired). Someone who is better at math can tell me if the increase in gas mileage works out to even the gap in prices. I'm too tired for algebra this morning.
posted by desjardins at 8:43 AM on August 23, 2008

The one thing that may happen is that the Check Engine warning indicator will light. If the ECU must retard timing to compensate for fuel with low combustion resistance, and the timing change is enough that the ECU considers it to be extraordinary or perhaps even a little out of scope for continuous operation, then the generic response will be "Check Engine."

Perhaps that won't bother you but it surely would bother me. Especially because, even though I may have suspicions about why the check engine warning is on, I won't know for sure.
posted by bz at 10:58 AM on August 23, 2008

It may depend on your engine. I have a 98 Passat which shared the same platform and engines as Audis around the same time. I was basically told (and can confirm, since I've done it) that the V6 engine has good anti-knock doohickeys that will allow you to use Regular gas, so I have when times are tight. I just remember to fill up with premium every few fill-ups to cleanse the system a bit.
However, I've heard it a bad idea to do it with the Turbo 4-cyl engine.
posted by ijoyner at 7:50 PM on August 23, 2008

Stay with premium. Your compression ratio is above 10 (specifically, it's 10.5). If you start using regular, your engine will probably start making an awful racket.
posted by Citrus at 8:38 AM on August 27, 2008