Glad somebody called him on this. Pretty much pegged my BS meter but this is turning into a piddling contest and that's when I sign off.

 

This is the situation we are speaking of. Using a fuel of lower grade than the manufacturer specifications. Which in some extreame cases can cause engine damage. Using the recomended fuel will work 100 percent of the time. Using a lower grade fuel will work most of the time, but its not going to work in every situation. I was just trying to make people aware that using lower grade gasoline isnt ALWAYS the best situation.

 

Just to chime in a note, when I had my acura, I did a lot of research on this and what it mainly came out to was the compression of the engine since that determines the rate of combustion. Higher compression is higher combustion so if your engine has a high compression, like v-tec engines, then youd need a higher octane so it can combust quicker. Feel free to change anything around if Im wrong...

 

I guess the fact that I prefer to drink Guinness to Coors Light means I better start treating my 2003 A6 2.7T better and splurge on an occasional tank of premium. I did find mileage to be slightly better using premium over midrange, but many other factors affect mpg as well (tire pressure, air filter, pedal pressure, outside temperature, cruising speed, starts). I would never use regular.

I'm surprised at some of the attitudes among fellow Audi owners. I think we need to find a petroleum engineer (and fellow owner) who can comment on the differences in the products and the additives. I'm just happy to find the occasional station that sells midrange for the same price as regular.

BTW, I own a 05 Crossfire that also "requires" premium and use midrange. It runs great and returns 27-28 mpg highway (once had a high of 31). No ping and I've had it to 140 before chickening out. It's as fast as the stock A6 but it gives away a lot of weight in spite of having only 215 hp.

 

I'm not a petroleum engineer, but a mechanical engineer who occasionally has to look at specifications and quality testing for the fuel oil used for running the emergency diesel generators at my plant. During my research I learned some interesting things about gasoline too, plus my 22 years of driving, riding, and turning wrenches on just about everthing under the sun.

Regarding differences in the products and additives, as I mentioned in a previous post, there are essentially none. All gasoline is essentially the same from company to company for regular and premium fuel, and has to be because the fuel is transported in pipelines. The quality of the gasoline going into the pipelines by the various oil companies is closely monitered and tested by an independent industry group in order to ensure that any single oil company is not cheating the others by putting a lower quality fuel into the pipeline. There is no way to ensure that a company gets its own product out of that pipeline at the terminal end you see. The standards used in the U.S. is the RON + MON / 2 octane testing method. Testing RON is controlled by ASTM standard D2699, and MON is ASTM standard D2700

As far as additives go, by law the additives that are put in regular and premium are the same. The additives themselves are propriatary and closely held trade secrets, however I really don't think it matters for the intent of this discussion, as the additive packages do not impact the octane numbers.

Mid-grade fuel is simply a (supposedly) 50 / 50 mixture of regular and premium fuels, mixed at the pump while you fill your car (though some stations do in fact have mid-grade tanks and the fuel was mixed elsewhere, most notably where they sell 85 octane as regular). I sometimes wonder if some stations don't cheat on this some. All it would take is to just throttle down the premium pump a bit from the volume that the regular pump runs, and it would be hard for a state agency to detect unless they took that sample to get octane tested. Typically though all the state inspection agencies check is that the pumped volume agrees with the indicated volume.

And again, as mentioned before, many western states in the Rockies sell 85 octane as regular unleaded due to the elevation effectively lowering the compression ratio of the motors. In fact this works for carburated motors, but for fuel injected motors not so much. Audi's minimum RON + MON / 2 number is 87 PERIOD.

The bottom line for me is this: Under controlled condtions, I have determined that for my car I get better gas milage using premium fuel, and this means that while I may pay more at the pump during a fill up, I do it less often and thus save money. Also, while my car has run fine on mid-grade, which I did often before I did my calculations and experimentation, you take your chances that you are getting ripped off when you pump it and it really isn't 89 octane, but something less. And, while I'm sure my car would run just fine with no damage on 87 octane, I simply won't do it because I have calculated that it is a waste of money.

My Isuzu Rodeo, on the other hand, does not get any better milage running premium fuel. Isuzu specified 87 octane, and that is what I run in it, with the following exception: When towing a trailer uphill, I have noticed some pinging on regular fuel, and that motor does not have knock sensors. So for trips when I'm towing a trailer, either my tent trailer or boat, I will pump either mid-grade or premium to prevent knock. (it also depends on what time of year it is and what the weather is like, and where I'm going...it knocks worse in the hot summer and going up long grades.)

 

i filled up with albertsons gas before i left for my 200 mile trip this weekend and got 21.6 mpg. when i filled up on my way back i used shell gas. on the way home i got 24 mpg. the weather was about the same and i drove roughly 75 the whole way down and the whole way back. i reset the trip, mpg and the avg speed both times. this kind of makes me want to buy shell gas from now on. just thought id share.

 

Personal preference aside. I own a 4.2 and the manual says premium fuel 89 octane or higher. I live in Seattle, which is at sea level and 89 is the mid grade and 92 is premium. I believe that 87 won't hurt your car but the engineers at AUDI obviuosly mad the recommendation for a reason. I use 92 and have experience no problems. Now, with a change of altitude there will be a difference in Octane and the computer will compensate. Spending time in Colorado, I noticed a difference when using thier Octane Ratings. After a couple of fill-ups I didnt notice the lag in performance any longer....My .02 cents says stick with the manufacturers rating and for peace of mind I agree with Southboroaudiguy...The little more you pay for premium will help your motor and your mind. BTW--Chevron is the best Gas around in my area..I only use this gas and never had a problem with any of my cars.

 

Interesting. I have also always had good luck with Shell gas, as I have never found water in it. In the past when I used to race my dirt bike I use to take samples of gas from various stations around where I live and tested for water. You put some in a glass jar in a dark place over night and look for seperation. The worst gas for water I've ever found was at Arco. I never buy Exxon gas for political reasons (Yes, that's right, Exxon Valdez... I have a very long memory, and I have personally seen the environmental damage they were responsible for). I once found water in Chevron gas, but that station was kind of sketchy anyway, and if memory serves they had to dig their single walled tanks out of the ground and soon went out of business.

I'm not sure that 200 miles is an adequately long test. It is just over two hundred miles from my house in eastern Washington to Seattle. I live at about 430 feet above sea level, and for some reason, it always seems like I get better milage coming back home from Seattle (essentially at sea level) than I get going there. Maybe it is because the western side of Snoqualmie Pass is steeper than the eastern side, so I'm climbing for longer going there and coasting more coming home... in any case, elevation changes make a huge difference in gas milage, as does prevailing wind directions.

 

I use the highest grade I can get in my car. I don't make a lot of money, but I won't go broke from spending and extra $2.25 to fill my car up. It runs better and and I don't have to worry about it. I would'nt pay $42,000 for a car and be worried about spending 15 cents extra a gallon on gas. That's just me. A lot of people say that it will run fine on 87, Good Luck.

2000 A6 2.7T

 

I always use Shell 93 octaine (V-Power) gas. If I can't find one of those I will use Exxon or Moble~