silverd2

ONLY if I was stranded (out of gas) in central Afghanistan, with a US flag on my car, would I consider putting 87 octane in my tank.

Yes 87 it will get you from point A to point B, with reduced HP and mpg's....and a human can live indefinitely on dog food and toilet water.

nyrobbie77

seeing as i'm getting 22mpg with the 87 octane in mixed driving, i have yet to see a decline in mpg. I was getting the same mpg with 93 octane prior to the switch. sure, hard throttle its not the same, but normal driving there is no difference.

can you provide any detail/proof as to how the 87 octane is bad for the car? fuel injector failure, clogged cats, etc? our compression ratio, while slightly higher than say an old chebby, isn't extremely high (10.8:1). silverD2, you've helped me out quite a bit in the past and i really respect your opinions/knowledge with these cars.

i've done 5 tank fulls of 87 octane, my next 5 will be with the 93 and will plot the results.

silverd2

Even though this a thread is about gas mileage, my point has very little to do with mpg's, which I originally left out (but added last minute) of my post.

Yes, at steady highway speeds, the mileage may be the same with high and low octane. Your original post said "mostly highway", but then you called it "mixed driving"...that could mean anything and has never meant "mostly highway" to me.

Our engines are plenty high enough compression (and performance) to be optimized for the grade of fuel used, right up to (and maybe beyond) 93 octane...not just the instant response of knock sensors, but built-in ignition timing and engine management maps utilized as the ECU "detects" the fuel rating's limits. Even my 1990 Passat (2 liter 16V), at just 10:1 compression, has built in ignition maps for low to high octane ratings and can not reach full advance without premium gasoline...Info straight from VW.

I'm not going to link to any specific source, because they're all over the map and include intelligent opinions on both sides of the issue. But almost all agree that unscientific home testing is no definitive answer...too many variables, including the driver knowing what type of fuel they are using and "maybe" being a little light on the throttle (esp during accel), trying to squeeze out more mileage. In true mixed driving (lots of speed change and some stopping), if a robot was driving your car in the same conditions, achieving the same speeds and rate of acceleration (all things equal & unbiased) I think the results might be a little different.

Most opinions (including manufacturers) also agree that under load, 87 not only loses power (due to not only knock sensor input, but the present retarded map in use by the ECU) but is "less efficient"...maybe just meaning lower power for the amount of gas being consumed (sounds like less mpg's to me).
There are also many statements out there, some from manufacturers, about non-specific engine life longevity issues possible when using low octane gas in a modern engine designed to optimize premium fuel...top efficiency = longest life(?). These engines also come from the factory with a recommendation for premium fuel...that is not some conspiracy with the oil companies...it's a considered opinion from an engineering viewpoint.

You also mentioned "for normal driving, I don't notice any difference". I think we have 2 very different views of what "normal driving" is. With respect, either there is something wrong with your ECU or you don't have much feel for your engine. Just pulling away (not "flogging") from a stop is a huge load on a motor, the same with gaining speed from 65-75 mph...plenty enough to be using a different map matched to the octane rating in use. By design, it does NOT have to trip the knock sensor to be running in a retarded state under any size load.

I've recently tried 87 octane in my Passat...it was a strain to even make up my hill...10 mph slower under full throttle. Also a while back, during a local fuel shortage, I was forced to put mid-grade in my D2....HATED IT. In virtually every aspect of driving, except steady highway, it felt like it had been re-tuned to run on 6 cylinders.

I did not buy a 2 ton car with a 300 hp V8 to worry about the price of gas or mpg's...however, I DO wish it was cheaper...I'm not "well to do" by any stretch. I bought it for the engineering and fun, which includes a motor running at the full, powerful efficiency it was designed to provide.

Like I stated, 87 will get you from here to there, probably without blowing up...but I just don't like eating dog food

Peace & Cheers,

Sam

ty4sound

I have a 99 a8 and I get around 27 mpg HW and 20 City. As for I use 91 or 93 oct. and you can tell the differance. At 65 mph my rpm @ 1700 with lower oct gas you can feel you have to give it more gas than with higher oct gas. An on other note not all 91 /93oct gas are the same. I can fell the differance so watch which company you use.

nyrobbie77

Thanks for the responses. sorry about the confusion of mixed vs highway driving. my trip to work is 40 miles each way, approx 5 miles of city roads till i'm at steady highway speeds, approx 70mph. half of the remaining 35 miles are b-roads with some hills but maintain 70mph with very little to no traffic. weekend errands are local but i use my tundra typically.

what i do notice on the 87 grade, hard-full throttle is def lacking compared to 93, and possibly it seems that the trans is a little "busier", upshifting too soon for throttle input, having to downshift, then upshift again. perhaps this is due to retarded timing, not producing enough power for given throttle input vs gear. also, trans seems slow to down shift if i give it the beans to pass someone. at times i'm nearly at their drivers door before it downshifts fully. with 93 it seems to reduce this effect. I had an '89 100 quattro awhile back, and noticed right away if i put in 87 vs 93. that car of course was stick shift, so i think throttle input and response is better felt compared to our D2's busy automatic.

i'm trying to gather some data, using the same driving routes and styles, measuring the mpg. i haven't seen any mpg change, was getting the same 22mpg average with the 93. then again, i was logging during winter months on the 93, and with a very bad cv axle. i had just switched to 87 octane before i replaced the cv, mpg's had dropped to 18mpg, but jumped back to 22mpg after changing the cv shaft.

5 gas tanks each is a small sample size statistically speaking, but so far the data has been fairly steady. i'm doing this to see if i can see a cost/benefit, and also to shut my dad up about the difference between a premium recommended vs premium required vehicle. he is a big chevy guy (has worked for GM all his life), so he is used to vehicle than can basically run on goat ****. his last position before his motorcycle accident was brand quality manager for the Duramax Diesel team. he is always talking about the quality of (diesel) fuel and its affects, which is strange that he doesn't see the correlation with premium fuel requirements.

anywho, i do appreciate the input guys. i will let u guys know in a couple weeks what the results of my tests are. i'm just about done with my last 87 octane tank and will refuel the next 5 with 93 and post my results.

twentysevenlitres

Late to the party (which is not unusual!)

Ausralians (like Canadians) have followed the developed world into the metric era... (just a little tease guys, please don't take offence )

As such I had to do a little conversion of my l/100km figures:

My 2001 FL S8 gets approx 24mpg highway, but drops off to about 16 (yes, 16!)mpg city cycle.
However, I am handicapped by a short commute to work (1.5 miles - I really should walk, but when it's raining I can't be bothered!), which essentially leaves the car in Open Loop (AKA Choke) the whole trip.

When I've spent a week doing longer trips, I have got around 18mpg climbing up and down the local hills!

ThaboTheWuff

My 99 gets about 15-16mpg in very short trips 10 miles around here at 5000Ft. On the highway, flat not mountains, I can get 25Mpg but you've gotta drive normally and really not stomp on the gas pedal so I rarely get this. Other day we did a 2 hour sprint on a flat straight road between 85+ mph and I averaged 22 I think. I think for the age of these cars and their complexity and AWD system it's pretty good. If you're going to commute 100 miles a day then these cars are not really for the budget minded.

twentysevenlitres

I was just reading the post about using 87 Octane.
I didn't know you guys could buy fuel that bad?

Our "crap" everyday fuel is 91 RON.
"Premium" starts at 95 RON for about a 10c/l more (minimum for the S8)
Then there's 98 RON at 14-15c/l more
Shell claims 100 RON for Optimax at certain outlets

Some 7/11's (which took over Mobil retail stations a couple of years ago) sell a 94 RON (I think) E10, and United sell a 100 RON E10 also.

silverd2

^^ Love it! ^^

93 is the normal premium where I live...anything below that feels horrible to me. Some states (or areas?) don't allow higher than 91 (92?), as I understand it.

93 alcohol-free is available at rare locations (most 93 here is 8-10% alcohol) and has a little better kick to it...and a little better mpg's (alcohol doesn't put out as much heat as real gas)....but it runs up to 15 cents higher per gallon.

twentysevenlitres

Ah, here we go - we're not comparing Apples with Apples!
According to Wikipedia (the last word in everything factual and correct!):

USA and Canada use an Average of RON and MON which give an "Anti Knock Index" number, which is what you quote as "Octane", or AKI.
Whereas in Australia, everything is quoted using RON only.
RON and MON measure fuel behaviour as a percentage of a baseline using pure isooctane (which is a specific hydrocarbon fuel molecule).
RON is essentially a measure of self ignition under compression
MON is a work measure using a preheated fuel in an engine under load and advancing the timing until it knocks (begins to pre-ignite).

Octane / RON
87 / 91 - "Regular Unleaded"
91 / 95 - "Premium"
93 / 98 - "Premium" aka "Optimax" (Shell) "Ultimate" (BP) etc
94 / 100 - Usually branded by whoever is offering it

Almost all "Premium" fuels here are ethanol free. In Victoria (where I am) regular unleaded is mostly ethanol free, and must be labeled if it contains it. In NSW, all regular unleaded is E10.

Start adding Ethanol and the variances get bigger.
Ethanol has a high RON, but a low MON.

In the USA/Canada AKI Octane measure, you would get a more representative figure of the effect of Ethanol has on the work available from the fuel.

In Australia, where RON only is quoted, we get the wool pulled over our eyes a bit as Ethanol will tend to produce a higher figure.
In NSW all regular unleaded is E10, but down in Victoria where I am, most fuel has no Ethanol, and where E10 is available it's often quoted as "94 RON", when based on what I've just read, it's probably equivalent to your 87 Octane 10% Ethanol Blend.
In fact, one Fuel company here offers E10 at 98 RON, now I know why it's so cheap (8c or 9c premium on regular unleaded), as it's only equivalent to 95 RON, or USA 91 Octane!

So in conclusion, 91 RON is still "crap", but 87 Octane isn't quite as "crap" as I thought it was