Is Audi being funny or are they saying I can technically use jet fuel?

September 15, 2008 12:01 PM

What's 95 octane gas and where do I buy it?

I've got an Audi A4 and it says the minimum gas rating I can use is 91. Which is premium. And which I use. But the max is octane 95. Which I don't use because I haven't seen it. Lord knows I'd love whatever extra performance I might get out of it with the higher octane but do any places SELL that kind of gas? In Southern California?

posted by rileyray3000 to travel & transportation (25 comments total)

Octane is measured differently in the US and Europe. They are probably referring the the European octane measurement.
posted by Fuzzy Skinner at 12:09 PM on September 15, 2008 [1 favorite]

I believe octane rating is an indication o the temperature at which the fuel ignites. From what I've been told by mechanics, you should always use the lowest octane at which your engine doesn't knock. I'm sure someone will expand on this or correct me, which I'd love to happen. But it sounds like you're A-OK.
posted by piedmont at 12:09 PM on September 15, 2008

Octane Ratings - I am not convinced you understand what the deal is with them.

If you want 95, you'll probably need to buy it from a race shop or call around and ask them...
posted by iamabot at 12:11 PM on September 15, 2008

I used to work for VW of America and drove Audis and VWs with that octane rating. I now own a Nissan Murano with a premium octane rating as well. I have never put anything greater than standard gasoline in any of those cars and they all ran fine. Nobody else I worked with did either. It's a farce... put in the "cheap" stuff. Your car won't run any differently and you won't do any damage to it.

If you don't want to take my word for it, here's an article in USA Today that says the same thing.
posted by fusinski at 12:12 PM on September 15, 2008 [1 favorite]

Some fuel sold for high-performance boats (cigarette boats & the like) has elevated octane numbers. Likewise, if you have any car tracks in the area, they'd be able to point you towards a vendor.

Local race-tracks and marinas would know about high-octane fuel and where to obtain it.

I would guess, though, that California has some regulations about the maximum octane of fuel that can be used in a street-legal automobile, since the emission control technology on your car might not work with the higher octane fuel.
posted by Crosius at 12:12 PM on September 15, 2008

Watch for lead if you mess around with high-octane fuel, for example 100 octane aviation fuel (AKA av gas). It uses tetra-ethyl lead to raise octane, but if you run lead through your new car you will probably wreck the catalytic converter.
posted by exogenous at 12:20 PM on September 15, 2008

Not sure it's totally related, but I have a 1969 Pontiac Grand Prix that wants to run on leaded premium. In Minneapolis, I buy 100 octane leaded fuel ($6.99 a gallon). Meant for off-road use... Perhaps you could find someone selling something similar for unleaded?

$6.99 ? yep!
posted by thilmony at 12:24 PM on September 15, 2008

95 is the european equivalent of 90-91 in US figures. The difference is that the european octane number is the straight Research Octane Number (RON), whilst the US number is the average of the RON and the Motor Octane Number (MON).

As a rule of thumb, the US octane number is 4 to 5 points lower than the european one. So the stated range of 91 - 95 equates to a range (at a US pump) of about 86 - 91. Therefore the premium you are using is at the high end of the rating, not the low end.

Hope that makes sense.
posted by le morte de bea arthur at 12:28 PM on September 15, 2008

The difference in octane ratings is between Motor Octane and Research Octane. Here in the states, the number on the pump is the average of the two, and should have something below the number like "(R+M)/2 method".

If its a US Audi, it should have both sets of numbers listed behind the gas cap.

Does your car have a turbocharged motor? If so, I would strongly suggest not running anything but the recommended grade.

The reason for premium: higher octane fuel has a higher threshold for preignition (knocking/"pinging"), allowing the engine management to advance the timing (trigger a spark a longer period of time before TDC) and allowing the manufacturer to design the engine with a higher compression ratio. This has the effect of increasing specific power output (power/displacement).

Modern engine management systems have sensor to detect knock, and dial back the ignition accordingly. So if you're out in the boonies and can't get premium, you'll survive with regular. You're going to lose power, though.

That said, turbocharged vehicles are much more prone to knock, hence the use of premium fuel. Running regular is asking for trouble. Severe knock can break rods, pistons, and other engine internals.
posted by kableh at 12:32 PM on September 15, 2008 [1 favorite]

Also, that USA Today article is kind of misleading. For one, they seem to be talking about people putting premium in vehicles that only call for regular. That is a waste, certainly. They then go on to say that premium is only _really_ necessary in supercharged vehicles (turbocharged vehicles aren't forced induction, then?), going into a half assed explanation of why. Basically, the article seems to be filled with half truths and conjecture, which is worthless, even if it comes from some oil industry exec.
posted by kableh at 12:41 PM on September 15, 2008

76 stations sometimes carry it. It is generally considered racing gas, you don't need it. I tried it once in my Subaru, it didn't seem to make much difference. Here's a map of locations in Southern CA.
posted by doctor_negative at 12:43 PM on September 15, 2008

and if you do get actual aviation fuel, don't put it in a gasoline engine. it's actually highly refined diesel.

Av gas is gasoline for aviation use. It isn't the same thing as Jet A, which is similar to diesel.
posted by ssg at 12:44 PM on September 15, 2008

Octane, at the root, is the resistance of a gasoline fuel to preignition, aka "knock". Higher performance engines use higher compression ratios (which make the fuel-air ratio more compressed, and thus hotter), so they need higher octane.
I say "need" higher octane. But there's another variable - spark advance (actually, there's a ton of variables, but...). Modern engine controls can change the spark timing -where in the engine's rotation the spark occurs - to reduce knock. If you put in regular unleaded, and did not have modern engine management, your car would likely knock like my Impala when I accidentally put the wrong thickness head gaskets and wound up with a compression ratio that is about 30% higher than you can run on *any* street gasoline. Anyway. When your computer senses the knock due to cheaper gas, it retards the timing, and the knock goes away. Some of your power goes away, too; I usually recommend to my friends with high-power vehicles to use regular unless they plan on driving the shit out of their cars.

Regarding thilmony's Grand Prix, back in teh day it was relatively easy to get leaded gas with a high octane rating, or to just drive down to the regional airport and get some airplane gas. SO a number of cars were made with this in mind, and had insane compression ratios from the factory. Leaded gas was also designed for in other ways in those days: it acted as a lubricant and a cushion for the valves, so on pre-catalytic-converter cars with original engines need to have a lead additive to keep from destroying your engine.
posted by notsnot at 12:44 PM on September 15, 2008

The "octane rating" article on Wikipedia is pretty good.
posted by notsnot at 12:45 PM on September 15, 2008

Let me summarize this for you:

1) It's against the law in CA to fuel your car with >91 octane and then run it on the street.

2) Higher octane gas, up to 100, is sold at a few gas stations and race shops. It is sometimes leaded (which will ruin your catalytic convertor and make your car incapable of passing smog) but most often not.

3) The higher octane gas is extremely expensive. It confers no benefit to your engine or your horsepower rating unless you've had the car extensively modified. Your engine control unit, which is a computer and a bunch of sensors, is paying attention to make sure you haven't put in really crappy 87 octane gas, in which case it will detune the engine a bit, but your engine control unit is certainly *not* going to tune your car to get the most out of 91 octane gas. It is instead going to burn 91 octane gas using a tuning table with a huge safety margin to prevent any knocking.

4) Aftermarket modifications to your car are generally focused on reducing this margin of safety, exchanging it for increasing horsepower.
posted by ikkyu2 at 12:46 PM on September 15, 2008

It's also probably worth noting, and this is somewhat of a derail, so please excuse, that Audi and other manufacturers have other priorities besides 1) generating power from the engine 2) leaving a safety margin against knocking. They also need to 3) ensure aggregate-fleet fuel economy numbers that won't result in cash penalties from the US Gov; 4) preserve engine life and reduce component wear and tear (very important for the German automakers that have inclusive service warranties); 5) Keep the hp ratings low enough that the buyers won't be dinged with huge insurance premiums; and 6) keep the hp rating low enough that the performance of the car you bought won't be better than whatever is next up on the rung of the price/performance marketing ladder.

Because of all these things, aftermarket tuners have a lot of room to maneuver to increase a car's hp. There's a huge market for Audi aftermarket mods. MeFi me if you need a pointer to a forum where people discuss such things.
posted by ikkyu2 at 12:50 PM on September 15, 2008

Your engine control unit, which is a computer and a bunch of sensors, is paying attention to make sure you haven't put in really crappy 87 octane gas, in which case it will detune the engine a bit

The thing is, engines don't have "octane sensors", they have knock sensors. That is, the only way you engine management knows that you've put in lower octane fuel is because knock is detected. In a Mazda rotary, _one_ ping can blow an apex seal. Chances are your engine will survive mild knock, but again, in a turbocharged vehicle I wouldn't take the chance.

but your engine control unit is certainly *not* going to tune your car to get the most out of 91 octane gas.

Nonsense. Modern Audi ECUs (in the B5 S4, at least) are constantly tweaking the ignition timing (as well as fuel trim, etc.), whether retarding timing to prevent knock or advancing timing when knock subsides. If the engine is designed for premium, it will, in fact, "tune" itself to take advantage.
posted by kableh at 12:59 PM on September 15, 2008

I own a VW R32 and it does in fact reduce timing in response to lower octane gas.

ikkyu2 has the science, otherwise. You can get US 93 octane gas in the more, erm, "pollution tolerant" parts of the country.
posted by rhizome at 1:35 PM on September 15, 2008

Lots of good and some odd/bad advice above. The quick answers:

1. Use the 91 octane - Higher octane gas will not improve your car's performance and may degrade performance or fuel efficiency.

2. Octane indicates the ignition temperature - high octane means it's harder to burn which is necessary is some high compression motors. Your's is not one of them.

3. High octane gas for generally specialized motors (boats, race cars, planes, etc) can be found where those specialty vehicles are operated (marina, race tracks, airports). It is very expensive and often contains lead which will damage your vehicle.

4. You may use lower octane gas but it is not recommended. Even though you may not hear an engine knock, it may be a result of your engine degrading performance to accommodate for the improper fuel (by getting feedback from the knock sensor). You may use higher octane gas but there will be no performance increase and you may damage your car, also fuel efficiency may suffer.

5. You will likely see a substantial increase in performance and fuel economy by properly maintaining your car - change the filters, fluids, properly inflate tires, clean the throttle body/intake, etc.

* The VWVortex.com forums are a great place to investigate how to maintain and improve your Audi; it's a very pleasant community, I wish there was something like it for Honda (ToyotaNation.com is the Toyota community, generally they're very together and nice).
posted by unclezeb at 1:42 PM on September 15, 2008 [1 favorite]

All good advice. And even if I don't get to use the locations for racing fuel, it's good to know where they are.
posted by rileyray3000 at 2:28 PM on September 15, 2008

Some cars designed for regular fuel will achieve a slight improvement in power with higher octane fuel. The Accord six cylinder is one such vehicle. When a new version came out in 2004 a Honda engineer mentioned this and then a bunch of gear heads tested it out and found that indeed the premium fuel gave about 7 extra hp due in part to the ECU allowing a little bit richer fuel ratio and advancing the timing. Who knows whether that works with your car, but if you have access to higher octane fuel and a dynamometer you could test it out. Better yet follow ikkyu2's advice. He is right about the performance enhancement industry for Audis. As for getting higher octane fuel, auto parts stores sell octane enhancers.
posted by caddis at 2:38 PM on September 15, 2008

kableh: If the engine is designed for premium, it will, in fact, "tune" itself to take advantage.

Right. And as I said, it will not take full advantage. The factory instructs the ECU to leave a giant margin between "all the power that could be extracted from premium before knocking" and what the engine actually does. The ECU does not, for instance, advance the timing and boost and lean out the mix until the engine knocks, and then back off 1%. It instead uses a pre-set table of values that is quite conservative. This is not nonsense, it is something that must be understood if you want to get the most out of your engine.

That's why you can get aftermarket mods that do nothing but tweak the ECU settings and get more power out of 91 octane gas.
posted by ikkyu2 at 3:25 PM on September 15, 2008

The thing is, engines don't have "octane sensors", they have knock sensors. That is, the only way you engine management knows that you've put in lower octane fuel is because knock is detected.

Not true. They have O2 sensors, and these will also allow the ECU to establish the quality of the fuel. Cars designed to run on higher octane fuel run pretty lumpy and with a much blacker exhaust colour (ie, colder and less efficient burning) and this will only be corrected (as ikkyu2 states) within the granularity of the pre-programmed table of values in the ECU.

Stay as close as you can to the recommended grade - run lower if you really have to, but I'd certainly suggest adding the odd tankful of the proper stuff - especially on long journeys - to try and burn off any odd deposits that have formed from the lower quality stuff. My car demands premium, but I'd happily run one premium, one not and expect no issues. Maybe even less often - say 3 then 1 tankfulls. If I can afford it, though, I burn the better stuff - hotter fuel burns cleaner and helps long term health of the engine as deposits from incomplete burning clog things up eventually. The only disadvantage of the higher octane stuff is cost. The disadvantage of the lower stuff is in engine life - either minor or not so minor depending on the disparity in fuel qualities.

Engine knocking: If you can hear it, rather than detect it with the sensor, it is already very much damaging your engine for the longer term. You cannot tune for knock accurately with your ears.
posted by Brockles at 5:20 PM on September 15, 2008

Right. And as I said, it will not take full advantage. The factory instructs the ECU to leave a giant margin between "all the power that could be extracted from premium before knocking" and what the engine actually does. The ECU does not, for instance, advance the timing and boost and lean out the mix until the engine knocks, and then back off 1%. It instead uses a pre-set table of values that is quite conservative. This is not nonsense, it is something that must be understood if you want to get the most out of your engine.

That's not necessarily true. They may not tune "up" to a value, but they will back off from their optimal settings if knock is detected. So if a vehicle's optimal settings are based on 91 octane, putting in 87 will cause a loss of power and potentially a loss of MPG, as the fuel is being used less efficiently.

So if the engineers did a good job of tuning the ECU to take full advantage of premium gas, it may well take "full advantage". (As an example, the Buick 3800 supercharged engines generally have some KR or knock reduction during normal operation. It does indeed attempt to extract as much as possible out of the fuel.)

So a vehicle designed for 91 will suffer, but one designed for 87 may not gain. But it could- if the ECU is detecting knock with 87, putting in 89 or 91 will allow it to run with its optimal settings.
posted by gjc at 6:06 AM on September 16, 2008

One correction to what has been said here - O2 sensors only read whether the engine is running rich or lean, which is not a function of the octane level of the fuel. If the engine is running lean is will be more likely to knock, but that is a side effect.
posted by rfs at 6:46 AM on September 16, 2008