Octane Gas
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Really?! I'm surprised that you need to ask this question. The higher octane fuel you put in your car, the better the engine will perform.
You'll hear people say that octane ratings are a scam, but this is simply not the case. These people are in denial (& tend to think that octane ratings are a oil company scam).
I don't want to write a book (and this subject is somewhat complicated and has many variables), so here is primer:
I'm assuming that you don't understand what an octane rating is, so a simple explanation is that the octane rating of a fuel describes the fuel's ability to resist detonation. Detonation is the fuel/air mixture beginning to burn before the spark plug ignites the mixture. This is caused primarily by a combination of high heat and pressure. That's why forced air induction is greatly enhanced by an intercooler - the intercooler reduces the temperature of the charge air and allows greater advancement of the engine's timing before detonation occurs (as well as increasing the amount of oxygen available - cooler = more molecules in the same volume). Pressure is related to compression ratio (& the actual amount of the air fuel charge. Forced induction = more air/fuel).
The higher the octane, the more resistant the fuel is to detonation and consequently, the long the engine can wait to ignite the fuel on it's own (called "advancing the timing"). The 3.2L motor has a compression ratio of 12.5:1 (which is pretty high for a production car). The Engine Management Unit of the car will advance the timing (which increases available torque) of the ignition until detonation is detected at which point timing will be retarded a little. Low octane gas will cause detonation to occur sooner so the EMU will be forced to retard the engine far more than is optimum to prevent knocking (which if allowed, will quickly burn holes through the tops of the pistons).
A couple of examples:
My '99 Mazda Miata is supercharged (w/ an intercooler). The temperature of the air charge at the intake manifold reaches a maximum temperature of about 140 F at full boost. I can safely run the car with the timing advanced 11 degrees without fear of detonation with 92 octane fuel. Experimentation has shown that detonation begins to occur at about 13 degrees of timing. I have never run lower octane fuel through the car because the car is not equipped with an anti-knock sensor and under boost, the motor could literally be destroyed in seconds.
The dragster I crew on is normally aspirated and we run the ignition advanced 20-22 degrees with 125 octane racing fuel (you couldn't really call the stuff gasoline).
The bottom line for you and your Audi is, you can (probably) safely run lower octane fuel in your car. Your performance and gas mileage will suffer.
Audi also strongly recommends that you run TopTier fuel in your car to prevent engine deposits. This essentially means that you need to avoid "bargain" gasoline (like Arco).
I hope that helps. I'll try to answer any other questions if you have them.
Of possible related interest. I took these pictures at Infineon Raceway earlier this year. Basically a self-serve Sunoco gas station. People were coming in and filling up their cars throughout the weekend. BMW's, Audi,s, and lots of muscle cars. (Apologies for the size if you're on a 56K connection, but really, isn't it time you got broadband?)
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You'll hear people say that octane ratings are a scam, but this is simply not the case. These people are in denial (& tend to think that octane ratings are a oil company scam).
I don't want to write a book (and this subject is somewhat complicated and has many variables), so here is primer:
I'm assuming that you don't understand what an octane rating is, so a simple explanation is that the octane rating of a fuel describes the fuel's ability to resist detonation. Detonation is the fuel/air mixture beginning to burn before the spark plug ignites the mixture. This is caused primarily by a combination of high heat and pressure. That's why forced air induction is greatly enhanced by an intercooler - the intercooler reduces the temperature of the charge air and allows greater advancement of the engine's timing before detonation occurs (as well as increasing the amount of oxygen available - cooler = more molecules in the same volume). Pressure is related to compression ratio (& the actual amount of the air fuel charge. Forced induction = more air/fuel).
The higher the octane, the more resistant the fuel is to detonation and consequently, the long the engine can wait to ignite the fuel on it's own (called "advancing the timing"). The 3.2L motor has a compression ratio of 12.5:1 (which is pretty high for a production car). The Engine Management Unit of the car will advance the timing (which increases available torque) of the ignition until detonation is detected at which point timing will be retarded a little. Low octane gas will cause detonation to occur sooner so the EMU will be forced to retard the engine far more than is optimum to prevent knocking (which if allowed, will quickly burn holes through the tops of the pistons).
A couple of examples:
My '99 Mazda Miata is supercharged (w/ an intercooler). The temperature of the air charge at the intake manifold reaches a maximum temperature of about 140 F at full boost. I can safely run the car with the timing advanced 11 degrees without fear of detonation with 92 octane fuel. Experimentation has shown that detonation begins to occur at about 13 degrees of timing. I have never run lower octane fuel through the car because the car is not equipped with an anti-knock sensor and under boost, the motor could literally be destroyed in seconds.
The dragster I crew on is normally aspirated and we run the ignition advanced 20-22 degrees with 125 octane racing fuel (you couldn't really call the stuff gasoline).
The bottom line for you and your Audi is, you can (probably) safely run lower octane fuel in your car. Your performance and gas mileage will suffer.
Audi also strongly recommends that you run TopTier fuel in your car to prevent engine deposits. This essentially means that you need to avoid "bargain" gasoline (like Arco).
I hope that helps. I'll try to answer any other questions if you have them.
Of possible related interest. I took these pictures at Infineon Raceway earlier this year. Basically a self-serve Sunoco gas station. People were coming in and filling up their cars throughout the weekend. BMW's, Audi,s, and lots of muscle cars. (Apologies for the size if you're on a 56K connection, but really, isn't it time you got broadband?)
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Thanks for the explanation Stimpyvan... It's a pity I haven't got a clue what you're talking about, being a bit crap when it comes to cars! 
"Just put the best petrol in the car that you can rather that put in the cheap stuff."
I think that's what you are trying to say!
"Just put the best petrol in the car that you can rather that put in the cheap stuff."
I think that's what you are trying to say!
Ah, I could have saved myself a lot of typing!"Just put the best petrol in the car that you can rather that put in the cheap stuff."
I think that's what you are trying to say!
No doubt someone will come along and post a better (& contradictory) explanation.
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Ah, I could have saved myself a lot of typing!
No doubt someone will come along and post a better (& contradictory) explanation.
For someone ilke me who hasn't got a clue what that technical stuff means, yes! I just noticed that I didn't inlcude anything about the ratio of fuel to air in the mixture. This also plays a pretty important role when discussing octane rating as a high ratio of fuel to air ("rich") has an overall negative effect on power, while a lower ratio of fuel to air ("lean") improves power up to the point that the engine runs so hot (exhaust gas temperature higher than about 1550 F) that no amount of timing retardation will prevent the motor from self destruction...
Just follow Highrisedrifter's summarized advice.
Just follow Highrisedrifter's summarized advice.
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If you had an S5 and it was UK you would say use Tesco Super wherever you can. Shell optimax as second choice and then if you really have to use BP or one of the other brand's super (but only fill up enough to get you to a shell or TescoThanks for the explanation Stimpyvan... It's a pity I haven't got a clue what you're talking about, being a bit crap when it comes to cars!
"Just put the best petrol in the car that you can rather that put in the cheap stuff."
I think that's what you are trying to say!
).With the A5 i dont think it makes any difference.
Although you should always buy from busy stations so the fuel is fresh. Especially in the case of Super.
Actually makes as much of a difference, if not more. The S5's compression ration is "only" 11.0:1 compared to the V6's 12.5:1. The V8 is actually probably more tolerant of [email protected] fuel.
This is very true. Also, how often the station replaces filters in the pumps makes a difference. If you ever notice that the pump you're using seems to be unusually slow, it probably is. Clogged filters will slow fuel delivery and the quality of the fuel you're pumping can be compromised.
What stimpyvan said is mostly true, except for a coiuple of little nitpicks I must add.
Detonation and pre-ignition are not exactly the same thing. Detonation is when the fuel burns too fast and the flame front moves supersonically. This happens more readily at higher compression ratios, so high octane fuel guards against this by burning more slowly under all conditions. Pre-ignition is what you call it when combustion starts before the spark due to compression alone. Higher octane fuel also guards against this. But they aren't the same thing. I will admit that any time you have conditions for pre-ignition, the resultant combustion will almost certainly be detonation, and not the slower deflagration. But you can have detonation caused by the spark (and not as the result of pre-ignition), as well.
The other issue is with the first line in your post, where you say
(In fact, it using high-octane fuel in a car that can't take advantage of it would hurt gas mileage by a tiny amount, since higher-octane fuel actually has slightly less energy density than lower-octane fuel. In reality, that difference in energy content is small and justifiably ignored.)
Detonation and pre-ignition are not exactly the same thing. Detonation is when the fuel burns too fast and the flame front moves supersonically. This happens more readily at higher compression ratios, so high octane fuel guards against this by burning more slowly under all conditions. Pre-ignition is what you call it when combustion starts before the spark due to compression alone. Higher octane fuel also guards against this. But they aren't the same thing. I will admit that any time you have conditions for pre-ignition, the resultant combustion will almost certainly be detonation, and not the slower deflagration. But you can have detonation caused by the spark (and not as the result of pre-ignition), as well.
The other issue is with the first line in your post, where you say
I know what you're getting at, but it's misleading to say that without exception. For cars whose engine management systems are not designed to take advantage of higher octane fuel (i.e. they won't advance the timing enough), boosting the octane level does nothing. This doesn't apply to cars like the A5/S5, and most other high peformance cars, because they are tuned to be able to make the most out of high-test. But your typical 1997 Chevy Cavalier will derive ZERO benefit from filling it up with 93 octane.stimpyvan said:
(In fact, it using high-octane fuel in a car that can't take advantage of it would hurt gas mileage by a tiny amount, since higher-octane fuel actually has slightly less energy density than lower-octane fuel. In reality, that difference in energy content is small and justifiably ignored.)
Aerodave is correct about detonation & pre-ignition being different and I should have made a comment about that in my first post. I was trying to avoid writing a novel and the point seemed trivial.
I will nit-pick back about his second point (regarding a '97 Cavalier gaining any performance from higher octane fuel). Since I know next to nothing about 1997 Chevrolet products, I'll assume that the Cavalier's ECU won't automatically advance timing when higher octane fuel is used, however, if the owner of the car decided to run higher octane fuel exclusively, he could advance the timing himself.
So, Aerodave is correct that pumping high octane fuel into the car won't improve engine performance on its own, but advancing the ignition timing along with higher octane fuel will. The power gained from a couple of degrees of timing advancement can be substantial, depending on the motor.
I will nit-pick back about his second point (regarding a '97 Cavalier gaining any performance from higher octane fuel). Since I know next to nothing about 1997 Chevrolet products, I'll assume that the Cavalier's ECU won't automatically advance timing when higher octane fuel is used, however, if the owner of the car decided to run higher octane fuel exclusively, he could advance the timing himself.
So, Aerodave is correct that pumping high octane fuel into the car won't improve engine performance on its own, but advancing the ignition timing along with higher octane fuel will. The power gained from a couple of degrees of timing advancement can be substantial, depending on the motor.
I doubt your Cavalier example would show any improvement with premium fuel. Like all modern cars, even the Chevy has a knock sensor, and will advance timing providing that detonation is not taking place. The compression ratio on a mainstream Chevy engine is not high enough that knock will occur under normal operating conditions when using mid grade fuel. Raising the octane level further will not result in an increase in performance, and as pointed out previously, because of the slightly lower energy content of high octane fuel, would probably result in a very small decrease in performance.
By the way, all bets are off if we start talking about forced induction engines. The potential for increasing power by increasing octane is very big in forced induction engines, although one may have to alter the engine managment program to fully exploit this.
I hope someone is paying attention to an old thread....
I own a '09 A5 3.2L, I live in Alaska and the highest octane I can get at any pump is 90 Octane. I just recently learned what the octane rating was for and that the engine has a compression ratio of 12.5:1. Of course I'm scouring the internet worried sick that I'm going to blow up my engine and thankfully I came across this and I'll sleep slightly better know the engine can overcome the danger of lower octane.
Has anyone tried those octane boosters you can buy at the auto store?
I own a '09 A5 3.2L, I live in Alaska and the highest octane I can get at any pump is 90 Octane. I just recently learned what the octane rating was for and that the engine has a compression ratio of 12.5:1. Of course I'm scouring the internet worried sick that I'm going to blow up my engine and thankfully I came across this and I'll sleep slightly better know the engine can overcome the danger of lower octane.
Has anyone tried those octane boosters you can buy at the auto store?
I recently filled up my 3.2 with 120 octane fuel. Let me say, the difference wasn't dramatic at first, but it soon became apparent that the engine ran and felt much smoother throughout the rev range. Silky is a good descriptor. It didn't feel considerably more powerful, honestly, but I did notice it felt more comfortable maintaining higher than usual speeds..
I worry a lot about the quality of the pump fuel here in the Dominican Republic. There have been reports of fuel being watered down, or octane ratings not being real, so despite my regular filling up with a station's "best" available fuel, I now make it a point to throw in a tank of some of that high-octane jungle juice every once in a while. I'm not so sure I'll make it 120 all the time (there's 99, 105, 110, 120..., and I'm not sure it can take full advantage of 120 ) since it's rather costly, but it gives me peace of mind to use some genuinely clean, high-octane fuel every once in a while.
I worry a lot about the quality of the pump fuel here in the Dominican Republic. There have been reports of fuel being watered down, or octane ratings not being real, so despite my regular filling up with a station's "best" available fuel, I now make it a point to throw in a tank of some of that high-octane jungle juice every once in a while. I'm not so sure I'll make it 120 all the time (there's 99, 105, 110, 120..., and I'm not sure it can take full advantage of 120 ) since it's rather costly, but it gives me peace of mind to use some genuinely clean, high-octane fuel every once in a while.
120 RON? My God, didn't know it existed? Are your RONs the same as ours? - in the UK, anything over 99, i.e. 'Super' pump fuel is deemed specialist/racing and has a price tag to match. As an example, the Sunoco distributor I've recently purchased my measly 102 octane fuel from charges £700 for 200 litres of 109 RON fuel (and £100 for 25 litres!)
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I believe BP were doing 102 octane for about £2.00 per litre. The trial was started down south in selected stations but I havnt heard much about it lately.
Personally I use Shell V Power which seems to deliver good results for a reasonable premium over regular
Personally I use Shell V Power which seems to deliver good results for a reasonable premium over regular
Good God! I found a sunoco distributor up here and balked when he quoted me $10 (USD) a gallon for 104 octane. If I got my conversions close that comes to roughly £1.6 per a liter.
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Most of you seem to have a pretty good grasp on this subject, however, you are missing out on one important point. Gas comes from oil, which in turn comes from melted dinosaurs and dead plants and stuff. So, you want to make sure the fuel you put into your car came from the cleanest melted dinosaurs and only the highest quality dead plants.
Also, the first half of octane is oct, which is short for October, which indicates that month is the best month to find the cleanest melted dinosaurs.
Also, the first half of octane is oct, which is short for October, which indicates that month is the best month to find the cleanest melted dinosaurs.
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There are two methods how to count octane number, European is different than US. Research Octane Number (RON) and Motor Octane Number (MON).
I have found something about it here: http://en.wikipedia.org/wiki/Octane_number
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