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View Full Version : O.T. sort of...premium fuel


richard
12-26-2002, 04:39 AM
I got in an argument at a party concerning using premium fuel. A friend told me I'm just wasting my money using it. I argued that even if the computer compensated for the lower octane my gas milage would suffer. I know there are some other reasons for using premium...and help?

Zaphod
12-26-2002, 05:32 AM
Your argument was right on the money. A lower octane fuel than what the manufacturer recommends will result in the knock sensor retarding timing in order to prevent knock. That will reduce power and fuel economy.

Next time you talk to your friend remind him that we have relatively high compression engines (10.5:1 in my MY02) necessitating the higher octane. 87 octane worked ok in the 8.7 or 9.0:1 motors from the 70's but doesn't cut it with the higher compression motors out there right now.

Jspeed
12-26-2002, 01:38 PM
A high compression engine such as ours requires premium fuel even if it's equipped w/ knock sensor. The knock sensor is there to prevent damage in case fuel w/ inadequate octane rating is put in the car. Show your friend the page below if they want proof that you're right.

http://www.ftc.gov/bcp/conline/pubs/autos/octane.htm

brave1heart
12-26-2002, 02:36 PM
I've always used 93 in both my car and my wife's A4. My understanding is that higher-octane fuel (up to a point at least) burns more efficiently and you get slightly better gas mileage and better performance, so it pays for itself. And I recall reading somewhere that the E46 adjusts the timing based on the fuel grade, so although 91 is the minimum recommended, 93 definitely gets you better performance.

Kaz
12-26-2002, 05:13 PM
Actually higher octane fuels do NOT burn more 'efficiently' in and of itself. In fact, the fact that it is harder to burn is exactly why high-compression (and boosted) engines need it to prevent knocking/pinging (predetonation).

Predetonation occurs when the air/fuel mixture that gets sucked in during the first stroke, then compressed in the 2nd stroke, is ignited by some hot spot in the combustion chamber BEFORE it is supposed to (i.e. when the spark plug is fired). This is not good for obvious reasons. A higher octane fuel is more resistant to being lit off by these hot spots. Its often said that cars tend to need higher octane as they age. This is because of carbon and other buildup in the combustion chamber that create more of these 'hot spots.'

One of the ways this can be compensated for in modern cars is to make the engine run cooler, in a sense, by messing with things like ignition timing, A/F ratios or valve timing, but these all tend to decrease the performance.

Pinecone
12-26-2002, 05:52 PM
Originally posted by Kaz
Actually higher octane fuels do NOT burn more 'efficiently' in and of itself. In fact, the fact that it is harder to burn is exactly why high-compression (and boosted) engines need it to prevent knocking/pinging (predetonation).

Predetonation occurs when the air/fuel mixture that gets sucked in during the first stroke, then compressed in the 2nd stroke, is ignited by some hot spot in the combustion chamber BEFORE it is supposed to (i.e. when the spark plug is fired). This is not good for obvious reasons. A higher octane fuel is more resistant to being lit off by these hot spots. Its often said that cars tend to need higher octane as they age. This is because of carbon and other buildup in the combustion chamber that create more of these 'hot spots.'

One of the ways this can be compensated for in modern cars is to make the engine run cooler, in a sense, by messing with things like ignition timing, A/F ratios or valve timing, but these all tend to decrease the performance.

Wrong. Higher octane fuels resist detonation, which occurs when the heat and pressure in the cylinder causes additional flame fronts to start. This actually occurs because the heat and pressure breaks down some molecules into less stable molecules and they ignite. High octane fuels are not inherently more or less easy to ignite. Unless you are talking about alternative fuels such as Nitromethane.

Wrong. That is PRE-IGNITION. There isn't any "predetonation". Higher octane fuel is no more or less likely to suffer from this problem. But many higher octane fuels are formulated with extra additives to help prevent such buildup and thus help prevent pre-ignition. But if there is a hot spot, high octane fuel will suffer from pre-ignition.

Sort of wrong. Changing the ignition timing, VANOS cam timing, A/F ratio all does change the combustion temp, but it also changes the combustion PRESSURE, which is what really helps reduce detonation. And based on the gas laws (PV=nRT) lower pressure= lower temperature.

Also realize that the engine is designed and tuned to produce the rated power at a given air pressure and air temperature. So in order to produce close to rated power in other temp and pressure combinations, the ignition timing, cam timing and A/F ratio can run a little "hotter" to produce close to normal. Also in some conditions with the rated octane the ignition timing and A/F ratio is adjusted to reduce power to avoid detonation.

So if you run higher octane fuel it allows you some extra timing advance in some conditons resulting in slightly more power or at least full power in conditions that otherwise would result in reduced power.

The E46 M3 seems to respond to octane increases up to about 96 PON (Pump Octane Number).

Pinecone
12-26-2002, 05:53 PM
Oh, and some higher octane fuels DO burn more efficiently since they are made up of different molecules (post lead ban). In the days of leaded fuels, this might not be true.