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Old 02-03-2006, 02:44 PM   #10
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Join Date: Sep 2005
Location: Minneapolis/St. Paul, Minnesota, USA
Posts: 3,308

The reason you want 93 Octane or better Fuel is that higher Octane Fuel can be compressed more before spontaneously combusting. It does not have any more Power (read Energy) than Regular 87 Octane, in fact in some instances it may actually contain less Energy depending upon which Octane Boosters are used.

When your Engine takes in a mixture of Fuel and Air, the piston compresses it. This raises the Temperature of the Mixture a couple hundred degrees above ambient Temperature. The engine is designed to compress the Mix to just Under the point where it will spontaneously combust and the spark from the ignition supplies the final amount of energy required to get it to burn. It's important that the Fuel/Air Mix doesn't combust prematurely as the rotating Engine is in the wrong part of it's Cycle and serious damage can occur. By differing the Chemistry of the Fuel, one can change it's combustion point to match the compression which the Engine produces.

The Boxster M96 Engine has a Compression ratio of 11:1 which is really quite high for a Naturally Aspirated car. You need to use a Fuel which can withstand this degree of compression without spontaneously combusting.

Because these Cars are run in different climates, temperatures, and altitudes, throughout the World, the Octane actually required differs slightly as well. But, to be all-encompassing, Porsche has specified that 93 Octane Fuel be used. This Octane level will not detonate until the right moment in all circumstances.

But, this level Fuel is not always available in all markets, so the DME is capable of compensating for lower Octane Fuel - down to 89 Octane. It does this by literally listening for pre-detonation (or knock) to occur with a Knock Sensor located on each bank of Cylinders. A Knock Sensor is really a sort of microphone and when it picks up the sound from a knocking Cylinder, the DME retards the Spark, that is, makes the Spark occur earlier in the Engine Cycle to control the detonation of the Fuel. You can retard the timing only so much on an Engine. and in this case, it does so sufficiently to use 89 Octane Fuel. The downside to this is that when you retard the Spark, you also reduce the Power the Engine can make (in this case, somewhere in the neighborhood of 5-8HP).

You can use whatever Octane Fuel which does not produce Knock. The problem is, you can not always tell when your Engine is Knocking so using the recommended Grade is a smart thing.

Also, all Gasoline is essentially the same. The Various Brands do not operate their own refineries. All the Gas is produced in Bulk at one of a limited number of refineries and then shipped via pipeline to various Terminals throughout the Country. It is at these Terminals where the various proprietary additives, including some Octane Boosters, are mixed with the Raw Gasoline and it is then trucked out to the Stations to be sold. BTW, while many Companies have proprietary Additives, these vary only slightly from one another, and are in the same Chemical Families and do the exact same things. The differences are TOTALLY for Marketing Purposes - so you buy Brand X instead of Brand Y. There are no definitive tests proving one Brand is better than any other.

There can be wide variations in the actual Octane Rating of a Fuel. For instance, here in the Mid-West, and other parts of the Country, Fuel changes with the Season. Here, they add up to 10% Ethanol (grain alcohol) to reduce pollutants in the Winter months because the Cold inhibits their dispersal and concentrates them at Ground Level. Now, Ethanol is an Octane Booster, so when it's added, the Fuel's Octane raises about 0.5 - 1.0 points (even though the Pump still says 87, 90 or 93 Octane). But, Ethanol, while it will raise the Octane of a Fuel, contains less Energy than an equal volume of Gasoline, so you experience a slight drop in MPG as well.

The Rating on the Pump is the Minimum Rating for that Fuel. In actuality, the Octane can be several points higher as a result of different Crude(s), or Batches produced - there is a significant variation in refining it. This is why you can sometimes get by with using lower grade Fuel (because it's actually higher grade than the Minimum Rating it carries), but again, you can never be certain beyond the Minimum Rating posted for that Fuel, this variation can differ from Truckload to Truckload, or Tank to Tank.

Considering the minimal cost difference, the drop in Performance and potential for serious damage, in a Pinch it's OK to use a lesser grade, but I would not recommend a steady Diet of it for your Car...

Happy Motoring!... Jim'99

Last edited by MNBoxster; 02-03-2006 at 06:21 PM.
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