Even with the price of oil dropping quickly, the prices at the pump reflect this change much more slowly. Premium gasoline (91 to 93 octane) in my part of the world is still well north of $4.00 per gallon. Imagine my surprise then after reading a NY Times article that it just might be that I’ve been wasting my money for a very long time. Sure, there are some performance trade-offs, but if the article is to be believed, they are very minor. Let’s explore this topic some more.
My most modern Porsche is a 1997 993 C4S. According to the manual I should only use a “premium” grade unleaded fuel with an Octane rating of 91 or higher. Because I love my car and I don’t want to hurt it, and, I had no reason to believe I could do anything but what the manual said, I have always run premium fuel in my car. In most instances 93 octane. When I can’t find 93 I put in at least 91. What is an Octane rating? Good question.
Octane Ratings Defined
According to How Stuff Works:
The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. Lower-octane gas (like “regular” 87-octane gasoline) can handle the least amount of compression before igniting.
The name “octane” comes from the following fact: When you take crude oil and “crack” it in a refinery, you end up getting hydrocarbon chains of different lengths. These different chain lengths can then be separated from each other and blended to form different fuels. For example, you may have heard of methane, propane and butane. All three of them are hydrocarbons. Methane has just a single carbon atom. Propane has three carbon atoms chained together. Butane has four carbon atoms chained together. Pentane has five, hexane has six, heptane has seven and octane has eight carbons chained together.
It turns out that heptane handles compression very poorly. Compress it just a little and it ignites spontaneously. Octane handles compression very well — you can compress it a lot and nothing happens. Eighty-seven-octane gasoline is gasoline that contains 87-percent octane and 13-percent heptane (or some other combination of fuels that has the same performance of the 87/13 combination of octane/heptane). It spontaneously ignites at a given compression level, and can only be used in engines that do not exceed that compression ratio.
For some, that’s probably more than you ever wanted to know about Octane; for others, it’s probably not enough. Stay with me as I’m not trying to turn this into a technical post. To sum things up, we use a higher Octane rated fuel in our Porsches to avoid spontaneous combustion that could cause knocking, thus potentially damaging our motors. This begs the question…
What is Engine Knocking and How can it Damage a Motor?
Almost all cars use four-stroke gasoline engines. One of the “strokes” is the compression stroke, where the engine compresses a cylinder-full of air and gas into a much smaller volume before igniting it with a spark plug. When gas ignites by compression rather than because of the spark from the spark plug, it causes knocking in the engine. Knocking can damage an engine, by creating excessive and premature wear to the piston and cylinder walls, so it is not something you want to have happening.
Today’s newer cars, including Porsches, have sophisticated computerized engine monitoring. These electronic brains monitor the activity in our motors and can actually advance or retard the timing (when the spark fires to ignite the fuel and push down the piston) in order to curtail any potential knocking before it happens.
What about warranty issues you ask? Won’t running lower than the recommended fuel octane ratings void your warranty. Tony Fouladpour, as spokesman for Porsche North America, was quoted in the NY Times article as saying,
“If you want the car to perform at its maximum capability, the best choice would not be 87, but we do not forbid it.”
I’m not suggesting you run out and start putting 87 octane in your Porsche, in fact just the opposite. I realize the title to this article may be a little misleading, but it is not intentional. In fact, it’s the question I wanted to answer, it just took me a long time to get there. So, how much money have you wasted buying premium fuel for your Porsche? My answer is none.
Let’s face it, sure you might save a couple of bucks a fill-up, but do you really want to take the risk? Curiously enough, Porsche doesn’t set a cut-off date for the statement made by Mr. Fouladpour. Clearly you don’t want to run anything than what is recommended in anything older than a 996 model. I just wouldn’t trust the engine control systems to do a good enough job. To me, saving a few bucks at each fill up just isn’t worth the potential of a five figure engine rebuild.
There are quite a few threads on both Rennlist and Pelican on this subject. I’d be curious to know what each of you think? What are you running for gas in your car? What about Ethanol and its effect on your motor and performance? I’ll have more on this subject next week.
[Source: How Stuff Works and NY Time online edition]