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.
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]
View Comments (16)
I drive an 04 BMW 325ci and I have always used premium. I tried doing some research when I first got the car and finally decided it wasn't worth taking a chance to only save a couple bucks a fill up.
@Cruzr,
I think you made the right decision.
I use the highest octane, that is what it says to do in the manual for my 997 so thats what i do :) . Gas isnt THAT expensive to where i would get the lower grade to save a few bucks. Perhaps a few less Starbucks per week would be a better place to cut costs, not on a $80,000 car! :) .
Here in the Midwest I buy most of my gas from an independent (non-brand name) marketer. The company puts out a brochure, basically stating that US formulations of gasoline are much different than European fuels, including additional anti-knock compounds. Allegedly, octane has not much to do with performance, just knocking. So, they suggest using the lowest octane (and least expensive gasoline) you can without experiencing knocks. I have run both 87 and 89 in other Euro brand cars, with no knocking and no perceptible difference in performance. Like the author, though, for the relative little driving the 993 gets and the smallish tank, I put 91 in. Less than $2 difference per tank. Just my 2 cents worth.
@Randall,
It's funny, I run 93 or 91 in my 993 (depends on what state I'm in) but don't mind putting 87 in my MB E class. In fact, I basically put 87 in for two out of ever three fills. I've noticed no performance degradation and of course, no knocking. I would never consider doing this in my Porsche.
The downside to having an engine knock can be a rebuild - I didn't see mention of when anti-knock sensors became standard equipment. I suspect the 993 Turbo has the sensors but I'm not so sure the Carrera does. I'll have to research that. But I know my 74 911 which has a new, modern EFI Brain that monitors fuel mixture definitely does NOT have a anti-knock sensor. The only sensors it needs are the O2 (for mixture) and temperature - again for mixture.
As others have said, taking the chance and having a *very* expensive engine failure would be an expensive way to save a little...
Larry,
Very true. I think the title of this post is very misleading and we might need to change it. As we state at the end of the article.
"So, how much money have you wasted buying premium fuel for your Porsche? My answer is none.
penny-wise-pound-foolishLet’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."
Let us know what you find out about anti-knock sensors becoming standard equipment. We'll take a look as well.
I have a 2005 Boxster S which I tried regular in for a few weeks. I did not really notice much difference in performance. I did notice that my gas mileage dropped by 1,5-2 miles per gallon. I wen back to premium and the mileage returned to what it had been. It seems to me there was no worthwhile saving and a possible reduction in performance.
L.T. Your experience seems to mimic most. A small reduction in performance and mileage. We've seen this in almost all our cars (Porsche or otherwise) when we've tried anything but the recommended fuel.
when anyone spends so much $ on any car the gas price is nothing you buy one of the best cars you buy the best gas. if you cant afford the gas then you shouldnt have the car.
The issue of fuel octane was a topic of discussion at The Racing Group (TRG) tech session our region had there and the Porsche trained engineers said there would not be an issue using mid grade 89 octane fuel. I have used it and notice no performance difference and no knock. I only use Chevron or Shell gas so I can't speak for some of the discount stations.
@RL, thats OK for you to say, but in the UK, gas prices are currently around $9 US per gallon. Costs me $150 to fill a 997 with premium, and that lasts about 3-4 days.
It only costs $120 to fill with regular, and I can't tell the difference in performance or mileage.