True Confessions - Premium Gas

[JG986]

Curious how many Boxster owners are faithfully using Premium gas on a regular basis?

[bhduxbury]

assuming 12k miles per year, 20mpg, $3 premium, and $2.75 regular, a switch from premium to regular would save you a whopping . . . $150 -- per year.

premium always.

[rdancd816]

Premium...Period. If the book says so I will comply!

[BoxsterSbob]

Usually, premium but sometimes super or plus (the mid grade). From what I can tell there's no difference in performance.

'04S
bob

[Biz-z Z]

I use premium always. Dux above points out the marginal savings in going to regular which is usually .25 less. Respectfully, this is no place where you'd want to skimp.

[blinkwatt]

If you use anything other then premuim make sure that you use Chevron Fuel System Cleaner every 3k.

[RandallNeighbour]

Nothing but the finest premium unleaded Chevron fuel for my baby. Same goes for my Lexus LS400. The Honda Accord, however, gets the cheapest unleaded I can find in town!

Porsche knows a lot more about the car than I do. So it will be premium.

[cyclocross]

Chevron Supreme 90% of the time, Shell Premium 10%.

[MNBoxster]

Hi,

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

[sooner]

After purchasing my 98 4 weeks age, I started using 93 (Shell and sinclair) and the engine immeadiately ran smoother. I was surprised by the difference.

[nautiquelfe]

I agree this is no place to skimp. With the considerable cost of maintenance on a porsche, premium fuel is the only way to go. I prefer chevron 93, only go w/ another brand if I must.

[denverpete]

Jim has some good points. In my previous life I had to inspect bulk fuel terminals. As Jim stated, they don't have separate tanks for each vendor. The only separate tanks are for the different Octane Ratings. Chevron tankers load up right next to Wal-Mart and others. Next to the loading stations is a VERY small shed containing tanks with each vendor's "proprietary" fuel additive. These tanks couldn't have been more than a few hundred gallons each. Compare this to the bulk tanks which are in the millions of gallons range. So, you know there's only a miniscule amount of additive per gallon. I'm not saying that some additives aren't better or more effective than others - but there is absolutely no difference in the base fuel.

As to getting more power from higher octane (or less power from lower octane), I'll defer to Jim on that one. But, I did see an interesting Speed Week last night where they actually took a 997 and dyno'd it with regular and then with premium. They reported a 5hp difference between the two.

[Pilot2519j]

Originally Posted by MNBoxster

Hi,

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.


Happy Motoring!... Jim'99

Very instructional reading. You must be a mechanical engineer! Appreciate your insight on the subject. I do use premium all the time. Money wise it does not hurt the bottom line in my daily usage of 20 miles a day therefore I use the premium gas. I am a firm believer of what the manufacturer recommends for the car. Whether it is tires or gasoline I follow the prescribed recommendation.

[eslai]

Originally Posted by denverpete

As to getting more power from higher octane (or less power from lower octane), I'll defer to Jim on that one. But, I did see an interesting Speed Week last night where they actually took a 997 and dyno'd it with regular and then with premium. They reported a 5hp difference between the two.

The point there is that you're not getting MORE power from the higher octane fuel, you're getting LESS than you're supposed to get by using the lower octane fuel as the computer retards timing to compensate for the lower quality of the fuel.

[MNBoxster]

Originally Posted by Pilot2519j

Very instructional reading. You must be a mechanical engineer! Appreciate your insight on the subject. I do use premium all the time. Money wise it does not hurt the bottom line in my daily usage of 20 miles a day therefore I use the premium gas. I am a firm believer of what the manufacturer recommends for the car. Whether it is tires or gasoline I follow the prescribed recommendation.

Hi,

Thank you for your kind words. I am not a Mechanical Engineer, but do have an Engineering degree, also a Science degree and I took waay more Chemistry than any Human Being should ever have to.

The truth is, I've hung around Motorsports for so long, that you learn this stuff by osmosis if nothing else. Also, I have a longtime friend with whom I co-owned and raced a Formula Ford some years back. Coincidentally, he is the Safety Director at the Ashland Oil Company Refinery at Pine Bend here in Mpls., where about 80% of all the Gas consumed in the State is refined, so I've had the $0.25 Tour several times...

Happy Motoring!... Jim'99

[Jeph]

This would make a good survey, though it seems most people here are following their manual's recommendations. The manual notes that the less octane can throw the check engine light.

[MNBoxster]

Originally Posted by Jeph

... The manual notes that the less octane can throw the check engine light...

Hi,

Sure, makes sense. If the DME cannot adequately compensate for the lower Octane it would cause a MIL.

An Engine may operate fine on the Lower Octane, with the DME compensating for it. But, put the Engine under Load (High Speed, Passing, Going Uphill), or raise the Ambient Temperature (lowering the detonation threshold) and suddenly it cannot cope, which it signals to you in the form of a MIL. I suspect that you'd feel the loss of Performance just as the MIL came on too. Also, an Engine which is Knocking has to cause Havoc with the Emissions Readings from the O² Sensors too...

Happy Motoring!... Jim'99

[9000RPMan]

I use premium gas at every fill up. Why buy a premium sports car only to use cheap gas? If Porsche recommends 93, then 93 is what my car drinks.

[tqtran]

93 octane all the way, and if I can't find it I will add octance boost.
The way I see it, if you can't afford the $0.20 extra per gallon then sell your car and get a Honda.