RJFabCab

Some of the Exxon stations around here have been changing over to Mobil, and I got to thinking... are their fuels one and the same? Same parent company, yes, but same refineries?

I usually use Exxon 93 supreme, but reputation for quality and brand recognition are the only 2 good reasons I could come up with. Go Valdez!

What do you use and why? Any CHEMICAL ENGINEERS out there with some scientific explanation as to why I should choose one fuel over the other?

We've all done it. You're on fumes and just pass that Citgo like it didn't even exist!

Bill, where ya at? No flames from the Royal Dutch supporters, please... hahaha

Riad

Mobil 93 (R+m)/2 method, and blondes

Sloth

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Le Chef

It seems to thrive on this diet, along with full throttle sprints. Cities make it listless and sulky. Interstates make it restless and resentful.

Top trainers recommend this diet, with an occasional quart of 0W40 as a supplement.

safetywork

I use Cheveron (because its the cheapest $/gallon) around my home. If I'm near work, I use the local Unocal (because that's the cheapest fuel nearby).

However I have noticed that I seem to get more mpg on the Unocal gas vs. the Cheveron.

DonW-Cape Cod

I read somewhere in a post, or Excellence, or Pano, that it's a good idea to use different manufactures every couple of fill-ups....the theory being that the buildup from one fuel recipe additives over time can be neutralized by a different manufacturer's recipe. IOW the different formulations have a beneficial effect. (93+oct/and no ethanol, of course)

rrys

Having worked on Gasoline and Diesel Blending systems at several different refineries in the US, CA, and Europe, I can basically say that Gasoline is a complicated product and is changing a lot as we speak. Fuel is different in various US zones based on the measured air quality. I.e. Los Angeles has special RFG (Reformulated) where rural areas can still sell conventional gas. Most zones get different volatility fuel in Summer, Winter, and Spring/Fall. Engine design (compression ratio for example) and fuel design are evolving together to meet demand for cleaner air (CO, NOx, unburned HC and many other pollutants) and the demand for more power and economy. In Jan 2004, for example you should be seeing big Sulfur reductions. High Sulfur in the US was thought to be the main reason for the Nikosil engine failures at BMW in the mid 90's. Remember the carbon build up issues on intake valves a few years ago, I think that new additives and modified engines solved that one. Your engine has become a chemical reactor with a sophisticated reactor control system needed to meet emissions qualifications. Nearly all of the RFG fuel contains the controversial MTBE (15% of RFG fuel) and several states are phasing it out with the result that alcohols (Ethanol, Methanol, Isopropanol) or ethers are taking it's place.

Some of the important issues for Porsche owners are:
1 Gasoline over 6 months old is junk, especially without added fuel stabilizer
2 Gasoline Octane can be important for perfomance as the latest Motronic FI will detune the engine if Octane is too low.
3 Once minimum Octane is achieved higher Octane does not help performance or fuel economy. Regular fuel has the same energy per gallon as premium. But so called convential fuels have a few percent more energy content per gallon than RFG fuels. High Octane fuel merely resists combustion by compression. Higher Octane is achieved by using slightly more expensive components like so called Alylate and Reformate named after refinery processes.
4 Some refiners make fuel that exceeds the rated (pump) Octane (can be highly variable i.e. batch dependent).
5 Additives are important especially to preserve the fuel and to prevent corrosion on cars used infrequently
6 Racing fuels can improve power but remember that engines were designed for a particular fuel. A Formula 1 engine is designed together with the allowable fuel. I don't have much experience with racing fuel, but would strongly suggest you don't try to mix your own.
7 As far as brands go, I think that most of the majors like Exxon/Mobil, Shell, Chevron etc. will make a very consistent and precise gasoline with good additives, it's hard to tell who is best this week. There is a lot of fuel swapping in the distribution system but usually additives will stay brand specific. Watch out for water in the gas especially at the onset of Winter or during cold snaps, and avoid filling up when the delivery truck is re-filling the station.


For further reading try:
http://www.faqs.org/faqs/autos/gasoline-faq/part1/

Riad

I noticed the Mobil station near mean states the gas "Now contains Ethanol" I was wondering about that.

Thanks for the post!

Carlos from Spain

Great post rrys

One question: 95 or 98 oct on a 996? (european octane units that it)
I always used 98 octane but often wondered if there is a difference in real life as to performace, milage, engine life....

RJFabCab

What kind of filter are you running?

rrys, excellent info. Thanks.

MKW

rrys , great info. Is it also true that all the different gas companies in one region fill up at the same refinery and just dump in some of their " brand" of additives as the only way to differentiate them ? I heard that here in the Bay Area , most if not all the companies use base gas stock refined at Tosco in Benecia .
Would you happen to know which refiners have some of that " more octane than listed " base stock ?

rrys

Carlos/RJ,

Both European and US refineries measure RON (Research Octane Number) and MON (Motor Octane Number) the same way i.e. by using a pair of specialized single cylinder Waukesha engines, one for RON and one for MON. In the US we post the "Road Octane(R+M)/2 at the pumps i.e. Premium is typically 93 Road Octane (also called CLC or AKI (Anti Knock Index)). In Europe they post the RON on the pumps and premium is 98 Research Octane. Typcially, but not always, the RON is about 10 higher than the MON. The RON correlates best to knocking under mild city driving and MON correlates best to high load highway driving. Originally there was only the RON number. I've heard it was the Germans in the late 60's that insisted on an alternate octane measure as they were blowing up engines on the Autobahn even with the proper RON. When the MON test came out it seems the US took the compromise approach to post the Road Octane, but the Europeans including the Germans stuck with the RON posting. European tradition? It would be much better if the complete specifications for the fuel were posted at the pump (RON, MON, ROAD, Reid Vapor Pressure, Oxygenate type, Sulfur, MMT amount, Aromatics.....) as I think that Porsche owners would be most interested in high MON values and owners of older cars might be interested in other compatibility issues.

Porsche (my manuals) indicate there is no problems with ethanol or MTBE but does not seem too keen on the Octane Enhancer MMT. Lead (Tetra Ethyl Lead can have an Octane of 10,000 for the first drop) will wreck your catalyst and O2 sensors and simply cannot be used. We've all heard of the various racing fuels (some are closer to chemical explosives) in dragsters etc. like NitroMethane, Nitrous Oxide, Methane, Propylene Oxide, Aviation Gas, and various other racing fuels. While some of these fuels can destroy engines there seems to be a very large number of firms that make racing fuels and many racing fuels are made by the major Oil companies for NASCAR and various other race circuits for example. I'd guess these might have some improvement in a stock Porsche, but are not that likely to be very cost effective. Just search for "racing gasoline" in Goggle and you will find lots of suppliers.

I was working with a number of enthusiasts in the UK recently, and they swear by Shell Isomax Gas. I find it interesting that the Porsche Shop Manuals recommend fuel filter change interval of 60,000 miles and the O2 sensors (4 of them) seem to be good for more than 100,000 miles as they don't even show up in the maintenance schedule.

Hey I guess with so little required maintenance we need to drive more. I would however be curious to hear any observations on racing fuel in a stock 996 engine.

Tool Pants

I am also in the SF Bay Area. There are 2 refineries. All gas comes from the same pipe. The only difference is the additive package that is squirted in depending on the brand. Jeff

ignacio

what's wrong with citgo? seems to work fine in my cars

RJFabCab

Just poking fun at Citgo name recognition.

rrys, great discussion. What is Shell Isomax fuel? Does this refer to the refining process and demetallization?

I understand that Techron may be a leading additive.

Chevron Techron