Fuel

I've noticed on a few US based forums, that the quality of fuel over the pond seems to be pretty poor. Is it true that you guys are putting 92 or even 91 octane fuel in your Porsches? I always put 99 octane in my cars, but will drop down to 97 if I can't find a Shell garage in time. Regular fuel in the UK is 95 octane, but my cars have never tasted a drop of anything that low. My question is (yes there is one), does the low grade of fuel diminish the power output of your car? So, a Cayman S is rated at 320bhp by Porsche, but does it actually put that out when it's being fed with 92 octane fuel? In my experience Porsches tend to over perform when put on a dyno, but obviously that's when using high quality fuel. There is a garage near me that sells 102 octane fuel, but that costs almost £3 per litre, so I haven't tried it

the octane rating on your pump is calculated differently from the octane rating on our pump, IIRC. Yours is a straight Research Octane Number (RON), whereas ours is the average of RON and Motor Octane Number (MON) ([R+M/2] method). Therefore, your octane number will be several points higher than ours, but still have the same anti-knock properties. Your 98 octane is roughly the same as our 93 octane. Not from a difference in quality, but from a difference in calculation.

Ahh - thanks for clearing that up

yep, just add or take 5% when converting.

Great info, thanks! I'm moving this to the technical section.

In some states (like here in California) the highest octane fuel we can get is 91, save for a few places that sell 100+ octane race gas. Porsche engines will run fine on 91 with a negligible decrease in performance.

Your other problem with most gas in the US is that it has ethanol in it which has less energy than petrol. Ethanol-free gas stations in the U.S. and Canada Wish I could get 93 octane in Vegas. I can get really crappy 91 and really expensive 100.

oh yeah, and 91 octane gas in 120 degree weather is really lame.

the highest octane that i've been able to find in the 808 state is 92. plus state law requires that 10% ethanol (e10) be used.

Speaking from past experience... If your car has a software upgrade and you dont specifically have a race gas map, when you put in race gas (105 RON) its make absolutly no power increase over normal premium fuel 98RON. I would have expecting some improvement, but NO not to be.

Not to be picky but a lower octane rating is not an indication of "poor" quality gas. I think the ethanol has much more of an impact on the quality than the octane.

Kehr is correct. Octane rating is not an indicator of fuel "quality", but rather a design parameter. Higher Octane fuels do not have greater specific energy per unit volume than lower octane fuels (including fuels using tetra ethyl lead or other octane enhancers). Feeding your engine higher octane fuel than what is recommended in the owners manual most likely will not result in higher power output. (Although there is evidence that some high performance production cars are designed to take advantage of higher than standard octane fuels.) However, using lower octane fuel will result in some level of power reduction because the engine control computer will retard the timing to prevent preignition (aka "knocking"). The impact of ethanol content is (like many things in life) complicated. Originally, ethanol was added to gasoline as an Octane enhancer and source of additional oxygen for combustion, to replace MTBE (Methyl Tert Butyl Ether). E85 (gasoline with 85% ethanol) has an Octane rating of 105, although the pump may not be labeled as such. Gasohol mixtures also burn cleaner than pure gasoline. Unfortunately (for us motorheads) the energy content is less, as a previous posted has noted. It's possible to overcome most of this "energy deficit" by redesiging the engine to take advantage of the higher compressibility (Octane rating) and other combustion properties of the fuel. I don't know if Porsche has tweaked the engine control computer codes to partially do this for US cars. (probably not, but you never know...). Given the financial incentives for using Ethanol in the US, I wonder if some refiners are supplying mid-range fuels that actually have octane ratings higher than what is labeled on the pump. (I believe the pump labels are minimums for octane). I don't have data to support this, other than casual discussions with a BMW technical service advisor about acceptible fuels. It would be great to see some solid dynometer data showing the different power outputs with different fuels. Maybe the folks at TPC Racing could do this? (Hint, hint!)

The worst part of adding ethanol to gasoline in the US is that it is a big net energy loser. Unlike Brazil, which uses cane leavings, in the US expensive and energy intensively farmed corn is used. If MSNBC is to be believed, 41% of the US crop - that is 15% of the world crop - is now burned in our cars. Take away the blender subsidy, the farm subsidy and the mandate and the market would collapse immediately. All we need is a widespread wheat/corn blight/rust and people everywhere will be asking whose bright idea this was.

I agree that the ethanol idea is monumentally insane and stupid from many points of view! They are now talking about levels up to 15% in our gasoline. That will cause even more corrosion and water absorption problems in motor fuel. Renew-ability is not the issue here, but look closely at the net energy cost of this energy! We should be eating corn or drinking it, but not driving with it.

The biggest problem with ethanol is just how stupid it is financially. Silly subsidies. But the problems for cars and boats are real too. The 3+% drop in fuel economy is also real. Classic Mini Cooper - Pictures of ethanol gas separation - www.MiniMania.com Looks like the last picture (at the bottom) died. My dad did some experiments with the crappy stuff a while back. Leave it sitting around very long and you get clear separation and/or water.

Other than loss of power, is there any damage that can be done to the engine when running 91 octane? -Callahan-

Theoretically the ECU is programmed with relaxed enough timings that 91 octane will work ok - and the detonation/knock sensor will prevent damage. Any time you can hear the detonation... well, it certainly doesn't give me warm fuzzies

You can add motorcycles to the list. Ducati is having big problems with deformed plastic tanks. My Paul Smart is in the shop waiting on its third fuel tank. Aprilia is having problems with leaking plastic fuel tanks. Last week, while I was getting my car serviced, I picked up a new Cayman brochure. In the brochure it says that Porsche is addressing 10% ethanol fuel by coating the fuel tank and using aluminum fuel lines (among other things). What's going to happen when we have to use 15% ethanol fuel?

Regarding the use of lower octane fuel: I agree that an occasional tankful wouldn't be harmful because the engine control computer would retard the ignition timing to prevent knock. I would NOT feed your car a steady diet of fuel rated lower than specified in the owners manual. Running the car for long periods with timing retarded may contribute to carbon formation inside the combustion chamber. Spark plugs have an optimal electrode operating temperature between 500 deg. C to 800 deg C. They at least need to run above about 450 deg. C to "self clean" themselves of carbon. The retarded timing may reduce combustion chamber temperatures enough to cause the plugs to operate below the "self cleaning" temperature. Regarding E85 and ethanol economics: I agree with the other posts. It makes no sense environmentally (at least from corn) or economically (without substantial subsidies), at least in the US. Even Al Gore is no longer supporting corn-based ethanol. Emails and letters to our governments asking for rational, science-based energy policies is a good place to start.

E85 (gasoline with 15% ethanol) has an Octane rating of 105, although the pump may not be labeled as such. Gasohol mixtures also burn cleaner than pure gasoline. Unfortunately (for us motorheads) the energy content is less, as a previous posted has noted. It's possible to overcome most of this "energy deficit" by redesiging the engine to take advantage of the higher compressibility (Octane rating) and other combustion properties of the fuel. I don't know if Porsche has tweaked the engine control computer codes to partially do this for US cars. (probably not, but you never know...). I believe you weren't thinking when you wrote that, E85 is 85% ETHANOL, and 15% gasoline. Otherwise, I agree totally with your post.

I will regularly use 93 oct fuel for street use and 100 for track. 2 comments from experince over and over (CS 2006 some mods) *100 octane will last a lot more *Automatic 8 to 10 WHP gain (on a mustang dyno) tested this twice with the same results. Jose

E85 is 85% ethanol. E85: Will it Save You Money? - Cars.com E85 - Wikipedia, the free encyclopedia

Good catch, Mitch48... Thanks. E85 is indeed 85% ethanol as you state. Amazing how I can edit for typo's then miss the obvious factual error...

Interesting discussion. I got here because I don't have my Cayman yet and I was wondering what Porsche specifies for fuel. I ask that because I had a MB that specified 91 RON and I always used 89 US because it would be 91-94 RON. Ditto for my wife's Acura RL. OTOH, had it been higher, I would have had to use the 91 or 93 US. With the difference in grades approaching $0.15 now, it is a consideration.

My Cayman S owners manual specifies "unleaded premium fuel with an octane rating of 98 RON (93 CLC or AKI)". CLC/AKI is the method used to obtain the numbers displayed on US fuel pumps. It goes on to state that "Be assured that your vehicle will operate properly on unleaded premium fuels with octane numbers of at least 95 RON (90 CLC or AKI), since the engine's "Electronic Oktane knock control" will adapt the ignition timing, if necessary" The manual also says "Do not use any fuels containing more than 10 percent ethanol by volume".

Then 89 US will be too low. About the best one can expect there is maybe 94 RON.

The CLC or AKI octane numbers mentioned are obtained by averaging the RON and MON numbers which is what is quoted on US fuel pumps. At least here in the mid-West finding the requisite 93 hasn't been an issue. If the ethanol lobby has its way keeping to the less than 10% ethanol might be another story. Does anyone know what would have to be changed to run the 15% mix that is being lobbied for now?

Nothing. IIRC vehicles made from 2007 on will/can run 15% ethanol with no problems. Not too long ago (Oct 2010?) the EPA 'approved' E15 gasoline for use in modern vehicles. Sincerely, Rockster.

I disagree, several carmakers including Porsche and BMW say 10% is the max. The EPA in their brilliance, has decreed that newer cars can use the 15% mix while older ones should continue to use the 10% mix. How that is going to be done is the big question, since no stations will carry both blends. Several groups have filed suit against EPA over this decision, and I certainly hope they prevail. One of the problems is that an advertised 10% can sometimes be 12 or 13%, so an advertised 15% can also exceed that limit. My '06 would definitely suffer from 10%, and I rarely use ANY alcohol in my gas. How much longer I can continue that I don't know, but stores selling 100% gasoline are becoming fewer and fewer. I currently buy from the company that bought my oil distributorship, and they continue to stock 100% 93 octane gasoline.

IIRC there is a caution in the owners manual recommending against using anything above E10 and even not using this if the engine manifests any untoward behavior. Supposedly while 10% is the max. allowable (now, before the new waiver) most gasolines were blended to come in below that number, down to just 7% or 8%. There is I guess a concern if someone fills up with ethanol free gasoline, then fills up a nearly empty gas tank with E15 that the engine controller's ability to control emissions properly can be compromised at least until the engine controller can readapt/relearn the proper fueling required to cleanly burn E15. Or vice versa. The EPA claims it has tested this higher E15 blend in a variety of older cars (circa 2001 and newer) and is supposed to release the results of its tests soon. There are too as you stated some pushback from some involved parties in this debate (gas resellers, distributors, car makers, etc.) and we'll see how this works out. Sincerely, Rockster.

There are several ways to look at the impact of ethanol-containing fuels: - Emissions - Material compatibility (swelling, corrosion etc.) -Performance Based on my reading of the literature, the EPA's evaluation of ethanol blends focused primarily on emissions and the performance of the emission control equipment, not the long term effects on the other fuel system components. So material compatibility is another issue. Swelling of certain polymer parts (primarily urethane, polyacrylate or ethylene acrylic elastomers) can be a problem. In addition, ethanol can include aggressive impurities like chloride and can degrade to form small amounts of acetic acid over time, both leading to increased metal part corrosion in fuel systems not designed to handle these materials. It's not a major technical hurdle for auto manufacturers to specify fuel system components fully compatible with a wide range of ethanol concentrations. I'm trying to confirm that new Porsche fuels systems ARE fully compatible and will post here what I find out. (this could take a while...) I would bet that they are, since ethanol blends have been around in the US for a few years and the additional cost for compatibility is relatively low. Performance is the last issue. Vehicles designed to run on E85 (85% Ethanol) usually have a sensor that measures the ethanol content and fuel temperature to allow the engine control computer to adjust air/fuel mixture and timing. Without this type of sensor, it's not clear how the engine control could optimize combustion for ethanol. (I don't think normal Porsche's have them) I'd also like to get more information on Porsche engine ability to deal with fuels in the 10 - 15% ethanol range. I'll bet they've tested it. I'd just like to know what they found out!