Why am I getting better mileage from regular fuel than premium?

Hey 996 fans, I make the same 40 mile drive to work every day (on the highway) and when using Premium Exxon fuel, I average 17 mpg. As an experiment, for three fill ups I put in regular Exxon fuel and am getting 19 mpg. Not only that but my 996 is driving just as good if not better on regular fuel than premium. How is this even possible? Anybody else ever has this experience?

why would you even try a lower fuel than the octane level recommended by Porsche?????? If you are trying to get more mileage to save money then you should not be driving a Porsche.

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Although I agree with you, I M CURIOUS AS TO WHY ALSO! I was putting 91 ethanol free in for a while since 93 was hard to find and I was always told the ethanol was not good for our engines. I was also told the 93 would give me better mileage so I go out of my way to put that in. It does not. I notice no difference at all...

My guess is because the 91 octane you are using is not truly ethanol free, and the 87 that you experimented with does not contain as much ethanol, or any at all. Regardless, your mpgs are not that great either way. Unless you are in a ton of stop & go traffic on you commute or hammer the car at every light and exit ramp?

Is the 93 ethanol free too? I doubt it. If you want the best mpgs, go back to the 91 without corn. If you are doing a track day, use the 93.

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Nope, unless it was the only choice and I was stranded, I would never put low octane fuel in a Porsche. These engines were designed and engineered around high octane gasoline. Compression and timing ratios on our Porsches are subject to the fuels we attempt to burn in them. If you burn fuel outside this threshold, you could potentially damage the motor over time. Why risk it over a few miles per gallon? Don't convince yourself you're doing a noble deed. Remember, no good deed goes unpunished. Put top tier gasoline in your Porsche and be thankful you get to drive an incredible car.

Saw a hottie filling up a 991 Turbo at Costco...with regular. I wanted to call her boyfriend and tell him to dump her. Don’t care what she looks like, that’s inexcusable! TC

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I did it as an experiment to see what would happen, not to save money. I have done this with other cars for the last thirty years and none of them blew up yet. But now I am curious about the ethanol in the fuel. So the question is: what is better for a 1999 Porsche 911> Premium fuel with ethanol or regular fuel with no ethanol?

I suppose the OP is getting away with it, as noted (in a Mk2 Owner's manual): "Porsche also recognizes that these fuels [93 Octane] may not always be available. 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 TMknock control' will adapt the ignition timing, if necessary." The gas mileage is perhaps a tradeoff for performance.

To the root of the question, nominally higher octane fuels are less energy dense then lower ones. As a result, you need a higher volume for the same work. For reference, go dig into using E85 or Ethanol for drag or Indy (respectively). But that misses the benefits of higher-octane in that you can get higher total engine output because you can run higher compression and/or later timing. This is because they are less volatile. So your ECU retards the ignition and for very modest driving, it's barely noticeable. But you will also get no where near your peak power and you'll experience pre-detonation/knock at off-throttle and partial load. Go find a nice long hill you can run at a regular speed at partial throttle and compare the sound your engine makes between the two tanks. It's fairly obvious which sounds better.

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^^^ This is the answer... Alrhough, to the OP, just 2mpg difference (19 vs 17mpg), that�s too close to call, as the real world commute is always a little different every time, even if you try to make it as close as possible. Temperature differences, how people brake in front of you, etc. Too close to call. I�ve heard people claim the same 2mpg for different types of oil. Possible, sure, but not scientific. Also, the idea that lower octane fuel is not suitable for an engine designed to run on higher octane depends on how you drive. Feathering the throttle and not using all available torque does not always have a negative effect - if at all. So if you are running across the flat country plains and want to cruise at 75mph on the open road, using about 50-60HP for that requirement, 91 or 89 octane is likely to have zero negative side effects. There are some fuels that have better fuel additives built-in to the higher octane, such as 92 or 93, and those additives are sometimes beneficial. But all else being equal, if your engine isn�t under load (high torque), there is no downside to lower octane. The people that say, �I�m paying for a high performance Porsche, why run low octane?� have a point, too, but again, depends on how you are driving and what your goals are.

My guess is the ECU is adapting to the lower octane.

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This post is useless without pictures.

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TC

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Yeah, I was aghast and forgot my primary duty. TC

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You run 87 octane in your Porsche 996 for 18 years??? Just trying to clarify this statement. I run 87 in my Subaru.

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The 93 was NOT ethanol free only the 91 at Stewarts (small gas/convenience chain Northeast NY) I got 93 at Cumberland Farms yesterday did not even know they carried it!

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Buy the highest octane you can find, (91 or 93) from the BUSIEST national chain you can find. I'm not a huge fan of these small mom & pop stations, because they just don't move enough volume of high octane fuel to insure freshness. Merry Christmas.

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Normally I would agree with you on small mom and pop ops. This is not a small mom and pop chain They are a major regional chain and sell more gas than most of the Sunoco stations around here.

The knock sensor will protect the engine regardless of octane however, the lower octane will increase the propensity to pre-ignite during a hot start ( you can clearly hear it when it happens). There probably isn�t much effect on the motor during the hot start pre-ignition but it puts quite a bit of stress on the starter pinion gear.

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And you assume it's her boyfriends car why?

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I know this may seem like I'm drinking the Porsche Kool Aid, but when it comes to maintaining high performance sports cars, I'm going to weigh the advice of the manufacturer along with other well respected groups (e.g. LN Engineering) when it comes to fuels in these cars. I believe there is good reasoning behind this advice. (see attached article) But at the end of the day, it's YOUR car and you can put cat **** in it if you want. Meowww! Cheers! NuttyProfessor

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Wait...didn't Porsche recommend 15,000 mile oil changes on our cars??

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To avoid being even more aghast. TC

Here in BC , 94 ethanol free is widely available at Chevron , I think Shell has it too . I use that not sure if it's better for the car , but it makes me feel better . Mind you I'm not using my car daily , I get the privilege of driving my diesel sprinter van most days ��

Here in central Texas I don�t see much ethanol-free �real gas� but have bought it in Oklahoma on a road trip. The price there wasn�t worth the extra gas mileage and it was going in an F-150 Ecoboost that had a stock 87 octane tune. I run a 93 octane tune in it normally but for long roadtips outside TX I�ll put the 87 tune in it as I�m not sure I can get 93 elsewhere. Back to Porsche content: just because our cars have knock sensors that will down-tune the engines when pre-knock is sensed doesn�t mean it�s a good idea to run 87 in them. If they offered a 996.2 with a detuned 3.4L 87 octane engine it would be the least loved, lowest resale value 911 of this family of cars. People pay thousands to get another 10-20HP. With all due respect to those running low octane, my personal opinion is that you are optimizing around the wrong thing.

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Please enlighten us on all these new sensors that prevent damage from knock. Newsflash, it's the same type of sensor they've been using for over 30 years. There is currently no technology that can sense what octane fuel is in the gas tank. Flex fuel vehicles have a sensor that can measure what percentage of alcohol is in the fuel. This is NOT the same as measuring octane. As for knock sensors measuring knock...... One tiny little problem with this mentality, the sensors only pick up the knock event after it has happened. The computer pulls back timing or adjusts the fuel mixture until the knock events stop (assuming there is enough margin to compensate, if you get a tank of really old gas it may fall outside of the adjustable parameters, this is very rare though). Once the knock events have stopped, the computer waits a designated amount of time and starts to restore the stock ignition / fuel parameters until the optimal tune is reached.....or it starts to knock again. This happens over and over and over again. Long story short, you are subjecting your motor to constant knock events (or detonation) which is never a good thing. Sure driving around like Miss Daisy is in the back seat with a bowl of hot soup may prevent the engine from ever having a knock event with the 87 octane, the next big question is: Why are you driving a Porsche??? Are these engines robust enough to survive never ending knock events from running on 87 octane? Maybe, maybe not. Detonation is the #1 cause of engine failures throughout motoring history. The negative aspects of detonation come in many forms, some minor like excessive blow-by, and some major like bent rods or holes in engine blocks. Others are long term like head gaskets that prematurely fail. None of this is worth risking unless it's a leased vehicle and you have no concerns about life after the lease is up. Frankly I think when cars are turned in for leases, a tech should check the knock count history and if it's over X they should be charged just like excessive mileage. Porsche first starting using such technology back in 1987.