Which fuel?

Neighborhood gas station now offers ethanol-free 89 octane gas. Should I stick with 91 octane + 10% etoh or move over to 89 octane with no etoh added. I am in Hawaii where the temps vary from a low of 65 to a high of 90.

The octane rating is important. It is what keep the fuel from igniting during compression earlier than expected and causing knock. Your car can compensate for lower octane of 89 but it will not be happy and degrade performance. Do not use 89 octane. Your 996 can handle the ethanol 91 octane. It's those of us in cold regions where we store the car over the winter where the ethanol is an issue. Is there no 93 octane on the island?

No there isn't.

Quotes source post: 19279849

^^^^ This. Buy octane for your 911. Use the ethanol free stuff in your lawnmower and other applications where it sits for long periods. You can also use it in vehicles that are okay with that octane level. But not the 911.

I would stick with anything over 90 octane. Here in Houston we have ethanol free 92 that I run in both my 991.1 C2S & my 996 C4 without issue.

I posed the question to Flat-six innovations a while back, Raby said to stick with the higher octane.

In my area I can buy "Ethanol free" in 87 and 92 Octane, The 87 goes in lawn mowers and Weed Eaters, the 92 goes in my Boats and Porsche....But these stations are not common, so the Porsche is driven often and usually gets a E10 in 91 up Octane "Top Tier".

We have E10 in 89, 91 and 93 octane. I always fill up with 93.

I am fortunate that I can get 93 non-ethanol top tier gas locally. Cars run good on it. My M9Experience motor sure hated the crappy California gas whe we took it out west last fall. I had to add Octane booster to every tank while on the Left coast.

Quotes source post: 19281445

That crappy California gas has made an enormous reduction in smog over the last 40 years. I wonder how all those high performance cars there survive. When I watch Youtube videos from there I forget the abundant amount of exotic vehicles they have. I guess that's because they have the 5th largest economy in the world. They must be doing something right.

Agree with everyone else, stick with higher octane to ensure there is no pre-ignition. If you think about what pinging (pre-ignition) is (the fuel igniting too early, while the piston is still on the upswing ), it's easy to see how that can create super-heated temps in the cylinder and wear from opposing forces within the engine. That's bad for all engines, but far worse for the 996 because the M96 cylinders aren't encased in a solid block like most conventional engines and therefore are easier to go out of round when superheated (causing bore scoring). Octane doesn't add any power at all -- 110 octane is no better than 91 for a car that only needs 91 octane. But you need enough octane for what your engine demands -- and should not count on the internal anti-pinging compensation to cope with running lower octane. The above said about additional octane not adding power, fuel composition does affect both power and mileage -- how much depends on how much Ethanol is mixed in. Gas contains 114,100 BTU's per gallon, while Ethanol only contains 76,100 BTU's per gallon. Depending on how you do the math, gas has 50% more energy than Ethanol, or Ethanol has 1/3rd the energy of gas -- either way it is a big difference. Running pure gas would have considerably more power and mileage than running pure Ethanol in a car (not a real scenario for street cars, since pure Ethanol isn't available at gas stations). When Ethanol is used in the smaller percentages typically seen at the pump (E10 or E15 - 10% or 15%), the power/mileage difference isn't that meaningful (though definitely not zero). For "Flex Fuel" cars that are designed to allow running either E85 (85% Ethanol) or pure gas, they will be noticeably faster and have much better gas mileage when running pure gas versus E85. Pure Ethanol is naturally very high in octane (109), but, as stated, octane doesn't help performance at all in a street car.

Quotes source post: 19284379

But higher octane fuel has a more efficient burn meaning the motor will actually run cooler which ultimately results in power gains so I'm slightly confused on your response.

Quotes source post: 19284391

As long as you have enough octane to prevent the cylinder from firing from heat & pressure alone (before the spark plug fires), the differences in octane are trivial. Lower octane is easier to ignite, so "theoretically" having just the minimum octane would perform slightly better since you want it to ignite exactly when the spark plug ignites, and excess octane could make it slightly more difficult to burn, so it might be ever so slightly cooler, but really the combustion is essentially immediate with either fuel once the spark plug fires, so I don't think there is any real difference between low and high octane as long as it is ignited by the spark plug and not self-combusting due to heat and pressure. I'm happy to be corrected if wrong.

Quotes source post: 19283764

I would surmise that those Cali Hi-Po cars are running on E85 or buying canned fuel from VP or Sunoco..and using it up fast enough for it not to go bad...( alcohol goes bad really quickly) Ethanol or Methanol does have a lower BTU of energy per gallon than gasoline, but at a AFR of about 9 to 1 for Alcohol vs 13 to 1 for Gasoline the power output is much greater ( although the MPG is much worse),, that why Alky cars make more horsepower .. As far as Octane goes, the higher Octane gives a slower much more controlled burn that allows more timing advance( BTDC) , for ideal power you want the combustion process to be completed by 22* after TDC....Lower Octane gives a more of an explosion instead of a strong steady push....Ideally you want a very strong steady push from TDC with the burn rate/maximum pressure completed by 22* after TDC and the push to continue... I ran a race car in the 80's on pure Meth, back then you could buy a 55 gal drum for $210 ....but you are burning twice the amount of Methanol, than you do Gasoline so it = $4 a gal worth of high Octane Gas...but Alcohol makes more power and also has a cooling effect in the intake as it vaporizes, The intake will actually be cool to the touch...

What about adding 100 octane to 91 octane? What ratio? Pointless? There's one gas station in Cali I know of that has this.

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Blending fuels is very common on Race Cars and Dirt Bikes, but is a PITA for an everyday street driven car....

Quotes source post: 19284535

I used to do this for my supercharged Miata. It was indeed a pain.

Quotes source post: 19284527

Does high octane gas really burn more slowly once ignited? I've seen it said that it burns more slowly in various articles, but usually the context is talking about resistance to pressure/heat combustion -- not slower burning once ignited by the spark plug. I definitely don't know one way or the other -- so I'm not trying to say I'm right -- just never seen any indication that it burns slower than lower octane once ignited by spark.

Quotes source post: 19287767

This is a highly dynamic/complicated subject with numerous variables and design considerations, but to answer the question simply and directly, YES, higher Octane fuel burns more slowly/controllably.. And to answer a question you are thinking but didn't ask, If an engine is designed and tuned "perfectly" to run on 87 Octane fuel, putting 100 Octane fuel in it WILL lose horsepower...( although run a bit cooler) It's all about splitting hairs....

Does it burn more slowly, or is it just a tad harder to ignite in the first place?

Quotes source post: 19287847

Thanks -- that's the tradeoff I was expecting in post #13, just didn't think there was a meaningful difference in burn rate once ignited.

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Higher Octane fuel burns more slowly/controllably, when the a/f mixture is lit, the expansion increases the pressure on the un-lit portion of the a/f mixture as the flame travels across the chamber, if the fuel does not have a high enough Octane it is unstable and the un-lit portion will "explode" violently instead of a steady strong burn/push.... This is equivalent to a hammer strike on the piston to rotate the crankshaft compared to a strong steady push on the piston over a number of degrees to rotate the crankshaft... It's called ":detonation" and can be very detrimental to the engine....

SKIP FOR PRESIDENT 2024

Quotes source post: 19289013

Gotcha, thanks Skip. I guess I was thinking of pre-ignition, where the gas ignites before the spark plug fires.

Quotes source post: 19289068

Right, pre-ignition and detonation get confused by all but the most proficient of tuners due to the similarities of the sound they make when happening... Pre-ignition is caused by running a spark plug that is too hot, an overheating combustion chamber, or carbon deposit, that ignites the fuel before the spark... They both end with uncontrolled explosions on the pistons but pre-ignition is even more damaging because it is started sooner...............pre-ignition happens before the intended combustion process, and detonation happens at near the end of the combustion process, therefore an engine can withstand a small amount of detonation, but pre-ignition can not be tolerated at all...

Quotes source post: 19280238

My mistake with original post. Premium gas here is 92 octane not 91.

All you can do is always use the highest octane you can get (91 is available almost everywhere) and always use �top tier� fuel if possible. If you can�t get anything higher than 87 or 89 you can always add octane boost to get you to the proper level.