[quote=jhbrennan]Technically...that is probably true but immeasurable in most cases as 91 octane will not cause the timing to be retarded and therefore there is no loss of power (except as noted above). Unless under extreme driving conditions, such as heat, heavy load, etc, most cars speced to run 93 octane will run fine on 91 octane with no problems whatsoever. Many of these cars also run fine on 89 octane.

I feel no difference between a stock 911 running 91 versus 93, but if you are going to push the car hard in hotter climates and 93 is available....use it for added protection from knock.

 

Around Philly...93 is all we have been using for quite some time...no problems whatsoever...

 

Last time I checked, the ignition system is in fact part of "the computer" that you referenced which controls/responds to the knock sensors.
Where are you getting your technical information regarding the advance/retard of timing ? What do you think would happen if you put 110 in a 997 ?

 

If you put 110 octane in your stock Porsche, it would most probably be faster. This is because your wallet would be lighter.

 

LOL - i like it, but I ain't buyin'

 

why would higher octane harm the car...

in fact 93 is recommended.

 

Yes, you are correct. I just drove back to that station you mention and realized that it is 100 octane, 93 is no longer according to the hard to understand woman who worked in the booth. So I assume that 100 is likely much to high for the 997S 3.8 engine?

 

The local gas stations carry 'Ultra 98' for 1.17$. I am sure that number is RON and based on European standards .. which would place it around 93/94 ((R+M)/2) American.

Either way .. who wants a group buy on gasoline from Kuwait?

 

Here in the UK the manual says the engine is designed to give optimum performance with fuels with 98 RON/88 MON and that the minimum is 95 RON/85 MON. There are other differences in the manual: e.g. the advice for the first 300km is to avoid high engine speeds especially when the engine is cold - no upper rpm limit stated and no guidance given on switching off regarding running at fast idle for a couple of minutes as per the US version

 

PEOPLE....LISTEN....octane is not power! Octane is a rating of how volital the fuel is. The lower the octane the more volital (ie quicker to ignite and burn) and the higher the octane number the less volital the fuel is. A engine manufacture will suggest the best octane rating based on the engines compression. High compression engines need a less volital fuel thus the higher octane rating. So with that in mind using the lowest octane rating and not causing the engines knock sensor to kick in will give you the best results for you engine.

 

Sorry man, but I think you are confused. You're octane/volatility relationship is flawed. Volatility is the rate at which gasoline vaporizes at a given temperature. The volatility of a gasoline is important because the liquid gasoline must be mixed with air and vaporized in order to burn in the engine.
Higher octane fuel requires a higher temperature to burn. As compression ratio or pressure increases so does the need for higher octane fuel. Most engines today are low compression engines therefore requiring a lower octane fuel (87). Any higher octane than required is just wasting money, unless of course you have a high compression motor, like the 997.

 

Flawed???think NOT! What you said is pretty much what I said...just different wording. Like what you said I also agree that using a higher octane rating will not make more power...in fact you will lose power.

 

LOL - hardly. Octane rating has absolutly nothing to do with volatility. Please cite something or provide a technical reference otherwise.

 

Modern ECUs (esp. in performance cars) most definitely take advantage of higher octane fuels. While the ECU can't "read" what the octane is, it can sense that it is not getting knock and will advance timing until knock is detected (within reason). Once knock is detected, the ECU will dial the timing back a little. This advance will add HP...

As long as the fuel is unleaded, it won't hurt your 997S. I used to run VP 104 in my Evo - it allowed me to crank up the boost and it smoothed out my torque/power curves. In a N/A car, it will advance timing and add HP. When I moved from CA to VA and 91 octane to 93 octane, the power definitely went up.

Bottom line, if you can get higher octane fuel and can afford it, DO IT!!!

 

You can't really have "too much" octane in any engine, at least so far as conventional, commonly available fuel goes. After 115 octane or so, some race fuel starts to have specific gravity and burn speed issues, and can actually reduce performance unless you're running enough boost to put the car into space.

The old adage of lower pump grade fuel making just as much relative power, so long as you don't run into detonation, is true to a point. The problem is, you can't hear anything other than severe detonation, and if you're at that point you're doing significant damage to the engine as the computer can't pull enough timing to prevent it.

93-96 (R+M/2) for a high compression street-run car is good insurance against detonation of any kind, especially in a turbo application. On a road course, especially in high temps, it's mandatory so far as I'm concerned.