that makes no sense. im new and ive never seen this thread, plenty of good info in it. i wouldve missed it if he didnt bump it to the top.

 

What a bunch of crap. A state as reputable as CA doesn't even have 93 octane. Shameful. Schwarzenegger needs to TERMINATE 91 crap and give you people 93 !

Hell, he should team up with Obama and make it a 50-state law, no exceptions.

Sad, all those exotics in Southern Cali ... on 91 for the most part.

 

To anybody: you'll learn more about octane at wikipedia.org than you'll ever learn here. While I always use 93, 91 or 93 or 95 doesn't make a bit of difference to those of us driving street cars - whether on the street or on the track.

 

I'm confused as well. I've had a long-standing informed belief (which I may have misunderstood) that modern cars with electronic injection, spark control and sensors (including knock) that the engine will adjust certain runtime parameters (e.g. spark advance, fuel/air mix, etc.) to ensure that there is no knock. However in doing so, performance will be commensurately degraded or enhanced in coping with this variance in octane (or knock resistance).
Thus higher octane will allow the engine to adjust and deliver higher power. But only up to a point. Similarly and only within limits, lower octane will be safe but power output will be reduced as the engine parameters are adjusted to mitigate knock. However there is a range of adjustments beyond which, ether higher or lower, the engine cannot cope. This will result in either dangerous knock (on the low end), or wasted octane ratings and cash (on the high end).
FWIW, I have a 2005 Ford GT that has no knock sensor. There is a lot of background discussion as to why Ford chose to omit it. But as a result, choosing a fuel with a higher octane rating will not deliver any changes in performance (though it's safe when it's a higher octane rating) and a lower than spec'd rating will produce knock that cannot be corrected without fresh fuel. Either of these conditions can also be addressed by downloading a new set of maps in to the ECU, but that's not something that I would frequently do.
Again, this is my understanding but may be invalid. Comments are welcomed.


[QUOTE=jhbrennan;710040]

 

Porsches are optimally tuned for 93. They will run on 91 and the ECU will compensate but performance will be reduced. OTOH, octane higher than 93 will not add more performance, the ECU will just compensate.

On Carreras w/ 16.9 gallon fuel tanks one can blend 4 gallons (3.8 actually) of 100 and top off with 91 to end up with the ideal 93.

 

+1, to add to this 91 octane reduces performance by ~8hp and not only will greater than 93 not give you gains it can actually be detrimental as higher octane burns hotter.

 

What about all the aftermarket ECU companies that claim they can tweak for higher octane to give more performance?

Also I forget who it was (EVOMSit, GIAC ?) who claim the Porsche ECU has multiple timing maps that are selected based on the fuel and other conditions.

Yeah it's EVOMSit who talk about the multiple maps :

http://www.evomsit.com/Porsche_MAIN.htm

 

Yes, you can have it tuned specifically for an octane. Typically EVOMS, etc. will tune for either 91 or 93 based on your specific market availability of those fuels. But stock, the car is tuned for 93 octane and will adjust itself for 91.

 

I had been running a mix of 100 and 91 to get 93, but I was out of town and had to fill up with 91. I didn't notice the difference UNTIL I filled it back up with the mix (93) after the one tank. There is a noticeable difference in performance with 93 vs. 91 in this car.

 

Most agree that there is only a negligible difference in power between 91 and 93 octane gas. Why opt for the 91, presumably cost, right?

I didn't see anyone mention that use of higher octane gas (in a high compression engine) results in measurably increased fuel economy. From personal experience, the improvement in mileage more than pays for itself with highway type travel. But if you drive mixed use (urban and traffic free highway) then it could just about balance out cost wise. Also, all other things being equal, higher octane burns hotter and thus keeps engine internals cleaner.

I don't know about other parts of the country, but in Connecticut there is a difference of literally only a few cents per gallon between 91 and 93.

So there isn't much of an argument for 91 in our cars unless you sit in traffic all day (such a shame in a 911) or its much cheaper in your market.

DRP

 

At the track we have 93 no -ethanol and 98 no-ethanol unleaded. I have used a few tanks of 98 just to see if there is any difference because if there was you would notice at the track.

There is no performance difference in a stock 997S with 98 octane.

 

I have a friend that tried 100 octane in his 996 GT3 and he didn't notice any improvement either. I would imagine that on a Turbo there would be noticeable gains, but not on a NA vehicle.

 

Depends on the tuning of the Turbo. If the car does not have preignition issues then probably no difference.

The higher the Octane the less volital the fuel. In theory you want to use to lower Octane number just before preignition...this all depends on the timing advance, compression and so forth on the engine.

 

I don't see why you would expect a difference without an ECU tweak. The stock ECU isn't tweaked for over 93. GIAC for example offers a button to switch the ECU to 100 octane mode which adjusts the timing, etc. I'd like to see reports of someone who actually switched the ECU and tried the race gas.

 

Same comment as I made a month ago; do some research on octane and you will find the following:


"Many high-performance engines are designed to operate with a high maximum compression, and thus demand high-octane premium gasoline. A common misconception is that power output or fuel mileage can be improved by burning higher octane fuel .... The power output of an engine depends in part on the energy density of its fuel, but similar fuels with different octane ratings have similar density. Since switching to a higher octane fuel does not add any more hydrocarbon content or oxygen, the engine cannot produce more power."