Fuel Octane and engine power

So, I can't get "good" gas here. 91 Octane is the best I can get. Shell gas, no Ethynol. It's not bad, relatively speaking....however, since my 996 C4s says it specifically wants 93 (minimum) I asked a few people for feedback. I've been told that while 93 is preferred....the 91 octane fuel won't really hurt the engine as it will "adjust" to compensate for the lower octane. I "might" lose a little power, not much...but the car will be fine. So, what I would like to know is how I can see what my engine is doing with the current fuel I'm using. Is it running at 100%? I have a Durametric unit...and there is TONS of awesome info I can pull up...but I'm not sure which info I can check to see if indeed my engine is "self-retarding" (btw, that's my word of the year) for the fuel I'm using or not. Any tech heads that can tell me what I'm looking for? There are so many ignition #'s I can see in the Durametric...but I'm not sure which ones could help me determine if my engine is running optimally or retardedly cheers

I think I would rather have 91 with no ethanol than 93 with ethanol (all I can get around DE/PA. Ethanol is just bad. Even at 10%. Someone on some site said that the 996 could advance/retard the timing so dramatically that it could run on rhino ****. Not sure I would try that.

Higher octane fuel is less prone to ignite by itself under compression than a lower octane fuel. In other words, high octane fuel has less tendency to auto-ignite. AFAIK there are a few external factors that help fuel (any gasoline) to be more prone to auto-ignite: - high humidity - high ambient / intake air temperature - lower altitudes vs. higher altitudes Your vehicles knock sensor will retard the timing to save your engine from failure if you use lower octane fuel, but it's not bulletproof, nor recommended. You will have lower performance with lower octane fuel. Given the above I am very finicky about octane, especially during Houston summers (sea-level altitude, brutal temps and high humidity). Toluene or Xylene, available at your local hardware store, will increase your octane number by about 2 points at a 1:10 or 11 ratio. Pure ethanol is about as effective in boosting your octane number, but has a lower energy density so your mileage would go down. JP

In California ( where 91 is only available) I do 2 things to assure my octane is near 93: 1. I add about 1 +1/4 gallons of toluene ( buy it at the Vista paint store) or 2: I find a Union 76 station that sells 100 octane (@ $9/gal) and add about 3 gallons to each fill of about 12-14 gallons. The car does run better and smoother, especially at RPMs above 3000. This usually adds about $15 to each fill-up. I am glad the car is not a daily driver. it does get expensive.

Octane number is the anti-detonation rating of a fuel derived from actual engine testing. There are two common methods: Motor octane and Research octane. Motor octane is a more severe test and delivers lower octane rating numbers. U.S. fuel pumps list the combined Research (R) and Motor (M) octane numbers divided by two to give an average of the two test octane values. R+M/2 For reference Euro octane ratings are higher because they list only Research octane numbers at the pump. For internal combustion engine purposes, detonation is defined as the uncontrolled burning of the END gases of combustion - after the sparkplug has fired. Detonation is also known as pinging. Pre-ignition is not detonation. Pre-ignition is the spontaneous combustion of the fuel BEFORE ignition occurs. Pre-ignition rarely occurs in modern engines unless there is some unusual mechanical malfunction or sharp edge that causes a hot spot and ignition of the fuel mixture prior to the sparkplug actually firing. The link below should help. http://en.wikipedia.org/wiki/Octane_rating Typical auto ECUs can retard the ignition timing sufficiently to use a few points lower octane than the preferred octane. So in the OP's case 93 is preferred but 91 will function fine, with a small decrease in HP at any point where the knock-sensor hears detonation and retards the timing accordingly.

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About the only way I know to be sure is to dyno the car/engine running 93 octane gas then dyno it running 91 octane gas and compare the HP/torque numbers. Based on some fuel tests I've come across the writers used a chassis dyno (and track time with a professional driver at the wheel) to measure the engine's output throughout its rpm range. The difference from best to worst was approx. 40ftlbs of torque and IIRC 20 (or less HP). Oh, test vehicle was a new (but broken in) VW GTI with the 1.8l turbo-charged engine. Various brands of gas and various octane ratings of gas were used from gas with an octane of 95 in UK numbers to 98 or even 100. I believe the engine's octane requirements from the owners manual was 98. (This one article was in a UK version of Evo magazine published a few years back.) In your car's case, with lower octane fuel, the engine's engine's max. hp/torque will be down but not by much. Under high load large throttle openings the engine controller can go open loop and feed the engine a richer air:fuel mixture for max. power/torque. This compensates somewhat for the engine controller having to possibly retard the ignition timing to avoid detonation with the lower octane fuel. Not sure if Durametric can provide any metric that you can use to compare the engine's output with two different octanes of gasoline. Surprisingly the fuel tests found the same gasolines bought at different times at different stations differed in their performance. The reason was traced to stale gasoline. Thus the recommendation is to use a quality brand of gas and to buy it from a busy station to ensure the freshest gasoline for your money. Sincerely, Macster.