911Dave

Quite correct. Premium octane has but one purpose - to reduce or eliminate predetonation. You can also solve the problem by reducing the ignition advance if you are forced to run on anything less than 93 octane. The 928 had a circuit that you could switch on for this purpose. Of course, you sacrifice a bit of horsepower by doing so. The only cars that can take advantage of 100 octane fuel are race cars (hence the name "race fuel") or highly modified street cars, with more extreme advance curves that are prone to predetonation. Any octane that exceeds what your owner's manual calls for is a complete waste of money.

By the way, altitude reduces the octane requirement. The 93 octane that our cars need only applies to sea level. In Denver, the octanes offered are 85/87/91. The 91 is appropriate for the 997 at this altitude, and is roughly equal to 93 at sea level.

Dave07997S

You know the E46 M3 would adjust up to 95 octane, I don't know if the 997 will do the same.

Dave

1999Porsche911

With stock computer, the M3 runs as best it can with 93 octane. There is NO improvement in performance using any higher octane. The M3's stock computer will not adjust any engine setting different from factory determined levels with any octane higher than is necessary to keep engine from knocking.

dstrimbu

Dave, I remember reading that the 997's Motronic 7.8 can and will adjust for up to 95 octane ((R+M)/2), so it may be that all of us in the Americas _can_ benefit from a bit of an octane up-lift... 'specially Californians who are stuck with 91 octane near sea level.

I'll search the citation down; I know I captured it somewhere.

-d

1999Porsche911

The ONLY way that any engine will benefit from a higher octane is if the engine is currently knocking due to the fuel igniting from compression and not spark. Without knock detected on an otherwise properly running engine, the predetermined timing is set at its maximum and you have all the power you are going to get. Octane does not provide power, it reduces compression ignition.

Those of you who claim that there are computers out there that can somehow determine your octane level and will automatically increase power if you use a higher octane than your engine requires, are misguided (to be polite). There is a maximum timing setting on your engine for various engine speeds, temperature, etc. Everything else being equal, the only time your timing will be pulled will be when knock is detected.

If knock is not detected using an 85 octane fuel, then using a 90 octane will provide absolutely no increase in performance. The required octane rating of the fuel for your engine is not to provide power but to eliminate knock. Octane can only recapture lost power. It cannot create more power.

dstrimbu

Understood. However, you can't guarantee that a high-perf engine like the 997's will never run in the pre-ignition regime, under load, with 91 octane fuel; and chances are very good that the same conditions will occur (although less often) with 93 octane fuels.

If the engine knocks, with any octane-rated fuel, Motronic will retard the timing. If this occurs, you can benefit from a higher octane. The optimum environment is to provide a fuel with sufficient pre-detonation resistance (octane) to prevent the ignition timing curve from being retarded / "de-advanced" under any circumstances.

Cool? That being said, I think that octane ratings in the 93-95 octane range will help the 997 run in its design range. Anything over that is wasted.

Thanks for the clarification.

dstrimbu

Hey Joe: I understand the "perfect launch" scenario. With the C4S, it's always a tradeoff between a bog and clutch smoke. I'm not experienced enough to always hit it right, so I generally go for smooth and leave the "scared rabbit" launches to the experts.

That being said, I generally just step off the clutch around 1-1.2k and use the throttle liberally. <g>

-d

Dave07997S

Sorry disagree with you on this....

Dave

1999Porsche911

So, if what your say is true, then a stock engine is always knocking when running 93 octane gas and therefore pulling the timing?

Other than that, there are no other possabilities.

If you insist on disagreeing with me (which makes you wrong) , why not enlighten us to exactly what in the engine changes when you go from a 93 octane (which prevents knock) to a 95 octane. There must be some specific changes in the engine to increase performance.

dhasselhoff

Well,

I've learned this board is as bad as any. So far only the fella from Chicago was nice and helpful People could use a reading comprehension class or two. It wasn't a maybe it's faster question. It was a fact. The dealer checked my car. In fact, the O2 sensor is retarding my timing on 91 octance more then any car they have seen. The O2 sensor is used to detect knocking. I figured if you read my first couple of posts, you would understand I know about timing, knocking, advance and what not. I may be getting a new motor per the dealer.

I wish I could just filter out people based in New Jersey. I didn't learn to drive in a Camero.

Joe

500

What's a "Camero"? lol

1999Porsche911

There are separate knock sensors on your engine. The O2 sensor have nothing to do with detecting or controlling knock.

911Dave

See you in class. The reason you experienced better performance with higher octane is exactly what we've been saying - running too low an octane caused your car's computer to retard the timing to avoid knock. When you ran higher octane it allowed the computer to use the normal timing map, which increased power ouput. Personally I thought everyone was helpful. There are other forums if you don't like this one.

H20NOO

+1 That's the most concise and accurate explanation in the thread. These engines are optimized for about 93 octane. California 91 octane has the engine management constantly retarding the timing to avoid detonation. Horsepower and combustion efficiency ARE compromised. So, the feeling of more power with higher octane is legitimate.

As others have correctly pointed out, too much octane is a waste of money and can actually reduce combustion efficiency unless the engine management is tweaked to exploit it. Since octane provides slower and/or better controlled flame travel, 100 octane fuel in a 997S is too much. A blend of pump premium and 100 octane unleaded that results in a 93-94 octane mix should provide the best result.

Here is an excellent article that provides a wealth of information on this subject:

http://www.europeancarweb.com/tech/0...ted/index.html

MC

Rick in Colorado

Actually, I thought that my response was helpful as well, but I'm not from Chicago as my screen name will attest.