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Re: 93 vs. 87 Octane
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Tue, 28 Feb 2006 05:55:19 Share Post by Email
In Reply to: 93 vs. 87 Octane, Gary 900s, Mon, 27 Feb 2006 17:00:54
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Higher octane fuel only gives better performance and mileage if your particular engine can take advantage of the higher octane. Beyond a certain point, it's a waste of money.

Octane rating is simply a measure of a fuel's resistance to pre-detonation (knock, ping, etc). When you squeeze fuel, it has a tendancy to explode - the higher the octane, the less the tendancy to expode. Exploding fuel in the cylinder is bad, especially if it happens at the wrong time. The idea of the spark plug is to (1) cause the fuel to burn, not explode, in a measured manner, and (2) start that flame front at exactly the right time.

Knock is caused by either too high a compression for the octane of fuel, timing too far advanced, or hot spots in the cylinder. A turbo engine is essentially variable compression - the higher the boost, the higher the equivalent compression.

In many turbocharged cars, Saabs especially, if the engine control detects knock, it reduces boost. This effectively reduces compression. It also reduces power. To replace that power, you may run the engine at higher RPM, which uses more fuel. But if the engine control doesn't detect knock, it doesn't just keep turning up the boost until it sees knock - it only goes up so high. So if with a turbocharged car, if it doesn't knock at full boost on 89 octane, running 91 or 93 won't help - the boost doesn't get turned up any.

In your non-turbo car, the ECU adjusts timing to reduce knock. If it sees knock, it retards the timing. This also reduces performance, although it can be very hard to detect. There actually may even be a slight fuel mileage increase with retarded timing. Or not -depends on the individual engine.

So, you should run the lowest octane fuel that your engine runs happily on. Higher octane provides no value, only cost. Now, what octane to run? I can give this advice - don't pay attention to what anybody tells you is the right octane number for your car. Yes, the factory has a recommendation. That covers 90% of cars when they leave the factory. Your car is 13+ years from the factory. Compression level is different - it may be higher due to carbon buildup, or lower due to piston ring wear. You may be running a little leaner or richer. You may or may not have hot spots in the engine. So someone with your exact same year and model will probably need a different octane to avoid knock. And octane needs change with weather - engines are more resistant to knock in cold weather, more likely to knock in warm.

If your engine is running fine on 87, go with it. Around my house, 87 is 10% cheaper than 91; I don't see much 93, but I have to expect there is an even better factor. Most folks would kill for a 10% fuel savings. Absolute mileage counts if you're looking to conserve the world's fuel supply. I suspect that your aim is to reduce your fuel costs. So run the lowest octane that the car runs happily on.

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