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Re: Octane in 94 9k Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Tue, 15 Nov 2005 05:56:46
In Reply to: Octane in 94 9k, Eleximp, Mon, 14 Nov 2005 13:14:02 Members do not see ads below this line. - Help Keep This Site Online - Signup |
As Sterling 98 says, octane rating is the measure of the resistance to knock of a fuel. The octane is measured by running the fuel in a special engine, and adjustments made until it knocks.
Higher octane fuel does not burn hotter, cleaner, or have more power. It simply resists blowing up when squeezed more than lower octane fuel. As an example, diesel fuel runs about 15-20 octane, yet diesels get decent power. They run at low octane because they WANT the mixture to blow up when squeezed - that's how they work.
So higher octane fuel isn't any more powerful. So why use it, and why is it used in more powerful cars? Well, one easy way to more power from an engine is to run at a higher compression ratio. But that requires a higher octane fuel, to prevent the air-fuel mixture from exploding before the spark plug sets it off at the right time. Run low octane in a high compression engine, and the fuel explosion at the wrong time (versus burning at the right time) can blow holes in pistons.
Compression ratio is set mechanically by the stroke of the engine and the piston/cylinder geometry. That doesn't change. (Valve timing can affect it some). But a turbocharged engine can vary the effective compression ratio by compressing the intake charge more (or less) before it's stuffed into the cylinder. So a turbocharged car usually has a lower mechanical compression ratio, and makes up the rest with the turbocharger. Since the Saab system adjusts the turbocharger, it can adjust the compression ratio as needed.
Today, most cars have some very sophisticated knock control systems, so it is rare an engine will knock. The engine control can adjust a variety of things (ignition timing, fuel-air ratio, valves (some cars) and turbocharger pressure (some cars) to keep the engine from knocking. But if the engine control is making those adjustments, it is usually doing something that is reducing performance. Maybe it retards the timing a little, or cuts back on turbo boost. The engine won't knock, but it won't produce full power. Can you notice? Depends on how much it adjusts.
Back to your question - what octane to use? That's easy - I can't tell you a number, and NOBODY else can, either. Oh, yes, there is a number on the fuel filler door and in the manual. That was determined by the manufacturer to be the right octane reading for all (or nearly all) new engines as they come from the factory so that the engine would not knock when the engine is running at full rated power. A higher octane fuel won't help - The engine control has the wick turned all the way up - there is no more adjustment up.
So that covers the broad range of manufacturing variation for that particular engine when new. Each engine is slightly different from the factory, and differences only grow as the engine ages. Some engines may be a little tighter when new, others looser - some may rev easier, breathe a little freer, or some not. And as they age, some will lose compression due to ring wear, others will increase compression as carbon builds up. Most likely, combinations of both and many more things.
You've got a 12 year old car (it was probably sold in 1993). That engine has changed a lot since it left the factory. It may require a higher octane fuel now, because you've got some carbon on the piston heads, and maybe a hot spot on the cylinder head that promotes knock. Or maybe there is a little wear on the rings and the compression is lower, and the injectors are running on the rich side, both promote less knock. I don't know your engine. I don't know how you drive your car, or what altitude you drive at or the ambient temperature (that affects it, too).
I can tell you this - you can run the lowest octane that reliably prevents knock. If the engine isn't knocking, it isn't being damaged. Period.
Next, with a higher octane, you may see an increase in performance. MAYBE. Only if the engine control can take advantage of it. If the Trionic is retarding the timing slightly in a cylinder or two (yes, it can do it cylinder by cylinder) on 87, it may be able to run at the rated timing on 89. So why not run 91? Becausse the Trionic isn't going to advance the timing any more to take advantage of it.
So, how do you tell? At a low tank, put in half a tank of 87, and see how the car runs. If you can't tell the difference between 87 and 91, run 87. If the car runs worse, fill the rest of the tank with some 91 to get an average of about 89. (that's why half a tank - you aren't stuck with a full tank of gas where the car runs poorly). Keep trying it until you find an octane rating that makes the car run where you want. There is no need to run higher - YOU GET NO ADVANTAGE. If you can't tell a difference, there is none. The best test is to find a nice, long, steep hill, and go up it. At lower RPM you'll have the most load, and the most chance of knock, or for the engine control to have to adjust for it.
I've had Saabs from the same year, and they took different octanes. I had a 9000 turbo that gave full boost on 87. I had a 900T with the same engine that required 93 for full boost. But the great thing about the Saab engine is that it adjusts for octane. Run a lower octane, and it'll adjust. So you can give up some peformance to save money, or because that's all that is available at the time.
posted by 192.249....
, Tue, 15 Nov 2005 05:56:46 <-- Viewing This Message, Tue, 15 Nov 2005 08:58:14
, Mon, 14 Nov 2005 15:01:57
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