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Re: Octane in 94 9k Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Tue, 15 Nov 2005 08:58:14
In Reply to: Re: Octane in 94 9k, Eleximp, Tue, 15 Nov 2005 07:33:44 Members do not see ads below this line. - Help Keep This Site Online - Signup |
The air/gas mixture in the cylinder is supposed to burn when the spark goes off. Burning produces gas, which expands and pushes the piston down. The timing of when this happens is very critical - too soon, and the piston is in the wrong place. If you compress an air/fuel mixture, it gets hot. If it gets hot enough, it explodes. Not only is this more violent than the controlled burn caused by the spark plug, but it also will be happening at the wrong time. This can cause damage to the engine, and also robs it of power.
This process is called 'detonation', also known as 'knock' or 'ping', from the sound it makes. It sounds like a metallic rattle, like hailstones or sleet hitting a car roof.
It's pretty common for a Saab engine to rattle a little at idle. That's normal, and it isn't knock. Engines rarely knock at idle, and if they do, they'll REALLY knock at speed. The most common place to hear knock is if the engine is at low rpm (say below 2500) and under a big load - say if you're trying to accelerate up a hill.
So if you are accelerating up a hill in too low a gear, and you see the revs are around 1500-2500 rpm, and you suddently hear a rattle from the engine, that would be knock. Or you're going up a steep hill on the highway in top gear, and you press the gas pedal to keep up speed, and you hear a rattle (or that hail sound), that would be knock.
However, with your Saab, it's unlikely that you'll ever hear it. That's because the engine control system can detect the knock very early, well before you can hear it. The control system adjusts the fuel and timing so the engine doesn't knock. But those adjustments will rob the engine of some power.
With a turbo engine, it's easy to determine what octane you need, because the engine performance is very sensitive to octane. You find a nice long hill, and charge up it in a higher gear. If the boost needle doesn't go all the way to the red, the system is cutting back on boost. Higher octane should provide more boost.
I'm providing that info, so when you get a turbo, you'll know. For your non-turbo, if you're running Waaay too low octane, you may hear knock on the hill test. Otherwise, running slightly lower octane would be very hard to tell - there will be a slight performance drop, but unless you do some well instrumented, back to back tests, you won't notice it.
That 'you won't notice it' is probably the biggest thing to understand with your normally-aspirated car. It's not very sensitive to fuel octane. And that's good. There is no need to run a higher octane than your car needs, and it only wastes money. And if you can't detect the difference in how your car runs, then higher octane fuel is still just wasting money.
Is higher octane fuel better? After all, it is called 'premium'. The highest octane fuel usually includes a few extra additives, like fuel line cleaners. But at 20 cents a gallon extra, it's no deal. If you drop a bottle of Techron into your tank every 3 months, that'll cost around $5. Buying premium to get the cleaner is a bad deal after about 2 1/2 tanks, assuming a 20 cent difference in regular versus premium.
Bottom line - if the car runs well on lower octane fuel, if you can't tell the difference in how it drives, if you don't hear a sound like hail or sleet on a car roof when driving up a hill, you're fine. You won't damage the car, the engine won't run hotter, colder, or sideways.
And the line below the bottom line is not to obsess over it - don't spend your time listening for knock - you'll know it when (or if) you hear it. And don't go crazy trying to detect slight performance differences. If it isn't obvious to you, it doesn't matter.
posted by 192.249....
, Tue, 15 Nov 2005 05:56:46, Tue, 15 Nov 2005 08:58:14 <-- Viewing This Message
, Mon, 14 Nov 2005 15:01:57
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