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87 octane burns faster and is slightly more volatile than 91, which is slightly more volatile than 93, etc. etc.
In high-compression applications, 87 can and will ignite itself due to the heat of compression by itself, if compression is high enough. This is one form of preignition, or "knock." This condition will be aggravated by the presence of any hot spots in the the chamber, via carbon buildup or sharp metal edges which will stay at higher temperatures than their surroundings (both conditions are relatively common as the car gets older).
The trouble with preignition is that it subjects the piston to tremendous expanding (i.e., downward) force, while the piston is still completing its upward travel. This is a recipe for damage as you can imagine.
While base compression in Saab turbos is set to 9.5:1 or thereabouts, NOMINAL compression will remain higher, as the turbo is always making up for that lower compression, bringing it AT LEAST to the compression ratio a non-turbo car operates at. Throw in a heavy foot, and that compression ratio goes sky-high.
It's for this reason that high-compression engines recommend higher octane ratings, to stave off this quicker flash point. If the manufacturer believes that things in the combustion chamber are going to be especially intense, they may not have confidence in the hardware's (not the software's) ability to compensate adequately. That circumstance is true for SPGs and Aeros. It's why premium gas is listed as a requirement.
Herein lies the issue about knock sensing. In order to compensate for knock, the brain listens for it, and then dials back systems incrementally to "detune" the engine. This includes bleeding off turbo pressure (which turns your base low compression into mighty mighty high pressure), interrupting fuel supply, adjusting ignition timing, and so forth.
However, there's a limit to the adjustability of some hardware -- for example, and crucially, the turbo itself in Aeros, which produces higher pressure than a stock turbo. Therefore, though an Aero's software senses knock and attempts to compensate, the hardware may not be able to provide adequate compensation, or do so fast enough.
Garden variety Saab turbos such as your 9-3 example ARE designed to compensate for 87; SPGs and Aeros are NOT, or at least on a routine basis. That doesn't mean that on a given day they won't adequately do so, but you should not assume that they will.
On fuel economy: you'd expect fuel efficiency to be negatively affected because in detuning itself to prevent damage, the engine is necessarily throwing away some of the fuel's potential energy. So the extra few miles of travel on a tank of 89 will offset the additional cost per gallon.
Interestingly, on the reverse end of the spectrum, I drive an 89 535i with a low-compression engine. This car is designed to run on 87. If I were to run 89 or higher, I'd be throwing my money away, because by the time the fuel burns completely (if it does), it's on its way out of the combustion chamber and doesn't contribute all its energy to driving the piston. I would (and did) experience power loss, fouling of plugs, etc. running anything other than 87.
I don't doubt that many people run their Aeros and SPGs with no ill effects for zillions of miles, but I would still stop short of recommending it, given that Saab is very specific about the required grade for these motors. One facet of these highly-tuned cars is that they're, well, highly-tuned. Thus, less adaptable to variations if fuel grade than you might think.
No acrimony intended in these messages, just hope I'm explaining my position fully and convincingly.
posted by 155.212.49...
, Tue, 18 Jul 2006 09:26:53, Tue, 18 Jul 2006 09:29:37
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