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A/F ratio is not a static, linear, or proportional thing when compared to the ouput of any oxygen sensor because A/F ratio is controlled-by the fuel map and processor and not the oxygen sensor. Yes, the sensor is one of the main inputs that the processor consults when determing fuel-load, but it's far from the only one; the Saab processor also considers things like: Did my ion analysis show some pinging indicators in cylinders 1,2,3 or 4 during the last combustion cycle?, What is the current throttle position?, What are my temperatures?, etc.
From what I've read and understand, maximum sustainable power is allowed-for when the A/F is around 12.6:1 to around 13.0:1. That is running on the rich-side to begin-with, anything below 14.7:1 is rich.
The A/F charts I've seen from actual dyno runs with stock components suggest that the actual ratios can go below 12.6:1, and that has been with good, 93 octane gas--which also supports info that I've read. Why would a modern computer controlled car purposely run more rich than it had-to for maximum power delivery, even with good fuel? Because extra fuel is added to assist in internal temperature and detonation control. Ever notice that many turbo cars have more black around the tailpipe-exit than the pipes on other cars? I know mine does. With low octane gas you're compounding the problem because you'll be asking for boost from the car when it's not safe to have boost. If everything is working correctly, the car just won't give you as much boost and you'll be short on power, and that's the best-case scenero. If the right combination of things fail electriconally then you'll melt a piston.
Now that I've written that, let me also say that I feel like there is also some gray room in the above when calibrations, etc., are considered, and that I plan to get to the definitive-bottom of things with my particular 9-3 HOT. I'm pretty curious about things in general, and this discussion ties-in nicely because I just ordered a wide-band 5-wire oxygen sensor, electronic boost gage with independent MAP sensor, oxygen gage, and an installable data logger last night. So in a week or two I'll be able to post some real-world data I've collected directly from my car.
Also, I will take-back some of what I said in the previous post because when I wrote I was thinking about the way I drive my turbo charged car; I suppose if a person were to drive their car extremely gently, infrequently reving the engine above, say 3000 rpm, then they obviously wouldn't see the benefits of higher octane because they wouldn't be using the boost that Saab has given them. But why bother driving a turbocharged car then?
I still maintain however, that anyone with a little bit of a hot-shoe will see a benefit in performance, engine safety, and fuel economy with an octane increase in their fuel. The hotter your shoe, the more likely you'll see finincial justification too.
posted by 67.10.24...
Partial Autospeed article/fuel cooling quote
, Fri, 22 Oct 2004 13:54:59
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