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TG disproves his own "theory"
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Posted by Greg Netzner (more from Greg Netzner) on Mon, 5 May 2003 10:00:23 Share Post by Email
In Reply to: Re: Well thought out....., TG, Sun, 4 May 2003 13:57:37
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Engine management is what throws the equation off. In a non-managed engine, the "ECU" is the driver. With a mechanical (or vacuum) spark advance & no computers, it's up to the driver to protect the operating condition of his car. The faster he drives, the more advance is produced. If "icipient knock" (as my '90's manual puts it) is present, the driver backs off the throttle, thereby retarding timing AND reducing fuel consumption. (The other proven correction for this condition is to use higher octane gas.) However, with a managed engine, the ECU can, to a limited extent, retard timing without relying on the driver's input. On my 16v N/A, this is done across all cylinders at once. On the DI turbos, this is done only to whichever cylinder is knocking. A Bosch "performance" engineer admitted to me that Saab does more with engine management (to maintain power) than any other make, keeping boost going to the engine long after others would back off. (See? I can quote ghost sources too.)

This is why your "theory" doesn't apply to modern engines. If I accept your "equation", then ANY octane should work in an ECU-managed car, & my high-altitude 85 octane would give the best fuel efficiency with the only concern being poor performance. And as I continuously drove up that 10,000 ft pass for 45min, & the knock got worse, & the air got thinner, the ECU would keep adjusting the timing & a/f ratio to the point where there wasn't enough power to drive the car. But that's not the case, is it? And that's why high-altitude Saab dealers now hand out turbo-specific "Mountain Driving" bulletins warning of using too-low octane.

As you once told me on this subject, you now "Need to update your old files".

Your comment that "Drivers generally 'say' that 87 gives lower mileage, and my reply is posted over with the 9-5 discussion to the effect that after driving on 93/92 octane, switching to 87 is a loss of torque. Drivers find themselves pressing much harder on the gas pedal to get the power back which results in lower fuel economy," is EXACTLY WHY premium provides better mpg. Loss of power = more accelerator = more gas = less mpgs. It doesn't matter how many times you repeat the phrase, "Seeing 2 identically equipped SAAB turbos in the lab--one with 87 fuel and the other with 93 fuel shows the results scientifically," because people don't drive their cars on a lab's test bench. Congratulations, you've just disproved your own outdated "theory".

As for your ill-chosen analogy to truckers wanting to drive 70mph rather than 55mph, the reason mpg's are lower there is wind resistance, not optimum engine operation. I don't have the exact numbers to give you, but as your ground speed increases linearly, your wind resistance increases exponentially. So going 27% faster (55mph to 70mph) creates much more than 27% air drag. When it was imposed as law, 55mph was considered to be the "break even" point for balancing wind resistance with engine efficiency. Hopefully, someone with the numbers will post them. Obviously, you'd need to be driving in a vacuum to eliminate this factor.

You have no "proof" of any of your claims; only the most tenuous grip on fleeting comments made by phantom "credentialled professionals." Bring some documentation to the discussion, rather than repeatedly flaunting the unsubstantiated "facts" of those you mistakenly expect us to idolize.


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