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I thought maybe Nick T from Genuine Saab would
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Posted by Drew in Houston [Email] (more from Drew in Houston) on Thu, 9 Nov 2006 22:10:21 Share Post by Email
In Reply to: WOW !! 40+mpg with aftermarket ECU, STICKMAN, Thu, 9 Nov 2006 12:11:26
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chime in here, but one option for increasing the conversion efficiency of your Saab would be to convert it to run on E85 (85% ethanol, 15% gasoline blend). In fact, I believe he has a trial car running around that's been converted.

You end up getting nearly the same MPG's as running straight gasoline but your overall efficiency is greater because there's less energy per unit volume in ethanol compared to gasoline. Like Dean says below, increasing the effective compression ratio (ECR) of an engine increases the overall conversion efficiency. Regular pump gasoline with it's relatively low octane rating becomes the limiting factor when increasing the ECR, BUT, by nature, ethanol has a much higher octane rating than gasoline so as the concentration of ethanol to gasoline increases a car that can vary its ECR can be optimized to convert the fuel more efficiently and effectively.

That said, Saabs have a natural advantage here because they are turbo charged (for the most part). On a car without either a turbo or super charger the Compression Ratio is fixed and can't easily be changed without performing some substantial engine modifications. In comparison, by the nature of it's design, a turbo charged car can change it's ECR by electronically controlling the amount of boost that the turbocharger introduces into the intake.

So, when you hear companies touting that their cars are E85 compatible, their statements generally just mean that the alcohol won't melt any of the seals or other materials and that their fuel injection system can supply the larger volumes of fuel necessary to compensate for the lower energy density of E85. It doesn't mean that they can burn the fuel efficiently.

Anyway, it would be possible to limit the maximum engine output electronically (limit airflow), especially with Trionic 7 cars, but doing that wouldn't necessarily gain anything. Peak conversion efficiency for an SI engine is at peak torque production. The other thing you could do is run the car leaner and hotter overall by reducing the amount of fuel introduced per volume air and probably increase timing. Honda does that with their lean burn technology, the problem with that is that it can become dangerous for your engine internals pretty quickly if it's not well controlled. I'm not up on my emissions, but I think too lean/hot can also cause some emissions problems too.

All of that said, I've read more than once on here and other boards where people install lower staged software and actually see an increase in around town fuel economy as well as increases in power when driven more aggressively.

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