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Turbo's impact...makes it more complex
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Posted by moeman (more from moeman) on Sat, 22 Apr 2006 12:20:10 Share Post by Email
In Reply to: Re: 9-3 SC 2.0T manual minimum rpm to drive?, Stephen Goldbergers, Thu, 20 Apr 2006 11:23:47
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Remember turbo's impact is essentially like incresing the displacement of the motor...So there are two variables. For a normally aspirated motor lower RPM always leads to less gas consumed, with turbo thats not always the case.

Its easy to see the "load" on the engine at any RPM look over to the turbo dial and see how much boost is applied. Low RPM going downhill will show very little boost. Going up hill may have it near the top of the range.

So the volumetric efficiency is somewhere in a sweetspot with low RPMs being one of the main fuel efficiency parameters. I agree with the comments of shifting early for better gas mileage but really think the 4 cly. cars won't do well below 1500 RPM and the V6 may be OK slightly lower. The V6 really don't rely on very much boost. Too bad SAAB doesn't have AWD becase they could easily get 300 HP out of the V6 with more boost.

I've messed around shifting my auto at very low shift points but even a slight grade sends the boost up immediately. So if i try to tweek the gas mileage up I think up shifting around 2300 rpm and keeping the higher gear engaged when decellerating is the only improvements that are 100% winners. You guys with manuals have way more control.

After all that hot air I blew ;)

...I will say that with my automatic transmission car one interesting observation is that cruise control does deliver better mileage even if I'm vigalent at keeping the speed constant--what that tells me is SAABs computer is very good at finding the balance of RPM and boost when it comes to saving gas.

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