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Old 01-04-2008, 02:47 PM   #2 (permalink)
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Join Date: Nov 2007
Location: 1000 Islands, Ontario, Canada
Posts: 22,316

Blackfly - '98 Geo Metro
Team Metro
Last 3: 70.09 mpg (US)

MPGiata - '90 Mazda Miata
90 day: 52.07 mpg (US)

Winter beater Metro - '00 Chevrolet Metro
90 day: 61.98 mpg (US)

Fancy Metro - '14 Mitsubishi Mirage top spec
90 day: 58.72 mpg (US)

Even Fancier Metro - '14 Mitsubishi Mirage top spec
90 day: 70.75 mpg (US)
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Cold air has more oxygen by volume. So you can stuff more fuel into the cylinder while maintaining the proper air/fuel ratio. That's one reason why the go-fast people like it.

The idea behind using warm air for efficiency is to intentionally depower the motor. Less oxygen by volume means you must open the throttle further to generate a given amount of power. A wider throttle opening means reduced pumping losses.

BTW, the O2 sensor will always strive to maintain the proper air/fuel ratio, so it's not running lean.

In a way, it's similar to driving at high elevations (less O2 in the air), which can be more efficient for a number of reasons.

In practice though, whether it works seems to depend on the specific vehicle and how it's programmed to respond (or not) to the increased air temps. Some ECU's may back off timing to prevent knock, so you end up with a zero gain. The Saturn drivers seem to have good luck with it though.
Project MPGiata! Mods for getting 50+ MPG from a 1990 Miata
Honda mods: Ecomodding my $800 Honda Fit 5-speed beater
Mitsu mods: 70 MPG in my ecomodded, dirt cheap, 3-cylinder Mirage.
Ecodriving test: Manual vs. automatic transmission MPG showdown

has launched a forum for the efficient new Mitsubishi Mirage - fuel efficiency info for Geo Metro owners - electric car conversion on a beer budget
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