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Old 01-04-2008, 03:06 PM   #3 (permalink)
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Originally Posted by MetroMPG View Post
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.

Thanks I always wondered about it.
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