The best data I've seen is on Metrompg's website and he didn't see a great difference with a WAI, which he attibuted to the fact that he believed the Metro's ECU wasn't adjusting the AFR due to the intake temp. While in theory, hotter intake temperatures may reduce pumping losses, I would think any gains made from this would be offset by the loss of power, which would then result in a larger throttle setting.
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