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Originally Posted by Kyle2k
Are you talking about fuel efficiency or emissions, because it kind of sounds like an emissions thing.
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Fuel efficiency. Here's a couple of examples.
It is common to use a rich fuel-air mixture to cool down the air charge, making it less prone to pre-ignition. Tuning your engine so that the air charge isn't as hot (by lowering boost or easing off on the compression ratio) lets you lean out the mix a little bit. When you do that, you get less power but you waste less fuel in the meantime. BMW is supposed to be introducing a new turbo engine that uses water injection (under certain circumstances) in order to improve the fuel economy (and possibly emissions too) by not having to use as much gasoline to achieve the same air charge temperature.
With cam profiles, a high performance cam has more overlap. That means that both the intake & exhaust valves are open at the same time. Inevitably, this leads to unburnt gasoline exiting the cylinder -a complete waste of fuel.