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after market cams and cam grinds are usualy made for NA and have a higher over lap then stock cams. Which mean that there is a point in which the exhaust and intake valves are open at the same time in between the exhaust stroke and intake stroke. Having the intake valve open before the exhaust valve closes helps in expelling burnt exhaust gasses in NA engines. This is also what makes cams emissions unfriendly. As for for turbo and supercharging, u don't want that overlap because pressure created will be lost going out the exhaust valve. if u want cams with ur turbo or SC make sure they are made specificaly for positive pressure.
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