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You are correct Mech, it is to help reduce windage. By lowering the pressure inside the crankcase below atmospheric you allow the pistons to travel down toward the crankshaft easier. It is a minimal gain on a street car, especially in an instance where you are running lower RPMs, such as driving conservatively. It is highly effective on high horsepower race engines though. Those engines spin at high rpm with tons of pressure inside the top end of the engine. Reducing pressure in the crankcases allows them to work more efficiently and use more of the power the engine is capable of making.
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