Quote:
Originally Posted by bandit86
By increasing the vacuum in the crank case you can do the same.
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I think these are actually slightly different effects. The notches permit air to more easily flow from under the pistons going down (for example 2 and 3) to under the pistons going up (1 and 4).
Pulling a vacuum in the crankase acts equally on the pistons going up (impeding their motion) and those going down (helping their motion). The lowering of pressure overall reduces air density, which reduces windage.
So one effect is permitting easier cylinder-to-cylinder flow, and the other is reducing the drag effects of air.
This is splitting hairs, of course.