[Christ]

Vacuum and pressure in the block really doesn't affect the pistons' travel at all, actually...

I say this because the engine is designed to run in a balanced configuration, so that while one piston is going down, another matches it's speed on the upstroke. The result is that all the air in the block gets moved around alot, but the pressure doesn't change (other than in minute variations) inside the block as a result of piston travel.

The primary benefit of this is that when you force something into a vacuum, it just goes easier than if you force it into 2 atmospheres of pressure. I.E. The pistons are affected by the density of the air inside the block, not the pressure. Vacuum has much less density... in fact, if you pull true vacuum, there is no density... that's the definition of vacuum in this case. A complete lack of atmospheric density.

So instead of the piston fighting to move air around (not compress it), it's just going up and down in what is essentially a lack of atmosphere, which is much easier, and accounts for a reduction in pumping losses.