Quote:
Originally Posted by mechman600
Don't forget that while it is difficult to pull the piston down on the intake stroke with high manifold vacuum, the volume of air on the piston that you are "stretching" acts like a giant spring and pulls the piston back up on the compression stroke, helping turn the engine and effectively cancelling out any effort it took to pull the piston down.
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But, if you're considering a butterfly valve method of trying to deactivate cylinders, that above is only true for the intake and compression strokes. The 3d stroke (that would have been the combustion stroke on a normally working cylinder) would again be subjected to intense vacuum that would be akin to stretching a spring (just as with the 1st stroke). However, the 4th "exhaust" stroke would immediately get rid of that vacuum as the exhaust valve opened up, causing exhaust gases to rush in just as the piston pushed back up to TDC.
Not good.