Even if the compressed mix did not light off that preassure that resisted the pistons upward movement will still push it down on the power stroke, this is why a Diesel engine will not slow you down much going down a hill unless you have a jake brake, a diesel has very high compression and NO SIGNIFICANT VACCUME in the intake like a gasoline engine.

I came up with some experiments for you to do stumpy. Get in your car, prefferably manual trans, go to the top of a big hill and let your car start coasting down the hill, engine off with the clutch in and the trans in 2nd or 3rd then once you get rolling real good let the clutch out, you will feel the engine rev up and the car will begin slowing drastically, now with the engine still off stomp the gas so the vaccume goes away and full compression can now be achieved, you will notice the car will not be brakeing near as hard as it did with the throttle closed and very little compression but high vaccume. That should prove beyond a shadow of doubt to you that vaccume has a much larger loss of energy than compression.

Hope this helps