Adding cylinders normally looses eficiency due to the increased surface area exposed during the power stroke, allowing heat to leave the mixture as it burns.

Hence 3 cylinders and even 2 cylinders in the new fiat "twinair" engine.

Only a boxer engine's arangment of cylinders can be truly balenced in the 1st and 2nd order, hence porches love of this arangment.

P.S. This should be an interesting thread