Once the rotating mass is balanced, whether 270 deg. or 360 deg., the main source of vibration becomes the power applied from the radial force of the explosion acting on the crankshaft. This is why the more cylinders in an engine, the smoother it runs (ex: an 8 cyl engine will apply force every 45 deg. of rotation - a four cylinder ever 90 deg.) The 270 deg. crank spreads the radial force out a little than the 360 deg. crank over full rotation, making it run smoother.
