I am trying to develop a four cylinder stationary engine that normally uses balance shafts. The shafts have been removed for cost savings and the engine will be ran at a lower speed and derated horsepower to compete in a different  market.  My problem being that when I run the engine on the dyno and measure 2nd order vibration the engine shows a dip in vibration at 1500 rpm. When the driveshaft is removed the engine vibrates horribly at this rpm and it's cycle seems to shift. My question is, how can I predict the effects of a balanced rotational mass to the rpm at which it will help dampen the vibration of the engine. Would a larger mass move my curve upward in my rpm range. It is my understanding that this engine will be used in belt driven as well as direct drive applications so being able to predict various "driveshaft" w will be necessary.

thank-you
Allen