Engine Balancing - Tech - How To - Terminology - Circle Track Magazine
How To Balance An Engine - Engine-Balancing Basics - Car Craft Magazine
You can balance it, but you are really just picking the rpm band you don't want vibration in.
There's also another factor - windage from
oil. Smokey Yunick spent some time in the 1960's getting high speed photography of a working crankshaft. In his book, Power Secrets, he publishes one of those photos. It shows a crank at high rpm with 3-4 quarts of
oil wrapped on it.
We spend a lot of money to get things within a quarter gram, but then even a quart of
oil will throw it completely off. It's exactly why racers use scrapers, windage trays, and have 7 quart pans, to make up for the loss of oil that sticks to the crank. Another more modern technique is to use oil resistant coatings on the lower rods, crankshaft cheeks, etc. to make it let go and reduce it.
I've asked a lot of guys who race, and machinists who make race engines, they all agree, balancing and the expense is largely something street car guys think they need. It's not that critical. What's 3% of a 1600 gram bob weight? 48 grams. Plus some fraction of a quart of oil, which weighs 800 grams. For some portion of weight that takes it out of balance, you have another that puts it back in, the whole thing is a spinning mass of changing volume dependent on rpm.
Again, production tolerances on factory parts do well enough. There's plenty of other sources of vibration, the primary and secondary nodes even caused engine designers to add shafts to four cylinders so the driver wouldn't feel them. Those don't stop the engine from having vibration, they just add counter vibration. It's still doing it at the journal level - the block is pushed back at the same time to offset it, adding even more force on the oil wedge.
If the engine is internally balanced, add a neutral harmonic and flywheel, consider it done. With hard motor mounts, the primary and secondary vibrations could likely be felt anyway.