borrowed from aaicar.com - original web article found here.
"Crankshaft Balance
Okay, so how does all this scientific mumbo jumbo translate into the real world dynamics of a spinning crankshaft? A crankshaft, like a flywheel, is a heavy rotating object. Whats more, it also has a bunch of piston and rod assemblies reciprocating back and forth along its axis that greatly complicate the problem of keeping everything in balance.
With inline four and six cylinder engines, and flat horizontally opposed fours and sixes (like Porsche and Subaru), all pistons move back and forth in the same plane and are typically phased 180 degrees apart so crankshaft counterweights are not needed to balance the reciprocating components. Balance can be achieved by carefully weighing all the pistons, rods, wrist pins, rings and bearings, then equalizing them to the lightest weight.
On V6, V8, V10 and V12 engines, it is a different story because the pistons are moving in different planes. This requires crankshaft counterweights to offset the reciprocating weight of the pistons, rings, wrist pins and upper half of the connecting rods.
With "internally balanced" engines, the counterweights themselves handle the job of offsetting the reciprocating mass of the pistons and rods. "Externally balanced" engines, on the other hand, have additional counterweights on the flywheel and/or harmonic damper to assist the crankshaft in maintaining balance. Some engines have to be externally balanced because there is not enough clearance inside the crankcase to handle counterweights of sufficient size to balance the engine. This is true of engines with longer strokes and/or large displacements.
When rebuilding an engine that is internally balanced, the flywheel and damper have no effect on engine balance and can be balanced separately. But with externally balanced engines, the flywheel and damper must be mounted on the crank prior to balancing.
Customers should be told what type of engine balance they have (internal or external), and warned about indexing the position of the flywheel if they have to remove it later for resurfacing. Owners of externally balanced engines should also be warned about installing different flywheels or harmonic dampers and how it can upset engine balance."
borrowed from ImportTuner Magazine's online Power Pages article w/ a 2008 Honda Civic Si - original article can be found here:
"Notes
One of the great debates raging on inside Internet forums and debated over bench racing bouts, is whether or not lightened pulleys can harm an engine. The theory goes that lightweight pulleys aren't balanced to specific engines' harmonics the way their OEM counterparts are, and using them can increase torsional vibration that causes premature bearing failure. But, Honda K-series engines are balanced internally, and do not use balanced pulleys from the factory. And today's newer engines are constructed with tighter tolerances than those that were around back when this rumor got started. Bottom line is: If your engine has been properly maintained, using high-quality, balanced, lightweight pulleys like NST's will not harm it"
+
=
a debate between MR2 4AGE owners on the factory crank pulley being a harmonic damper or not can be found here under the 4AGE pulley group buy heading (11 Sep 2006 15:52:53)
one of the contributors sited a Club4AG tech reference section pertaining to the factory crank pulley. There is too much to re-print in this thread and would be better if the reader were re-directed to the Tech reference page which can be found here under the 4A-G Front Pulley heading/section.
Essentially, the front crank pulley is balanced BUT balanced by itself. the crank and the rest of the rotating assembly is balanced separately from both the FW and pulley from the factory. Throughout the 4A-G's production run, varying crank pulleys are found inter-mixed between all of the 16v to 20v engines, between the various FWD & RWD configurations. TRD themselves developed a pulley (p/n 13471-AE801) with NO rubber isolation ring (usually found on stock front pulleys) intended for their class of Formula Atlantic/N2 race engines.
TRD 4A-G Front Crank Pulley (p/n 13471-AE801)
Front View:
Back View:
again, to re-iterate: the oem crank pulley IS balanced but balanced apart from the rotating assembly. the pulley itself is spun and balanced the way your wheels and tires are one by one on a wheel balancer. instead of slapping on weights to balance the pulley in full rotation, material is taken away from it in the form of a hole or 2 in strategic spots to counter excess weight on the opposing side.
suffice it to say, this tuner/reader finds an overwhelmingly large amount of proof coupled with a basic knowledge & understanding of the natural physics of rotating objects and the effects of centrifugal force that the 4A-G crank pulley is NOT a harmonic balancer/damper. so, as long as after market pulleys are themselves balanced prior to installation, then all worries and concerns can be laid to rest regarding lightened, non-damping pulleys for our beloved 4A-G.
exigo
!