Originally Posted by dogwater
I'm going to have to get another harmonic balancer, I been using a 6 3/4 in. balancer on my sbc internally balanced. The outer ring is moving towards the timing cover. I don't know if this has anything to do with but I did put on a new flex plate when I changed over to a 700r4. Any how I read that a 8 in. balancer wasn't really needed,it just added more weight to the crank. The 6 3/4 was bought new but it was a cheap one only about 70.00. But now I'm thinking I might be better off with an 8 in. as this engine does see 6 grand some times but not alot. When walking around the junk yard I see 6 3/4 balancers on V6 engines, all V8's have 8 in. I like the idea of having less weight on the crank but maybe an 8 in. would be a better choose. Please inlighten me. And yes I know that a Rattler is the best but I didn't want to spend 300.00 plus for a balance.
If you're using an OEM style bonded damper the 8 inch is more effective in dealing with a wider range of frequencies than the 6.75 or 7 inch. Certainly a lighter damper allows faster shaft acceleration but you need to be careful that the damper does in-fact snub the frequencies in the operating range where the crankshafts critical harmonic response also occurs. You will find this level of technical information is one of the biggest secrets on the planet, so it is difficult to know if you got this correctly till time passes and nothing happens, or a short time after putting the damper on you're greeted with a broken crank snout or a set of blown rods on the number one throw. The bonded rubber dampers are application specific and are tuned to unique frequencies needing to be snubbed on that particular engine, using a V6 damper on a V8 could be an invitation to sweeping up a lot of expensive pieces of parts from the pavement, or not, like I said itís really difficult to get engineering data from the OEM or the aftermarket as to what the generated frequencies are on the shaft and what the snubbing frequencies and effectiveness of the snubber is for those frequencies.
Things like the Rattler and Fluid Damper snub a much wider range of harmonic frequencies on the shaft than the bonded rubber type damper, for the money they bring, me at least, a lot more peace of mind that inside parts of the engine won't be hanging out of the block. The larger and heavier damper also snubs a greater frequency range and a larger amount of amplitude from those frequencies.
I have had failures happen that could be directly attributed the failure to the damper to adaquatly squash harmonics on the shaft so this isn't just theory. These failures occur in the nose extension, the number one main, and the number one rod journal. The classic sign is the undamped harmonics are causing the snout to wander and orbit off the center of rotation. This carves out the front seal one of the first signs is an oil ooze from the seal. This on a cast shaft will most likely followed by a fracture where the snout steps up the main bearing. Usually a forged shaft will survive this but the orbital motion will be transferred into the number one main where if carves out the bearing. Oil pressure eventually is lost in the number 1 main which then starves the number 1 rod on the number one throw. You can guess what happens next.