6.75" - Lets the engine rev a small fraction faster, Less weight on the
crank snout, smaller diameter makes it easier to fit in some engine bays.
8" - better damping due to the higher weight, easier to set timing due
to larger markings, larger bolt circle allows a larger pulley for
accessory drive (larger alternator, plow pump, HD stuff).
Just make sure that whatever size you use that you get the matching
timing tab AND set it correctly.
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