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From Another C3 Forum:

"I have a 1968 with a 383" stroker, Holley Sniper, and MSD ignition. When I started it yesterday morning I heard a terrible squealing coming from under the hood that got worse as I revved it but then the squealing slowly subsided. I took it for a short drive and she seemed to run fine. When I started it this morning the terrible squealing started again. My alternator is only about a year old. Any ideas"?

On a cold start the alternator will go into a full charge mode and then slowly taper off until the battery is fully charged. The lower the battery voltage is the higher the charge rate will be. When the battery's charge is low a 63 amp Delco Remy alternator will consume around 5 hp so its 15/32" drive belt must be tensioned correctly or the belt will slip and squeal.

A C3 alternator belt needs to be tensioned until it will only deflect 1/2" when a 25 pound weight is placed on it between the alternator and the water pump. His belt is likely glazed from continuous slipping so it should be replaced. Note: The early C3's used 2-5/8" diameter alternator pulleys and 15/32" drive belts whereas the later C3's used 3-1/8" diameter alternator pulleys and 17/32" drive belts.

Many years ago I was in an alternator rebuilding shop and watching a 50 amp 24 volt Delco Remy alternator being tested. The test machine used a single 15/32" wide drive belt and under full power the belt started smoking even though it was tight. That's why those big alternators use two 15/32" belts to drive them.
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