Have a 72 Cutlass with a 455, MSD Ignition w/ the MSD 6AL. The alternator is a stock with I think 40 amps and an underdrive pulley on the crank. Problem, when the headlights are on the alternator will not put out 12 volts at idle so when I let off the gas the motor will stall. Typically in a turn which is always a lot of fun. Also the longer I run with the headlights on the battery will drain down and the motor will skip especially as the rpm's lesson (caught in traffic). Not sure if the problem is the underdrive pulley, the alternator is to small, or other wiring issues causing excessive power draw?

 

40 amps? ouch. And its underdriven?? I'd say you've got electrical woes. Ideally you'd want to see 13.8 to 14.2 volts, with a load ( engine idle at 700 rpm, high beams on, heater/ac running, ect) are you running an electric fan? Even more load. You want about 80 to 100 amp single wire alternator and it will solve all your charging issues. don't underdrive if you can avoid it.

 

The problem is all of the above Too little alt too much draw and not enough alt speed. I suggest a cs130 or cs144 alt with the smallest alt pulley you can find.One wire alt is easiest,but not as effecient.

 

Underdrive is the main issue. I bet a previous owner was trying to take advantage of that 1/10HP gain the underdrive provided... Or, the wrong pulley was inadvertenly installed...
Go back to a stock pulley.

The highest output stock alt that year was 63 amp, mainly if it came with A/C or defogger. If you have these, you need a minimum of 63A.

If your car has the above AND newer updates like electric fans or high power stereos, then the CS130 or 144 would be more sufficient.

I did some performance tests on my '72's 63A alt and found it to be ~almost~ a little underpowered itself with high beams on, A/C on, A/C fan on high, and stop lights on. When I turned on one of the seat heaters on high, that extra 5A started coming off the battery.

If you ever change the alternator, go higher.

 

Will start with the correct pulley. Any place better that OPGI or year one to get one?

 

I think I got my alternator at RockAuto for about half the price OPGI or Parts Place wanted for the same. I believe it's an AC Delco as well. You might want to check on the pulley there as well.

Oh, ALSO, and I learned this from experience, I find it to be a good idea to replace the voltage regulator while you're at it if you either don't know the last time it was replaced or has been a long time. I was having some issues similar to yours on my old '68 LeMans and thought it was the alternator. Well, one day I was driving and stopped at where I was going. There was this terrible sulfur smell eminating from underneath the car. We thought we hit a skunk or something. Well, long story short, after checking under the hood, it was the battery--that had heated up to the point it was about to explode. The side was bulging. My voltage regulator was bad. So, I replaced the battery and voltage regulator, and all was fine.