Having a problem with my alternator at the track. I bought a rebuilt factory one for my 72 Charger. The problem I am having is that it works ok in the morning for the first couple of passes. Shows about 13 or so amps. By the middle of the afternoon it is only making about 10 if i'm lucky. I'm spinning the motor to about 7000 using the stock pulley. Is this the problem?? Not sure or if I need to get a different alternator. I charge the battery between rounds. I also have electric fans, water pump. This has been a continuing problem. Any suggestions as to why this is happening. Jeremy

Sounds like it`s on it`s way out. My 60 amp denso start between 14.5-14.7 and only drops at idle at night w/lights and everything going.........elec. fan, water pump and fuel pump. I`d look into the late model stuff myself............

Full field your regulator to rule it out

I beilieve he is saying amps not volts.Unless he made a mistake.The alt will only put out as many amps as the battery will take.They also drop as the temp gets hotter.Rocky

I'm sure he meant volts as 13 amps is nothing, 2 fans would draw that much. I bought the 100 amp Tuff alternator as it looks stock & fitted a race regulator to control it. anything above idle & the ammeter is showing +. no lack of power on my car now.

What they said BUT !!!!
I don't think I have ever seen an engine compartment that CLEAN!!!
Mike

Thanks for the help everyone. I think I'll go with a 75 AMP power master. How do you field the regulator as was mentioned? Jeremy Actually the engine bay is always that clean.

Suggest you get a smaller crank pulley or a bigger than normal alternator pulley. 7000 RPM with a stock crank pulley is way fast. Assuming the alternator has a 2.5" pulley and the crank pulley is 7" that's close to 20k alternator RPM (7000*7/2.5=19,600 alternator RPM). I think max is around 15k... Powermaster will know and advise.

Also, a square back can work but a newer style Denso or even GM case would have better idle output and maybe take a few more RPM.

Holy patriotic engine, Batman!!

Yes I'm running stock size pulleys. What size alternator pulley would you suggest? Jeremy

On the pulley ratio... stock was in the range of 3 or as much as 4:1. Powermaster recommends at least 1.75:1 for drag or 1:1 for circle track (see below). It may be easiest to change the crank pulley. I switched from a stock (~7.25") to a March aluminum 6.25" to slow everything down a bit for my warm 496 B engine with 6000 RPM redline. My alternator pulley (stock 70's "big Chrysler" 100+A) is 3" so this is a nominal 2.08:1. I get plenty of idle charge and don't think I am overspeeding the alternator... which has great idle output and is waaay heavy.

I'll suggest something in the same ballpark for your situation. A 6.25" crank pulley and 3" or minimum 2.5" alternator pulley as it sounds street/strip oriented. 7000 RPM with a 6.25" crank and 2.5" alternator pulley puts you at 17,500 alternator RPM which is probably OK for short periods. Again, the square back derivatives are not very good at idle so a Denso or GM case would be better. I switched from a squareback to the "big Chrysler" for this reason. I didn't use the GM derivative as it did not fit with Ed heads and AC. The late 70's "factory high amp" did fit.
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The type of racing will determine the pulley ratio. For example, an overdrive pulley ratio is recommended for drag racing because it is best to charge while in staging and on the return slip. This enables the battery to be fully charged for optimum ignition when you pull to the line. In most cases, a pulley ratio of 1.75:1 or more is recommended for drag racers (see also "Power Pulleys"). For circle track racing, charging while on the track is necessary for long periods of time. For this reason a straight 1:1 pulley ratio is recommended. Powermaster has different styles of pulleys..