Engine throws alternator belt

[CITRON YELLA 71]

I have a 71 Dodge Challenger with a 340 engine (450hp/500tq) and when I go wide open throttle or rev the motor stationary, the alternator belt comes off. I have Powermaster single wire, 140 amp, GM style alternator. Fan belt alignment is good. I have a turn buckle style alternator bracket from March Pulleys. I 've noticed when I rev the engine, the alternator shows some flex. I am considering going back to the Chrysler style alternator and original mounting bracket.
Any thoughts?

 

[moparlee]

Go back to the original mounting brackets. Mopar engineers had it right.

 

[pschlosser]

My first thought would be make a video of the belt coming off, to see if I can identify where the cause originates. Particularly from the vantage point on the passenger side, looking down the long way of the fan belt with the camera aligned with the belt and all the pulleys. By watching it a few times, one may be able to figure out what's going on.

But going back to stock is a play I've used many, many times in the past. When I work on a Mopar, and something isn't right about it with aftermarket parts or a change from factory, if I can't figure it out, I'll find the stock parts and put those on or "put things back to factory", and in most cases, resolves my problem.

But then, I'm talking about electric fuel pumps, intake manifolds, carburetors, electrical systems, and other things changed by previous owners. We may sometimes give up power, performance, or some other function by removing aftermarket parts and returning to factory conditions, but it pays for itself with less heartache and frustration. In the end, I just want it to function correctly.

 

[moparleo]

Pulleys come in different widths, angles and depths. The belt must match the pulley specs. Also pulley diameters.

 

[Chryco Psycho]

If the brackets are flexing you found the issue .

 

[Xcudame]

The alternator should not flex at all with the correct brackets!

 

[MoparMur]

Also make sure you're using a top-quality belt. Could be briefly stretching when you rev it up.

 

[Bret Schneider]

I have the exact same set up as you on a small block. No issues, works great. Make sure you have the spacers that come with the March pulley set installed correctly. Guessing maybe you are missing a spacer or have one in the wrong place. That would account for the movement in the alternator.

 

[CITRON YELLA 71]

Thank you all for your comments/suggestions. I think I'm going to look into going to a Chrysler style alternator and bracket. Tuff Stuff makes a 130 amp alternator that is a single wire hookup (part # 9509RDPDP).

 

[Cratos]

bet it is an alinement problem I had to shim out a bracket to stop mine from popping off.

 

[TIMINATOR]

You don't need a hi output alternator unless u have efi and a big sound system.
A quick revving 340 needs a larger diameter alternator pulley. That slows it down to where it normally RPMed with a stock engine. Gears and a stall convertor also needs a larger pulley. Stock alternator pulley ratios are about 2.5 to 1 overdrive, some are more.
So divide the alternator pulley diameter into the crank pulley to get the ratio, then multiply that times your shift RPM.
A while back we watched a big block with a 6400 shift point explode an alternator! He lit it off in front of my shop (no hood) and he launched the rotor, front half of the case and pulley about 20 feet in the air! Also destroyed was the radiator, and fan. We never found all of the alternator parts. The calculated alternator RPM was a bit shy of 18,000!
Not only does a bigger alternator pulley slow the RPM but it provides more belt contact area (wrap).
If you must run a higher amp alternator (most don't NEED it)
you also need a larger size charge wire to the battery. A 140 amps on a welder will properly run a larger than 1/8" stick rod. Would you use a 10 or 12 ga. wire for that?
TIMINATOR