Last night at the track, the factory steel cooling fan separated and shot through my fiberglass hood.

What caused the alternator fan to fail?

 

I've seen a few steel alternator fans come apart as well as the plastic ones. What kind of pulleys do you have? It happens more when you have a large amount of overdrive. If your crank pulley is say 8" and your alt pulley is 2 1/4"-2 1/2" you might rev your engine to 6500 but your alternator will see as much as 15000. Get your self one of those Moroso or Moroso type aluminum deep groove alternator pulleys to slow the alternator down a bit. But to answer your original question, I ran an alternator with no fan for 3 seasons on my 1/4 mile car with no issues... All you can do is try it, but I still recommend the larger pulley! D. Miles

 

2-3 mins at a time and you should be ok. Lots of variables though.

 

you will be fine without the fan. however, that fan came off for the reason posted above, the alternator is spinning way too fast.

A good guide line is spin the alternator no more than 25% - 35% faster than engine speed. [this is for race only combinations]

7000 engine RPM x 1.25 = 8750 alternator RPM
7000 engine RPM x 1.35 = 9450 alternator RPM

A crankshaft pulley of 4" OD combined with an alternator pulley of 3" gives a 1.33 overdrive of the alternator [4" crank pulley divided by 3" alternator pulley = 1.33]

You can shuffle the math and pulley diameters to arrive at what you need. Most alternators will put out full voltage at about 1800 alternator RPM so 1400 engine RPM will give full voltage output when overdriven by 35%

 

you will be fine without the fan. however, that fan came off for the reason posted above, the alternator is spinning way too fast.

A good guide line is spin the alternator no more than 25% - 35% faster than engine speed. [this is for race only combinations]

7000 engine RPM x 1.25 = 8750 alternator RPM
7000 engine RPM x 1.35 = 9450 alternator RPM

A crankshaft pulley of 4" OD combined with an alternator pulley of 3" gives a 1.33 overdrive of the alternator [4" crank pulley divided by 3" alternator pulley = 1.33]

You can shuffle the math and pulley diameters to arrive at what you need. Most alternators will put out full voltage at about 1800 alternator RPM so 1400 engine RPM will give full voltage output when overdriven by 35%

Most aftermarket "race" alternators are not even turning on at 1800 alternator rpm. 6000 rotor rpm is used to make the calaculation for maximum output of an alternator on every tester I have seen. There is no problem spinning an newer automotive alternator to 20,000 rpm during a drag race. Large case automotive or truck style alternators could haver issues, and big truck alternators are limited to 10-15,000 rpm.
99/100 a race application should have a pulley ratio of 2-2.5 to one. Having a steel fan come off the front of the alternator, super, super rare. I have had over 30,000 alternators out there, never had it happen. Seen some CS144 plastic/steel combinations come apart, and some steel interanal fans separate from the the rotor, but a 1/16" thick steel fan on the front of an alternator just split in half? Not yet personally.

 

also, just to compare, a factory pulley set will spin the alternator as much as 300% faster than the engine.

A stock crank pulley could be in the 6.5" range and the stock alternator pulley about 2" [6.5" divided by 2" = 325% overdrive]

7000 engine RPM x 3.25 = 22,750 alternator RPM, you can see the potential problem.

 

Why not switch to a late model alternator with a internal fan.

 

Thanks for all the replies. I think I will try it without the fan for awhile. Here are some pics

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Wow, and that is a new style fan too. Are there scrap marks around the front housing from the outer edges of the fan. It looks like it came loose before splitting. If so, that would mean your pulley spun too, if that is the case you will need a new pulley, and install it with bearing retainer onto the rotor shaft before putting the pulley nut on. If the gouges are deep in the housing, I would replace the rotor altogether.