Can the alternator be spun too fast?

[Chepsk8]

Is there a point where the alternator can be turning too fast, or does the regulator do it's job no matter the RPM?

Thinking about my pulley set-up, and driving the alternator vith a 5" dia crank pulley instead of the current 4"

[Mike D]

Alternators are rated way above what you could spin any sort of pushrod motor to.
The old 289 Hi-Pos used a large pully on their generators, but that was to prevent over-charging the system. That won't happen with an alternator.

[Jack Wojack]

You might find info on the PA-Performance site, a mfg of HP alternators. I would think if you spin it too fast you could shorten it useful life...bearing life or do damage to something.

Jack

[cChrisM]

Not sure if this applies to you, but this is discussed a lot on the Terminator section on SVTperformance, where guys change the lower pulley to spin everything faster and many put on a larger alternator pulley.

This is from the FAQs section:

Found the cure to our alternator woes!!! - SVTPerformance

....As most of you know, the 03-04 cobra are notorious for having alternators go bad. I have been hunting down a voltage problem myself for over a year now and I have found some very interesting evidence that I think ALL terminator owners should know.

1. The car, as built by Ford, is over spinning the alternator.

Through my year of testing more of our alternators than you could shake a stick at, I have found that the alternaotrs in our cars all turn on ( as in start producing voltage) at about 650-700 rpms. That is the speed of the alternator, NOT the engine speed. So, if the crank pulley is roughly 7.5 inches and the alternator pulley is around 2.5 inches, the alternator spins about 3x what the engine does. So on any given car with an idle speed of 800 rpms, the alternator is now spinning at 2,400 rpms, 1,700 rpms past when it actually turns on. This, according to my testing, is too high. At 6,500 rpms you are now spinning the alternator at 19,500 rpms, seriously over spun. If you install, for example, a 3.2 inch alternator pulley, this will now produce an alternator speed of around 2x the engine speed. Alternator idle speed is now dropped to 1600 rpms and the 6,500 rpm engine speed has now dropped the alternator down to 13,000. Knocking 6,500 rpms off of the top speed of the alternator is going to seriosly increase the life of our alternators! Now for those worried about voltage problems, I have tested this on my car as well as a couple other mustangs. This has shown to still produce 13.9-14.2 volts at idle once warmed up. Data logging has also shown a much more steady voltage in the high rpms as well as much less of a drop off. I truly believe that under driving our alternators is the key to making them last longer.

2. Our alternators are not rated high enough to power our cars.

This is where it gets interesting. Our stock alternator is rated to 105 amps. Any given alternator is only capable of producing 80% of its rating continuously, so a 105 amp alternator can safely continuously produce 82 amps. Being an electrician I happen to have certain tools which help to prove my point here. If you place an amp meter (Amp clamp to most who know) on the outgoing wire of the alternator you can tell exactly how much amperage the car is drawing. Our cars, with the fans running and the light on pull about 70-75 amps. Turn on the A/C and radio and you are now at 90-93 amps. This is 8-11 amps above what the alternator is capable of doing. this shows the stock alternator is not even powerful enough to safely and efficiently power our cars even in stock form! I would seriously suggest EVERYONE run at least a 140 amp rated alternator.

3. Heat kills our alternators as well.

I have found that heat has been a huge factor in the death of our alternators. During my testing i found the alternator becoming almost too hot to touch when run for 10 minutes. This all changed though after I slowed the alternator down. Once it was slowed down the alternator did get hot, but never too hot to touch. Although this is not the most scientific test, it gives us an understanding on what's going on. As for more accurate testing, I give you this. With the stock pulley on the cars tested would idle and produce a voltage of around 14.3. Once warmed up they produced only 13.7-13.8 volts. After changing to a bigger alternator pulley the cars now idled when cold at around 14.2 volts but when warmed up, only dropped to 14.0-13.97. This seems to be a much more stable setup.

In conclusion. A bigger alternator pulley on a stock unit should greatly increase the life of our alternators but a bigger alternator with a bigger pulley is the ideal setup.

So take it for what it's worth but I can tell you I have tested more on these cars, in terms of the charging system, than ANYONE has ever tested. I believe even more than Ford tested themselves......

[Mustang Man]

The Hi-Po Mustangs used alternators and they had a larger pulley to slow the alternator down. It wasn't just the early generator cars (like Hi-Po Fairlanes). You CAN overspeed an alternator with the wrong pulley combination. Stick with stock pulleys or an engineered pulley kit, do not just mix and match from a pile of spare parts or the junkyard...

HTH...
Mark

[turbonut48]

Cheapskate:

I think youre going the wrong direction. You need to increase the size of the alternator, or decrease the size of the engine sheave.

The Nut

[CobraDreaming]

I fried a few of them back in the day in my 70 cougar 351C spinning up to around 7000 RPM. Of course they were rebuilds but the engine revved high then things stopped charging. Man I miss that car / engine

[Big Blocker]

"Thinking about my pulley set-up, and driving the alternator with a 5" dia crank pulley instead of the current 4".

If you are thinking of installing a larger crank pulley, you will increase the speed of the alternator.
The closer the crank and alternator pulley sizes get together, the more the alternator will slow down. 5" crank and 5" Alternator = same speed.

Either make the alt pulley larger or the crank smaller, remembering that there is a water pump involved here too. The same effect happens at the WP, if you were to make the pulley smaller at the WP, the pump would spin so fast it would cavitate(sp) and your engine would overheat. This is the effect that drag racers get when they put a 3" pulley on the crank for a race, slows down all the accessories, adds HP, but at a cost of cooling and charging.

Larger pulleys were installed on Hi-Po 289's to slow down the Alternator.

HTH

Doc