Originally Posted by 68 fastback

Just some thoughts..

Generaly the electrical system design is such that the alternator can, at idle, satisfy all likely-concurrent loads: lights, a/c, stereo (at a subset of max output), ECU, etc.

Underdrive pulleys are a two-edged sword: spin the belt and/or driven devices slower (alt, compressor, etc) and you WILL experience:
-reduced accessory losses (a few more HP, especially at higher rpm)
-reduced accessory output

If you have a tune on the car, you can use the programmer to set the idle higher to compensate for some of the pulley reduction. This is a good idea since it will not give back any of the power gains at but will keep the battery better conditioned.

If you're lucky, running underdrive pulleys will just shorten battery life (can be considerably shorter -- from 6-8 years to 1-3 years DEPENDING ON YOUR DRIVING SITUATION.

If you have a Shaker 1000 and like it LOUD and BASSY, drive in stop and go traffic for much of your commute in pre-dawn hours in Phoenix, don't be surprised if your batter life is even shorter than 1-year.

If your daily driving is typically a freeway commute with little or no dead-stopidling, your battery life may not be affected much at all, even with heavy accessory use: a/c, lights, stereo...

Most people think of a battery kinda like a jug of water, you drain it down and the alternator fills it up, and as long as it doesn't run out (you get ahead of the alternator for too long a period) you're ok.

But that analogy is not entirely a good one. Battteries are generally only elastic in the top 10% of their capacity -- the deeper you dip beyond that starts to shorten their life by more and more (disproportionately). Once you get to the point where you're discharging up to 40% or so (depends on the quality and design of the battery), life span is greatly reduced, especially if it sits in the reduced state overnight (as opposed to being recharged before shut-off. However, DEEP cycling of even a quality automotive battery beyond its design-point even with a 'full' recharge will greatly reduce its life (a deep-cycled impaired battery will never reach full capacity again). Marine batteries are designed for deeper cycling but give up some other capabilities in the trade (marine batts are not recommeded for cars).

A severe service battery (e.g. optima yellow/red-top) will prevent spills and take much more vibration, but will not last any longer than a conventional battery of similar capacity and cycle-design.

So the net is, IMO, asking others if pulleys worked ok for them is not usefull in predicting how pulleys will work for you. The specifics of the car (alternator capacity at idle), the climate, the battery design/capacity, the actual daily driving conditions and the actual habits of the driver ALL INTERACT UNIQUELY to determine if the battery is operating [too] deeply into its reserves or not on a regular basis.

To the extent it is, is to the extent your battery WILL fail prematurely, and how prematurely.

The engineers who designed the car took great pains to assure that you don't inadvertantly, and on a regular basis, dip into that reserve deeper than the modelled use scenarios of the car and working design parameters of the total electrical system permit.

When you change pulleys you fundamentally change those relationships: some under most conditions, but mostly under idle/slow-speed conditions. Depending on the above use factors, two identical cars can experience vastly different lifespans and failure rates for various electrical components with altered pulleys. Fortunately most will only see a shortened battery life and some will see almost no change, but others can and will see battery and component failure ---

---IT ALL DEPENDS...

Originally Posted by Nick85

I had Steeda UDP's installed at about 10,000 miles. At 20,000 miles it caused one of the scariest problems i've ever encountered driving. The car slowly shut down piece by piece on the interstate. Started with the battery and ended with the speedometer buried in 0 mph while I was doing 65 mph. It fried my alternator. Ford replaced it no questions. Guess what happened at 35,000 miles....yup. Pullies were removed and the AC gets extremely cold now.

Originally Posted by Nick85

I had Steeda UDP's installed at about 10,000 miles. At 20,000 miles it caused one of the scariest problems i've ever encountered driving. The car slowly shut down piece by piece on the interstate. Started with the battery and ended with the speedometer buried in 0 mph while I was doing 65 mph. It fried my alternator. Ford replaced it no questions.

Originally Posted by dustindu4

that's exactly what my car did