What do you guys think? My tuner says it's not necessary with the 4 LB. He only recommends it with 6LB and up. I have read various opinions...what's yours? I will be going 2.93 upper with a stage IV Steggy. I would also like to know WHY you feel the way you do. Thanks!
Alternator pulley with 4 LB lower?????
- Thread starter wtsn8ke
- Start date
3.5" with 4# lower and underspin the alt slightly. I have been doing it for months now with no issues.
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Lethal performance helped me with mine. No probs yet. I have 4# lower 3.25 upper w140ax
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+1 w/3.5" alt pulley w/2.93 upper & 4# lower
The alternators in our cars turn on at around 800 to 1000 shaft RPM, not engine RPM. The ratio between a 8.5" lower (4lb) and a 3.5" alt pulley is about 2.42:1. So with a 4lb lower, at idle of say 800 rpm, the alternator is going ~1936 rpm, which is way above it's turn on speed. At that shaft rpm it should be charging just fine.
For comparison, the factory lower is 7.65" and the factory alt is 2.6" which has a ratio of 2.94:1, or ~2352 shaft rpm at 800 rpm engine idle.
Also for comparison, adding a 8.5" lower (4lb) and retaining the factory 2.6" alt pulley gives a ratio of 3.27:1 or ~2615 shaft rpm at 800 rpm idle.
The reason that these alts pop so frequently is that their supposed redline is around 16,000 rpm. Even with factory pullies and factory 6500rpm rev limiter you will spin your alt to over 19,000 rpm.
With an 8.5" lower and stock alt pulley you will take the alt over 21k rpm. :dw: It is somewhat unlikely that the alternator would even survive one trip to this speed.
With an 8.5" lower and a 3.5" alt pulley you're going right about 15.7k rpm, just below alternator redline at 6500rpm on the tach.
It's all about finding a balance between alternator idle rpm and alternator max rpm. Going a little bit over 16,000 isn't nearly as bad as going to 19k or higher. Even with a factory lower, you could put a 3.0" alt pulley on (2.55:1) and be in the clear for both the low and the high.
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Excellent analysis Matt!
+1. Thanks for the great information on this subject.
i have a 3.5 pulley, NIB i would sell to someone cheap (somehow i have an extra one). .
While all the information above is sound, it's not the reality of the 03/04' alternator "situation". Alternators have been failing prematurely on these cars from their first year of production, regardless of modifications or usage. They have failed with stock pulley setups, with overdriven, with underdriven, you name it. There is no conclusive evidence that slowing the alternator down on these cars will prolong their life. That being said, it certainly makes sense that it could be a benefit to alternator longevity.
I ran a 4 lb lower with the factory 2.6" alternator pulley for 2 years with no issues. When I swtiched over to a 6 lb lower, went ahead and installed a 3.2" alt pulley. That was two years ago and the alternator is still going strong after nearly 8 years and 39,000 miles.
To be accurate, with normal driving habits, the alternator will be "overspun" very, very infrequently when compared to total usage. Probably less than 1% of the time actually.
I ran a 4 lb lower with the factory 2.6" alternator pulley for 2 years with no issues. When I swtiched over to a 6 lb lower, went ahead and installed a 3.2" alt pulley. That was two years ago and the alternator is still going strong after nearly 8 years and 39,000 miles.
To be accurate, with normal driving habits, the alternator will be "overspun" very, very infrequently when compared to total usage. Probably less than 1% of the time actually.
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