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The Terminator
Engine/Tuning
Alternator pulley with 4 LB lower?????
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<blockquote data-quote="thelpb" data-source="post: 9654459" data-attributes="member: 36649"><p>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.</p><p></p><p>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.</p><p></p><p>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.</p><p></p><p>The reason that these alts pop so frequently is that their supposed redline is around 16,000 rpm. <u>Even with factory pullies and factory 6500rpm rev limiter you will spin your alt to over <strong>19,000</strong> rpm.</u></p><p></p><p>With an 8.5" lower and stock alt pulley you will take the alt over <strong><u><span style="color: Red">21k</span></u></strong> rpm. :dw: It is somewhat unlikely that the alternator would even survive one trip to this speed.</p><p></p><p>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.</p><p></p><p>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.</p></blockquote><p></p>
[QUOTE="thelpb, post: 9654459, member: 36649"] 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. [U]Even with factory pullies and factory 6500rpm rev limiter you will spin your alt to over [B]19,000[/B] rpm.[/U] With an 8.5" lower and stock alt pulley you will take the alt over [B][U][COLOR="Red"]21k[/COLOR][/U][/B] 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. [/QUOTE]
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Alternator pulley with 4 LB lower?????
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