underdrive pulley....please need help?
Thread Starter
Pro
Joined: Mar 2006
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underdrive pulley....please need help?
When you buy one of these...do they come with the new belts (would be different size correct?)....and does anyone have any recommendations on which one to go with...I'm doing a H&C swap and thought I would do the pulley while they are in there. Any help would be greatly appreciated...thanks guys
Tech Contributor
Joined: Mar 2003
Posts: 12,094
Likes: 197
From: Colorado Springs, CO
St. Jude Donor '08-'09-'10-'11
I think you meant the alternator pulley.....typo I suppose.If you do the crank pulley and not the alternator pulley, the belts will be smaller.
Belts came with mine from Tbyrne but they tend not to. I paid 260 with the belts. A powerbond which looks great doesnt come with the belts and they are under 200.
Thread Starter
Pro
Joined: Mar 2006
Posts: 620
Likes: 0
Um what? I think you meant the alternator pulley.....typo I suppose.
If you do the crank pulley and not the alternator pulley, the belts will be smaller.
Belts came with mine from Tbyrne but they tend not to. I paid 260 with the belts. A powerbond which looks great doesnt come with the belts and they are under 200.
I think you meant the alternator pulley.....typo I suppose.If you do the crank pulley and not the alternator pulley, the belts will be smaller.
Belts came with mine from Tbyrne but they tend not to. I paid 260 with the belts. A powerbond which looks great doesnt come with the belts and they are under 200.
Burning Brakes
Joined: Mar 2006
Posts: 800
Likes: 0
From: Las Vegas Nevada
Um what? I think you meant the alternator pulley.....typo I suppose.
If you do the crank pulley and not the alternator pulley, the belts will be smaller.
Belts came with mine from Tbyrne but they tend not to. I paid 260 with the belts. A powerbond which looks great doesnt come with the belts and they are under 200.
I think you meant the alternator pulley.....typo I suppose.If you do the crank pulley and not the alternator pulley, the belts will be smaller.
Belts came with mine from Tbyrne but they tend not to. I paid 260 with the belts. A powerbond which looks great doesnt come with the belts and they are under 200.
Drifting
Joined: Jul 2005
Posts: 1,533
Likes: 0
From: az
spin is always pulling the slack, keep us straight. 
Tech Contributor
Joined: Mar 2003
Posts: 12,094
Likes: 197
From: Colorado Springs, CO
St. Jude Donor '08-'09-'10-'11
if you want to use the same belt you have to underdrive the alternate pulley or some other pulley. with both underdrive crank & underdrive alternator pulley it may cause you to have charging problem if you have big amps. i would just change the crank only, since it already under drives everything around 20-25% ish already.
When you change an UD pulley it is likely that it is with a cam change. When you swap cams, you raise the idle. If you go to 800rpm from 625-650rpm you raised the speed of the alternator back to the speed it would have been with the stock pulley/stock idle and hence there will be absolutely no change in the alternator output. No charge issues. The alternator pulley is used for cars that retain the same idle or near it. The alternator would then be spinning slower than stock and only at idle would you notice a dimming if the voltage regulator cant keep up. So in summary 25% reduction in gear +25% increase in idle rpm = no change in alternator output at idle.
Regardless, if you use an UD pulley with or without a alternator pulley, the AC belt is shorter (40 inch).
Last edited by SpinMonster; Jun 24, 2007 at 09:37 AM.
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Drifting
Joined: Jul 2005
Posts: 1,533
Likes: 0
From: az
The alternator pulley isnt underdrive, its overdrive, to spin the alternator faster if you dim with stock idle speeds.
When you change an UD pulley it is likely that it is with a cam change. When you swap cams, you raise the idle. If you go to 800rpm from 625-650rpm you raised the speed of the alternator back to the speed it would have been with the stock pulley/stock idle and hence there will be absolutely no change in the alternator output. No charge issues. The alternator pulley is used for cars that retain the same idle or near it. The alternator would then be spinning slower than stock and only at idle would you notice a dimming if the voltage regulator cant keep up. So in summary 25% reduction in gear +25% increase in idle rpm = no change in alternator output at idle.
Regardless, if you use an UD pulley with or without a alternator pulley, the AC belt is shorter (40 inch).
When you change an UD pulley it is likely that it is with a cam change. When you swap cams, you raise the idle. If you go to 800rpm from 625-650rpm you raised the speed of the alternator back to the speed it would have been with the stock pulley/stock idle and hence there will be absolutely no change in the alternator output. No charge issues. The alternator pulley is used for cars that retain the same idle or near it. The alternator would then be spinning slower than stock and only at idle would you notice a dimming if the voltage regulator cant keep up. So in summary 25% reduction in gear +25% increase in idle rpm = no change in alternator output at idle.
Regardless, if you use an UD pulley with or without a alternator pulley, the AC belt is shorter (40 inch).
