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Discussion Starter · #1 ·
I've got an Infiniti alternator in my car, and it has just occurred to me that the overall gearing of the pulleys is off, that the alternator is not being spun as fast as it needs to be.

My brain isn't working too well at the moment - would I have to swap the pulley on the alternator for a bigger one, or a smaller one, to spin it faster?
 

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V=RW... linear velocity = radius x angular velocity.

think of a bicycle. one size gives you more speed but less torque, the other gives you more torque less speed.

a bigger sized pully = higher speed...

a smaller sized pulley = more torque.

i should mention!:... the alternators are designed to have a maximum spinning speed... if you spin it faster, i promise you will blow the diode packs instantly causing alternator death. nissan alternators are famous for weakling diodes. i know first hand, from my 93 altima beater (now Rests In Pieces..lol) i could litterally blow the diodes in it just by doing some spirited driving and keep the RPM's really high.
 

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in short - smaller pulley. However, archaeic speaks the truth. you are better off putting in a high-output alternator - if you can find the max rpm it's allowed, then you can work out the maximum pulley size you can run by working out the ratio between the crank pulley diameter and the alt. pulley diameter. 5 minutes and a cup of tea should see that math sorted :)
 

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Discussion Starter · #5 ·
thanks. it's a 160 amp alternator, which is quite a step up from the stock 85 amp. It just doesn't seem to be putting out maximum power, and I also spent a LOT of time idling in traffic, so I was thinking of bumping up the RPMs slightly, 10% or less.

But that's a good note about the diodes. Throwing the alternator belt was all it took to kill my last alternator. Maybe I should really just bite the bullet and buy a true high performance alternator (mine are rebuilt and modified Infiniti units, maybe not the most reliable things on earth).
 

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You'll find modern alternators will offer full output from quite low shaft RPM's, so you may not need a bigger pulley. On a quite high revving engine like an RB26 you may want to run the alternator at a slightly LOWER overall gearing than on the Infinity, depending on what max RPM that was capable of. You need the crank pulley diameter of the parent Infinity engine to do any meaningful calculations, or the output / RPM graph of the alternator, which you probably won't get without physically measuring it I'm afraid.
 
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