Alternator Pulley - Subaru WRX Forum
 
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post #1 of 15 Old 05-09-2005, 09:36 PM Thread Starter
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Alternator Pulley

If I have a lot of electrical stuff running throughout my car, would an underdriven alternator pulley help out the alternator so that it recharges the battery quicker? Also would it help out with my crowded electrical system?
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post #2 of 15 Old 05-09-2005, 10:31 PM
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An underdrive pulley is only going to make it worse. What you need is an overdrive pulley and I am not even sure if they make them.

Last edited by WRXtreme; 05-10-2005 at 12:46 PM. Reason: Don't know what I am talking about
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post #3 of 15 Old 05-09-2005, 11:01 PM Thread Starter
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but wont the underdrive pulley make it spin and work faster?
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post #4 of 15 Old 05-09-2005, 11:02 PM
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My buddy has an underdrive pulley on his car and he does not like it. As soon as he has more work to do on the engine he is going to take it off.
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post #5 of 15 Old 05-09-2005, 11:13 PM Thread Starter
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is that the underdrive crank or alternator pulley....ive heard GREAT things about the unorthodox racing crank pulley as well
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post #6 of 15 Old 05-10-2005, 12:48 PM
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Underdrive pulleys are a bad idea in general. If you want a pulley set, get one that is lightened but still the same size as stock.

I believe underdrive pulleys are larger then stock(thats a little confusing to me) and there for makes the alternator spin slower.
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post #7 of 15 Old 05-10-2005, 01:19 PM
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Quote:
Originally Posted by WRXtreme
Underdrive pulleys are a bad idea in general. If you want a pulley set, get one that is lightened but still the same size as stock.

I believe underdrive pulleys are larger then stock(thats a little confusing to me) and there for makes the alternator spin slower.
K I'll try to explain this for everyone. Underdrive pulleys are smaller and require shorter belts than stock.

Think of 2 circles one with 1" Radius and 1 with 2" Radius. Circumference = 2*Pi*R (R=Radius) (Pi = ~3.14)

1" Radius has circumference of 2*Pi*1 or ~6.28

2" Radius has circumference of 2*Pi*2 or ~12.56

So in 1 360 degree turn the 2" radius pulley would move the belt twice as many inches. Since the other pulley is a fixed circumference this means the belt would turn the alternator pulley twice as far. Of course the difference in UD pulleys is nowhere near this extreme but it should help you understand.

Hope this helps.
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post #8 of 15 Old 05-10-2005, 06:27 PM
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I could very well be wrong, but.....

I disagree, I think that a 2" pulley is going to spin half as much as a 1" pulley. My thought pattern is like this: Think of a 5-speed bike, with 1 gear at the pedals and 5 gears in the back. The biggest gear makes the back tire spin the slowest and the smallest gear spins the fastest. Maybe it is totally different with pulleys for a engine, but that is my thought pattern and I am sticking with it.

-Adam
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post #9 of 15 Old 05-10-2005, 10:47 PM Thread Starter
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Okay so should i be lookin at the Perrin Crank Pulley i've also heard some good things about that...?
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post #10 of 15 Old 05-10-2005, 11:01 PM
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yes, The crank shaft pully is a good idea. I instaslled a perrin crank shaft pully which in soppost to be the same size as stock only much lighter and was very impressed with it. The engine reved quicker and it improved throtle responce. def a good pick up.
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post #11 of 15 Old 05-11-2005, 12:37 AM
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Quote:
Originally Posted by WRXtreme
I could very well be wrong, but.....

I disagree, I think that a 2" pulley is going to spin half as much as a 1" pulley. My thought pattern is like this: Think of a 5-speed bike, with 1 gear at the pedals and 5 gears in the back. The biggest gear makes the back tire spin the slowest and the smallest gear spins the fastest. Maybe it is totally different with pulleys for a engine, but that is my thought pattern and I am sticking with it.

-Adam
Go back and reread. I am saying the larger pulley will turn more or drive the accessories faster than the smaller one.

I understand where the thought comes from. The difference here is you are changing the "turned" pulley/gear versus a UD pulley you are changing the "working" pulley.

On your car the crank pulley is what actually moves (on a bike the pedal gear). The belt /chain is turned by the pulley which makes the accessory(back wheel) turn. If the path of the pulley on the accessory was enlarged by using a larger pulley the result would be the accessory is turned slower. The speed you pedal being the same for every turn of the pedal 360 deg you get distance X of chain moved. If the path around the wheel driven gear is enlarged it will take more length of chain and there fore more pedaling to turn it 360 deg.

Remember the idea of a UD pulley is to lessen the workload of the motor so it wants the accessories to be turned less than stock. Also as I said look at any underdrive crank pulley and it ALWAYS says shorter belts required.

Isn't Geometry fun!?
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post #12 of 15 Old 05-11-2005, 07:29 AM
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Are you talking about a crank pulley or say an alternator pulley, because I was talking about a alternator pulley which would be the same as the rear gear on a bike. I think you are talking about the crank pulley which would be the front gear on a bike. If thats the case then everything makes sense and it was a miscommunication, if not, then I am more confused then ever.....
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post #13 of 15 Old 05-11-2005, 10:18 PM
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here think of it this way.(this is the same for wheels) a smaller pully is going to have to turn twice as much to reach the recharging point of the alternator. example, going back to the bike, put the bike in the smallest(easiest to peddle) gear possible. then mark were u have started. make one revolution with the crank and stop and mark that. then put it into a bigger gear(harder to peddle) then do the same. the bigger gear will have moved the bike farther than the smaller gear would have. which makes underdriving a car "take a load off the engine."
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post #14 of 15 Old 05-11-2005, 10:38 PM
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Quote:
Originally Posted by WRXtreme
Are you talking about a crank pulley or say an alternator pulley, because I was talking about a alternator pulley which would be the same as the rear gear on a bike. I think you are talking about the crank pulley which would be the front gear on a bike. If thats the case then everything makes sense and it was a miscommunication, if not, then I am more confused then ever.....

Ha actually it looks like that is what happened. IDK of any underdrive alternator pulleys available but hey they may be out there. A smaller pulley on the accessory will make it spin more. I'd bet it would burn up the alternator though. Also, the alternator is designed to make peak output at typical driving rpms ~2500.
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post #15 of 15 Old 05-12-2005, 08:38 AM
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Now that we have that cleared up, it all makes sense.....
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