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Old 04-24-2001, 10:02 PM   #1
Legacy777
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Question Underdrive Pulley questions Pros/Cons

Ok, I've done some research on these. Been considering one for a while. I know about how the lighten weight and smaller dia helps the engine, faster revs, more hp, etc.

I guess my main concern is the reduction in rpm's of the alternator. So at an idle there is less juice goin to the car's electrical system. I have read what their site says, and everything. I have talked to two friends, that have done it, one has an RS, and it does dim the lights when at an idle, until the revs get to about 1200-1500. The other friend has a early 90's turbo touring wagon....he said he didn't notice any difference or dimming in the lights.

Sooo, my question....what's everyone else's experiences, observations, Legacy, impreza, svx owners..???

I have a 90 Legacy. I do have a 500+ Watt stereo system, however I have it tuned where the gains are fairly low. I also have an Optima Yellow top deep cycle battery.

I've got more specific details on my stereo and other mods on my website Check it out.

The main reason I am asking is that uncle sam decided to give me some money this year So I was thinkin....what better way to spend it, then on my subie...haha

Thanks for any input.

Josh
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Old 04-25-2001, 12:30 AM   #2
suberboy
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i have had an underdrive pully for about 9 months now and havn't had any problems. i have 500+ watt stereo and use the ac occasionally and havn't noticed a difference from stock. some people on this board have noticed a difference with ac though so you never know. hope that helps.
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Old 04-25-2001, 05:35 AM   #3
tmat3
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If you're worried about not enough electricity being generated by using UDP, you have an option of getting the same size pulley as the stock ones, but a lot lighter. I think JC Sports sells it.
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Old 04-25-2001, 08:37 AM   #4
Matt Monson
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I've had the unorthodx pulley for about 3 months. I haven't noticed any problems, and my gas mileage went up about 1 mpg.
My car is a MY00 RS w/cobb intake and stromung cat back.
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Old 04-25-2001, 09:26 AM   #5
LangstonSubaru
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I have them, haven't seen any difference. It took a couple days to get used to the new belt sound, after listening to the belts on the steel heavy pullies, the aluminum sounds different. I say "go getcha sum!"
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Old 04-25-2001, 10:21 AM   #6
edkwon
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Ahh so the trick to avoid the idle/stalling out problem/ with a racing underdrive pulley is to find one that is same sized as stock but lighter? I've heard problems from other ppl as well about the smaller sized pulleys and inadequate electrical delivery to the alternator.

Ed
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Old 04-25-2001, 11:43 AM   #7
adamsrs
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I love mine!
adamsrs www.unnatural.org
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Old 04-25-2001, 11:45 AM   #8
Legacy777
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The thing is that when you reduce the size of the main pulley, you are effectively changing the "gear" ratio between the main pulley and the alternator. and/or other pulleys.

This helps, because it gives the engine less resistance and is easier to rev, hence little more power, and throttle response.

However it decreases the final drive ratio by the alternator pulley.

Easy way to think of this, is with a mtn bike. When you're in the big gear in the front and little one in the back you can go very fast, but it takes alot more leg power to get moving. However, when you're in the small gear in the front and big one in the back, you have alot more torque, it's easier to get going faster, but you can pedal like crazy and you only go so fast. Granted this example is on the extremes. But should give you a better picture.

The best solution would be to make a smaller alternator pulley to compensate for the decrease in size of the main pulley. A friend of mine was going to do this for me, but backed out at the last minute, because of time.

I've got it calculated how much the rpm of the alternator drops. I'll post the difference when I get back home and have the papers.

Josh
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Old 04-25-2001, 11:55 AM   #9
John2.5RS
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I have an UDP and notice that the power steering is also effected. Anyone else?
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Old 04-25-2001, 01:27 PM   #10
Legacy777
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What does it do to the power steering? Just harder to turn at idle speeds?
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Old 04-25-2001, 01:39 PM   #11
Legacy777
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Ok, here's the info on the speeds of the pulleys (mainly alternator and main pulley)

Here's the test or given info, if you want to call it that.

Engine speed(rpm)=500rpm
Stock crank pulley dia (edge to edge)=5.25"
Stock Alternator pulley dia (edge to edge)=2.375
Underdrive crank pulley dia (edge to edge)=4.375

For those parameters, engine speed 500 rpm

With the stock crank pulley the alternator pulley would be goin around 1105 rpm's

With the underdrive crank pulley, the alternator pulley would be goin around 921 rpm's

So....Ultimately...you're loosing about 200 rpms in speed to the alternator pulley.

Without knowing more info about the alternator, it's hard to tell how much less voltage & amperage is coming out with a loss of 200 rpm's.


If anyone cares to know how I got my info, here's the calcs I used

D=dia driving pulley
d=dia driven pulley
S=speed of driving pulley
s=speed of driven pulley

d = (D*S)/s

Have any questions about the above stuf....drop me a line
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