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I am thinking about getting an under drive pully..

I have a Gen4, and I am wondering what it will do..

How much HP gain would it get?

Will it decrease my MPG?

And how much does one cost?

Thanks
 

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Since we're talking about UDP's, I'd like to ask a question, too.

I've had a lot of free time while on winter break, and I was just thinking about that UDP I have. When I had it put on, the stock belt would slip REALLY bad during the 1-2 shift, so I put on a shorter belt. But I was just wondering if that smaller belt would screw up some of the effect of the UDP.

Because the whole point of the UDP is to increase the number of rotations for the crank to spin the serpentine belt(like a gear reduction), but now that I've shortened the belt it now takes fewer rotations for the crank pulley to spin the belt, increasing resistance and accessory drive. Does this make sense? If I'm right about this, why would companies advertise a shorter belt with their UDP's?
 

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The pulley is larger and therefore will pull the belt less inches per revolution. This results in a slowing down of the accessories driven rpm. You would actually need a longer than OEM belt.
 

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Yeah, the belt length has no effect on pulley speed. Any belt that fits without slipping will have the same speed/performance.
 

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what stock pulleys do UDP's replace? If they replace the pulleys on the accessories, then they should be larger, if they replace the pulley on the crank alone, then they (it) should be smaller - if my thinking is correct.

edit: i had it backward
edit2: had it right originally - getting too confusing
 

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Sorry, on mine it is the driver crank pulley. Most use the smaller driven accessory pulleys which would take a smaller belt.
 

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Does anyone have a stock crank pulley from a vulcan? I am willing to give that person 30 bucks for one oif they have it, see I have a friend that is willing to sell them rather nicely priced on here for all the vulcan guys, they come in shiny finish...we have everything need to make em, just the damn pulley, anyone who has one let me know asap! at [email protected]
 

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Originally posted by golovko@Dec 30 2003, 01:33 PM
what stock pulleys do UDP's replace?  If they replace the pulleys on the accessories, then they should be smaller, if they replace the pulley on the crank alone, then they (it) should be larger -  if my thinking is correct.

edit: i had it backward
Wait... I think you had it right the first time. Larger driver, smaller driven would make the driven spin faster. Smaller driver, larger driven would make the driven spin slower, which is what you want. Right? Car audio nuts will stick a "racing pulley" on their alternator. It is smaller in diameter, and therefore, while the surface speed (controlled by the speed of the belt) remains the same, the alternator operates at higher rpms, increasing output.

SHOZ123 is confusing me!
 

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DeltarMN or anyone else who has an UDP on their duratec. What model do you have from SHO SHOP is it one for the contour SVT or the GEN III SHO (there are three different bolt patterns for the Gen III if you could specify.) Just in case I run into a used one.

Thanks
 

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Damn I want one... I've wanted one for awhile.. but I'm soo impatient. DRAXAS is making one soon but that'll probably be awhile from now.
 

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If you change the crank to smaller diameter, all of the other pulleys will drive SLOWER due to a SLOWER belt speed. The crank turns a certain rpm (which does not change). Let's say the circumference of the old pulley is 12 inches. At 1000 rpm the belt travels at 1000 feet per minute. If you change the crank pulley to a smaller diameter, let's say 10 inches circumference, the belt speed would be 833 feet per minute. The crank UDP actually UNDER drives the entire system. If you are turning all of the other pulleys slower, it takes less torque to accelerate all the accessories which in turn frees up some additional horsepower.
 

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Depends on if is the driver pulley or the driven pulley. sable_owner has it right as far as the driver pulley is concerned. It is the opposite for the driven pulleys. Sooo...

Driver pulley (crank)

smaller= slower speed
larger = faster speed

Driven pulley (accessory)

larger = slower speed
smaller = faster speed


The problem with SOME crank pulleys is the self destructed. And a business can make a lot more money selling individual accessory pulleys rather than one crank pulley.

If you contact ASP they will make any pulley you want.
 

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Don't forget about weight reduction. The SHO shop UDP for the Duratec weighs about 5lbs (guesstimated) and the stock one weighed about 15lbs (guesstimated) (with that stupid bracket that held up the stock heavy a** pulley). So I'm sure we can all agree that a lighter pulley will not take as much power to spin right away. Just like a light weight flywheel helps with performance.

I have the UDP on my car and can't really notice a diffrence but, I KNOW it did add some performance. And from what I heard (somewhere) the belt SHO shop sent w/pulley was a stock size belt even if they say it is smaller. Because the tensenior pulley will make up for the smaller pulley. That's why it has so much play, you can add a much alrger belt and the tensoiner pulley will addjust to the belt. Of course it is possiblie to go to far with the size big, or small. The pully can only adjust so far. But the UDP isn't small enough to add a belt that wont fit the tensioner pully.
 

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Another BIG thing to consider is the amount of under drive. This is usually referred to in a percentage less than normal. If you select a size pulley that is too small (or large depending on if it is a driver or driven) cooling, charging, A/C and power steering could be affected to the point of disfunction. 10% underdrive is a healthy starting point for any street driven car but even at this little of a change the operations of the cooling system and the charging system may be affected.

Aluminum will reduce the amount of inertia required to turn them which in turn will net some extra hp. Just changing the pulleys to aluminum will reduce the weight in the car.
 

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I don't think any of us have any choice on which size UDP we want. Right now it's, find it and buy it. Cause if you don't someone else will and your left with nothing.
 

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I can appreciate that but if you get too much underdrive, you could damage an alternator, A/C compressor, or something worse. It's worth asking the question about the size of the pulley before making that jump. You can calculate the amount of underdrive from the pulley diameter as measrued on the outside of the pulley (not the flange).

One thing on belt size; the only way to get the right belt is to measure for it yourself. I learned this the hard way from working on my 'stang.
 

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HOw do we know if their is too munch UD?? How much UD can the alt and ac compressor handel?? It sounds like some trial and error, and if you don't get it right the first time than you'll have to replace a few things. I usuall trust a company to make a product that works. Now if some individual made it for the first time, than i would be a little worried about correct size.
 

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So whats the formula for figuring out the under drive?

My stock pulley had a diameter of 6 1/2", minus two 1/8" lips, so the belt would ride on a 6 1/4" pulley.

The ShoShop UDP has a 5 1/4" diameter, minus the two 1/8" lips and the belt rides on a 5" pulley.

So I went from 6 1/4" to 5", how do I figure out how much under drive I got?
 

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Crank Pulley Underdrive Formula=(1-(new diameter/old diameter)) x 100

(1-(5.00/6.25)) x 100=20% underdrive

Your accesories are running 20% slower than normal. This amount of overdrive may be fine for racing SHO's. Have you had any problems with charging or cooling? Do your lights brighten when you rev up the motor and dim when you go down to idle. Any power steering noise at low engine rpm? All I'm saying is that I've heard of problems with engines at 15% or more. I currently have a 10% underdrive crank pulley on my mustang and I have not seen any problems.
 
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