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Old 06-04-2015, 09:50 PM   #1
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Under Drive pullies?

Do under drive pullies really free up horsepower or is that a myth? Me and a buddy are debating I told them they do nothing but look good, he swears that they help. Neither of us have them. And we're going to do a little comparison he ordered his last night.

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Old 06-04-2015, 10:27 PM   #2
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The do free up horsepower by taking away some of the horsepower taken by belt driven accessories. Not much, but some.

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Old 06-04-2015, 11:08 PM   #3
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All of those accessories that are driven by your serpentine belt take work to spin. So some of the work that your engine does is purely to spin those accessories. They dont spin themselves you know. So underdrve pulleys do just what they say. They "under"drive the accessories so they are less parasitic. Instead of your alternator spinning 20,000 RPM, it will only spin half of that which takes half of the work. Those numbers are made up but the idea is there. Bigger pulleys means they spin the **** less. Its like the big sprocket on your mountain bike. It takes very little effort to pedal when you are in 1st gear, but you accomplish less and you have energy left over to do other things.
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Old 06-05-2015, 09:59 AM   #4
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Its like 2hp if anything. They are not worth the money and the potential cooling or charging issues. Like... you are not going to gain anything power wise that is going to do anything. If you are getting beat by someone at the track putting on a set of pullies is not going to put your car over the edge or something.

Now if you are going for looks... pull your stock pullies off and send them out to get blasted and coated.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.

Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.
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Old 06-05-2015, 10:59 PM   #5
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Serpentine belts take HP to run, and I don't remember the HP loose per pulley but when you add them up it is enough to feel it in the seat of your pants............and for us old guys that's at least 10hp to this well educated butt.
To make a comparison, my 97 Cobra had under drive pulleys and one the chassis dyno it said there was just under 3hp gain at 6000rpm. When the same engine was out of the car and put on an engine dyno without the complete serpentine system it put out 356hp. The factory rating was 305. Granted it was only running the FPRP headers and no cats, but I don't think the cats accounted for all of the 50hp difference.
As was said earlier, I would spend any money on them.
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Old 06-08-2015, 07:49 AM   #6
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I remember reading they are good for 5-7 HP. I have them on my GT and the car felt much 'lighter' in street driving. I had to use less gas pedal when going from a standing start. I like them and would do it again.
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Old 06-08-2015, 10:49 AM   #7
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Let me know how it goes. Debating if I should get them or not.

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