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SR Performance Underdrive Pulleys (Late 01-04 GT)

Market Price $159.99 You Save 25% ($40.00)


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  • Item 388722
  • MPN# 388722

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Features, Video & Details

  • Gain Horsepower
  • Complete Kit
  • Durable, Lightweight Aluminum
  • Brilliant Polished Finish
  • Premium Quality
  • Fits Late 2001 to 2004 GT

Review & Installation Video

Review & Install Video

Hey I’m Justin with, and I’m going to be breaking down the SR Performance underdrive pulleys, fitting the 4.6L V8 motor found in all 96-04 Mustangs. Engine accessories like the AC compressor, water pump and alternator can really rob your car of horsepower, a great example of how an accessory can really eat up is whenever you turn your car’s AC on, and instantly everything feels a little more sluggish. This is referred to as parasitic drag. The job of an underdrive pulley is to turn the different accessories at a different speed by using a different sized pulley, thus requiring less amount of energy from the engine to turn the accessory belt. This reduction in drag on the crank shaft will allow the engine to work more efficiently, and should equate to more power at the rear wheels. The SR Performance kit does include a brand new crank pulley, a new water pump pulley which will differ depending on your year, a new alternator pulley, and all the hardware and spacers needed for the install. All 3 pulleys have been made from lightweight aluminum and feature a nice bright polished finish, which should definitely help the appearance side of things underneath the hood. As far as the install is concerned, it really shouldn’t be all that bad, ideally, you should have a torque wrench on hand before you get started, as well as some high temp RTV sealant and a breaker bar for that crank pulley. Again this isn’t the hardest job, but you should expect to spend about 2 hours getting everything bolted up. If you should have any problems, you can always reference the detailed instructions along with pictures located on this product page. So if you’re looking to take some of the drag off your accessories, in turn freeing up some horsepower, and improving your throttle response, then you should definitely check out these pulleys from SR Performance, available right here at

Untap Hidden Horsepower. Add some more bite to your GT's bark by reducing the amount of power you're losing to accessories and A/C. When you install this underdrive pulley kit, you'll free up up to 15 horsepower - and it's pretty much all gain and no pain, since you won't sacrifice your charging characteristics or A/C to do it.

Premium Quality. Don't settle for inferior parts. These premium quality pulleys are made from lightweight aluminum for durability and performance. Made using precise, state-of-the-art CNC manufacturing, you can count on these pulleys to perform at the highest level.

Easy Installation. This Underdrive Pulley kit installs quickly and easily in under 2 hours with no modifications required.

Application. These underdrive pullies are designed for the Late 2001 to 2004 GT with the short water pump pulley with a bowed in face. Re-uses factory serpentine belt.

Technical Note. Midway through the production run for the 2001 model year Ford changed the design of the water pump on the 4.6L engine. The older style pump produced from 1996 to mid 2001 had a longer shaft. In January of 2001 the design of the pump was changed to a shorter shaft. This necessitated a change in the design of the pump pulley.

Click here to see pictures of the different pulley styles.

Fitment: 2001 2002 2003 2004 Details

What's in the Box
  • (1) Crank Pully
  • (1) Water Pump Pully
  • (1) Alternator Pully
  • (2) Spacers
    Hardware included:
  • (4) Bolts
  • (3) Washers
  • (1) Crank Key
Installation Info
Tech Specs
Underdrive Pulley Specifications
Year/Model: Late 01-04 GT Manufacturer:
Material: Aluminum Finish: Polished
Crank Pulley: Piggyback SFI Certified: No
Will it fit my Mustang?
  • GT - 01, 02, 03, 04

Most Helpful Reviews

Stack Underdrive Pulleys

Got the product and forgot it was a piggy back system, so i was little worried. i installed the water pump pulley with no problem. the alternator pulley was a little more difficult. the instructions had ...

Read More
Helpful (9)

good definatley not great

I bought these under drives to put on my mildly modified 03 GT automatic, when i got he part i noticed first off that the pulleys, particularly the crankshaft and alternator pulleys were scratched ad scored ...

Read More
Helpful (3)

Overall Rating 4.3 out of 5 stars
Read All 16 Reviews

Customer Photos (3) Submit Your Photo

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1 Questions / 1 Answers

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how much does this kit change change the ratio at the crank? What is the diameter of the stock crank pulley and what is the diameter of the BBK? Thanks.


  • It does not change the ratio at the crank the ratios are changed at the power steering, air conditioning, alternator and smog pump if you have one. They supply a pulley to maintain the rpm at the water pump so you don't over heat. And the HP gain is a bout 5 to 8 HP. Get a tuner or a CAI or both if you really want to feel a increase in power when you hammer it down!!!


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Customer Reviews (16) Write a Review

Overall Rating 4.3

out of 5 stars

Installation Time:Afternoon

Installation Cost:$100-$250

  • Performance Gain 3.8 3.8
  • Fit 3.8 3.8

  • 5/5

    May 09, 2011

    Stack Underdrive Pulleys

    Got the product and forgot it was a piggy back system, so i was little worried. i installed the water pump pulley with no problem. the alternator pulley was a little more difficult. the instructions had nothing in it about using the brass spacers. plus the bigger spacer that i had to use was a little small, so i just filled it out a little and it worked prefect. i wanted to put those 2 pulleys on myself to keep the mechanic's labor down. i then tried to put the belt back on to drive it down to my mechanic and the belt does not fit on the stock crank pulley with the new water pump and alt. pulley. bottom line if u cant install the crank pulley let the mechanic put them all on. and please use the piggy back system and not just the new crank, u will throw off the balance of the engine off if u do not bolt the stock crank and balancer to the new stack racing crank pulley. after all that i did notice a big difference, i did run on the dyno before and after the install. i gained 18 HP and 21 FT-TQ. I think thats awesome gains. i dont have much done to the car. if you want proof email me and ill show you the dyno print outs. i Think its a great product and am recommending it to all my friends. PROOF DOESN'T LIE.

    jason L.

    Year: 2002 Submodel: GT

    Helpful (9) (1)

  • 3/5

    Installation Time: Full Day

    August 29, 2011

    good definatley not great

    I bought these under drives to put on my mildly modified 03 GT automatic, when i got he part i noticed first off that the pulleys, particularly the crankshaft and alternator pulleys were scratched ad scored like they were factory seconds, this displeased me due to the fact that the polished aluminum is supposed to be a bit of a cosmetic under the hood. installation was pretty good, the instructions were not helpful, it had no mention of brass spacers, (larger one goes on alternator) and another thing was even with the automatic there is no need for impact tools, a breaker bar will do just fine. the bolts on the harmonic balancer are a bit fragile so be careful torquing them down. i broke one and it was a pain to drill out. extremely dissatisfied with installation. however the performance was decent, the car pins you back in your seat above 3k rpms, enjoyable gain but if i did it again i would go with Steeda


    Year: 2003 Submodel: GT Greenville SC

    Helpful (3) (0)

  • 5/5

    January 20, 2011


    Instructions although printed with color photos it is lacking information. I was able to figure it out however. Pay close attention to how far your pulley is away from the alternator because there are different sized bushings which is not mentioned. The stock crank pulley has one key way and one supplied has two. Use plenty of RTV. Other than that everything lined up good and runs true. The video on the tech article supplied by American muscle is for a Steeda kit and is only useful for general information. What do I like... winds up quicker and third gear is now useful. Car pulls harder. If in doubt go for it. Real gain for very little money from Stack Racing. Thanks American Muscle.


    Helpful (4) (1)

  • 4/5

    February 26, 2011

    Stack Underdrive Pulleys

    Fit and finish on product is great and price is reasonable. Improvement is noticeable at higher RPM's. Instructions are simply adequate. Getting the crankshaft nut off can be difficult so be prepared for that. If you don't have a torque wrench, air tools etc. you might want to just have your mechanic put these on. I have the tools and install only took about 30 min. Don't forget to use the RTV.


    Year: 2003 Submodel: GT

    Helpful (3) (1)

  • 4/5

    Installation Time: Less than 1 Hour

    August 05, 2013

    Good gain for the money!

    Installed in no time! You can feel the difference if you pay enough attention to your car, it's subtle but you can feel it.


    Year: 2004 Submodel: GT Virginia

    Helpful (1) (0)

  • 4/5

    Installation Time: Afternoon

    December 22, 2012

    2004 Mustang GT

    The pulleys are good quality. The performance improvement was very noticable in both throttle response and HP. The only problem is that the alternator pulley is a few hundredths too small for the factory alternator shaft. You need to be able to bore out the pulley slightly and evenly for installation.


    Year: 2004 Submodel: GT Tolland, CT

    Helpful (1) (0)

  • 5/5

    March 22, 2011

    felt gains right away.

    I liked the product for the look and how easy it was to install. I choice these pulleys for the cost and so far they are running great. felt a gain soon as i put them on.


    Year: 2003 Submodel: GT

    Helpful (1) (1)

  • 5/5

    Installation Time: Afternoon

    December 31, 2012

    Great Product!

    I bought these pulleys for my 04 gt, to start the instructions in the box were horrible! the kit comes with 2 brass washers and the instructions say nothing about the washers. for the alternator pulley the brass washer didn't even fit, good thing i work in a machine shop i had to Bore the washer out a good amount (.006") to get it to fit nice and snug. other then that putting them on went smooth. i felt a big difference over 3000 rpm's, you can feel the car throw you in your seat a lot more and i could most defiantly felt a difference. i had NO problems with my battery dying because of the alternator not charging. GREAT product i recommend 100%


    Year: 2004 Submodel: GT

    Helpful (0) (1)

  • 3/5

    Installation Time: Less than 1 Hour

    December 22, 2012

    its ok

    I bought this underdrive pull set installed it was less than impressed with the outcome so I got a more expensive set from steeda it was much easier to instal no bushings to choose from for the alternator and no need to reuse the stock harmonic balancer as it came with a one price balancer/pully and gained a little more power:)


    Year: 2004 Submodel: GT east texas

    Helpful (0) (1)

  • 5/5

    Installation Time: Afternoon

    January 27, 2012

    great price

    These underdrive pullies are a great price and look really good. Well worth the money!


    Year: 2004 Submodel: GT Schuylkill Haven, PA

    Helpful (0) (1)

  • 5/5

    September 21, 2016

    Just what I needed.

    Look great. Smaller and Lighter then Factory Pulleys. My car just hit 176,000 miles and I recently paired these pulleys with some other engine repairs and my car is running great! Better than it ever has. Much more responsive, really pulls through every gear better and harder and gives a better exhaust note through every gear.. Also All that power that's taken when you turn on your AC is gone with these. Which is why I bought them. I am very satisfied.


    Year: 2002 Submodel: GT

    Helpful (0) (0)

  • 4/5

    July 12, 2016

    worth it

    Throttle response is quicker feel a noticeable power increase just suckes there piggyback


    Year: 2002 Submodel: GT

    Helpful (0) (0)

  • 4/5

    Installation Time: Afternoon

    April 22, 2016

    Great Replacement!

    These are a great Replacement for the OEM pulleys. I would recommend a friend help install them. And pay attention to the instructions!


    Year: 2004 Submodel: GT

    Helpful (0) (0)

  • 4/5

    Installation Time: Less than 1 Hour

    April 11, 2016

    Effective performance

    Solid product although I didn't like the piggie back pulley on the crank but all in all noticeable hp looks great and fairly simple to install I've owned probably 15 mustangs from fox bodies up and I have to say the 99 to 04's I've had a blast building more hp from these than any of the rest just by bolt on performance, thanks AM.


    Year: 2002 Submodel: GT

    Helpful (0) (0)

  • 5/5

    Installation Time: Shop Install

    Installation Cost: $100-$250

    January 26, 2016

    Amazing product

    These under drive pulleys are a great product and for the price they can't be beat. My harmonic balancer was going out so these were just what i needed to fix the problem and add alittle horsepower as well as improve the look of my engine bay. I don't know exactly how much power was gained from them but i did notice the car was able to rev faster due to less parasitic loss from the engine components. Overall this is a very good product and i would buy it again.


    Year: 2003 Submodel: GT Birmingham, AL

    Helpful (0) (0)

  • 4/5

    Installation Time: Shop Install

    Installation Cost: $100-$250

    October 05, 2015

    Great buy

    Very happy with these pulleys. Didn't take too long to instal and the gains were noticeable right away.


    Year: 2002 Submodel: GT Orlando, FL

    Helpful (0) (0)

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