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Fox Body Engine Pulley & Accessory Options

Written By: Connor MC

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In the modern world of muscle cars, the Fox Body Mustang can easily be considered an underdog. Even the score with a set of underdrive pulleys and increase power and throttle response by reducing the parasitic drag your Mustang's accessories have on your engine.

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Fox Body Mustangs can benefit from reworking the stock pulley and accessory system. Aftermarket underdrive pulleys are one option that will help your Mustang rev faster, but there are other routes you can explore to increase your Fox Body's power and efficiency too.

All The Drawbacks To Stock Accessories

Fox Body Underdrive Pulley Set

 A fully loaded 5.0L Foxbody Mustang came with a truck load of parasitic accessories. A power steering pump, 95 amp alternator, counter-clockwise rotation water pump, A/C compressor and a smog pump all hang off the front of the motor and are driven via a serpentine belt powered by the crankshaft pulley. Now, it is widely known (this may be an understatement…) that the engine provides power to the wheels of the car, but it also provides power to these accessories. The fact of the matter is, these accessories suck up some of the available engine power, resulting in less getting to the wheels.

If you’re considering a budget-friendly way to regain some of this power, you have two options:

  • Slow the accessories down so that they do not draw as much power and
  • Remove any unnecessary accessories altogether

Slowing Down Your Mustang's Engine Accessories

Fox Body With labeled Pulleys

As already mentioned, slowing the engine accessories will recover a few horses. Logically speaking, the faster an accessory is spun, the more power it requires to be driven. Thus, slowing down the rotation of an accessory is a simple solution to recover some horses. How do we go about slowing them down, you ask? With underdrive pulleys!

The underdrive pulley is basically a replacement pulley of a different size for:

  • Water pump pulley
  • Alternator pulley
  • Crankshaft pulley
  • Supercharger pulley

The idea behind underdrive pulleys is to change the revolution ratio between the accessory (driven pulley) and the crankshaft pulley (driver pulley). It works exactly like a gear ratio. If you have a 2” driver pulley connected via a belt to a 2” driven pulley, they will rotate the same number of revolutions per minute. (a 1:1 ratio). Now, if you swapped the driven pulley with a larger 4” pulley, the smaller driver pulley will have to rotate twice to complete one revolution of the larger driven pulley. Another way to look at it is that for everyone one revolution of the driver pulley, the driven pulley completes a half revolution. Thus the driven pulley is turning at half the rate. Keep in mind, that for this application, the driver pulley is actually the crankshaft pulley, which is turning at the same RPM as what is indicated on the tachometer. Thereby the crankshaft pulley is spinning like always, but the accessory pulley is now being rotated at half the rate of the crankshaft pulley at any given RPM, thus reducing power consumption.

A 3-piece underdrive kit from Ford Racing, BBK or MAC Performance all feature different-than-stock pulley sizes that are optimized for the best ratio between the crankshaft pulley and accessory pulley. The change in pulley size allows for the accessories to turn at a lesser rate and thus drawing less power to operate. See below for pictures of stock pulley sizes.

Underdrive pulleys are a very easy way to pick up some horses (5-10, at a minimum), but they can also pose a couple of problems. Because they slow down the operation of the water pump and alternator, particularly at idle, overheating (rather, above average heating due to a diminished coolant flow rate at idle) and low voltage can occur if your Foxbody already shows signs of these problems before installing underdrive pulleys. If you’re concerned about this happening to you, make sure the cooling and charging systems are in tip-top shape. If either system is less than optimal in stock form, it is likely one of the above issues will show face at idle with underdrive pulleys installed. In a way, underdrive pulleys can weed out existing problems in the electrical and cooling systems.

Fox Body Engine Accessories That Rob You of Power 

Smog Delete Pulley For Fox Bodies
SR Performance Smog Delete

As far as option #2 goes, A/C (if equipped) and the smog pump are both expendable accessories that will not really have a noticeable impact on driveability and will free up a little power, not to mention some space in the engine bay. There is the option of removing the power steering pump and switching over to a manual rack, but in all honesty this isn’t too practical and best suited for a track only car.

If you decide to regain some of that lost power and don’t mind living without A/C, Ford Racing offers an A/C removal kit that allows you to remove the air conditioning compressor and relocate the power steering pump in its place for easier access. Regarding the smog pump, Ford Racing and SR Performance both offer a smog pump eliminator kit. Whether you pick the billet aluminum SR Performance offering or the Ford Racing cast aluminum piece, they both eliminate the power robbing smog pump and replace it with an additional idler pulley to retain the stock serpentine belt length and path.

Mustang Underdrive Pulley Perfromance Gains

Fox Body mustang Pulley Diagram

One note of caution: when changing pulley sizes or removing an accessory all together, the drive belt length and path can change. Some kits come with the correct belt length for the new setup, whereas others may require that you measure and purchase one yourself.

All in all, an underdrive pulley system is good to pick up some hidden horsepower and reduce the parasitic draw the stock pullies have on your Fox Body's engine performance. And, if you choose to drop any of the expendable pumps as well (A/C or smog), another 5-7HP can be expected. The numbers really don’t seem that high, but when put into a cost per horsepower perspective, it rounds out to approximately $10/HP, which any other modification will be hard pressed to beat.

Fitment includes: 1979, 1980, 1981, 1982, 1983, 1984, 1985, 1986, 1987, 1988, 1989, 1990, 1991, 1992, 1993, GT, Cobra, LX, SVO