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Mustang Underdrive Pulley Guide

Written By: Andrew Cilio

Underdrive pulleys are often one of the first modifications many people make to their Mustang. There’s a good reason for this, as they’re a relatively cheap upgrade when comparing performance return per dollar spent.

American Muscle

Mustang Underdrive Pulleys - History

Underdrive pulleys have been around for decades, but it wasn’t until the mid-to late 80’s when they came to the attention of mainstream enthusiasts. Prior to that, underdrive pulleys were only commonly seen in purpose built race cars dating back to the 50’s and 60’s. These cars used highly modified engines that ran at a high RPM for longer periods of time than just a simple trip down the drag strip. The pulleys were used to prevent the vehicles accessories for being driven too hard at the higher RPM’s, which would damage them. A damaged accessory, such as an alternator or water pump, could potentially cost the driver the race. 

With the introduction of a single serpentine belt becoming common in the 80’s, the idea of using underdrive pulleys to enhance the performance of street cars was born. While the underdive pulley is most commonly associated with the 5.0 Fox-Body Mustang, they were also available for several other vehicles as well.

2005-2010 GT Mustang with Underdrive Pulleys Installed

How do Mustang Underdrive Pulleys Work?

It’s common knowledge that your engine powers your car. Many tend to forget, however, that the engine performs other functions as well. It charges the battery by spinning the alternator, cools the engine by spinning the water pump, and makes steering easier by turning the power steering pump. The end result is that you lose a little bit of horsepower to your wheels due to the fact that the engine also has to power these accessories as well.

Obviously, you can’t avoid powering these accessories on a street car; at least not without adverse side effects. So you’re left with the possibility of reducing how much power the engine uses to turn the accessories. This is exactly what an underdrive pulley is designed to do.

There are two ways to reduce the power the engine is using to turn the accessories. These methods would be to either increase the diameter of the accessory pulley, or decrease the diameter of the crankshaft pulley. To break this down so that’s easier to understand, let’s use the following diagram that shows a simple, two pulley setup for illustrative purposes. 

2006 GT Mustang at the Track

Ratio 1 to 1

In this simplified example, we see a two-pulley setup, where both pulleys are the same size. If the crankshaft (the drive pulley) makes one revolution, the belt will spin the accessory pulley (the driven pulley) one complete turn as well. So, you have a true 1:1 ratio between the two pulleys.

Now, let’s move onto the second example… 

1:1 Ratio Mustang Pulley Diagram

Underdrive Ratio

This time, the crankshaft pulley is smaller than the accessory pulley. For simplicities sake, even though this diagram isn’t to scale, we’ll say the crankshaft pulley is half the size of the accessory pulley. Since the crankshaft pulley drives the accessory pulley, it must fully rotate several times before the accessory pulley rotates a full turn. So as the engine is turning at 3000 RPM, the accessory is being rotated slower, which in turn, reduces parasitic drag from the accessory, as the engine doesn’t have to spin that particular accessory as fast.

Mustang Underdrive Pulley Diagram

Finding The Right Ratio


As mentioned earlier, to accomplish an underdrive ratio, you can change either the crankshaft pulley or the accessory pulleys. As there is more than one accessory that often needs to be underdriven, pulley kits start with a smaller crankshaft pulley. This gets all of the accessories close to the underdriven goal, with a single pulley change.

There’s a little more to the equation, however, than simply changing a single pulley and being able to tear up the road while enjoying your newly found power. This is why kits include more than one pulley. You can only underdrive your accessories to a certain point without potential side effects. So, in order to fine tune the ratio between the crankshaft and accessories, you are often provided with one or more additional pulleys, to either bring the accessory speed back up slightly, or reduce it down a little more than the other accessories, if it’s still possible to do so and still have them operate properly. 

2010-2014 Mustang on a Dyno

What Are The Benefits of Mustang Underdrive Pulleys?

  • Underdrive pulleys are used to reduce the amount of power required of the engine to run vehicle accessories
  • Underdrive pulleys increase torque and horsepower by reducing drag caused by belt-driven accessories
  • Installing underdrive pulleys have little downside and should be an easy upgrade for even the more novice Mustang owner
  • Overall, you'll be reducing the amount of power your engine needs to run everything from AC, lights, and windows, while still getting the same quality of use with no ill effects on other electrical systems

Underdrive pulleys usually are a performance enhancing item that increases the torque and horsepower output of an engine by reducing drag caused by belt-driven accessories. Horsepower gains from underdrive pulleys alone are usually around 4-7 hp. These are some of the best HP improvements enthusiasts can find for their money.

Steeda Mustang Underdrive Pullet Set

Potential Issues with Underdrive Pulleys

Underdrive pulleys add HP but in some cases can cause issues with overheating. There are a few specific steps a customer can take if this issue occurs to help:

  • Coolant Additive -  This should be step #1 and the quickest and most cost-effective solution. Mishimoto Liquid Chill and RP Purple Ice are both brands we carry  
  • Parts Upgrade - Look at upgrading the water pump and or radiator to a higher capacity version

Underdrive pulleys divert otherwise wasted HP away from the accessories, like the water pump, alternator to drive the crank faster, making more power. That said, there is always a give and take. AUTOMATIC cars, especially when running A/C and headlights, will see the most drastic issues. Underdrive pulleys may also cause charging issues as well. Things to do:

  • Upgrading the alternator
  • Running a capacitor for upgraded stereos
  • Running a secondary battery
  • Rev the car when it stops/idle to spin alternator and acc. faster (in a pinch)

Installing Underdrive Pulleys on Your Mustang & Potential Side Effects

So now that you understand how a set of underdrive pulleys work and you’re ready to place an order. Before you load up your Shopping Cart and whip out your credit card, first step back and analyze your situation a little.

For most automotive enthusiasts, underdrive pulleys should not cause any issues at all. This is due to the fact that the auto manufacturers provide a certain level of accessory operation that exceeds normal needs. If you have installed aftermarket parts that would force the accessories roles to be more important, however, you may be utilizing more of that extra “buffer” than the manufacturer provides. A perfect example of this would be high-powered aftermarket stereo equipment, which can tax the factory alternator or even require a higher-powered alternator. So it’s important to weigh your particular situation first.

When it’s all said and done, most people will find that there is no downside to installing underdrive pulleys. For those enthusiasts, a pretty straightforward pulley swap is all that stands between them and a car that has an increased responsiveness, as well as a little more power! 

Powermaster Mustang Alternator

How Do I Know Which Serpentine Belt I Need After Upgrading?

Measuring a belt is quite simple once done once. The easiest way to determine a belt length is to have all the pulleys you plan on running installed on your Mustang. Next, take a piece of string and run the route of the pulleys. When the string meets the original starting point, make a mark and then measure the distance on the string. This would be the belt length to fit around the system. Belt length doesn’t have to be exact provided proper tension is maintained on the system with the tensioner in its resting position.

Fitment includes: 1979, 1980, 1981, 1982, 1983, 1984, 1985, 1986, 1987, 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, GT, V6, Cobra, ShelbyGT500, Mach1, Bullitt, Boss, LX, SVO, EcoBoost, ShelbyGT350