Agustin Jimenez
Technical Editor
August 4, 2015

One of the questions we often get asked is what are the best quick, cheap, and easy-to-install performance part for late-model Mustangs? That’s not easy to answer because there are many things to consider when upgrading your pony in search of more power. When it comes to horsepower gains that don’t break the bank, nothing’s quite as effective as a set of underdrive pulleys, but you need to understand how they work as well as what the benefits and drawbacks are.

Underdrive pulleys are perfect for track cars that don’t require a lot of electrical power to make a few passes. For those of you who like to hit the road course tracks, underdrive pulleys will allow you to get a little extra power and throttle response out of your Mustang while keeping your charging system working. The area that might be up for debate is a street driven/daily driver Mustang that sees a fair bit of rush hour traffic with the air conditioning blowing and the loud tunes going. You can still run them, but you need to be aware of the effects that a smaller crank pulley will have on the overall performance of your accessories.

How they work

In a late-model Mustang, the crankshaft pulley drives the serpentine belt, which spins the accessories, such as the power steering, A/C, water pump, and alternator. The objective of an underdrive pulley is to slow down the accessories in order to reduce the parasitic power loss on the engine. This is achieved by using a crankshaft pulley with a smaller diameter than stock, which slows the serpentine belt. Many underdrive pulley sets are also available with a slightly smaller alternator pulley to help increase the operating speed of the alternator to keep the charging system functioning at an optimum level.


Underdrive pulleys will yield a small increase in horsepower anywhere from 8 to 15 hp. This is accomplished by reducing the amount of power needed to drive the engine accessories that eat up precious horsepower. A smaller crank pulley ensures that the accessories will spin slower, which results in less parasitic power loss than the stock-sized pulley can offer. On top of that, underdrive pulleys are ideal for high-rpm track engines since they can keep the alternator from overcharging as well as eliminate overheating issues that can occur when the water pump spins too fast, leading to cavitation of the coolant. On top of these benefits, underdrive pulleys can also increase fuel economy by 2 to 3 mpg since the engine is not working as hard to spin the accessory drive.

Things to consider

Underdrive pulleys are an awesome and effective way of freeing up horsepower, but before you decide to pull the trigger on this type of upgrade, you need to consider the needs of your particular charging system as well as your cooling system. Your charging system might see a larger amp draw than a typical stock configuration if your car features a large stereo system, electric fans, or other electrical accessories that suck up a lot of juice. The key thing is to figure out how hard the alternator is currently working. If you are driving your Mustang in hot weather with the A/C on full blast, you might have to upgrade to a higher-amp alternator in order to keep your charging system operating at an optimum level. In some cases, the stock alternator will only produce enough power to charge the battery while you’re running down the road. Factor in a rainy night when you need your lights, windshield wipers, and heater, and you can see just how easy it is to overwhelm the charging system.

Beyond that, you have to consider the cooling system and whether or not you will be using a radiator fan clutch or an electric fan. A fan that spins slower could make your engine run hot, especially in bumper-to-bumper traffic where airflow through the radiator is reduced. A simple remedy for this is to upgrade to an electric fan to keep the engine running cool.

Since a picture is worth a thousand words, we decided to prove to you that underdrive pulleys will absolutely work on most Mustangs out there. Our Mustang features a set of underdrive pulleys as well as an electric radiator fan; the alternator is charging at just over 12 volts at idle, which is perfectly acceptable for our needs. The voltage quickly goes up as soon as the car pulls away from a stop.