What Are Underdrive Pulleys

How Do Underdrive Pulleys Work?

Underdrive Pulleys have been a tried and true method for years to help eliminate parasitic loss or engine drag on your engine's accessory drive setup. Typically, a set of these pulleys will include a replacement for the crankshaft, water pump, and alternator pulley on your engine. The engine in your Ford Mustang creates power at the crankshaft. The pulley attached to the crankshaft will turn other accessory drive components. This includes the power steering pump, alternator "charging system," the water pump "cooling system," air conditioning compressor, and other associated drive and accessory pulleys.

The crankshaft has to turn with the serpentine belt; this will rob engine power and diminish overall horsepower and torque output. Underdrive Pulley's will help with combating this power loss and will typically result in horsepower gains. Whether a daily driver or a race car, most of us want to get every bit of power we can get out of our Mustang's engine for optimal performance.

These underdrive pulleys will work by decreasing the size of the drive pulley and then increasing the dimensions of corresponding accessory pulleys. In Lamens terms, if both the crankshaft and the accessory pulley are the same diameters, they will turn at the same speed at a 1:1 ratio. The ratio will change if the crankshaft pulley or drive pulley is smaller than the accessory pulley or driven pulley. This will result in a drop in overall parasitic loss or drag on the crank pulley because it does not have to turn the corresponding driven pulley as fast.

Pros/Cons of Underdrive Pulleys

The best improvement is going to be the improved torque and overall horsepower your engine will produce after installing an aftermarket underdrive pulley set. Usually, these will be machined from lightweight billet aluminum or steel, which in turn will also help with parasitic engine drag. Even when it comes to minor weight differences, this can add up to help with power gains, in the long run.

The removal and installation process is a straightforward task that most gearheads can complete with basic hand tools.

While horsepower gains are typical with these pulley sets, there can be a few drawbacks. As we previously discussed, since the drive and driven pulleys are different sizes, these now will turn at different RPMs. If the ratio is incorrect between the crank pulley, water pump pulley, and the alternator pulley, overheating and charging problems can arise.

These issues can be easily combated by simple upgrades such as a high output water pump and upgrading your alternator to a higher overall amperage application in most cases. Pulley sets like the 5.0 Resto 1993 Cobra style kit, keep the alternator and water pump turning at "stock" RPM (That way charging and cooling system are unaffected from stock) and underdrives the power steering, A/C, and Smog 11.78%

Serpentine Belt Guide

One common question that comes up after installing aftermarket underdrive pulley kits is 'Will my belt length change?" Typically, the OEM belt can be reused, but the belt length can change depending on the ratios and if you eliminate any other accessories while performing this install.

LMR has you covered with a vast selection of OEM and aftermarket belts for pretty much any application. We also have our trusted belt guide and the how-to measure serpentine belt, articles to help you find out what size belt you need.

Other Underdrive Pulley Applications

OEM vehicles like the Ford SVT Lightning, 03/04 SVT Cobra Mustang, and the 07-14 Shelby GT500 came equipped with a factory supercharger. Making the supercharger turn at high RPMs will result in a higher boost output, resulting in more horsepower to the rear tires. This can be achieved by either using a larger drive pulley on the crankshaft or a smaller driven pulley on the supercharger. These pulley systems are often paired together to optimize the best overall combination for power output.

Other parasitic loss and power gains can come from free spinning accessory pulleys or bypassing specific drive components. One popular upgrade is eliminating the air conditioning compressor. Depending on your application, an idler pulley can be used in the place of the compressor, or relocating other accessory drive components like the power steering system can result in eliminating engine drag.

Overall, underdrive pulleys are one of the best first mods for anyone that owns a late model Mustang. As always, for the most up-to-date product information, pricing, or availability on anything in this article, please reference LMR.com.