18 Underdrive Pulley Pros and Cons

The underdrive pulley is one of the most common ways to improve the output of an engine. It reduces the draw of the engine’s accessories by slowing them down. That reduces the amount of horsepower being used. The gains achieved can be as much as 15 horsepower at the wheels, though the actual results depending about the make and model of the vehicle, as well as the engine involved.

Underdrive pullies refer to either an accessory pulley, such as the water pump, alternator, or power steering of the vehicle or the crankshaft. Although there are many benefits to the underdrive pulley design, there are certain disadvantages to consider when a poorly engineered design is implemented.

That’s why the pros and cons of an underdrive pulley must be taken under consideration if you are building or fixing your own vehicle or kit.

A note on the pros and cons listed below: every vehicle is different. The experiences of an underdrive pulley, including its advantages and disadvantages, are applied to individual vehicles only. Some owners suspect their UD pulley was responsible for certain issues without evidence. The same can be said about those who suggest using this upgrade for benefits.

Although Ford Mustangs receive specific guidance on using underdrive pulleys, other street car makes and models treat the upgrade as an after-market option. That means you could void certain warranties under specific conditions.

Proceed with caution when taking any mechanical advice from the internet.

List of the Pros of an Underdrive Pulley

1. The gains offered by this technology add to vehicle acceleration.
When you’re able to increase the diameter of accessory pullies on your vehicle, then you decrease the revolutions per minute required of the underdrive. That process cuts down on the drag demanded by each component. It is through this process that horsepower gains are achieved. There are noticeable acceleration gains to be found if you can replace the stock air conditioning, alternator, water pump, and power steering assist with this technology.

2. You will reduce the amount of belt wear experienced during operation.
When you use an underdrive pulley, then you’ll reduce the amount of belt wear your vehicle generated. That process improves because the belt makes less contact with the pulley guides. When you have fewer rotations occurring during operations, there are fewer contact points for the belts to encounter wear and tear. It is possible to extend the life of the belt past the recommended specifications of the manufacturer because weathering, flexibility, and durability risks are all reduced.

3. There is less component wear to worry about also.
Some underdrive pullies turn 30% less than other versions of this technology. That means you have less wear and tear happening on your sealed bearings and the bushings for your vehicle. When properly engineered, you can extend the life of your seats and races as well. Your accessory bearings for the alternator, power steering pump, and the air conditioning unit see the best results with this change.

4. It increases torque because it reduces drag.
The underdrive pulley was introduced as a way to reduce fatigue on the water pump, alternator, and steering mechanism of the vehicle as it raced around a track. Too much stress on these components on a standard belt-driven system could cause them to fail prematurely, which would cause the driver and team to lose the race. With the pullies in place, you’re able to reduce the drag on these components because you’re slowing the output achieved through the system before it reaches each accessory.

5. The cost of underdrive pulleys is relatively low compared to the gains received.
You can install an underdrive pulley on your vehicle in less than an hour. They create power on virtually any system you have, even though the biggest enthusiasts tend to own Ford Mustangs. Most are priced for less than $200. You’ll find some underdrive pulley kits are even priced below $100.

Even if you factor in another $200 in labor to install it, you’ll find that there aren’t many options to provide better power efficiencies at a dollar-for-dollar rate than the underdrive pulley.

6. You can fix overheating issues with coolant additives.
If you find that your underdrive pulley is causing your engine to run hotter than you’d like, then a coolant additive may resolve the situation for you. American Muscle recommends using RP Purple Ice or Mishmoto Liquid Chill to resolve the situation. For most owners, this will be the fastest and cheapest way to resolve the situation. If that doesn’t work, you can still resolve the issue with a radiator or water pump upgrade to handle a higher capacity.

7. For many owners, there is no downside to the installation of underdrive pulleys.
In most situations, owners are swapping pulleys and that is it. They receive improved responsiveness from the vehicle, along with a little extra power to enjoy. For automatics that do encounter issues, most of the problems tend to be minor. The vehicle may struggle when in idle, which is a situation that can be rectified by revving the engine a little to prevent voltage levels from dropping too dramatically.

8. There is excellent durability available with this type of system.
Most owners who have upgraded to an underdrive pulley have experienced benefits for 50,000 miles – and some even more than that. Although there is a general consensus that battery issues may occur if you idle your vehicle at city driving speeds for an extended time, most vehicles will experience a benefit with this modification.

List of the Cons of an Underdrive Pulley

1. It may not spin the vehicle accessories fast enough.
When an underdrive pulley is poorly designed, there may be several unwanted side effects experienced during driving. It may not spin the alternator fast enough, which can lead to premature battery death, even while driving. It may offer a weak air conditioning experience, if the system even works at all. There may be issues with the power steering assistance, especially when operating the vehicle at a low RPM.

2. There is a power reduction issue which must be considered.
When you have belt-driven components, then you are given a buffer zone for your performance parameters that includes additional output capacities. Some may even go beyond the maximum specs which are listed for the item. If you upgrade to an underdrive pulley, however, then all your components will run simultaneously. That creates a voltage drop in your charging system that reduces overall power access.

You might find that your headlights are not as bright as they used to be. Your engine’s electrical system might not receive the correct voltage to fire the spark plugs. In severe situations, it would impact driver safety when using the vehicle.

3. There are cooling issues associated with underdrive pullies.
Let’s say that you replace the water pump pulley with an underdrive pulley. Now let’s add the hypothetical issue that your cooling system has a leak or a clog. Because the system is now operating at a lower speed, you’ll have less coolant running through your pipes to reach your engine. That may result in a higher normal operating temperature for your engine. If the unit is oversized too much, you could experience an overheating issue.

4. It may affect the performance of your after-market additions.
If you made modifications to your vehicle that are designed for high-performance situations, then your after-market efforts may not work properly when you upgrade to an underdrive pulley. As with the other issues noted above, there isn’t the same voltage levels available across the system when you make the change. You are trading performance for availability, which means your speakers, amplifiers, and turbo chargers wouldn’t be able to run properly.

In severe instances, the addition of after-market electric fuel pumps or cooling fans have been impacted by the change to underdrive pullies, which affected the core functionality of the vehicle to be used.

5. It isn’t something that benefits the average vehicle.
If you’re thinking about an underdrive pulley, then it should be for performance reasons on a vehicle capable of the upgrade. Although it is a technology which has been around for 60+ years, it didn’t become popular until the 1980s. Before then, it was almost always used in race cars only. For the average vehicle, if you have a serpentine belt, then you’re already good to go with your vehicle setup.

For street car owners who want a small boost in performance, then the underdrive pulley is an option. Even then, what you’re doing is maximizing the horsepower at the rear wheels more than anything. It might be fairer to say that you experience fewer HP losses with this system instead of experiencing a “gain” with it.

6. Underdrive pulleys work best with a manual transmission.
Because an underdrive pulley diverts HP that would be wasted from accessory to drive your crank faster, you create more power that is usable. As you know, every time you get a little something, then you must also give a little something in return. When you have an automatic transmission, you’ll find that the drastic issues found in these disadvantages are more prevalent compared to a manual transmission.

You can avoid many of these issues by not operating your headlights and air conditioning system simultaneously. Some vehicles, however, require further upgrades to resolve the issue. You may need to run a secondary battery, install a capacitor for an upgraded stereo, and upgrade the alternator.

7. You may need a new serpentine belt installed after the upgrade.
One of the costs that isn’t incorporated into an underdrive pulley project is the need for a new serpentine belt. You must measure for the new belt once the work is done. Have all the pulleys you plan to run installed on your vehicle. Then take a string to run the route of each pulley. When your string meets the starting point, make your mark and measure the distance. Then you’ll need to have the belt installed. This process may add another $200 or so to the final project.

8. It may cause harmonic issues with specific vehicles.
On some vehicles, you may find that the presence of an underdrive pulley creates more vibration within the vehicle. There have been harmonic issues associated with ASP underdrive pulleys that have resulted in broken timing chains. Although many vehicles can have this issue countered with the installation of a damper, that is not a guarantee. Every car responds differently to the upgrade, so make sure you pay attention to any unusual symptoms that creep up.

9. It may create a deformation of your crankshaft.
Some vehicle owners believe that their crankshaft is large and strong enough to be rigid and inflexible. That simply isn’t the case. On some makes and models, the installation of underdrive pulleys can create twists, bends, and flexes in the crankshaft that are similar to what you could do with a rubber band. Although each movement is small, there can be significant impacts on the overall performance of the engine.

10. There are fatigue failures associated with underdrive pulleys.
With repeated bending and twisting, metals eventually experience fatigue failure. Think about what happens when you bend a metal paper clip back and forth enough times. It will eventually break. The same thing can occur to a vehicle after it receives an underdrive pulley upgrade. You may create an imbalance of the reciprocating or rotating parts. It is possible to create vibrations within the individual forces to create lateral oscillation. Even with a harmonic dampener in place, the effect may be strong enough to create ongoing issues.

The pros and cons of underdrive pullies requires you to look at your current setup first. If you’ve already made upgrades, especially to your A/C system, power steering, or your alternator, then there may not be enough voltage to go around for everything to work properly. It is possible to see performance upgrades if this is the first step you take, but it could see decreases if the system is not properly engineered. That is why this option should never be a rushed decision.