BMW Underdrive Pulleys
When it comes to car modification, enthusiasts are generally looking for ways to increase power and performance. While there are countless ways to add horsepower, everything from bolt-ons to full engine builds and swaps, not all upgrades are created equal. Generally speaking, the more horsepower you want to pull from your car, the more money you will need to spend. In the early days of competitive racing, teams figured out little ways to pull the last ounces of power from their engines, which brought us the concept of Underdrive Pulleys. Changing the ratio between the drive pulley and driven pulleys frees up horsepower and gives your BMW an easy boost in performance at minimal cost. The crankshaft spins a crank pulley, which drives the accessories, such as your power steering pump, alternator, A/C compressor, and water pump. Underdrive pulleys reduce the amount of work your engine needs to do in order to turn your accessories and allow that power to be sent to your wheels for more driving pleasure.
Ratios
Changing the ratio between the drive pulley and driven pulleys frees up horsepower and gives your BMW an easy boost in performance at minimal cost. The diameter of the drive pulley compared to the diameter of the driven pulley is written as a ratio and determines the amount of force required to turn the accessory. The pulley diameters define how many rotations the drive pulley has to make to turn a driven pulley. With a 1 to 1 ratio, the drive pulley rotates once, which rotates a driven pulley once. To underdrive your accessories, you will either increase the diameter of the driven pulley or decrease the diameter of the drive pulley. This allows the accessories to spin much more slowly, thus reducing parasitic drag.
Parasitic Drag
While it is more commonly associated with flight, parasitic drag applies to your engine as well. Your engine powers more than just your wheels. Every bit of energy that is “wasted” spinning your accessories is energy that could go into making wheel horsepower. The horsepower that you lose to those accessories is what we would refer to as sources of parasitic drag. That drag, or resistance to power delivery, must be overcome before your wheels start spinning.
Benefits
Of course, horsepower and torque increases are the goals and the most notable benefits experienced with underdrive pulleys. Our Turner Motorsport Power Pulley kits release somewhere around 11hp on most BMW engines by reducing the rotations of your water pump, power steering pump, and, depending on the car, your alternator by 10-20%. Not only are the power pulleys helpful in reducing parasitic drag through rotations, but they are also lighter weight than the factory pulleys they replace. The weight reduction further decreases the amount of energy needed to spin the accessories and devotes the most power available to your wheels with minimal sacrifices to your accessory functions under normal driving conditions.
Takeaways
The beauty of our Turner Motorsport Power Pulleys are their simplicity: by reducing the parasitic drag and decreasing the rotational inertia of driven pulleys, we are able to free up spare horsepower to be diverted to powering the wheels. With our pulley kits, your BMW can expect up to 11hp increases and extended lifespans of driven parts, like your power steering pump or water pump. Make sure you are getting everything out of your BMW’s engine you can with the Turner Power Pulley Kits!
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