The primary purpose of an aftermarket pulley is to reduce the size and/or mass of the stock unit. They serve two purposes: to reduce horsepower loss and increase throttle response. Notice the use of "reduce horsepower loss". Pulleys DO NOT add horsepower, rather they free up horsepower due to the reduction of rotational mass.
HP gain is 5-10HP. These HP figures are a range as there have been very few before/after dyno runs with lightened or under driven pulleys.
What is a lightened pulley? Generally, it is an exact copy of the OEM pulley only CNC machined out of aircraft grade aluminum and powder coated for corrosion protection and aesthetic appeal.
What is an under driven pulley? This is a pulley that mimics the function of the OEM unit, but is smaller in diameter. This will reduce the duty cycle of the pulley and proportionally the parasitic drag on the engine.
Are under driven pulleys bad? It depends on the amount they are under driven. Some under driven pulleys can lead to reduced A/C output, lighting problems, stereo problems, and other voltage issues. Some are lightly under driven and some more so.
Isn't replacing the stock harmonic damper (pulley) bad? Subarus do not come with a harmonic damper or balancer like some other vehicles do. For other manufacturers' vehicles, this is a legitimate concern. Proof:
"Thank you for your patience as I checked with our Technical Services Department regarding your message below. They advised that the crank pulley is a pulley and nothing else. It is not used as a harmonic damper/balancer.
Thanks for the opportunity to be of assistance. If you need any future assistance, please feel free to contact us again."
John J. Mergen
Customer Service Department
Subaru of America, Inc.
Are there any downsides to these pulleys? There have not been significant amounts of problems with pulleys. There have been some reports of high RPM issues with pulleys though. These issues tend to happen without regard to type or manufacturer. It possibly stems from rotational issues with the crank position sensor which is located near the pulley. The indication of error is a reduction of the fuel cutoff redline of the motor (false redline). The solution is to reinstall the stock pulley. As with any aftermarket part, even "perfect" ones, sometimes you run into a rogue issue. From research, this is not a widespread issue at all, only an advisory if this situation should occur.
Which manufacturer is best? This topic is highly debated. There have been no reported consistent "bad" pulleys on the market. Obviously, there may have been bad pulleys sold, but not enough to report as "bad" overall.
Which pulleys have the best gains? There is no irrefutable evidence that any pulley set has better gains than another. That being said, it stands to reason that a lighter pulley has more gain than a heavier competitor. Under driven would also free up more power than standard size.
Where do I buy a pulley or pulley set? Every Subaru/Import performance store sells pulleys. For purchasing, support your local economy or the NASIOC Vendors.
How hard is it to install a pulley or pulley set? Allow around one hour for install time. Professional installation, depending on your area, is around $75. This is one vehicle modification that is very simple and can be successfully accomplished by even the greenest shade tree mechanic.
How do I install a pulley or pulley set? Refer to the pulley manufacturer's instructions. For pulleys without instructions, many rely on the Vishnu instructions for installation. Below are links to some of the better known pulley installation instructions:
Vishnu's Instructions (contained inside the Stage Zero Instructions)
scoobymods.com instructions (with photos)
Should I reset my ECU after this mod? It is never a bad idea to perform an ECU reset after a mod. The traditional route is to disconnect the negative battery terminal, press the brake pedal for a few seconds to bleed the system of charge, and reattach. Some use the more advanced Vishnu Reset.
if you only replaced the crank pulley then yes, the volt. will drop... i replaced both the crank AND alt. pulley with the GFB ones... the crank is smaller, but the alt. pulley is also correspondingly smaller tooo... so no voltage drop