Chrome Steel vs. Polished Aluminum
We get a lot of questions about whether to use chrome or billet aluminum pulleys and what the differences are.
Let's start with the similarities. As long as you use 6061-T6 aluminum, both aluminum and steel are very durable and look great. Both materials are corrosion resistant and are an easy bolt on upgrade. But, you want to know about the differences, right?
The main reason to use chrome steel pulleys is for their mirror like finish. Aluminum pulleys can approach the same level of shine with polishing, but it's hard to beat a chrome finish for looks. Chrome pulleys also tend to be a little cheaper because they are mass produced on dedicated pulley splitting equipment instead of CNC machined from bar stock. However, the tooling required to make steel pulleys is expensive, so it only makes sense to produce steel pulleys in high volume. That's why there are fewer options available in chrome steel and it's also why most chrome pulleys are not underdrive (generally they are the same diameter as stock).
Billet Aluminum Pulleys:
Weight: One of the biggest advantages of aluminum pulleys is that they are 40 - 50% lighter than steel. When your building a high performance street machine or race car, there are 3 kinds of weight: unsprung weight, sprung weight and rotating weight. Rotating weight is by far the worst. That's because when you stomp on the gas, you have to accelerate rotating weight from 0 to 60mph and you also have to accelerate it from 800 to 3000 RPM, or 8000 rpm on an alternator. Serious racers usually upgrade their driveshaft and wheels to aluminum for this very reason.
Underdrive: When you run underdrive pulleys, you save 10-20 HP by recovering some of the parasitic losses from accessories. True Billet Aluminum pulleys are usually made on a CNC lathe from a bar of aluminum. This means that you can produce pulleys of different diameters with a simple change to the CNC program. That's why most underdrive pulleys are made out of billet aluminum instead of steel.
So, if you look under the other guy's hood and see chrome pulleys, you might want to race for pinks. If you see billet aluminum pulleys, proceed with caution