You should read this
http://www.harleyhelp.com/faq.html before you buy. This guy has an interesting take on the Hypercharger. See his response below. He says on his website he is factory trained & authorized Harley-Davidson Service Technician and started the site to help those who can't afford the high prices HD charges and for people who like to work their own bikes.
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Q: What do you think about those cool Hyper-Chargers?
Now here is an interesting item. It is possibly the most popular aftermarket product for sale, and yet could be the cause of a lot of the problems I have been hearing about.
Let's start with something you might not have noticed. One of the most attractive things about the Hyper-Charger are those cute little butterflies that flutter open and closed when you work the throttle. It's so cool, just like the blowers on the top fuel funny cars... except for one thing, the Hyper-Charger butterflies operate off manifold vacuum and the top fuelers are operated mechanically.
At idle and normal cruising speeds, the main air intake is through the butterflies but during acceleration the main air intake is shut off by the closed butterflies so there has to be another air intake area, and you will see a slot at the bottom rear of the Hyper-Charger body for this purpose.
The company claims that installing this air cleaner kit will improve performance and horsepower and that is true... under the right conditions. The reason it improves performance is because it utilizes a K&N free-flowing style filter, the same as you will find in the Screaming Eagle air cleaner kit and many others, even S&S is using the K&N style filter now. What the company does not tell you is that it will produce much better results on the dyno than on the highway. On the dyno it works great, but the dyno won't reproduce that 80mph wind forced into this air cleaner and out the rear like you get on the highway.
Because of the way the air travels into the front and out the rear of the Hyper-Charger, instead of forcing air into the carburetor, it actually siphons air (and fuel) out. As a result, you get what has been described as a "dead spot" or lack of power at highway speeds, not to mention the fuel breaks down the oil in your air filter hampering it's usefulness and the fuel mixed with air filter oil tends to deposit itself all down the right side of your bike when you go on long rides and then when you stop it all drips down on the cam cover, your foot, that cigarette butt you just dropped, and so on...