It's a known fact that engines need air cleaners if they are to have a long and useful life. An engine running without an air cleaner is a bit like one of us drinking water and then eating the glass, it can be done but isn't a great idea. Air filters for Harley-Davidsons vary widely in appearance and performance. Let use ignore the very subjective issue of appearance and only consider air cleaner performance. Air cleaner performance has two aspects, dirt removal and airflow capacity. If the filter does not remove small-enough dirt particles, the engine wears prematurely. If it does not pass a large-enough volume of air, the engine cannot deliver its maximum possible power output. The trick is in getting both, fine particle removal and generous airflow, while satisfying styling demands. Stock "paper," plastic foam and pleated cloth filter elements can all remove small particles of dirt. The main practical difference is the size required to both filter well and pass large amounts of air. The pleated cloth filter passes more air for a given size than either the factory paper or aftermarket foam filters. This is the reason for the near-universal use of the K&N-type filter design in high performance applications. The larger the filter surface area and the straighter the air flow into the mouth of the carb, the less restriction on air flow. If the filter is large enough and air flow into and out of the filter is free enough, there is no significant flow loss and the engine can realize its maximum power potential. Examples of such a filter design are the Mikuni 2.5" and 3" filters. These filters, made by K&N, are large in area. They point the air flow directly into the mouth of the HSR42/45 carburetor via a short velocity stack. And, they are shielded from the wind by a large chromed filter cover. No filter delivers more air to the HSR series carburetors. Small area filters, especially if they are both small and made of foam or paper, limit air flow and, therefore, power output. Even large filters, no matter what they are made from, cannot perform well if the filter cover is too close to the filter material. The ideal filter/cover configuration would allow air to flow directly (straight) into the mouth of the carburetor. There is an energy (flow) loss each time air must turn a corner on its way into the carburetor. This loss may be very small but it is still there. This is the fundamental reason why very high performance engines have the straightest possible intake systems. So, when you choose the filter assembly for your Mikuni HSR, S&S or any other high performance carburetor, please keep in mind that function could be compromised by styling. The Mikuni filter design is the standard by which to judge other filters. It flows a maximum amount of air and delivers that air, as directly as possible, into the mouth of the carburetor.