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 Mikuni, 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 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.