Why foam?
The principle of foam filtration
A foam filter is not a mechanical barrier to contaminating particles (dust). The filter works by presenting a sticky matrix to the flow of dust, such that it is very unlikely that dust will pass through the matrix without becoming attached to the matrix.

By contrast, a paper filter is a mechanical barrier. The paper has extremely small holes in it, so particles larger than the hole size cannot pass, but get caught on or very close to the surface of the paper.

Smaller and lighter than dust particles, air molecules pass with little impedance throughout the foam matrix. Dust particles, having more inertia, contact the sticky oil on the matrix and do not pass through.

When a dust particle becomes stuck to the matrix, it becomes a small additional material on the matrix. The hole it was passing through is not significantly reduced in size by its presence. Consequently, air flow is not measurably restricted as dust is arrested by the filter. This is why ITG filters operate for a long time with no change in restriction (measured as pressure loss) at the filter. In contrast, when a dust particle is caught on a paper filter, the hole it was passing through becomes completely blocked, so the restriction and pressure loss of the filter increases continually as dust builds up.

Over many years and many different programmes of testing ITG have learnt what foam types, oil types, and combinations of properties are required to reliably filter dust particles as small as 5,4,or 3 microns.

Many different grades of foam with larger or smaller holes forming the matrix are available. Foams used by ITG are good quality, have consistent and correct hole sizes throughout the volume of the foam. The best grade of foam is chosen for each application.

For filters for some on-track racing series (e.g. Formula 1), there is an imperative towards low air flow restriction, so very fine grade (small hole size) foams are ignored, and foam thickness is kept to a minimum. The balance between air flow restriction and the probability of damaging dust passing through the filter is influenced by the facts that engines are rebuilt frequently, and that race tracks are relatively clean environments.

In very dusty environments, such as desert rallying, or for trucks working in quarries, dust flows are high and engine wear minimisation is a priority, so thicker and finer foams are used and more air flow restriction is accepted.