In the early days of automotive engine development, most engines made do without any form of oil filtration, or at best a rudimentary gauze filter. Until the 1970s there were mainstream production engines operating without any real filtration, a notable example being the Volkswagen air-cooled boxer (although did eventually gain a filter in the 1980s). However, ever-finer tolerances and longer service intervals have made filters an indispensable part of both race and road engine lubrication systems.
Ultimately, debris in the oil – be it from external contamination or the breakdown of components such as bearings – is seriously detrimental to an engine’s longevity. The most common filter type, found on everything from a sedate shopping sedan to an 850 bhp NASCAR Cup car, is the ‘cartridge’ filter. First developed in the 1950s, these filters may look simple externally but the differences between the ones found at a local part factor and those developed for racing use are considerable. While the basics of their operation are similar, the needs of a race engine compared to a road engine are very different.
The most important difference relates to the level of filtration, and the impact this has on oil pressure and flow. Logic dictates that one would want the best level of filtration possible, in order to trap the greatest quantity of debris. As an example of the level of filtration available, a high-quality automotive filter will trap about 80% of particles between 8-10 µm in size. However, research by one manufacturer of racing oil systems has found that, in the vast majority of cases, this degree of filtering is not necessary and could in fact be detrimental to the overall performance of the oil system.
The research found that some very fine filters produced an unacceptable level of pressure drop across the filter because of the restriction of the filter element, which in some cases was sufficient to cause the oil bypass valve to open and feed unfiltered oil to the engine. Such occurrences would be almost undetectable on most engines, as the impact on overall system pressure (gauge pressure) would be minimal, and the first indication that something was amiss would be an increase in wear rates.
The company’s study indicated that particles smaller than 20 µm are not a particular concern for engines with short service intervals, which encompasses most race engines. With this in mind it was able to create a filter that provided complete filtration of particles down to 27 µm, resulting in a maximum pressure drop across the filter of only 2.5 psi.
Another factor to be considered in filters destined for racing use is the suitability of the filtering medium itself. Since the 1940s, cellulose-based materials have been the norm, with the oldest filters using simple cotton fibres. Most fibres used in mainstream oil filters are derived from wood pulp, with the fibres being made rigid by adding resin. For most applications, cellulose is still more than sufficient, providing filtration down to around 20 µm; for racing though, a synthetic filter material is the only real choice.
There are two key reasons why this is the case – flow and temperature/pressure resistance. Cellulose fibres are relatively thick, while synthetic ones are much thinner, so for a set mesh size the synthetic filter will provide a greater area for oil to flow through. Regarding the resistance of the fibres to temperature and pressure, synthetics maintain their structural integrity under extreme conditions much better than cellulose. Thus in race engines subject to elevated temperatures and running higher oil pressures, they are an ideal solution.
Written by Lawrence Butcher