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The fuel filter

Mention the word ‘filter’ and I guarantee that most people will think immediately of either the oil or air filter – very few, I am sure, will mention the fuel filter. Nevertheless, as important as both air and oil filters would seem to be, I might argue that for motorsport applications the fuel filter is perhaps the most important of all. Let me explain.

If an engine is designed and carefully assembled according to well-defined procedures and then generously run-in to acceptable standards, after a while the wear metals will reduce in size to those generally smaller than those the oil filter can trap. At this point, although I would never truly recommend it, the filter could be removed –I have tried it in the past, and it worked without any undue issues with the engine. So long as you change the oil frequently then the system will most likely remain mainly unaffected. Likewise with the air filter. Careful design of the intake system could produce a system where the larger dust particles can drop out, leaving only those that would pass through the air cleaner, so removing the air cleaner will have no effect.    

When it comes to the fuel filter, however, the risks these days are totally different. A simple carburettor for instance would have a small mesh screen. Preventing any debris from passing into the body of the device, any smaller detritus would pass through and be drawn through the engine with little apparent effect. Modern engines are far more sophisticated though. Using high-pressure fuel pumps with close-fitting manufacturing tolerances, the fuel is pumped up to the injectors. Here, at pressures up to 200 bar, the fuel is injected through a multitude of (often up to 12) tiny micron-sized holes, each carefully angled and positioned to deliver the correct amount of fuel into the turbulent air precisely as required. Any dirt or detritus interfering with the operation of the system, however little, could clearly spell disaster.

That is why the best fuel systems have at least two fuel filters. The first of these is more of a strainer. On the suction side of the in-tank mounted pump, this will remove much of the heavy sediment, scale or dirt which (despite every effort by fuel manufacturers) always finds its way into the tank. This filter is more to protect the pump, which will tolerate a lot of the debris but still has to be saved from the worst of it.

After the fuel but before the fuel rail or high-pressure pump, a more refined filter should be positioned. Designed to trap more of the finest dirt, the essential media doing the trapping can be synthetic fibre, cellulose or sometimes even a simple wire mesh or metal strainer, depending on the fuel used, the degree of filtration required or the time needed between cleaning/replacement. Motorsport filters for instance could use, say, a 55 micron mesh screen to get the flow rate necessary, but this would come at the loss of filtration efficiency and potential damage to the fuel system. Provided the flow rate necessary could be maintained, a better alternative could be an 8 micron pleated paper filter. With up to 55 sq in of filtration media for more modern vehicles, this should be a better solution.

Fuels for motorsport, by virtue of the fact that they are often stored in small containers, are more susceptible to the ingress of dirt or rust from the cans. Forecourt fuel on the other hand is likely to be much cleaner.

Surely therefore, in motorsport the last thing you want to do is to dispense with the fuel filter.

Fig. 1 - Inline fuel filter showing a rather old nylon mesh within a plastic frame. Later versions of this filter have an optional metal mesh or paper filter

Written by John Coxon

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