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Air filters

Been researching air filter for some years on motor bikes, rally cars and finally 4wds.

The poor old air filter is often blamed for all sorts of performance problems. Sometimes it's justified. Speaking specifically about the 80 Series, here's a review of my observations.

1. Factory air filters flow well but do allow fine particles to get through. For example run a clean rag inside the inlet tract down stream of the air filter and it'll be dirty. A well maintained foam filter won't let this happen. Obviously the particle size it traps is smaller than that trapped by the factory filter. (my experience here is limited to the Finer filter).

Also after blowing out the factory filter, pointing the air gun back into the filter will result in a cloud of dust back through the filter. This is scary stuff indeed and suggests that you do need to wash the filter in water (as per the instructions) before installing. That said, the dust particle size that does get through is probably small enough to not cause accelerated engine wear.

2. Regular maintenance of a foam filter is critical. By regular, I mean daily in dusty conditions (you're following someone down a dusty track) to say 3,000km max around town.

3. The light duty Finer filter flows better than the heavy duty version but is prone to leaks around the base when dirty. The heavy duty version incorporates steel plates top and bottom much like the factory filter and the inner foam is bonded to the plates. If you do run the light duty filter, a generous smear of water proof grease along the top and bottom
surfaces is encouraged.

4. The factory filter is designed to flow well enough with the standard inlet tract. If a Safari snorkel is fitted (which does away with the cyclonic unit in the front guard) then the filter becomes a source for restriction (however, Safari does recommend using the factory filter).

5. The amount of oil in the Finer filter has a big impact on the flow restriction but surprisingly little affect on its filtering properties. People often over oil the filter which is a mistake.

6. Almost all the horror stories about foam filters are due to improper service. You really do have to be anal about cleaning them often and taking extreme care when fitting.

7. You can use a second outer band over the filter for really dusty conditions and remove it say half way through the day. (A thin light duty outer element stretched over the heavy duty filter is a good choice).

8. Foam filters are messy and a pain to service. Have a spare in the cupboard.

9. Drying a washed factory filter is a pain. Have a clean spare and stick the freshly washed one (wrapped in a clean lint free cloth) in a cupboard to dry ready for the next service.

10. Spare foam outer elements can be stored easily in the car ready to be replaced as needed on a long desert trip.

11. For performance, the best oil for a foam filter is 2 stroke motorbike oil however it drains out of the filter too quickly. Don't be tempted to use it.

12. Fresh foam filter oil is mandatory. The oil is thin to allow even and thorough penetration. The thinning agents evaporate leaving a sticky goo that does the work. If your oil is old and thick, then you'll invariably have to over oil the filter to get it right through the filter.

13. The cost savings of running a foam filter are marginal.

14. The correct solvent used for washing a foam filter is critical in its performance. For example, washing in petrol will clean it but will break down the foam cells, drastically reducing the filter's effectiveness. It may also affect the bonding agents used to keep the foam in place. Mineral turpentine is recommended.

15. After washing a couple of times in turps, you need to wash it well in warm soapy water. This is what really gets it clean.

If you're still with me I'll share some interesting info with you... With all the industry hype about foam filters (some good and some bad) there's a lot of folk lore about the flow performance. There is no doubt however about the foam filter's ability to filter air. They do so very well indeed and filter better than the factory air filter.

So, in an effort to achieve a definitive answer on performance, I decided to measure the pressure drop across 3 different filters. Hooked up a pressure gauge with the high pressure side plugged into where the factory filter warning switch plugs into the air filter housing. This measures the air pressure before the filter. The low pressure side was plugged into where the factory filter warning switch plugs into the inlet tract side of the air filter top.

This effectively samples the pressure drop across the air filter and things like snorkel etc have no influence on the readings.

The filters tested were the heavy duty Finer filter (slightly over oiled), the standard Finer filter (oiled just right) and a 5,000km old factory filter which was blown out and washed.

Test performed on a 1HD-FT turbo diesel with Safari intercooler at 3,000rpm, full throttle in 3rd gear (up a bloody big hill).
Standard Finer filter 5 inches of water
Heavy duty Finer filter 6 inches of water
Factory filter  6 inches of water

The standard Finer filter flows slightly better than the factory and heavy duty Finer filters. It should be noted however that this motor pumps a lot more air than the 1FZ-FE, 1HZ and 1HD-T, so the pressure difference is more pronounced than that one would expect with the other motors.


'97 1HD-FT - Melbourne Oz.