Peter , I have tested the restriction of my OEM barrel filter on my 3.9 and it is 11 inches of water for the whole filter and snorkel.
This was measured at full throttle 5500RPMs in second gear up a very steep hill with a Minihelic vacuum/pressure gauge.
On light throttle the restrictiuon is less than 1inch of water, and with the snorkel vent facing forward is slightly positive at 80Kmh.
This is a non issue as far as V8s are concerned as 11 inches of water is a very small number.
I too thought that the filter may be restrictive before testing it, and had bought a Commodore filter and modified it. After testing I gave up the idea.
Regards Philip A

Thanks for reminding me - please pm/email me your address again Rick, and I will send your filters off.

The 2nd graph shows the efficiency with which a filter will remove a dust particle of a particular size. The first thing you need to know to understand the results are FILTERS ARE NOT SIEVES! So filters do not collect all particles above a given size and none below it (like a sieve), rather the forces involved in particle collection get weaker and stronger as particle size changes. A perfect filter (e.g. a HEPA/ULPA filter) would have 100% efficiency (a flat line) across the whole range. Filter which are less effective will start to deviate from the 100% line earlier (as the K&N does).

It should also be noted, that FILTERS INCREASE IN EFFICIENCY AS THEY LOAD WITH DUST! The dust actually helps the filtration process (again because filters aren't sieves). So most people change their filters FAR TOO EARLY. The donaldson filter rick sent me which has 20k km worth of dust in it could easily last another 20-40k km (depending on the conditions it is driven in). Also, because the K&N is a poorer filter to start with, the dust collected has a greater effect on filtration efficiency.


As Phillip posted above - his measurements seem to show that the standard filter have enough restriction to need it - as was pointed out in the other thread, the TDI flows more air than the V8, so needs a bigger filter. The only real advantage you would get by switching to a Tdi filter is that you would almost quadruple your filter lifespan (filter is twice the size and has a cyclonic pre-cleaner which should remove about 50% of the particle mass before it gets to the filter.


Not just donaldson, other reputable filter companies too. Isuzu-powered 110s come standard with such a guage. Both air filter elements in my 110 were 2nd hand when I bought them, and I have been waiting for the restrictor to pop up - I have done about 60k km on the current filters, and I think they were at leats 10k old when I bought the truck.

You could go to a wreckers and remove the complete round airbox assembly from an 80 series turbo diesel Landcruiser, complete with pressure switch and wire it to trigger an air filter warning light.

Well you have just confirmed what my brother who is an engineer has told me for a few years
a) The paper pleated elements are better and
b)That the paper pleated elements actually work better as they collect dust.
I never believed him as I thought how could they?
Oh well I suppose it is humble pie for me.

The differences between the Donaldson tested and the Coopers filter are because the Donaldson isn't a 300Tdi filter, but a Donaldson Cyclopac element that I found fits the 300Tdi housing.
I use them because
a) I can buy them locally
b) they are quite a bit cheaper (1/2 price wholesale)

Donaldson # is P182052
Coopers # is AEM2566 (equivalent of ESR2623)

The large closed area acts in a similar way to the Donaldson plastic sleeve/cyclone fins that aren't shown in the shots above.
It forces the incoming air to diffuse and feed through a larger surface area of filter and not just the point where the air box inlet is.
If you saw the dust build up on the K&N, it was all concentrated in a 3" circle at the base of the filter.
Part of the secret to good filtration is reduce the impact velocity of the particles hitting the media.
If I was to run the K&N again, I'd fit the Donaldson sleeve/fins to help a bit (it fits )

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To carry on from rick130's observations I tend to think if you are hooked on use oiled element filters like the K&N then it is essential that they are not the barrel design but ones with a large initial air contact area.

They then need to be in a filter box that will feed that inlet air over that larger surface area of the filter and not concentrated.

I have a K&N sitting in a square D1 V8 air box that has two inlets coming from the raised air intake (front and side).

IMHO I think some of the bad wraps oil filters like the K&N get is because they are probably not suited to barrel design air box design to start with??? isuzurover I would be interested to know your view as the master-tester and of course from others

Good point cockie, however I think it unlikely that filter efficiency would be significantly increased. There are competing factors in filter design - increasing surface area increases dust capacity, decreases pressure drop, and decreases flow velocity through the filter. The first 2 are good, the latter may mean that the filter efficiency decreases slightly, as larger particles are mainly collected by inertial effects. However, lower air velocity may also reduce the chance of particles bouncing (rather than adhering to the filter fibres).

Most flat type filters actually have MUCH LESS surface area than a comparable barrel filter. E.g. the Disco and defender 300Tdi filters are a case in point. The defenmder has a barrel filter and the disco a flat filter. The disco flat filter only has about 1/2 - 2/3 the surface area of the defender barrel.

Rick, I agree that the "uncovered" section of filter captures more dust, but this is mainly larger particles, As the filter loads with dust, more and more airflow would go through the other sections of the filter, until the flow eventually becomes fairly uniform.

Thanks Joel, I'll check that out.

Thanks for that advice; a guage is now on my "to do" list!

Ben, at what point would too high or too low air velocity actually be detrimental to efficiency ?

As a young apprentice, I was taught that too high a velocity could actually carry particles through the filter rather than have them be captured by the fibres (talking synthetic fibre air con filters in the 75-90% efficiency ranges)
Is this incorrect ?

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