"Performance" air filters - what do they really achieve ?

[chris7676]

Knowing how popular modifications these are, I was wondering what the real difference they give on relevant cars - from YOUR experience. I remember such a filter in my ex K-Series 1.4 (16v, 103hp) and it was acutally strangulating the engine at hight revs (!) - what a nice difference it was to go back to the standard one... Also the quality of filtration may be questionable which would be especially important when you have a turbo...

[H2DaE]

not much real difference from changing the filter part. the piping however, is a different matter. decent gains can be had from these

[dern]

All filters filter out particles of stuff and stop that stuff from entering the engine. As I understand it a standard filter does that using a paper filter with small holes in it. As it catches particles they block the small holes. This means that over time the maximum flow rate is compromised and that if you let them get too blocked then the engine can't get enough air. A performance filter doesn't work in the same way but has a number of much larger passages through the material through which the air and particles can travel on their way to the engine. However, these work because you apply oil to the filter and the result of that is that the particles will impact the walls of the passageways in the filter and stick to the oil. The result of that is that the ability of the filter to allow air to pass is largely uneffected over time. So basically the aim of a performance filter is to provide consistent rather than decreasing air flow through it. As an added bonus you can usually clean a oil based performance filter and re-oil it but a potential disadvantage is that oil mist can get in to the engine or (more importantly) the MAF if your car has one.

The size of the filter, the location of it and whether or not to keep the airbox are all different issues but the premise of a 'performance' filter is (as I understand it) as above.

[H2DaE]

They also get absolutely filthy too, will post a pic of my one which sits on the bottom of my bumper later

[Snake the Sniper]

On a stock engine, it used to be worth doing, and may have gained you 1-3 bhp, maybe. These days however, most filters are plenty big and free flowing enough to make no difference at all. Waste of money as a first stage of tuning these days. Put the money towards a better exhaust.

[grahambell]

Rolling road test showed about 3bhp increase on a little 3-cylinder Diahatsu engine just by replacing the standard air filter element with an ITG foam one.

[H2DaE]

You won't notice the gains from a filter tbh, and rolling road tests done by the company themselves are hardly accurate are they!

A decent full induction kit would give proper benefits, especially to cars with restrictive piping like a Fiesta ST for example

[Foolish Dave]

I've found that on a modern engine, they make a better noise, but no defference otherwise.

I've only used OEM filters on my own older cars, but I've seen plenty of race cars that run them - but then they seem to have exposed trumpets with no filter as their first option, so I'm sure they find the power is reudced with them.

The point of them has been well described by Dern above, I agree with what he's said

[grahambell]

H2DaE said:

You won't notice the gains from a filter tbh, and rolling road tests done by the company themselves are hardly accurate are they!

The rolling road test I quoted wasn't done by the filter manufacturers. Spent some time working with the guy who developed the 'performance' models for Daihatsu UK and that test was the first stage of engine tuning for the Charade.

Because the filter housing on Daihatsu's 3-pot is built into the cam cover and gets hot we also tried rigging up a cold air feed to a remote filter. And got no more power...

Things don't always go as you expect, that's why you need comparison tests on rolling roads to see what really works.

[Mr Whippy]

I bought a 'filter' for my car, some Pipercross foam thing, and yes it has lasted two paper filter services, but cleaning it properly is a PITA, and I'm sure it's filtration in absolute terms isn't as good as a paper, and I'm also sure that it's total flow restriction isn't that great either...

I prefer a new paper one every 12k and a good tap out for big debris every 6k... I guess the best test is as Autospeed magazine showed, to use a manometer and check the restriction before/after filter changes/cleaning etc. Or do your own independant tests to see the difference (plan to do a few 1/4m runs on my cleaned Pipercross panel and then a new paper one soon)...

All I usually see of 'performance' air filters is people forgetting about them, they are often open to atmosphere so likely clog up quickly, under-oiled and 'dry' looking so likely letting crap through too.

Without tests nothing can be certain. Manometer for restriction over rpm range, and then dyno and/or 1/4m tests etc to see if the performance has improved. Restriction may be lower, but with say an open cone it might now be sucking in hot air or not benefiting from ram-air features etc.

Dave

[Foolish Dave]

I saw an article way back before I got my air filter for the FTO - that car had a plastic box with the filter diagonally across it - drilling big holes in the sides before the filter gave almost as good air flow as the filter I went for, at a fraction of the cost, and better bhp improvments over the one I got.
The noise wasn't as good though.