Air filters

Fit a free-er flowing air filter

If you read around in the diesel world you’ll find a lot of varied opinion on air filters, and what is best. I’m not going to say which is best because I’ve not tried them all, I’ll simply point out a few things to bear in mind when fitting an air filter.

Open vs Enclosed fitlers

Well this is an ongoing debate which I see no end to in the near future, largely because people are comparing chalk and cheese and it is very application specific. Many people will say that an open filter is best for power, whilst common sense says that a filter which can suck cold air rather than hot engine bay air will perform better. What most people don’t bear in mind is that an enclosed filter is often smaller than an open filter (it has to fit in the same space and some of the space is needed to house the enclosure for the filter!).

If everything else is equal then a filter sucking cold air will perform better than an open one sat in a hot engine bay. You’ll notice that I said sucking cold air rather than an enclosed filter, it is possible to site an open cone filter in an area that receives a good supply of cold air, but it usually isn’t easy!


I’m afraid its true, size does matter. The bigger the filter area the less resistance to air flowing through it. The more eagle eyed will have spotted that I said filter area, not filter size, The surface area of the filter is the important bit rather than its physical size. This is why pretty much all original filters are pleated paper ones. The pleats increase the surface area so you can pack a good amount of airflow in a small space. The paper material keeps the cost down.

Filter material

Again this is a battle that rages on and on and these are my opinions so it’s up to you whether you believe them or not.

Paper: It is cheap, readily available and what pretty much all original equipment manufacturers use. It flows pretty well and filters pretty well too but does tend to get blocked up quite quickly, i.e. its dirt capacity isn’t huge.

Foam: This tends to be popular in the performance crowd. Indeed it flows very well and is reusable if cleaned and oiled properly. However it doesn’t really filter that well, infact is lets a lot of stuff through. Also there is the danger of over oiling which will contaminate and damage the MAF sensor on the 25/45 etc engines (the 200/400/600 won’t care!). Its dirt capacity is about the same as paper so it’ll need regular cleaning.

Cotton/gauze no oil: This tends to be more popular with the jap crowd. It flows reasonably well, but the filtration is pretty rubbish, probably the worst of all the types listed here. They also claim it is reusable but how you clean them remains a mystery to me! Vacuuming them doesn’t count as cleaning!

Cotton/gauze oiled: When you mention performance air filters most people will mention the K&N filters at some point. These fall into this material category but there are others out there. They filter reasonably well, and also flow reasonably well too. Where they have a big advantage is the dirt capacity of the filter, due to the oil trapping all the dirt this means that the filter has a longer life between cleaning, and indeed some of the dirt trapped actually helps improve the filtration of the filter. If you are fitting the filter in a dirty area or want to have long intervals between cleaning then this is probably the filter for you.

What do I use:

Well now there is a question and a half.

On my current 600 diesel I use standard paper filters in a standard airbox. I don’t have any smoke or performance problems (@130bhp) and I change them roughly every 12000 miles.

On the 220 turbo petrol I run an open cone filter sat right next to a red hot turbo. Not ideal but it still makes 212bhp. – It was there when we got the car and I can’t be bothered to change it!

On the maestro 2.0 turbo petrol I run a K&N filter in the original airbox. The airbox has a good cold air feed as standard and people have made 250bhp (100 more than standard), on this setup without any issues.

So as you can see it is as clear as mud! The only type I’ve not really used is the dry cotton/gauze ones as the filtration of these is pretty rubbish. The dry foam ones I hear about are also likely to be the same. In general get the biggest surface area one you can find and put it where the cold air is and you’ll not go far wrong.