AIR FILTERS have changed hugely over the past 100 years. If you go back to FJ Holden days, 60-something years ago, youíd open up the engine bay and come across something called an oil bath air filter.

Basically they were an oil bath where the air would get drawn through the fluid, the particles would get caught, and clean air went into the carburetor.

As weíve moved on, weíve mostly gone for pleated paper filters with different shapes and micron ratings If you look at the photo of the two white filters, the one thatís gone grey is only six months old, so about 10,000km. That gives you an idea of how much dirt and pollution theyíre picking up. You imagine trying to breathe when you get a cold Ė thatís what itís like for the car. The downside is the engine is working harder to do the same thing, using more fuel and its hurting your economy.

With a modern car, you know when you pull up behind and get that bad sulphurous smell? Itís because itís running rich and the cat isnít coping. With older cars, you pull up behind and thereís a big cloud of black smoke as they take off.

People will do silly things like pulling out injectors and cleaning them, or try getting into the tuning via the computer when Ė nine times out of 10 Ė itís just the air filter.

Itís a very simple system, but itís essential itís changed regularly. I would suggest every six months, if youíre using the car daily. As the example of the two white filters show, it doesnít take long for things to block up.

When it comes to quality, you get what you pay for. With the really cheap filters, youíre problem is the risk they start to break up and feed bits of filter through your fuel system. Itís not catastrophic, but itís a hassle you could live without.

When it comes to modifying the air filter, think of your engine as giant pump.

If itís able to draw in more air and fuel mixture, it should perform better.

In a lot of situations, a cold air induction system works a treat. Weíve all had that experience where the car feels better on a cool evening, drawing in that cool dense air. Done right they work.

Theyíre an expense, so you need to weigh up whether itís worth it.

If youíre going that way, itís advisable to check what itís doing to the mixture. So you put a Ďsnifferí up the exhaust before to get a base reading (assuming itís right when you start!) and then check it once the modifications are done.

A lot of workshops have the gear and it doesnít hurt to seek an opinion. It will vary from one car to another.

For example a lot of fuel injections systems are selfmonitoring and can adjust on the run.

Anyway, just remember to check your air filter. You might be surprised at the difference it makes. Itís people who donít check them who keep me employed!