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Ford Fiesta Aftermarket Air Filters

Ollie Andrews

Author: Ollie Andrews | Photographer: Ollie Andrews

K&N Breather Filters

My particular example of this car is fitted with a K&N air filter and matching breather filter, this will be of interest to anyone out there who is thinking of doing the same to their car. With the original air cleaner box there is a section of the drivers side where two pipes are connected, one goes to the rocker cover and the other goes to the inlet manifold. With a K&N kit fitted these pipes become obsolete, the rocker cover pipe is dealt with by the breather filter, the inlet manifold one however is not. I found that by simply this hose up I started getting slight roughness in the idling of the car and it seemed to lack the power it had before, I was also beginning experience starting problems whether hot or cold. There is an adapter plate that can be obtained from K&N that will allow this pipe to be reconnected to the side of the air filter, but even this needed some modification as the hole through the adapter allowed too much air through and made the car run too weak. I made this modification and the car ran fine after that.

However, due to the rocker cover breather filter I was also getting a strong smell of hot oil in the car which was making me feel sick and had started to give me a headache when ever I drove. I was, therefore still looking for a better solution to the problem. Then I went to Halfords, where I stumbled across a replacement PCV unit for the 1300 and 1600 engines. PCV means Positive Crankcase Ventilation and this has been built into the air cleaner on the 1400 whereas on the 1300 and 1600 it is a unit that plugs into the bottom of the air cleaner. What you do with this is connect the rocker cover pipe to the big side entry, connect the inlet manifold pipe to the small bottom entry and then attach the breather filter to the top where the unit would normally fit into the air cleaner. Hey presto, all pipes reconnected and all problems sorted.

XR2 K&N Breather Filter

Performance Filter Cool Air Ducting

Ever noticed a reduction in power from the engine when the weather is hot? On cars with performance filters such as K&N or Piper Cross the filter is mounted directly above the carb... and right inside the engine bay! This means that the air flowing to the filter has to come over the engine and through the radiator. So the air entering the carb is already baking hot. Although not a problem in the winter when the air is very cold outside anyway, this can cause a reduction in power in the summer, or in hot weather.

Technical reason: When the air is hotter the particles are expanded slightly and so less oxygen enters the carb. As oxygen is the main component required to make the fuel burn economically, this make the mixture richer. Which puts the tuning of the engine out slightly and causes the reduction in power!


The solution is to add a cool air duct, this is a very simple job to do and requires only a section of ducting and some cable ties. Ducting is available from most car accessory shops for about £4 for a 1 metre piece, or try DIY shops where it is probably cheaper and you might be able to get more than 1 metre. Decide where you want to route the duct before you buy it, or you'll end up with 2 bits taped together like mine!

Basically all you need to do is run the duct from the air filter round to a point where it can get air from outside, it needs to be pointing forwards so the air will be forced in and up to the filter. The easiest place to put the duct is down the drivers side of the engine and out next to the radiator, this will only require a 1 metre length of duct. However, I had mine here and it started to rub a hole in my bonnet sound deadening so I've moved it to the passenger side, where is runs down to the top of the gearbox and then down under the car and is cable tied onto the tie bar mount. Either method works, and on OHV engines it should be even easier to fit as there is more space. Don't make it too permanent though as you may need to take it out in the winter to avoid carburettor icing!

Page Last Updated: Monday 18th of November 2019