Performance air filters

Just like us, engines need to breathe. Restrict the supply of air and they won’t work as well as they should – something that’s usually reflected in an increase in fuel consumption. But although your power­plant needs just about all the air it can get, it doesn’t want all the debris which is normally carried in it – and this is where the filter comes in.

Tiny particles held in the air are trap­ped by the filter and prevented from attacking the pistons, bores and valve seals. Ignore the filter, however, and these particles can start to restrict the air flow, and in severe cases cut the engine’s power output by up to 10bhp. The key to keeping the airways flowing is regular servicing. Inspection schedules will always include a filter check, and usually recommend replacement – although you can renew it yourself in between normal intervals.

Your car’s handbook will point out where to look, and getting to the part often involves the simple loosening of a couple of fasteners. Original equipment filters are made from a material similar to tissue paper, and look grubby when they need replacing.

To renew a filter, all you have to do is pop in the replacement. However, you don’t have to use a paper filter as a replacement, as there are other options to choose from. You could fit a sports filter, which simply slots into the original housing and will never need renewing.

Alternatively, you could go for a complete induction kit; this replaces the original system, including most of the ducting. Both additions improve a vehicle’s driveability, throttle response and power – with some set-ups boosting the output by as much as 10bhp and around 14Nm of torque.

Many motorists ignore induction kits and sports filters because their image is tarnished by boy racers, but that can be a mistake. The improvements these add-ons give in performance and fuel economy are well worth the money.

What’s more, this is a modification you can make on any car, whatever the age, and it shouldn’t affect the warranty – although it is still worth consulting your dealer about cover and notifying your insurer of the change.

Click on the images in the right-hand column for product information.

Rating the performance gains

To see how well cars respond when the standard filter is replaced by a sports set-up, we took a Vauxhall Signum 1.9 CDTI diesel and a VW Golf GTI to tuning specialist Regal Autosport in Southampton.

Here, we hooked the models up to a rolling road and logged power data with a standard element in place, then with a sports filter and a full induction kit. We used K&N sports filters (£48) on both cars, a K&N Apollo induction kit (£190) on the Signum and an Evolution V-Flow (£249) on the Golf.

Vauxhall Signum CDTI Standard*/Sports filter/Induction kit: 150bhp/152bhp/156bhp

VW Golf GTI Standard*/Sports filter/Induction kit: 218bhp/219bhp/224.5bhp

*As tested at Regal Autosport (023 8055 8636, www.regal-auto.co.uk)