Ducting

Know how the engine measures its air

Before changing anything ahead of the throttle, find out how the ECU measures air. Some engines use an air flow meter in the intake pipe; others read manifold pressure. A hot-wire air flow meter effectively reads air speed and the ECU converts that into mass using the housing's fixed diameter. An intake that keeps the factory meter in a housing of the same size needs no retune; a larger housing or a different meter does. Kits made for a specific car keep the meter where the factory put it, which is the main reason to choose one over a universal filter.

Choose your car with the car selector to see the intakes listed for it. HKS, GReddy, APEXi, Blitz and ARC all make intakes and filters for many Japanese cars.

Filter types

• Panel filters replace the element in the factory airbox. The smallest change, the factory intake sound stays, and the easiest to live with on a road car.
• Pod filters replace the airbox with an open filter. More induction noise and freer flow, but an unshielded pod draws hot air from under the bonnet.
• Induction boxes enclose a filter and feed it from a cooler place, usually through ducting from the front of the car.

Cooler air is denser, so intake temperature matters as much as filter flow. A pod's neck is sized in millimetres and must match the outside diameter of the pipe it fits. Service filters as their maker instructs, and buy the maker's own replacement filters for its kits, since shapes and flanges differ between makers.

On a turbocharged engine

The intake runs from the filter to the turbo's compressor inlet, and its diameter and bends matter more as boost rises. Intake piping comes in aluminium, which holds its shape, and silicone or rubber, which absorbs engine movement; either needs secure clamps, since a pipe that slips off downstream of the air flow meter lets in air the ECU never counted.

Beyond the filter

Surge tanks and larger or individual throttle systems belong to engines whose other parts already flow more, such as big-turbo builds and high-revving naturally aspirated engines. They need an ECU tune and are fitted as part of a planned build rather than as a first upgrade.