Clamp-On Air Filters

A clamp-on air filter fits onto the end of an intake pipe, throttle body, carburettor or air mass sensor housing, held by a hose clip around its rubber neck. It replaces the airbox with an open element. This category holds universal clamp-on filters in a wide spread of neck sizes, element lengths and designs.

The neck decides what the filter fits; the element behind it decides how freely it breathes. Restriction rises with the speed of the air through the media, so a longer or larger-diameter element, with more pleated area, flows the same air with a smaller pressure drop. It also takes longer to load up with dirt, which stretches the cleaning interval.

An exposed filter draws whatever air surrounds it. Under the bonnet that air is warmed by the radiator, the engine and the exhaust, and warm air carries less oxygen per litre. A shield that seals against the bonnet, or a duct feeding outside air to the filter, recovers much of that loss. Heat soak is worst after the car has stood still or crawled in traffic.

To choose a clamp-on filter:

• Neck size: measure the outside diameter of the pipe or housing it will clamp onto. Some titles give inlet sizes in inches, such as 3.5, 3.75 or 4 inches.
• Element length: lengths such as 6 and 8 inches appear in some titles. Check the space to the bonnet, inner wing and radiator, with room left to reach the clip.
• Intended vehicle: some filters list the vehicles they were made for, many of them motorcycles and classic cars. Use that list as a starting point and check the dimensions.
• Media: confirm whether the element is oiled or dry, as that decides how it is cleaned and which care products suit it.

Clean the pipe end before fitting so the neck cannot slip, and tighten the clip evenly without crushing the rubber. A new filter on an air mass sensor housing can change the airflow the sensor sees, so check fuel trims or the mixture afterwards. Brace a long filter where the engine rocks on its mounts. For dusty events, a pre-filter wrap from the filter accessories catches coarse dirt before it reaches the element.