The MGB air filtration arrangement changed several times across production, and the correct air filter, housing, and associated gaskets depend on which carburettor type is fitted to the engine. Understanding these differences is important when ordering replacement elements or upgrading to performance filters. MGBs fitted with twin SU carburettors used two separate air cleaners, one per carburettor, of a truncated cone shape, painted black, and of the dry paper element type. On all cars with 18G-series engines up to 1971, the air cleaners were supplied by Coopers and carried labels with that name.
From 1971, cars with 18V-series engines used a slightly different design of air cleaner carrying Unipart labels. In December 1971, modified air cleaner housings were introduced on home market cars to comply with new noise regulations, a change that had already been applied to non-North American export models from the start of the 1972 model year. Slight differences also occurred between air cleaners on UK, North American, and other export models throughout the remainder of production.
Carburettor-Specific Filters
The carburettor type determines which air filter components are needed. The standard 1½-inch HS4 carburettor was fitted from 1962 to late 1973 with the HIF4 (Horizontal Integral Float) taking over from 1972 onwards on export cars and from late 1973 on home market cars. Both the HS4 and HIF4 are 1½-inch carburettors and share the same air filter element, but the mounting bolts differ, HS4 carburettors use a fine thread bolt, while HIF4 carburettors use a coarse thread. The air filter-to-carburettor gasket is also specific to the carburettor size.
Cars fitted with the larger 1¾-inch HS6 carburettor conversion require a different gasket and different performance filter sizes. The standard paper element is a straightforward service item and should be replaced at regular intervals to maintain proper air flow and engine performance. The alloy baseplates, sealing rings, and mounting grommets that form the housing assembly are also available individually for owners rebuilding or refurbishing their original air cleaner assemblies. A coupling seal between the two air cleaners is available for cars where this component has deteriorated.
Performance Filters
For owners seeking improved air flow and engine performance, a range of aftermarket performance filters is available. These include chrome-topped foam filters, SU-branded chrome filters, and high-efficiency reusable cotton gauze filters in both deep-element kit form and low-profile pancake styles. Performance filters are sized to match the carburettor throat diameter, 1½-inch for HS4 and HIF4 installations, and 1¾-inch for HS6 conversions. A filter is also available for the Weber 45 DCOE carburettor conversion.
When fitting performance filters, particularly high-flow types, it may be necessary to adjust the carburettor needle specification to accommodate the increased air flow. On fixed-needle HS4 carburettors a richer needle may be required, while biased-needle carburettors may similarly need attention. Modified cars should ideally be set up on a rolling road after any filtration change to ensure the mixture is correctly calibrated. Stub stacks, short velocity trumpets that fit between the carburettor and the filter, are available for 1½-inch and 1¾-inch setups and are recommended alongside performance filters for further improvements to air flow into the carburettors.
The MGOC Spares range covers replacement paper elements, complete housing components, gaskets, and a full range of performance air filter options to suit every MGB carburettor configuration.