Replacing the standard paper element air filters with high-flow performance alternatives is one of the simplest and most effective breathing upgrades available for the MGB. The original truncated cone air cleaners do an adequate job of filtration but are inherently restrictive to airflow, and on a modified engine they can become a bottleneck that limits the benefit of other work carried out to the cylinder head, camshaft, or carburettors. Performance air filters work by allowing a significantly greater volume of air to pass through the filter element while still maintaining effective filtration. The result is improved throttle response, a modest increase in power, particularly at higher engine speeds where the engine’s demand for air is greatest, and in the case of reusable cotton gauze filters, a long-term saving over repeatedly replacing disposable paper elements.
Several styles of performance filter are available, all sized to match the carburettor throat diameter. For the standard 1½-inch HS4 and HIF4 installations, options include chrome-topped foam filters, chrome SU-branded filters, deep-element filter kits with alloy cover plates, and low-profile pancake-style filters in mirror-finish stainless steel. For the larger 1¾-inch HS6 carburettor conversion, the same range of styles is available in the appropriate size. A dedicated filter is also available for the Weber 45 DCOE twin choke conversion.
Reusable cotton gauze filters require periodic cleaning and re-oiling rather than replacement. Cleaning kits and re-oiling sachets are available to maintain their performance over time, a filter that has become excessively dirty or has dried out will lose its filtration efficiency and may allow harmful particles through to the carburettor internals.
Stub Stacks & Tuning
Stub stacks, short velocity trumpets that fit between the carburettor intake and the filter element, are available in both 1½-inch and 1¾-inch sizes and are highly recommended as a companion to any performance filter. They smooth the airflow into the carburettor throat, reducing turbulence at the intake and improving the efficiency of the air charge entering the engine. The improvement is most noticeable in the mid-range and at higher rpm.
When fitting performance air filters, particularly high-flow types, it may be necessary to recalibrate the carburettor mixture by changing to a richer needle profile. On fixed-needle HS4 carburettors, a richer needle may be required in place of the standard specification. Biased-needle HS4 and HIF4 carburettors may similarly need attention. Modified cars should ideally be set up on a rolling road after any filtration change to ensure the fuel mixture is correctly optimised across the rev range.
The mounting hardware for performance filters differs slightly depending on the carburettor type, HS4 carburettors use a fine thread mounting bolt, while HIF4 carburettors use a coarse thread. The correct bolt type should be confirmed before fitting.