The original MGB exhaust manifold is a cast iron dual design, with the exhaust from cylinders one and four feeding into a Y-shaped manifold leading to the front downpipe, and the siamesed exhaust port from cylinders two and three feeding a single pipe in the centre leading to the rear downpipe. While robust and perfectly adequate for standard use, the cast iron manifold’s internal gas passages are relatively restrictive, and the design does not lend itself to efficient exhaust scavenging, particularly on engines running performance camshafts or modified cylinder heads. Two types of original cast iron manifold were used across production.
Early 18G-series engines (1962 to 1971) used a thick-flange manifold, while 18V-series engines from 1971 onwards were fitted with a thin-flange manifold which incorporated a hot-spot facility for the carburettors. The thin-flange type uses six identical manifold-to-head studs, whereas the thick-flange type uses two outer studs and four inner studs of differing lengths. Reconditioned cast iron manifolds in both types are available separately in the OE Exhaust Systems section for owners seeking a like-for-like replacement.
Stainless Performance Manifold
The stainless steel performance manifold offers a significant improvement in exhaust gas flow over the original cast iron item. These are gas-flowed tubular manifolds manufactured in stainless steel, with individual primary pipes for each cylinder merging into a collector, a design that promotes more efficient exhaust scavenging and smoother gas flow compared to the siamesed port arrangement of the factory casting. The result is improved throttle response, a broader spread of torque, and a modest increase in power output, particularly when combined with other breathing modifications such as a Stage II cylinder head or performance camshaft. The stainless performance manifold is available in right-hand drive, and is designed to couple directly to an existing exhaust system, meaning it can be fitted as a standalone upgrade without requiring replacement of the full system.
For owners seeking maximum benefit, it can also be ordered as part of a complete performance pack which pairs the manifold with a full stainless steel exhaust system in standard box, bomb box, free flow, or big bore configurations. These packs are available for both chrome bumper and rubber bumper applications and are listed in the Stainless Steel Exhaust Systems section. A mild steel performance manifold is also available for those on a tighter budget or seeking a performance improvement without the cost of a full stainless setup. This too is a three-branch tubular design that improves gas flow and promotes smoother running.
It can be fitted individually or as part of the mild steel complete performance packs listed in the OE Exhaust Systems section. When fitting any performance manifold, the existing manifold gasket and brass nuts should be renewed. The manifold gasket is common to both the original cast iron and performance tubular types. Brass nuts are recommended over steel to resist seizing onto the studs, a common issue on exhaust manifolds subjected to repeated heat cycles.