Headers! M50 engines come with cast iron manifolds, where S50 or S52 engines come with tubular steel manifolds. Some generalized differences are design and material. Starting with material, cast iron, is bulky, heavy, and rough (inside and out), and steel is lighter and smoother, and easily formed so as to make an efficient shape. Design differs from manifold to header as well. Manifolds are like a dumping ground that the exhaust port spills into that is then poured through a single exit. Headers are a separate exhaust path for each cylinder exhaust port and are then eased into each other for a much better flow and less back-pressure, which equates to more power. Because M50 and S50/52 share the same block and configuration, these bolt right up. When installing your new headers, be sure to replace gaskets, because there will be more airflow and old gaskets will only get worse and they're ~$25 (c'mon). The S50 headers bolt right up without any modification. S52 headers, require some light modification. Because the S52 headers are from OBDII vehicles, they have secondary vacuum lines at the manifold side, and O2 ports close to the cat-side flange. The O2 holes are easily plugged with M18 oil drain plugs from a 1985 Land Cruiser, found at any hardware store. The vacuum lines may have a plate welded over them, or with an OEM Plates that bolts over it.
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