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Metric Mechanic - Porting the BMW Manifold


 

We don't sell headers for the same reason that we don't sell side draft carburators. These items are no longer appropriate for street use. Their place is in racing. For example, many states require a visual inspection for emissions testing and a header will not pass.

Also, BMW makes very good cast iron manifolds for 4 & 6 cylinder models. Overall, cast iron manifolds offer the following advantages over a header:

  1. Properly ported, we feel a cast iron manifold will equal the performance you can get from a header.
  2. Header exhaust flanges often leak and exhaust nuts like to come loose.
  3. Headers like to crack up at the flange or down at the collector. A leak at the flange can lead to a burned exhaust valve. Headers usually last only about 3 to 5 years. Cast iron rarely cracks and seems to last forever.
  4. Headers put a lot more heat in the engine compartment and shorten the life of the spark plug insulators and plug wires.
  5. Headers are noisier.
Porting Cast Iron Manifolds
Once exhaust leaves the cylinder head, it shouldn't be allowed to return. The function of a header or a properly ported exhaust manifold is to keep exhaust from back flowing into the cylinder.Any excess exhaust that returns to the cylinder will reduce the amount of intake cylinder filling. A Metric Mechanic ported exhaust manifold prevents exhaust reversion from occuring because it doesn't match with the exhaust port of the head. Making the manifold port larger creates a "ledge" at the parting line of the manifold head. The ledge "traps" some of the exhaust as it attempts to back flow. This anti-reversionary system, combined with the factory's "Tri-Y" manifold and downpipe design, makes for a system that we feel is the equivalent of a header without the hassles.
Above, a ported anti-reversionary exhaust manifold for a 4 cylinder. Below, a stock 6 cylinder cast manifold.
 

09-02-2010 03:20:55 PM
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