In recent years, automotive exhaust systems have been the subject of dramatic design changes. Nowhere is this more apparent than the technology being aimed at the exhaust manifold. The drivers for change and the reasons for the current diversity can be summarized into four major areas: Exhaust temperature, start up emissions, aspiration, and refinement.

Cast iron is the material of choice for 60 - 70 percent of today's exhaust manifold market. While S.G iron will perform quite satisfactorily at temperatures of up to 850 degrees centigrade, beyond this temperature, the material's reduced yield strength and the associated thermal expansion problems make the material less attractive.

Ni resist iron will maintain adequate strength at temperatures up to 1000 degrees centigrade and has, as a result, largely replaced S.G iron as the choice of material for cast manifolds. It is, however, more difficult to cast, more difficult to machine, and more expensive.

Increasingly stringent regulations on startup emissions have led to a fascinating diversity of manifold design across the automotive spectrum. With current technologies, the catalyst light-up time is a crucial factor and so, therefore, so is its location in respect to the combustion chambers. Rear wheel drive cars like Mercedes Benz and B.M.W, have their catalysts located in the under-floor position, quite remote from the engine position. This has resulted in a need to insulate the gases passing through the manifold using a double wall air gap design. A double wall design can either be an inner and outer tube design or inner and outer fabricated steel pressings. Both designs use stainless steel capable of withstanding temperatures of 1000 degrees centigrade and above. However this rule does not apply to all designs of car. Generally, front wheel drive cars use fabricated steel single tube manifolds to accommodate exhaust temperatures, while being able to meet emissions regulations with suitable catalyst positioning.

So how does a car's aspiration have an effect on the choice of manifold design and material? Well, naturally aspirated engines still have a wide diversity of exhaust manifolds as described in the foregoing, but with turbocharged and supercharged gasoline engines the trend is toward fabricated steel manifolds, for temperature considerations. More specifically though, turbo-charged engines are moving towards fabricated steel pressings that permit the design of a manifold with the strength and rigidity to support the turbocharger, within the available space envelope. We are now seeing more and more implementation of steel manifolds, based purely on raising quality within the overall package.

So far, the view we have had of manifold development appears to follow a logical course concerning emissions, structural strength, and cost. You might be surprised then, to find a double wall air gap manifold, (using pressings rather than cheaper tubing), on a Toyota Yaris. The explanation is of course, that a double wall manifold offers the additional benefit of noise reduction, particularly in the car's interior, where engine sound quality is perceived.

The move towards steel manifolds is a progressive one, and CSM forecasts that within the next 4-6 years, this steel will surpass cast iron as the primary exhaust manifold material.