The list of processes for making exhaust manifolds includes several casting technologies. Selecting the optimal one entails considering quantities required, the physical size of the manifold, geometric requirements, details of the application, service life, and of course cost.

In most cases sand casting provides the best approach. This blog explains why.

How Function Dictates Exhaust Manifold Design

The exhaust manifold is the interface between the cylinder head or engine block and the exhaust system. It needs to handle hot, fast moving gases while imposing minimal resistance to their flow. This dictates large bores, generous internal radii, and smooth surfaces.

Combustion temperature and gas composition determine the material used as these can cause oxidation and thermal fatigue.

Most exhausts are produced in SiMo ductile iron which provides high temperature strength and oxidation resistance and has good castability and machinability. Stainless steel is the main alternative, although titanium, Inconel and even aluminum are used in some applications.

Physical Dimensions and Production Quantities

Engines range in size from compact units measuring just a few inches to multi-cylinder, high horsepower machines measured in feet, and the exhaust manifolds are sized to suit. For small engines the exhaust manifolds may be machined from billet but as they get larger only casting is practical.

Light vehicle engines are produced by the million but larger engines are built in smaller quantities. Thus, the exhaust manifold production process should be cost-effective over small, medium and large quantity production.

Beneficial Characteristics of Sand Casting

Given the characteristics needed, the optimal casting process must satisfy requirements for:

• Tooling accuracy and cost
• A wide range of part dimensions
• Production rates and quantities that can vary widely
• Satisfactory accuracy and surface finish

For tooling, sand casting uses expendable, single use molds and cores. Molds are made by packing sand around a pattern that is then removed while core production involves blowing sand into a core box (like a mold for sand.) Both patterns and core boxes are relatively inexpensive and are reused.

Sand casting can be used for single piece quantities with 3D printing finding a growing role in mold and core production. However, for cost-effective high-volume production both core-making and casting are automated.

In single piece production there is almost no limit to the maximum size of casting. Automated systems however have a maximum mold size.

Sand type influences geometric accuracy and surface. Historically, foundries used green sand, so-called because of the moisture of the clay binder that held the grains together. In recent years most have switched to resin sand. (Some offer both.) Using chemical bonding to form stronger molds and cores, this improves accuracy, enables thinner walls, and results in higher yields.

A generally accepted limit for sand casting accuracy is +/- 0.03” per linear inch. (Tighter limits are achievable with increased care in mold and core production.) Surface finish is a function of sand grain size and packing. In most cases 250 micro-inch is about the limit of what is achievable, which is sufficient to have only a minimal negative effect on gas flow.

Design Constraints Imposed by Sand Casting

The accuracy achievable also influences machining allowances. In general, sand casting needs more generous allowances than other casting processes. However, when finishing exhaust manifolds the impact on cost is negligible.

Sand cast parts do require draft, (an angle on vertical walls in the mold.) In some applications this can create challenges for part design but it’s rarely a problem for exhaust manifolds.

Alloy Compatibility

Alloys and casting processes aren’t universally compatible. The ideal process is one that produces satisfactory results with a wide range of alloys. Sand casting works with most ferrous and nonferrous metals. Cast iron and stainless steel both have good flow and solidification behavior.

Impro, For Your Sand Cast Parts

When producing exhaust manifolds it’s important to consider fitness for purpose and cost. The geometry and finish available from sand casting are almost always sufficient, and tooling is less expensive than that for other processes. Furthermore, the casting process itself is less expensive, especially when automated. Finally, considering quantities, sand casting scales readily from small to large order sizes.

Impro is a leader in sand casting and produces an extensive range of parts for automotive and high horsepower engines. If you need a source for parts like these, we can help. Contact us and let’s talk.