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On modern engines, the fuel injection ports are often covered by the air intake manifold, all of which sits on top of the camshafts and cylinder heads. In this configuration, the torque of the bolts that hold these things together is critical to allow both engine expansion and contraction and to keep things from leaking or creating gaps as that happens.

Typically, the intake manifold will be in two parts: an upper and lower.

The Upper Intake Manifold

This attaches to the lower and is where the air intake, usually from a hose attached to a filter and sometimes with a turbo or other device to force the air in faster. The upper manifold is charged with bringing the air into the air intakes and diverting it to the lower manifold for distribution to the cylinder heads.

The upper intake's primary purpose is to regulate the air flow and force it down to the lower manifold.

The Lower Intake Manifold

This is where the air that has been filtered and regulated is brought in to be distributed to the cylinder heads. The lower manifold's job is to push the air into the heads to give oxygen for the fuel burning process.

Some lower manifolds have control valves and devices, controlled by the camshafts or timing belt, that open and close air channels to move the air into cylinders. Most operate by using the cylinders themselves as the doorways to control air flow.

Assembling Air Intake Manifolds

While models will vary, nearly all modern intake manifolds are put together in roughly the same way. Most are made of aluminum to save weight on the engine and will bolt directly to the area in between or on top of the cylinder heads and fuel injectors.

The lower is added first, with a gasket usually buffering the intake from the engine block and heads. Specific torque will be applied to the bolts holding this lower intake to the block and heads. For most engines, this is in the 25-35 pound range and will need to be done in sequence to prevent gaps from forming. Check your engine's specifications for exact torque.

On top of the lower, the upper intake manifold is set. There is, again, often a gasket to seal the two together. It is most often paper or fiber. Again, the bolts connecting the two parts of the intake will have specific torque application requirements, usually in the 20-25 pounds range. Check your engine specifications.

Other attachments such as a throttle or valve control for intake may be separate parts of the upper, but torque settings are generally the same for these additions.

All in all while the intake manifold as a whole is a large bulk of the engine's overall makeup, it is one of the simpler parts of the engine's design. It has few moving parts and it's only common mechanical failure is in loss of seal between components rather than actual parts becoming broken.

Torque Specifications for Bolts