Cam-in-block

The cam-in-block valvetrain layout of piston engines is one where the camshaft is placed within the cylinder block, usually beside and slightly above the crankshaft in a straight engine or directly above the crankshaft in the V of a V engine. This contrasts with an overhead camshaft (OHC) design which places the camshafts within the cylinder head and drives the valves directly or through short rocker arms.

Placing the camshaft inside the engine block has a long history in its use in valve-in-block engines, in straight and V configurations, the Ford flathead being exemplary of the type. Pushrod overhead valve engines with the cam in the block were long used in Chevrolet and Buick straight engines from the 1930s through the mid-1950s and in various similar six-cylinder engines until the extensive employment of the V6 configuration in the 1980s.

There are three main cam-in-block designs:

L-head (flathead) refers to the pushrod valvetrain configuration in which the valves are placed in the engine block beside the pistons. The design was common on early engine designs, but has since fallen from use.

Generally L-head engines use a small chamber on one side of the cylinder to carry the valves. This has a number of advantages, primarily in that it makes the cylinder head much simpler. It also means that the valve can be operated by pushing directly up on it, as opposed to needing some sort of mechanical arrangement to push the valves down. It may also lead to slightly easier cooling, as the valves and operating rods are out of the way of the cylinder, making a cooling jacket simpler to construct (but see below). The line of intakes along the side of the engine lead to the name L-head, due to the cylinders having the shape of an upside-down L. This configuration is also known as sidevalve, as the valves are located beside the cylinders.

On the downside, the L-head engine also requires the airflow to make at least a 90° turn to enter the cylinder, which makes it less efficient; colloquially it's said that such an engine has poorer "breathing". Breathing was not greatly emphasized in past production cars because engines could not run long and reliably at high speed due to other factors. This was a minor concern given the benefits in simplicity.

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