The cam-in-block valvetrain layouts of piston engines are ones in which 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.

Cam-in-block designs are perceived to be "old fashioned" by most modern automotive press, although modern pushrod engines have become more competitive lately. The cause is historical: while both layouts are over 100 years old, the pushrod design engine came first. OHC engines were developed as more expensive high-performance engines and have largely replaced pushrod designs in countries where cars are taxed based on engine displacement. Having the camshaft residing inside the engine block instead of below or beside it, like early pushrods, is a relatively new wrinkle, as metallurgy of the very early 20th Century was not precise enough to allow such a configuration to be reliably produced. In 1949, Oldsmobile introduced the Rocket V8. It was the first high-compression, cam-in-block design, and is the archetype for most modern pushrod engines. Currently only General Motors, Ford, DaimlerChrysler, and Cummins produce cam-in-block or pushrod engines in any large numbers, although GM in particular has several OHV engine families.

There are three main pushrod designs:

• L-head, also known as L-block, flathead or sidevalve
• F-head
• I-head, also known as overhead valve (OHV)

L-head

Ford flathead V8 engine, modified for power, depicted on cover of Hot Rod magazine. The "flat head" is evident.

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.

A more serious concern is that the exhaust often follows a more complicated path to leave the engine. This virtually guarantees that the engine will overheat under sustained steady heavy use. It is sometimes possible to arrange the engine layout so the exhaust will be taken through a second set of similar chambers moved to the other side of the cylinder, in which case the layout is referred to as a T-block.

Although L-head inline 4 and 6 cylinder engines were frequently used for automobiles, tractors, etc., the best known L-head automotive engine is the early 20th century Ford V-8, which has both sets of valves (intake and exhaust) located on the inside of the "Vee," and which are all operated by a single camshaft located above the crankshaft.

Due to the heating and efficiency problems, L-head engines fell from high power uses such as aircraft engines fairly quickly, prior to World War I. They lived on for some time in the automotive world and were used in the World War II Jeep, for instance. L-heads are no longer used in automobile engines, although they are still used for some small-engine applications like lawnmowers. Because of their design, the size of valves and the compression ratio are limited, which in turn reduces available power and economy. Not all L-heads are cam-in-block engines; the location of the camshaft varies in this layout.

F-head

The F-head layout (not to be confused with flathead) can be thought of as a combination of L-head and I-head: the intake manifold and its valves are located atop the cylinders (in the cylinder head, as in an I-head design) and are operated by pushrods, but the exhaust manifold and its valves are located beside the cylinders (in the block, as in an L-head design). The exhaust valves are either roughly or exactly parallel with the pistons; their faces point upwards and they are not operated by pushrods, but by direct contact with a camshaft.

This was a more expensive engine design. Its advantages over competing L-head engines included more power from its higher compression, better intake mixture flow, less susceptibility to pinging, and greater reliability from its cooling of the exhaust valve and its spring (and having half the number of pushrods of an OHV engine).

For years the British motor car firms Rolls-Royce and Rover used this arrangement. From 1927-1929, the American firm Hudson used a 6-cylinder engine of this form as well, but this engine is not to be confused with that of the race-winning Hudsons of the 1950s. The last major use was the Willys Hurricane engine, used in civilian Jeeps in the 1950s and 1960s. It was replaced by the I-head design.

I-head

The I-head design is one in which the entry and exit valves and ports are contained in the cylinder head. It was developed by the Scottish-American David Dunbar Buick. It employed pushrod-actuated valves parallel to the pistons and is still in use today in some designs (notably several engines produced by General Motors).

It has several advantages over L- and F-head designs, but the most notable is the fact that the intake charge and exhaust gases have a more direct path into and out of the combustion chambers, increasing power, improving fuel efficiency and reducing noxious exhaust emissions.

Advantages

Cam-in-block designs have two distinct advantages:

• Smaller overall packaging
• Because of the camshaft's location inside the engine block, pushrods are generally more compact than an overhead cam engine of comparable displacement. For example, Ford's 4.6 L OHC modular V8 is larger than the "5.0" L OHV Windsor V8 it replaced (and, in fact, the double overhead camshaft variant of the Modular V8 is comparable in size to Ford's 7.0 L OHV V8s) and GM's 4.6 L OHC Northstar V8 is slightly taller and wider than GM's larger displacement 5.7 to 7.0 L OHV LS V8.
• Cost
• An OHC V-type engine can have up to four camshafts, this adds to the cost of the engine.

1994 Mercedes-Benz/Ilmor Indianapolis 500 engine

The Indy 500 race in Indianapolis each year bears some vestige of its original purpose as a proving ground for automobile manufacturers, in that it once gave an advantage in engine displacement as well as turbocharger boost pressure to engines based on stock production engines, as distinct from out-and-out racing engines designed from scratch. One factor in identifying production from racing engines was the use of pushrods, rather than the overhead cams used on most modern racing engines. The Buick V6 program that began in the late 1980s and continued through 1996 as the Menard V6 was designed to capitalize on these provisions. However, that engine was relatively strictly production-based, and reliability problems limited its major successes to qualifying. The breakthrough came when Ilmor realized before the 1994 race that they could very carefully tailor a purpose-built racing engine using pushrods to meet the requirements of the Indy rules and take advantage of the 'production based' loophole but still design it to be state of the racing art in all other ways, without any of the drawbacks of a real production-based engine. They entered this engine in 1994, and, as expected, dominated the race. After the race, the rules were changed to prevent a recurrence, and the engine became obsolete after just the one race, as Ilmor knew it would when deciding a victory at Indy was worth it.

Cam-in-block limitations

Three specific problems remain with pushrod engines:

• Limited engine speeds
• OHV engines have more reciprocating mass, suffer more easily from valve "float", and exhibit a tendency for the pushrods themselves to flex or snap at high engine speeds. Therefore, conventional wisdom says that a cam-in-block engine cannot revolve ("rev") at engine speeds as high as an OHC design. Modern cam-in-block engines generally rev to 6,000 rpm: compare this to modern OHC engines that can easily rev from 7,000 rpm in average engines to well past 10,000 rpm in specialty engines. High-rev pushrod engines can and have also been developed — in 1969, Chevrolet offered a Camaro Z28 with a pushrod V8 that revved to 8,000 rpm. Volvo B18 and B20 engines can rev to more than 7000 rpm. The 2006 Chevrolet Corvette features a 7.0 L engine capable of revving to 7000 rpm. Custom manufactured or modified engines can utilize oversquare crankshafts, and lightweight valvetrains to rev in excess of 8000 rpms, similar to those used in NASCAR racing.
• Difficulty in using crossflow cylinder heads in straight engine configurations
• A few straight cam-in-block engines have been manufactured with crossflow heads, such as the six cylinder Humber Super Snipe. These engines combined much of the performance of the overhead camshaft with the ease of service of the cam-in-block, but were more expensive to manufacture than either competing design.
• Limited valve flexibility
• The biggest benefit of an OHC design is the use of multiple intake and exhaust valves and variable valve timing. Most cam-in-block engines have two valves per cylinder, while many modern OHC engines use three, four or even five valves per cylinder to achieve greater efficiency and power. Recently, however, GM has begun offering an I-head V6 with VVT, and Cummins' ISB is a four valve per cylinder I-head straight-6.