Cylinder Head Design

Words: Larry Shepard

While the block is the foundation of the engine, the cylinder head is the most popular aspect of the engine that is discussed by racers, writers, customers and fans. To change the cylinder heads can be somewhat expensive and requires the engine to be partially disassembled, so this may not be the most popular change—in terms of shear volume—but everyone talks about them. Of the 20-plus main aspects of cylinder heads, most enthusiasts typically only want to talk about one—air flow! They have very little interest in the exhaust side or the actual flow curve and they only want to discuss peak-flow numbers on the intake side of the head.

Up until a few years ago, there were very few choices in actual cylinder heads or castings. The production engine used production heads but there were a few exceptions. The 1968 and newer 340 engine used heads that had bigger, higher flow ports and bigger valves than other similar “LA” engines—such as the 318. The 1962–64 Max Wedge big blocks had big ports but after a few years of being out-of-production, they became very hard to find (no longer true – use P5007494 resto or P5249824 hot rod). The 1965 A990 426 HEMI® used aluminum heads rather than the typical cast iron. These aluminum pieces quickly became hard to find until 1970 when they were re-done by Chrysler Performance Parts, now Mopar® Performance. This was probably the first cylinder head casting/machining designed outside of the production engineering process.

The cylinder head is designed for a specific engine and it tends to be unique to that engine or engine family. “LA” engine heads fit on LA-engines (small blocks or 318-340-360), B/RB heads fit on big blocks (383-400-440) etc. Typically “V” engines like V-8s, 
use two heads that are designed the same, so they can be installed on either side or bank. An in-line engine, like an in-line 6 (Slant Six) or in-line 4 only uses one head. The key aspect for the heads in this family grouping is the block’s bore centers. The big block family (B/RB engines)—361-383-400 and 413-426W-440—has 4.80" bore centers and the small block family (LA-engines)—273-318-340-360—has 4.46" bore centers. Another related aspect that 
is dictated by the block is the number of head bolts around each cylinder—four (4) for small blocks and five (5) for big blocks. On the small blocks, the attaching bolts are one-size larger than the big block. There also are two dowels in each deck surface that locate the head properly. The one unique aspect is that the 426 HEMI, currently called the Gen II, 
uses a short stud in the fifth (top) bolt location while the B/RB engines use a standard bolt. The actual hole location is moved about one bolt diameter. The stud threads into the bottom on the HEMI head’s intake port.

One of the first things that you might notice about a cylinder head is its material—cast iron is the most common and is used to make most production heads and aluminum is the lightweight option. This material decision has to be made early in the design process because it affects so much of the internal choices. The typical weight savings for aluminum over cast iron in a head with four cylinders is about 25 lbs; or 50 lbs. on a V-8 engine. The aluminum 426 HEMI head is P5153779 and the aluminum head for the standard LA-engine family is P5153849—uses 2.02"/1.60" valve sizes. All aluminum heads use valve seat inserts and valve guide inserts. These inserts can be replaced; the process should only be done in 
a cylinder head shop and not in the home garage. Seat and guide inserts can also be used to repair cast iron heads.

A coat of paint will cover up the most obvious use of aluminum but another aspect of the head that is easily observed is the valve cover rail. The LA-engine uses five (5) bolts, the B/RB uses six (6) and the Magnum® uses ten (10). The 426 HEMI (Gen II) has a very wide cover and the Gen III cover is almost flat – very shallow. There’s a couple tricks to this – the early (pre-’64) B/RB-engines used a 4-bolt cover. In 1964, production added a bolt on each end of the cover/head and that style has been maintained since. The ’64 426 HEMI and the 1966–71 street HEMI used a slightly different valve cover rail shape, but since there are so many more street HEMIs produced, all replacement heads like the cast iron version P4529898 and service parts have used the street HEMI pattern.

Another aspect of cylinder heads that can be seen with the heads installed on the engine are the core plugs. There may be core plugs on the ends of the head and also on the exhaust face. These features are more important to the restoration customer than the performance customer. Many 426 HEMI heads have blank core plugs on the ends machined to look the same as production. The newer W2 cast iron heads, like P5249769 have ribs added to the exhaust side for added strength which is important to the performance customer.

The spark plug location or style is generally a function of the engine. All of the new, HEMI Gen III engines use two or dual spark plugs per cylinder. This dual-plug feature was somewhat introduced on the 426 HEMI in the early 1970s in racing. Because of the valve relationship in the HEMI combustion chamber, there are two basic positions for the spark plug—
the A and the B. The production street HEMI heads could only use one but most replacement heads offer both, but only machine one. In the late 1990s fully-machined dual-plug heads were offered by MP but are no longer available. However the feature can be added by your engine/cylinder head shop. The big blocks (B/RB engines) sometimes use an angle-plug which moves the plug in the chamber and changes the plug’s angle. With these heads you must be sure that the plug locations clear your headers.

There are many, many things relative to the cylinder head design that are not visible once the head is installed. One of the more important is the combustion chamber. The chamber has size (volume) and shape considerations and the chamber can affect the port’s flow numbers and performance considerations like octane requirements and detonation. In a general sense, combustion chambers are discussed as open (large) or closed (small) and small block chambers tend to run in the 60-65-70 cc range while big block chambers run in the 80-85-90 cc range while the 426 HEMI comes in around 170 cc. The W2 small block heads have been built with both open and closed chambers—P5007445AB is a closed, 65-cc chamber. The top part of any combustion chamber tends to be formed by the valves. Therefore the valve spacing, the distance from one valve centerline to the other, and the valve angles, the angle of the valve relative to the cylinder bore centerline, are very important. These aspects directly affect the valve notch locations in the piston. Typically the B-engine uses one set of numbers and the LA-engine another set but race/performance heads change these specs. For example the standard LA-engine uses 18-degree valves while some race LA-engine heads use 15 or less. Valve size can also be a factor. For example, the standard B/RB production head uses 2.08" intake and 1.74" exhaust valve sizes while most performance applications that allow oversized valves to be used will install 2.14" intake and 1.81" exhaust valves P5249202. A valve seat insert, typical of aluminum head and also used in cast iron for repair purposes, is selected based on the valve-head diameter. Valve guides and valve guide inserts are selected based on the valve stem diameter. The typical wedge valve uses 3/8" stems while the 426 HEMI uses 5/16" stems. An 11/32" stem is also popular in racing.

The combustion chamber should never be larger than the cylinder bore. The bore size in the cylinder head gasket should also be larger than the chamber. The block controls or dictates the bore size. The head gasket should be selected based on that bore size. For example, the 4.500" big bore big blocks (426 HEMI) require a big-bore gasket – P4876071. The head gasket also controls the flow of water through the block and heads. Not all the holes in the deck surface—both the block’s deck surface and the head’s deck surface—allow water to flow through. This flow control must be done in the development stages and is typically only done by production engine engineers and the NASCAR/NHRA “pro” teams.

Perhaps the hardest aspect to design into a cylinder head is the water jacket. It is very important that the water get to the backside of the combustion chamber to cool it or you will have detonation problems. As the ports get larger for improved flow, the water jacket has to get smaller. This is a real challenge. Head bolts have to pass through it and valvetrain requirements have to be considered. When you are done, you want the water jacket large enough and smooth enough so that water will flow through it easily and the actual core that makes it will be strong enough so it will not break or you will not be able to cast it.

The actual ports are probably the easiest thing in the head to design by themselves. However, when you add in all of the functional constraints as discussed above, then it isn’t so easy. High-flow ports tend to get larger and have more cross-sectional area. For example, the aluminum head for the B/RB engines (P5153524) flows 290 cfm on the intake, which is a spectacular gain over the production cast-iron heads. More air flow tends to make more horsepower. The really good ports actually increase port flow without increasing the port’s cross-sectional area or at least only a minimum increase, like the aluminum Magnum P5153847. While making a better casting is one approach to improved air flow, you can also have a head cnc-ported. CNC-porting can be expensive. Heads designed to be CNC-ported tend to not be usable out-of-the-box but the hi-flow W9 small block head P5007065AB can be used as-is—porting not required, but optional.

The valve gear attaches to the head and basically connects the camshaft to the valves. Most of the production heads oil the valve gear up through the cylinder head from the block, so these passages must be open. In some cases, like the big-port aluminum LA-engine head P5153850, offset rockers are required to allow the pushrods to clear the larger (wider) ports and connect to the rockers. In this case, offset rocker set P5153995 is recommended. In some special cases or for special reasons the cast-in rocker stands used on the LA & B/RB engines can be machined off and then aluminum rocker stands, like P5249049 (LA-engine) are used to hold the rocker shaft/valvetrain in place. Most of the wedge-style heads including the LA-engine, Magnum and B/RB engines return the oil that has been used to lubricate the valves and valvetrain back down past the tappets. The exception is the 426 HEMI which has an oil drain-back hole on each end of the head and must have matching holes in the block and head gasket.

More cylinder heads and part numbers are being added to Mopar’s lineup. For additional applications, please refer to the latest Mopar Performance catalog or visit www.mopar.com Space limits my discussion; if you have specific questions, please call the Mopar Direct Connection Tech Line at 1-888-528-HEMI (4364).