Race-prepared cylinder heads are the most important component in a racing engine. Most of the power an engine makes comes from the cylinder heads and their ability to fill the cylinders and evacuate them efficiently. Not surprisingly, a substantial portion of a racing engine budget goes to cylinder heads to ensure a competitive build. The difference between short blocks assembled by a range of competent engine builders is relatively small, but the builder with the superior cylinder heads always excels, often to a considerable degree.
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There are hundreds of different sizes and configurations, each designed to meet very specific requirements dictated by their intended competition environment. The shape and specific dimensions of individual ports and combustion chambers exerts enormous influence on the shape and positioning of the torque curve and the overall powerband of the engine.
If maximum power spread across an application-specific powerband is the primary goal of competition engine building, cylinder heads have the greatest influence on the VE and cylinder filling ability that make this possible. Although the camshaft commands the precise timing of flow path events, the cylinder head flow passages (ports and valves) manage the rate and volume of flow into the engine based on engine speed and piston position as dictated by stroke length and rod length.
All of these components work in close concert and must be appropriately matched to ensure compatibility, optimum cylinder filling, and combustion efficiency. Incompatible components anywhere in the system not only fail to deliver the anticipated level of performance, they also restrict the optimum performance of other components operating within the flow path environment.
The cylinder head’s intake and exhaust flow paths lead to and from the combustion space (chamber) where air and fuel are processed into power. In many ways the cylinder head is the heart of the matter. It provides the all-important combustion space where energy is harnessed from the reaction of the air/fuel mixture and it provides the valves and flow paths that escort the air/fuel mixture into the combustion chamber and usher it out via the exhaust system after the magic happens. How all of this is coordinated, managed, and properly tuned for high VE is largely a matter of sizing the ports and valves to suit the application.
To fully appreciate the dynamics of gas exchange it is logical to start by examining the combustion space for the critical keys to its function. For the purpose of discussion, think of the combustion space as having a roof (chamber with valves), a floor (piston top), and walls (the cylinder). Within this high-speed environment, the roof and floor approach each other rapidly as many as hundreds of times per minute. This motion exerts a profound influence on the fuel mixture as it enters the combustion space, combusts, expands, and exits at a very high rate. Since the spark ignition is initiated a selected distance before TDC, the piston (floor) is still approaching the roof with rising pressure creating negative work.
Turbulence driven by rising piston motion and quench against the roof drives the denser fuel charge toward the spark plug where it ignites with gusto. In the best cases the flame expands smoothly depending on mixture homogeneity, piston area, and the effects of any obstructions such as compression domes. The higher the mixture quality, the better the burn, resulting in smooth combustion. Most combustion chambers are designed to drive the advancing flame forward toward the exhaust valve, hastening the burn and improving the exhaust cycle, which often increases power.
In recent years combustion chambers have grown smaller and specific shapes have been refined to encourage swirl and tumble within the fuel mixture as it enters the cylinder through the intake valve. Swirl is a rotational motion of the incoming fuel mixture that tends to assume a circular path defined by the cylinder walls. Tumble is a similar but vertical motion where the mixture tumbles into the cylinder in a waterfall effect.
The mixture-enhancing qualities of swirl and tumble encourage power improvements even though they tend to restrict net airflow to some degree. Along with advances in port design, spark plug placement, valve design and placement, and CNC machining processes, major cylinder head mods are no longer undertaken by many builders. The modern dilemma is to choose the ideal cylinder head from the vast selection offered by specialized cylinder head manufacturers.
With application specifics in mind builders tend to evaluate cylinder heads (when not specified or restricted by rules) based on mid-range and beyond flow numbers, port turbulence, chamber shape and efficiency, valve size and angle, spark plug placement, port shape, size and position, and other critical attributes that affect power production. Nearly all modern race heads now incorporate optimized valve placement, thicker deck surfaces, optimized cooling jackets, and better oil control.
It is commonly recommended to use the smallest ports and valves that support the power level you desire. Smaller ports with higher flow velocities offer superior flow and cylinder filling qualities accompanied by the more desirable mixture qualities that promote superior power. These qualities support strong low- and mid-range power even as they encounter a power ceiling based on the head’s flow capacity. Larger ports and valves are superior at high RPM and when suitably cammed, they create big power upstairs. There is a critical balance that must be struck for every racing application and experienced engine builders routinely probe the limits while trying not to exceed the desirable flow velocities generated by smaller ports and the enhanced mixture qualities that promote good power.
Choosing a Cylinder Head
With the exception of Stock Eliminator drag racing and various lower level circle track applications, most racing engines use aluminum heads. Some applications are restricted to the stock-type iron heads and valve sizes with no porting allowed, but most applications allow aluminum heads with a variety of valve sizes and porting configurations. When selecting a cylinder head for a particular application it is important to closely match the desired RPM range based on port dimensions that include port length and cross section and the mid- and high-range flow rates.
The primary factors affecting cylinder head flow rate include valve size, port shape and dimensions, the bend radius before the valve, and the surface texture of the port surfaces. This is particularly true of the port floor and the port roof to prevent fuel separation. Inertia tends to push the bulk of the fuel mixture above the port centerline toward the roof where the runner makes the turn to the valve. The bend radius influences flow rates considerably and it is one reason high port heads with shallower valve angles have evolved.
Most manufacturers publish valve sizes and flow figures for their cylinder heads and some provide port volumes. The best ones also provide port dimensions including height, width, cross-sectional area, and port location. Port length along the centerline of the port is also useful, but few provide it.
The more of this information you have, the easier it is to pinpoint the ideal head for your combination. Flow figures help determine the head’s ability to fill and evacuate the cylinders efficiently. Port dimensions help calculate the torque peak and the spread of the powerband. This information is particularly useful if you are brainstorming combinations on one of the many engine simulation programs available for your PC.
Valves and Valve Sizes
Valves are a necessary evil in a racing engine. They represent a variable-flow restriction that must be dealt with to effectively feed the engine. Bigger valves are generally better for obvious reasons, but valve size is always limited by the bore size. Under naturally aspirated conditions, intake mixtures must find their own way into the cylinders at relatively low pressure, hence bigger valves are required on the intake side. Exhaust gases are discharged under higher pressures so smaller valves can be used.
In most racing circles, the intake valve is typically about 52 percent of the applicable bore size, while exhaust valves are generally about 72 to 76 percent of the intake valve diameter. These sizes are not set in stone, but they yield an effective combination in most high-speed, two-valve-per-cylinder applications.
Part of the complication with valves is shrouding caused by proximity to cylinder walls and some deeper combustion chambers. Shrouding presents a barrier that closes off part of the valve curtain or flow window. It reduces net flow, redirects the optimum flow path, and often influences the combustion process with negative results. This is the main reason that many successful racing heads use canted or splayed valves that move away from the cylinder walls as they open. Normally, the largest valve that fits works best, but in the case of severe shrouding, smaller valves may flow better because they are farther from the cylinder wall and present less restriction.
Valves are also somewhat delicate compared to most other components. It is very easy to burn or otherwise damage a valve to the point where the head breaks off and you lose an engine. Show them plenty of respect in order to avoid more serious consequences.
Valve seats must be perfectly concentric to seal properly and the valvetrain must be configured to minimize the severity with which it opens and closes the valves, avoiding valve float at all cost. Valve angles are critical to maximizing performance. Many race heads use a radius seat or up to five different angles on each seat. While 45-degree seats are the norm, Pro/Stock–type applications now use 50- to 55-degree seat angles to take advantage of their superior airflow characteristics at higher engine speeds.
Valve weight is another important concern in high-speed engines. RPM potential and overall valvetrain dynamics are largely influenced by the mass of the valve and what it takes to open and close it with a perfect seal at very high engine speeds. Lightweight titanium valves are standard equipment in all serious racing engines except those applications where they are not permitted.
Cylinder heads are all about using ports, valves, and combustion chambers to manage and optimize airflow through the engine. To achieve the highest degree of success the cylinder heads must be carefully tailored to match the specific requirements of the desired powerband and its associated racing application. This is not difficult to achieve with the broad range of racing cylinder heads available today, but it is expensive. As a rule, top-performing cylinder heads are almost always the most expensive component of any superior racing engine.
Written by John Baechtel and Posted with Permission of CarTechBooks