How to Build Racing Engines: Cylinder Heads Guide

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.

Highly refined canted-valve aluminum cylinder heads such as this Pro 1 head from Dart represent the gold standard in commercially available racing cylinder heads. Available in varying degrees of preparation they can be further finished to any degree of precision necessary to meet specific racing requirements.

Highly refined canted-valve aluminum cylinder heads such as this Pro 1 head from Dart represent the gold standard in commercially available racing cylinder heads. Available in varying degrees of preparation they can be further finished to any degree of precision necessary to meet specific racing requirements.

 

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Chevrolet RO7 cylinder heads start with these raw castings and undergo extensive precision machine work to bring them to the level of Cup compe¬tition. In addition to detailed porting measures, the valvetrain is set up with exhausting precision to ensure maxi¬mum durability in the extended high- RPM environment of Sprint Cup racing. (Courtesy General Motors)

Chevrolet RO7 cylinder heads start with these raw castings and undergo extensive precision machine work to bring them to the level of Cup compe¬tition. In addition to detailed porting measures, the valvetrain is set up with exhausting precision to ensure maxi¬mum durability in the extended high- RPM environment of Sprint Cup racing. (Courtesy General Motors)

 

Dart’s 9-degree aluminum small-block Chevy race head illustrates the ideal simplicity of an inline-valve configuration with optimum port placement and compact 38-cc combustion chambers to achieve superior power potential.

Dart’s 9-degree aluminum small-block Chevy race head illustrates the ideal simplicity of an inline-valve configuration with optimum port placement and compact 38-cc combustion chambers to achieve superior power potential.

 

These high-end Dart race cylinder heads incorpo¬rate 50-degree valve-seat angles for enhanced high-RPM flow and exotic copper beryllium seats for optimum combustion chamber sealing.

These high-end Dart race cylinder heads incorpo¬rate 50-degree valve-seat angles for enhanced high-RPM flow and exotic copper beryllium seats for optimum combustion chamber sealing.


 

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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.

Dart Big Chief Pro 1 cylinder heads for big-block Chevys represent top-of-the-line rac¬ing cylinder heads. Enormous 2.400-/1.900-inch valves and large rectangular ports deliver outstanding flow for larger displacement engines with high-horsepower requirements.

Dart Big Chief Pro 1 cylinder heads for big-block Chevys represent top-of-the-line rac¬ing cylinder heads. Enormous 2.400-/1.900-inch valves and large rectangular ports deliver outstanding flow for larger displacement engines with high-horsepower requirements.

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.

Note that the machining marks inside this ported Dart cylin¬der head are perpendicular to the direction of airflow. This creates small rolling vortices along the flow path boundary layer. These vortices act like miniature roller bearings to encourage smooth flow without boundary-layer drag. (Courtesy Dart Machinery)

Note that the machining marks inside this ported Dart cylin¬der head are perpendicular to the direction of airflow. This creates small rolling vortices along the flow path boundary layer. These vortices act like miniature roller bearings to encourage smooth flow without boundary-layer drag. (Courtesy Dart Machinery)

 

Big Chief CNC full-port versions illus¬trate the extreme degree of detailed preparation required in a racing cyl¬inder head. Ports and chambers are fully CNC’d and the valveguide tapers are shaped and positioned to opti¬mize flow into and out of the combus¬tion chambers.

Big Chief CNC full-port versions illus¬trate the extreme degree of detailed preparation required in a racing cyl¬inder head. Ports and chambers are fully CNC’d and the valveguide tapers are shaped and positioned to opti¬mize flow into and out of the combus¬tion chambers.

 

Titanium valves with optimized seat angles and a fully CNC’d combustion chamber are the recipe for high-combustion efficiency as long as the ports and flow paths are optimized to the target application.

Titanium valves with optimized seat angles and a fully CNC’d combustion chamber are the recipe for high-combustion efficiency as long as the ports and flow paths are optimized to the target application.

 

CNC-ported small-block head shows the perpendicular porting troughs that encourage efficient boundary-layer flow. Note the aerodynamically tapered valveguides shaped to mini-mize flow restriction.

CNC-ported small-block head shows the perpendicular porting troughs that encourage efficient boundary-layer flow. Note the aerodynamically tapered valveguides shaped to mini-mize flow restriction.

 

This open intake valve shows the generous valve curtain, or flow win¬dow. Canted valve angles open the valves away from the cylinder walls to provide good flow with minimal valve shrouding.

This open intake valve shows the generous valve curtain, or flow win¬dow. Canted valve angles open the valves away from the cylinder walls to provide good flow with minimal valve shrouding.

 

SuperMod ported, spread-port big-block Dart head illustrates basic port matching in the first 1/2 inch of the intake ports. Ports remain as cast, but chambers are fully CNC’d to encour¬age combustion efficiency.

SuperMod ported, spread-port big-block Dart head illustrates basic port matching in the first 1/2 inch of the intake ports. Ports remain as cast, but chambers are fully CNC’d to encour¬age combustion efficiency.

 

 

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Titanium valves are essen¬tial for all high-RPM envi¬ronments. Their strength and light weight support impossibly high engine speeds without failure. Their minimal mass less¬ens valvespring require¬ments and reduces stress to related val¬vetrain com¬ponents.

Titanium valves are essen¬tial for all high-RPM envi¬ronments. Their strength and light weight support impossibly high engine speeds without failure. Their minimal mass less¬ens valvespring require¬ments and reduces stress to related val¬vetrain com¬ponents.

 

Dart’s oval-port Big Chief heads with full port CNC treatment are designed to enhance mid-range torque with¬out sacrificing top-end power. Note the carefully blended and tapered valveguide.

Dart’s oval-port Big Chief heads with full port CNC treatment are designed to enhance mid-range torque with¬out sacrificing top-end power. Note the carefully blended and tapered valveguide.

 

Here’s an intake port on a Big Chief Pro 1 head. Even in unported condi¬tion it offers strong performance potential. Minor sportsman-style port¬ing can be applied with good results even by amateur home porters.

Here’s an intake port on a Big Chief Pro 1 head. Even in unported condi¬tion it offers strong performance potential. Minor sportsman-style port¬ing can be applied with good results even by amateur home porters.

 

Valvetrain accuracy and stability are essential to take full advantage of well matched high-flowing cylinder heads. Valve gear like T&D Machine Products’ shaft rockers stabi¬lize the valvetrain and ensure the accuracy of valve timing in every cylinder. These rockers are the first line of defense against high-RPM engine damage.

Valvetrain accuracy and stability are essential to take full advantage of well matched high-flowing cylinder heads. Valve gear like T&D Machine Products’ shaft rockers stabi¬lize the valvetrain and ensure the accuracy of valve timing in every cylinder. These rockers are the first line of defense against high-RPM engine damage.

 

A full-race T&D offset rocker shows the amount of offset from pushrod cup to roller tip. These rockers are light¬ened on the top and they can be drilled for internal spring oilers for a small increase in price.

A full-race T&D offset rocker shows the amount of offset from pushrod cup to roller tip. These rockers are light¬ened on the top and they can be drilled for internal spring oilers for a small increase in price.

 

Unported big-block Chevy exhaust port exhibits good efficiency as cast, but it really wakes up when treated to full-port CNC machining. The port size and shape are good, but the sur¬face texture does little to support the effective evacuation of exhaust gases.

Unported big-block Chevy exhaust port exhibits good efficiency as cast, but it really wakes up when treated to full-port CNC machining. The port size and shape are good, but the sur¬face texture does little to support the effective evacuation of exhaust gases.

 

Here’s a fully ported small-block exhaust port. Note the contoured and tapered valveguide boss and the polished surface texture. Higher-pres¬sure exhaust gases don’t necessarily need the assistance of CNC-generated vortices to exit the port effectively.

Here’s a fully ported small-block exhaust port. Note the contoured and tapered valveguide boss and the polished surface texture. Higher-pres¬sure exhaust gases don’t necessarily need the assistance of CNC-generated vortices to exit the port effectively.

 

T&D Machine’s rockershaft sys¬tem for Dart Big Chief heads has three different rocker arms; left offset, right offset, and straight. Each one uses its own unique rocker stand and shaft and they all use assorted shim sizes to set the proper height.

T&D Machine’s rockershaft sys¬tem for Dart Big Chief heads has three different rocker arms; left offset, right offset, and straight. Each one uses its own unique rocker stand and shaft and they all use assorted shim sizes to set the proper height.

 

Here’s another view of the diffferent rocker offsets nec¬essary to obtain good valvetrain geometry and pushrod alignment on a big-block Chevy. Note that the left and right offsets are not the same.

Here’s another view of the diffferent rocker offsets nec¬essary to obtain good valvetrain geometry and pushrod alignment on a big-block Chevy. Note that the left and right offsets are not the same.

 

CC’ing the combustion chambers is an important step in race engine preparation. Today’s CNC-machined chambers are very consistent in shape and volume, but a good builder always checks everything.

CC’ing the combustion chambers is an important step in race engine preparation. Today’s CNC-machined chambers are very consistent in shape and volume, but a good builder always checks everything.

 

When using CNC-ported heads from top manufacturers you can be pretty sure the port volumes are close to advertised, but a good engine builder always checks every port.

When using CNC-ported heads from top manufacturers you can be pretty sure the port volumes are close to advertised, but a good engine builder always checks every port.

 

When cc’ing a chamber, it is important not to let any of the sealing grease press out into the chamber where it can alter the volume. This plate is perfectly sealed with the right amount of grease.

When cc’ing a chamber, it is important not to let any of the sealing grease press out into the chamber where it can alter the volume. This plate is perfectly sealed with the right amount of grease.

 

Most builders have a favor¬ite gasket for the engines they build. This Fel Pro head gasket illustrates how the gasket bore is typically larger than the cylinder bore with an irregular shape around the valves.

Most builders have a favor¬ite gasket for the engines they build. This Fel Pro head gasket illustrates how the gasket bore is typically larger than the cylinder bore with an irregular shape around the valves.

 

Smoothed or polished ports on the exhaust side are beneficial to performance. There are no fuel-separation issues on the exhaust side. You simply want to provide the best possible exit path for the exhaust gases which are still under pressure.

Smoothed or polished ports on the exhaust side are beneficial to performance. There are no fuel-separation issues on the exhaust side. You simply want to provide the best possible exit path for the exhaust gases which are still under pressure.

 

Here’s a typical CNC-prepped small-block combustion chamber with bowl blending. You can just see part of the contoured intake valveguide in this picture.

Here’s a typical CNC-prepped small-block combustion chamber with bowl blending. You can just see part of the contoured intake valveguide in this picture.

 

This is an as-cast Dart Pro 1 small-block combustion chamber with streamlined intake valveguide boss.

This is an as-cast Dart Pro 1 small-block combustion chamber with streamlined intake valveguide boss.

 

This is the as-cast Pro 1 intake ports with no modifications.

This is the as-cast Pro 1 intake ports with no modifications.

 

This Little Chief spread-port head shows similar porting and guide blending. The port to the right has a drilled and tapped hole in the roof for the rocker arm stud. Each port must be checked to ensure that threaded bolts or studs do not protrude into the port.

This Little Chief spread-port head shows similar porting and guide blending. The port to the right has a drilled and tapped hole in the roof for the rocker arm stud. Each port must be checked to ensure that threaded bolts or studs do not protrude into the port.

 

Here’s the same Pro 1 chamber after full CNC porting. Note the precise valveguide blending and multi-angle valve job.

Here’s the same Pro 1 chamber after full CNC porting. Note the precise valveguide blending and multi-angle valve job.

 

Here’s the same Pro 1 intake ports with full CNC porting. Note how the porting cuts are all perpendicular to the direc¬tion of flow to provide a roller bearing effect that keeps the air flowing smoothly while resisting fuel separation.

Here’s the same Pro 1 intake ports with full CNC porting. Note how the porting cuts are all perpendicular to the direc¬tion of flow to provide a roller bearing effect that keeps the air flowing smoothly while resisting fuel separation.

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

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