Fire in the Hole
How combustion chamber design relates to engine performance

By Ray T. Bohacz
Photography: Ray T. Bohacz

Most aspects of a cylinder head are relatively straightforward and easily grasped. Valve size, deck thickness and combustion chamber volume are often topics of discussion and qualify the design's ability to be a player. But there is much more to be considered. The characteristics of the combustion chamber will dictate the engine's power, octane tolerance and brake specific fuel consumption (BSFC). So if you thought the shape of the combustion chamber had little to do with a cylinder head's performance, you will be surprised.

Author's note: Due to the nature of this story, references to cylinder heads from manufacturers other than Pontiac are used to demonstrate different combustion chamber technologies.

Basic Combustion
The flame from a candle, a simple form, has a key element of combustion related to that of an engine. But taking place in the atmosphere, it differs from that of an engine where the gas exchange process occurs internally under higher than atmospheric pressure. A flame can have two distinct regions: pre-mixed and diffusion. A burning candle experiences a diffusion flame because it occurs at the interface between the fuel and the oxidant. With a candle, the fuel is melted and evaporated by the radiant heat of the flame and then oxidized by the air.

A more complex example of combustion is a Bunsen burner which has both pre-mixed and diffusion flames. It consists of an air regulator, fuel source and a cylindrical tube. The flame generated nearest the base is identified as pre-mixed. The air entering at the base of the Bunsen burner is not sufficient for complete combustion. Consequently, a second flame front above that point is established at the interface where the air is diffusing into the unburned fuel. This is responsible for the Bunsen burner's flame-within-a-flame appearance.

Even though the combustion event inside an engine is quite a bit more complicated, the basics still hold true. Gasoline, a hydrocarbon-based fuel, needs to be atomized and emulsified (broken down into small particles and mixed with air) to burn. It will not burn by itself in liquid form. When atomized, gasoline has a laminar burning velocity of approximately 0.5 meter/second (m/s) or 1.64 feet/second. As a comparison, acetylene mixed with air burns at a rate of 1.58 m/s or 5.18 feet/second. The slow laminar burning speed of gasoline poses an interesting problem when used as a fuel for an internal combustion engine.

Since this is best represented using metric measurements, ignore the dimensions but accept the concept. Given a cylinder with a 100 mm diameter and an ideal central location for ignition, the time for a gasoline-fueled flame to travel this distance is 100 milliseconds. The problem is that when an engine of this dimension is running at 3000 rpm, there is only a window of 10 milliseconds for the combustion event to take place. Obviously another force must be at work because we all know that a gasoline engine can operate at speeds substantially higher than 3000 rpm. The key is to increase the burn velocity.

It has been established that the flame in an engine travels across the bore at a rate of 10-25 m/s. This is substantially faster than the velocity stated earlier, but it is the reason why gasoline can be used as a motor fuel. To increase burn velocity, turbulence needs to be introduced to the combustion event. In an engine, this is accomplished by the induction and compression process along with the design of the combustion chamber. During pre-mixed combustion, the effect of the turbulence is to break up or wrinkle the flame front, creating burnt gases in the unburned region, and vice versa. This effectively increases the flame front area and speeds up combustion. Though diffusion is usually associated with a compression ignition engine, better known as the diesel, it can also occur in a spark ignition engine when stratified charged. The fuel would be injected in a fine spray and the turbulent air motion would sweep away the vaporized fuel and combustion products from the fuel droplets, speeding up the burn velocity.

The actual combustion event that causes a flame front to be established and expand against the piston is very complex. At this level a full grasp of the chemistry involved is not required, but the laws of thermodynamics, the study of energy and its transformations need to be touched upon briefly. Consisting of two statements deemed laws, the first says that energy cannot be consumed or destroyed; only its state can change. In simpler terms this can be applied to an engine and how energy is turned into heat, then motion, and back to heat. The second law is more complex but can be summarized thus: energy follows certain guidelines and never deviates. For example, heat will travel only from hot to cold without an external energy force being present. The laws of thermodynamics apply to a combustion chamber directly due to thermal transfer into the casting and engine coolant, along with the effect that the compression ratio has on thermal efficiency.

A common analogy that compares an engine to an air pump establishes the fact that the more air pumped, the greater the output. This cannot be denied but it's a one-dimensional statement ignoring the fact that without an efficient combustion event, the air by itself can do nothing. For this reason we need to examine the impact the combustion chamber has on an engine.

The Combustion Chamber
In 1673, Christian Huygen, King Louis XIV's water keeper, invented the first engine. It was developed as a better means to transport water from the Seine River to the grounds and gardens of the Palace of Versailles. This gunpowder-consuming single-cylinder external-combustion behemoth was welcomed by the peasants and oxen that were used as water carriers prior to its existence. As the internal combustion engine slowly progressed from these humble roots it was discovered that efficiency and power could be increased with a controlled process in a closed environment. The early combustion chambers were simply little more than covers for the cylinder. A major breakthrough in combustion chamber design was realized by Ricardo, who invented the turbulent cylinder head for a side-valve motor. Taking place in the early 1900s, it set new standards for compression ratio, running at 6.00:1. At that time, the fuel had an octane rating of only 60 to 70. Over the next decades the impact the combustion chamber had on the character of the engine was accepted and explored. A major breakthrough occurred in 1951 when the Chrysler Corporation introduced its hemispherical combustion chamber on its 331-cubic-inch V8. Today, combustion chamber design and technology are constantly evolving and producing smaller, higher specific-output, fuel-efficient engines.

Criteria that drive combustion chamber development involve many facets. The distance that the flame front needs to travel should be minimized. It can be accomplished by reducing the distance from the spark plug electrode to the incoming charge, called the end gas. This allows for higher potential engine speeds, which will produce more power. In addition, there is less time for something to go wrong. Abnormal combustion, better known as detonation, is more likely with a slow combustion process since it allows time for an additional flame front to start.

Each spark plug should be centrally located in the bore and nearest the exhaust valve because these are the most turbulent and hottest parts of the combustion chamber, respectively. Additionally, the exhaust valve should be as far from the intake valve as possible, limiting thermal transfer to the fresh incoming charge.

There needs to be sufficient turbulence to promote rapid combustion, but too much can create an issue, transferring heat away from the chamber and promoting noisy combustion. This turbulence is generated by design and can be induced either externally in the intake port, or internally using squish pads. The clearance between the deck of the cylinder head and the piston is identified as the squish region. It acts to cool the intake valve and is best located near it.

Valvetrain design and the number of valves impact the combustion chamber with concerns for valve placement, size and actuation. As you gain knowledge in this area you will see that many Pontiac engines have very poorly designed combustion chambers due to economic pressures. Another case of greed by corporations and the power of the bean counters and stockholders. No traditional domestic pushrod V8 engine with inline valve placement other than the old Chrysler Hemi allows for a central spark plug. What is often done though is to design the chamber to use a long-reach plug that places the electrode tip near the center, even though the entry point is at the perimeter. The General Motors LS1 and L31 Vortec castings are excellent examples of this method. Disappointingly, many cylinder heads place the spark plug electrode at the perimeter of the bore, and some early Pontiac V8s actually had a bias toward the intake valve, allowing function to take a back seat to ease of manufacture.

With the laws of thermodynamics presented, the ideal engine would have a high compression ratio for thermal efficiency and throttle response but would need to work in unison with a combustion chamber that has a fast burn rate. This is essential to increase the engine's octane tolerance and limit the production of the emission oxides of nitrogen (NOx). This poisonous gas is responsible for photochemical smog and has been the impetus for enhanced emissions testing such as I/M 240. It requires three elements to be produced: heat, pressure and exposure time. A high compression ratio increases the production of NOx by its elevated cylinder pressure and the heating of the charge as it is forced into a smaller region. This phenomenon can be cheated by the implementation of a fast burn rate, eliminating the third element, exposure time, in the recipe for NOx. The best production example to date for a balance between octane tolerance and high compression ratio with quick burn rates is the May Fireball combustion chamber, produced by Jaguar in 1982, which allowed 11.0:1 compression ratio on 87-octane fuel.

Other factors that come into play are the material used and the spark plug location. As mentioned previously, the start of the flame in the center of the bore allows for a quicker more even burn that translates into higher cylinder pressure in less degrees of the crankshaft's rotation past TDC. Looking beyond emissions and octane tolerance to produce power, it's necessary to have the cylinder pressure rise as quickly as possible, allowing it to be used to expand against the piston for as much of the stroke as possible.

Most performance aftermarket cylinder heads are aluminum castings due to their light weight and ease of porting and manufacturing, along with the ability to dissipate more heat and allow a higher compression ratio. But it is often overlooked that it's easier to produce power with a cast-iron head if all factors of design are the same, due to its superior thermal efficiency. When switching from iron to aluminum the engine will require about one additional point in compression ratio to maintain the same thermal efficiency. This is due to the cast iron's ability to hold heat and use it to expand against the piston.

Of great concern to the combustion engineer but never mentioned in the aftermarket is the surface-to-volume ratio. This minimizes heat loss into the casting and water jacket of the cylinder head along with reducing hydrocarbon production. It is desirable to have a surface area as small as possible, relative to the volume occupied by the chamber. It can be derived with the following calculation:

surface-to-volume ratio =surface area/volume of chamber

Hydrocarbon emissions are created due to the outer layers of the mixture being cooled in the region of the chamber walls of designs with high numeric ratios. The flame cools as it approaches the chamber wall, extinguishing and leaving a layer of hydrocarbons behind. The hemispherical combustion chamber offers the best surface-to-volume ratio and tests conducted by Chrysler in 1950 showed that to match the thermal efficiency of a 7.00:1 compression ratio Hemi engine, its previous combustion chamber would need 10.0:1 at 1200 rpm, 9.4:1 at 2000 rpm, 8.9:1 at 2800 rpm and 8.5:1 at 3600 rpm. The required compression ratio drops as engine speed increases due to gains in volumetric efficiency at higher piston speeds.

Types of Combustion Chambers
Most of us are familiar with the terms open and closed when referring to a combustion chamber, a term popularized by Chevrolet with its big-block engine series. In fact, most times Pontiac cylinder heads are classified as "closed" chamber up to 1967 and "open" from 1968 forward. There has been much confusion over these designations. In fact, they are not even engineering jargon to identify a combustion chamber, and are arbitrarily used to describe the squish-to-bore-area relationship. A combustion chamber is nothing more than a cavity in the cylinder head casting, with the exception being the bowl-in-piston designs used in many diesel engines. The relationship to the area of the bore that is consumed by the combustion chamber quantifies whether a chamber is opened or closed. The easiest method to determine this is to place the proper head gasket on the deck of the cylinder head to orient the bore position. If a large amount of the deck surface of the head is exposed to the bore, the chamber can be considered closed. The portion of the head's deck that is outside the combustion chamber but exposed to the bore is used as a squish region. Its function is to create internal charge acceleration that stimulates the end gas and increases the burn velocity as it rushes to escape this area as the piston sweeps toward TDC. It is considered internal charge acceleration because it's created in the bore.

To properly identify a combustion chamber, all its aspects including shape need to be considered. For our purposes we limit the discussion to those found on most production engines in America.

Hemispherical or Pentroof
A chamber of this design is considered to offer the least amount of compromise for the efficiency gained. The valves are placed at the bore perimeter and, in the instances of the original Chrysler Hemi, at an included angle of 58.5* from the crankshaft centerline. This position also allows for huge airflow gains since it moves the valve away from the wall and unshrouds quickly. This creates a more efficient cross-flow movement of the charge during overlap and limits thermal transfer from the exhaust valve to the fresh charge. As mentioned previously, this design offers the best surface-to-volume ratio and also creates a very short direct exhaust port, essential in limiting heat rejection into the coolant. Having a central spark plug, the Hemi offers excellent octane tolerance. At the perimeter of the bore across from the valves are small squish pads to help move the end gas over to the spark plug and increase burn speeds. With pushrod designs, the valve placement requires dual rocker shafts but lends itself very well to dual OHC configurations. An additional benefit is the distance between the intake and exhaust valves, which further limits heat transfer. The incoming charge also generates a high rate of tumble.

Mickey Thompson experimented with Hemi heads on Pontiacs in the 1960s and you'll recall that the division designed an experimental aluminum Pontiac Hemi engine, reported on in the March 2002 issue of HPP.

Wedge
Used over the years by almost every manufacturer including Pontiac, this chamber resembles an inclined bathtub recessed into the deck of the head. Inline valves are normally tilted to accommodate the sloping roof of this design. The spark plug is located on the thick side of the wedge and is usually positioned midway between the valves. The inherent steep walls work to mask the air/fuel flow path and deflect and force it to move in a downward spiral around the cylinder axis. During the compression stroke, the squish area reduces to such an extent that the trapped mixture is violently thrust from the thin to the thick end of the chamber.

Bathtub or Heart-shaped
The bathtub designation is generally reserved for any chamber that's not a wedge or hemispherical. Most domestic engines of pushrod design have used it in varying forms. In some instances the shape of the combustion chamber was almost oval, with the latest trends being the efficient heart shape. An example of this would be the current L-31 Vortec, LT1, LT4 and LS1, all by Chevrolet. The deck of the cylinder head that overlaps the piston forms two squish regions: a large area across from the spark plug and a smaller region on the opposite side. Its crescent shape has nicknamed it the heart chamber. The valves are inline and are partially masked by the chamber wall being more exposed on the plug side. The area across from the major squish region is generally tapered and does not have the steep wall of a wedge style. Spark plug location is maximized by biasing toward the exhaust valve and as central as possible, working under these limitations. Heat transfer from the close proximity of the valves limits volumetric efficiency and octane tolerance.

Bowl in Piston
To the best of HPP's knowledge, this style has not been utilized by Detroit on a gasoline engine in the last fifty years but is common in Europe. It consists of a flat cylinder head deck with a single row of valves facing a circular cavity cast into the piston. An annular squish region is created around the piston perimeter. Known for very turbulent combustion, it works well for diesel engines but was deemed excessively noisy for American standards.

Making Sense of It All
Since we don't have the means to create our own cylinder head, we're forced to work with what is available. The theory of combustion chamber design and function was touched on only briefly here; many have spent their entire lives studying this with new discoveries each day. Our reasoning was to establish that more than flow numbers need to be considered when choosing a Pontiac cylinder head. How the combustion chamber uses the airflow is just as important as the flow value itself. Even the worst combustion chamber design can be improved upon by smoothing the walls and surface of the chamber to increase flame speeds, reducing the volume of the squish region with a zero deck or thinner head gasket, and indexing the spark plug. The worth of these simple tricks is diminished, as the design of the chamber becomes better, but should not be forgotten.

Airflow numbers are easily obtained on a test bench but a trained eye is needed to identify a more efficient combustion chamber. A good rule is to query the manufacturer on the amount of spark advance his cylinder head would require with your combination. The more lead it needs, the greater the propensity for detonation and the slower the burn speed.

Head Games
Here is the evolution of the combustion chamber for high performance Pontiac engines. Special thanks to Jim Taylor and Mark Erney for their assistance in obtaining this photography.

As you can see, the "basic" design of Pontiac's fully machined combustion chamber was little changed over its history. It is actually a combination of the wedge and bathtub style. However, its size and the valve placement were modified as needed. The early 716 Tri-Power heads shown feature 1.92/1.66 valves and are referred to as "closed chamber" in the hobby. In 1967, as shown on the 670 heads, the "closed chamber" remained but the valve inclination was changed from 20* to 14*, which provided space for larger 2.11/1.77 valves. The chamber was also relieved on the intake side.

For 1968, the chambers were opened up, reducing the shrouding of the valves. In mid 1968, the round-port Ram Air II heads debuted and as you can see, the chamber shape was subtly changed as well as compared to D-port heads. It was done by opening up the area around the valves on the spark plug side and adding a small scallop above the plug hole. The same holds true for the 1969-70 Ram Air-IV heads and the HO and SDs of the '70s.

A special treat is to see a Ram Air V head. The chamber is exclusive to the Ram Air V and closely resembles that of a Tunnel Port Ford head of the era. Valve sizes would be a whopping 2.19/1.73.

Chamber size varied by year and application, with early 400 heads using a 71-72 cc chamber in most cases; some chambers were as small as 67 ccs. The 1971 and later heads exhibit a much larger chamber to reduce compression with the 96 400 head featuring a 96 cc volume and the 455 HO heads featuring large 111 cc chambers. The 1976 6X 400 head reveals the smaller 1.66 exhaust valve that returned in 1973 on all D-Port 400 and 455 4-barrel heads. Its chamber size can range from 95 ccs to 101 depending upon application, to provide a compression ratio as low as 7.6:1 in 350s and 400s.

The LS6 head displays the current thinking at GM regarding combustion chambers. Note the differences between this chamber and the vintage Pontiacs, as discussed in the text.--Thomas A. DeMauro