When discussing cylinder heads, lots of technical terms are thrown about. You can’t really understand heads until you know what these terms mean. When you do, however, you’ll probably find that some of the people who use them with apparent confidence aren’t entirely sure of their meaning. But pay attention and you will be.

First published in the March 2012 issue of Street Machine

DECK FACE

The flat area of the cylinder head that bolts to the top of the cylinder block, which is called the block deck.

COMBUSTION CHAMBER

Looking at the face of the cylinder head, the combustion chamber is the hollowed out area. In typical engines (see OHV & Valve In Block) this is where the valves and sparkplugs are located.

EXHAUST & INLET PORTS

The inlet port is the passage that runs from the inlet manifold, into the combustion chamber via the inlet valve. The exhaust port is the passage that runs from the exhaust valve and out into the exhaust system. The cross-sectional area and shape of a port at any given point along its length is vital in determining how much air/fuel mixture it will allow into an engine.

OHV & VALVE IN BLOCK

The vast majority of classic engines use an overhead valve (OHV) configuration. Basically, OHV means pushrod-operated valves and this is the only type of engine we’re referring to in this article.

However, in some early engines, the valves were set upside-down in the deck of the block, next to the bore, so only the sparkplug is positioned in the head. In these engines, the valve tips bear directly on the lifters — there are no pushrods or rockers. They’re referred to as valve-in-block engines, side valves (the valves sit beside the bore) or flatheads.

The first V8, Ford’s flathead, is the best-known example of this configuration.

SPARKPLUG LOCATION

It’s desirable to position the sparkplug as close to the centre of the chamber as possible, as this creates the shortest possible flame path between the ’plug and the edge of the chamber. It’s also advantageous to point the ’plug at the exhaust valve as this initiates burning in the hottest region of the chamber, reducing the chance of spontaneous ignition.

FLAME FRONT

Combustion is a chemical reaction that begins at the spark and progressively spreads through the air/fuel mixture as a rapidly expanding flame front. This happens very rapidly but it’s much slower than an explosion — explosive expansion is extremely undesirable as it creates destructive pressure waves.

PRE-IGNITION

Sharp edges and poorly selected sparkplugs may create hot spots inside a cylinder that can prematurely ignite the air/fuel mixture. Ignition then creates a second flame front which moves toward (and reacts with) the premature front, creating severe pressure waves.

SPARK-KNOCK

Overly advanced ignition timing will begin burning the mixture too early. Cylinder pressure then rises prematurely, resulting in spontaneous ignition in the unburned mixture and damaging pressure waves.

DETONATION

Pre-ignition and spark-knock are often mistakenly referred to as detonation but this is something quite different. Detonation occurs when conditions in the chamber cause unburned mixture to explode after the spark initiates burning, resulting in destructive pressure waves.

SWIRL

In a two-valve engine, the inlet valve is closer to one side of the bore than the other. Mixture entering the chamber contacts the side of the bore and is turned, creating a spiral motion known as swirl. This turbulence promotes faster flame travel, improving efficiency and power. In four-valve heads, the swirl on one side of the bore is cancelled by that on the opposite side. This turns the mixture downwards in a somersaulting motion described as ‘tumble’. Tumble is at least as beneficial as swirl, maybe even more so.

CLEARANCE VOLUME

This is the space (volume) remaining when the piston is at the top of its travel (TDC). It can be calculated by adding up the volume of the combustion chamber, head gasket thickness and the volume of the piston crown. Note that piston crown volume can be positive (dish top) or negative (pop-top) and it includes the area above the top ring between the piston and the bore. Swept volume is the volume uncovered by the piston at the very bottom of its travel (BDC).

COMPRESSION RATIO

Calculated by taking the total volume with the piston at BDC (ie, clearance volume plus swept volume) and dividing it by the clearance volume. Typical production cars run from 6.0:1 to around 10.0:1. High performance street engines can go as high as 13.0:1.

FACING/DECKING

Performed on the block as well as the cylinder head, it’s the process of machining the mating surfaces to make them perfectly flat for optimal head gasket seal. Decking the head reduces combustion chamber volume, thus increasing compression. In extreme applications the heads are machined at an angle to change the valve angle and chamber volume.

LAID-BACK CHAMBER/SHROUDING

When flow past one side of a valve is impeded by the side of the combustion chamber, the valve is said to be shrouded. In more efficient chambers, the area around much of the valve is shallower — laid back — which reduces shrouding. High performance chambers can be so smoothly blended that the transition from the sides to the roof can become indistinguishable.

VALVE SEAT ANGLE

To make the valve seal, the valve head features a 45-degree cut. Valve spring pressure pulls this into a matching 45-degree cut in the valve seat. Flow is assisted by adding a 70deg back cut (towards the stem) and a 30deg front cut. This is the basic three-angle valve job. High-performance cylinder heads may feature four, five and even six-angle valve jobs.

23, 18, 15, 12-DEGREE ANGLES

The valve angle is the angle formed between the face of the valve and the deck. More upright valves (smaller valve angles) have been found to improve performance. This is why small-block Chev valves have become increasingly upright from their original 23-degree position. The factory offered 18deg units, while the aftermarket offerings went as far as 15 degrees. In comparison, LS and Holden V8s feature factory angles of 12 and 6 degrees respectively.

WEDGE HEADS

SettingG the inlet and exhaust valves at the same angle results in a wedge-shaped combustion chamber. The greater the angle, the more pronounced the wedge shape. Small-block Chev heads had very distinct wedge-shaped chambers.

TWISTED WEDGE

Looking along the length of the head, twisted wedge heads have their valve stems stepped back towards the inlet side of the cylinder head. This allows the inlet valve to be positioned closer to the centre of the chamber. The upshot of this is when you look at the face of the head, the combustion chamber is rotated or twisted.

HEMISPHERICAL CHAMBER/HEMI HEAD

Dome-shaped combustion chambers are described as ‘hemispherical’, meaning half a sphere. Hemi heads (as popularised by Chrysler), allows the use of large valves for excellent flow.

OPEN/CLOSED CHAMBER

Round-shaped combustion chambers that follow the circumference of the bore are generally known as open chambers. In a closed chamber, the edges of the chamber are closer to the valves in some places. This reduces the clearance volume, raises compression and improves performance.

2V, 4V, 3V

Ford heads designated 2V were designed to be used with two-venturi carburettors. Models designated 4V featured much larger port volumes and were designed for larger carburettors with four venturis. Aftermarket developer CHI created a version that’s somewhere in between, which it dubbed 3V.

SQUISH/QUENCH

Wherever the deck extends into a combustion chamber it creates a flat area in the chamber, which the piston gets close to at TDC. On compression, mixture in this area is rapidly pushed or squished out, improving turbulence. During combustion the same region removes heat from the mixture between the surfaces and quenches or delays combustion, preventing detonation.

FLOW

The more air and fuel you get in, the more power you get out, which is why improving flow is so important. It is measured on a flow bench (page 106) which works by creating a pressure drop in the head, causing air (at atmospheric pressure) to flow through the port and valve. Flow is expressed in cubic feet per minute (cfm). The pressure drop is measured in inches of water, with 28 inches being the de facto standard. A given mass of air can be combined with a particular mass of fuel to create a theoretical horsepower rating. Because of this, flow is increasingly expressed in horsepower but it should be noted that this is a theoretical output. If the real-world horsepower is to match this figure, the engine combination and tune must be perfect.

SIAMESED PORTS

Small-block Chev heads are commonly described in the industry as having siamesed ports because the port arrangements for the inlets and central exhausts are very close to each other. However, a truly siamesed port is one in which valves in adjacent cylinders are joined by a single undivided port, a configuration seen in old and diesel engines.

BIFURCATED PORTS

Small-block Chev ports are closer to what’s known as a bifurcated port, although that’s still not 100 per cent accurate. A bifurcated port has a single undivided port at the manifold face that becomes divided some distance before the valves in adjacent cylinders, as with a Holden six-cylinder.

RAISED OR HIGH PORTS

As flow enters the head, it has to be turned to direct it against the back of the valves — this is why the short-turn radius is so important. Casting the ports higher up in a head (known as a high-port head) and turning the mixture higher allows it to follow a longer, straighter path before reaching the valve.

PORTING

Porting is the process of altering the cross-sectional shape and volume of a port by grinding or filling, to improve air flow. Changing the shape of a combustion chamber is also considered part of the porting process. Although hand-porting is still common, it’s increasingly used to develop new port shapes which are then reproduced by CNC (Computer Numerically Controlled) machining.

DIGITISING

This is the process of using a CMM (Coordinate Measuring Machine) to measure points on an object and create an extremely accurate three-dimensional digital model for use in CNC machining, along with CAD (Computer Aided Design) processes.

BOWL JOB/VALVE BOWL

The valve bowl is the rounded, bowl-shaped area directly behind the head of the valve. Reasonable power can be found by modifying just these areas, which include the valve-guide bosses. Doing so is the basis of a bowl job.

POLISHING

It was once common practice to polish ports and combustion chambers. A mirror finish in ports is not so common any more as some texture in the ports is said to be beneficial. Polishing combustion chambers was done to remove hot spots and in some engines it was vital and could result in as much as a 10deg timing advance without detonation. Well-designed heads don’t require this treatment.

So there you have it, a brief rundown on commonly used cylinder head terms. It isn’t possible to cover the whole subject in a single article, so we’ll bring you more info next time.

Huge thanks to The 308 Shop; PRE; Yella Terra; and CHI for their help and information in putting this feature together.

IN DETAIL:

1. An open-chamber Cleveland head. The large volume results in a lower compression ratio and consequently reduced performance.

2. This is a closed-chamber Cleveland head. Its reduced volume creates higher compression ratios and higher performance. Note the shrouding around the exhaust valve.

3. Ports are either equally spaced along the head or positioned side-by-side in a pseudo-siamesed arrangement, as with these small-block Chev heads. Equally spaced ports are more desirable — EFI Holdens, the LS-series and almost all big and small-block Fords use this configuration.

4. The three-angle valve job refers to the angles at which the valve seats are cut. Note also the laid-back profile of this chamber, especially near the seats.

5. Indexing sparkplugs means making sure the electrode is facing in the right direction. The open area between the central electrode and the earth strap should directly face the piston so that the spark is directed towards the top of the piston.

6. The short-turn radius is the tighter turn that curves around towards the back of the valve. Its shape is crucial to performance and modifying it poorly does more damage than good.

7. Scott from CHI digitising a chamber using a CMM arm. This is the latest in measuring technology.

8. Note the stock 23deg angle of the valves in this GM head (top) compared to the much straighter 6deg angle of the Holden head.

9. These days, head castings are turned into finished products on CNC machines. In addition to basic manufacturing these machines can reproduce the finest aspects of any design, including the subtle port and chamber shapes created by hand and digitised via CMM.

10. The 308 Shop often has pretty serious engines on the bench. On the left is a hemispherical head off a 500ci Keith Black — note the twin sparkplug holes. Compare the chamber and inlet port to the aftermarket small-block Chev (right).

11. A classic Ford flathead V8. Note how shallow the heads are due to the absence of valves and rocker gear.

12. Two different factory heads for Ford Windsors. Note the ’plug location in the more open chamber — it’s positioned more centrally in the chamber.

13. These head sections from Yella Terra are split through the inlet ports horizontally, revealing an unmodified short turn on the left and a modified type on the right. Yella Terra has the largest collection of sectioned cylinder heads in the country.

14. Note the difference in the valve angles when looking along this canted-valve big-block Chev head. They’re also at different angles when viewed across the head — Clevelands look very similar to this.

15. Most modern four-valve heads have chamber roofs that are set at pretty much the same angle as the valves. This creates a chamber shape known as a pent roof, as seen here in this current generation inline Ford six.

16. This PRE Pro Stock head features rotated chambers to create more of a cross-flow configuration. Due to the very upright orientation it uses a special multi-electrode ’plug to ensure the spark is still directed towards the top of the piston.

17. Creating chambers of equal volume results in even output from cylinder to cylinder. Chamber volume is measured using a burette — a tall glass tube with a tap at the bottom. They’re quite accurate and feature volumetric graduations in cubic centimetres (cc). Hence the term ‘cc-ing’ the chambers.