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    Producing Quality Computer Tools for Racers and Engine Builders since 1986







What are Cylinder Heads?


Air to Air Intercooler
A very simple Cylinder Head - Image courtesy of oldminibikes.co
To the novice, Cylinder Heads are basically the top of the cylinder. The purpose is to contain the air fuel mixture for the compression stroke and the power stroke. For this simple design, the only additional job of the cylinder head is to also hold the spark plug. The simple Briggs & Stratton engines (like most flat head designs) were designed this way for power equipment and Karting.  For these old flat head designs, the valves were contained in the block and the head's combustion chamber was quite large because it had to contain both the cylinder bore and the valves.

In 1949 Oldsmobile introduced the Rocked V-8, one of the first mass produced Overhead Valve engines.  Now the head located the valves over the bore, which greatly reduced the combustion chamber size.  The valves and the ports were contained in the heads in addition to the spark plug.  This is the basic modern cylinder head design.  

The performance of the cylinder head basically involve the following areas:

Combustion Chamber Design (how quickly and completely the air fuel mixture is burned).
Valve/Port Flow Capability (how free flowing the ports and valves are)
Mixture Motion (which involves both the chamber and intake port design)
Material (which involves the heat transfer rate)


YZF Cylinder Heads
YZF 450 Cylinder Heads - Image courtesy of dirtrider.com
It should be noted that the Overhead Valve (OHV) engine is typically a pushrod engine.  There are also Overhead Cam (OHC) engines which eliminate the pushrods and place the cam directly (or via a rocker arm) on top of the valve, in the head.  At Performance Trends, even though the cam is in the head, the cam, rocker arms, valve springs, etc (also located in the head) are considered to be part of the Valve Train, not part of the head.

Combustion Chamber Design is the size and shape of the chamber, and the location of the spark plug(s).  It also involves the shape of the piston top and how close the piston top comes to the head.  In general, the more compact the chamber and the more centrally located the spark plug, or the more spark plugs used, the faster the burn rate.  Fast burn chambers require less spark advance and produce a more efficient burn, or higher Thermal Efficiency.  They are also less prone to detonation or spark knock and produce better fuel efficiency.

Valve/Port Flow Capability  involves how much CFM flow can be produced by the ports for a given pressure drop.  The more flow for a particular pressure, the better the flow capability or simply Head Flow.  The head's Flow Capability is typically the major flow restriction in the engine's total Flow Capability.  Therefore the heads are often the focus of racing engine builders for improved total engine Flow Capability, Volumetric Efficiency and HP potential.

Flow Capability is determined by 2 things:

The number and size of the valves (like 3, 4 and 5 valve heads instead of just 2 valves)
The Flow Efficiency of the ports and valves (like larger, straighter ports and other porting details)

Click on this Head Flow link for more details. 

Mixture Motion  is the motion of the incoming air/fuel mixture, and is generally broken down into swirl or tumble.  There is also just the incoming velocity and turbulence.  Smaller valves and ports produce higher velocity and turbulence, but reduce the Flow Capability.  Then there is the motion produced by the piston as it approaches the head at TDC.  If some areas of the piston come very close the the head, it will "squish" the mixture to a larger area, like squeezing a tube of toothpaste.  (It happens so fast, its more like smashing a tube of toothpaste with a sledge hammer.)  


Front Mount Intercooler
Typical V8 Cylinder Head - Image courtesy of hotrod.com
All these motions can improve air/fuel mixing and quicken the burn rate.  However, these motions can also cause fuel to separate out (too much swirl) or actually blow out the flame being produced by the spark plug.  

Mixture motion is very much a "black art" and can only be improved with LOTS of "cut and try" dyno testing.  This testing is greatly enhanced with instruments to measure cylinder pressure in great detail to understand how the charge is burning, degree by degree.

Material is typically either cast iron or aluminum.  Aluminum is lighter and easier to port and modify.  However aluminum can "suck" more heat out of the chamber due to it's higher thermal conductivity.  The aluminum head may run cooler, which helps pack more air into the chamber because the incoming cool air does not pick up as much heat.  Both of these aspects can help avoid detonation and improve volumetric efficiency.  

However, loosing heat to the coolant rather than keeping in the chamber to push on the top of the piston should reduce thermal efficiency.  This is typically insignificant to the other cooling benefits to aluminum.

Depending on the actual warm-up time and other aspects of the engine's running conditions (quick drag race vs road racing vs street driving), cast iron can actually run cooler than aluminum in some cases.

In addition, modern technology has developed coatings which try to change the heat transfer rate.  They claim to be able to improve performance, fuel economy and reduce detonation.

Programs that work with the different types of Cylinder Heads

We have 3 programs that work with Cylinder Heads when conducting Engine Simulation: Engine Analyzer v3.4, Engine Analyzer Plus v3.4 and Engine Analyzer Pro v3.9.

Our Port Flow Analyzer v3.5 program can be used to figure out the Flow Capability and Flow Efficiency of your existing heads and can also be used to see improvements (or losses) in modifications to your heads.

Plus we have 3 tools that can be added to your Performance Trends EZ Flow System, or other flow benches. Starting with our Automatic Valve Opener. Allowing you to make 50 different lifts automatically. Plus our Swirl Meter allows you to measure intake mixture motion on your flow bench. And finally our Tumble Adapter makes it possible to mount our Swirl Meter sideways so you can use it to measure Tumble.