This month, we'll discuss cylinder heads and intake manifolds, including some of their relationship with valve events. The specifics of valvetrain and valvetrain components will be covered in some detail in the next installment.

To begin, we should discuss some theoretical concepts before getting into the practical applications of heads and manifolds. The purpose of this approach is to help create a mental picture of what is taking place in the combustion space.

An internal combustion engine oxidizes fuel in a specific volume (combustion chamber). Oxidation is a chemical reaction which adds oxygen to create combustion (burning). This reaction is exothermic which means that the reactants (fuel and oxygen) produce heat and a byproduct--exhaust. The gases created from the combustion process are at high temperature and pressure, expanding against all exposed surfaces, but most notably the piston. The defining feature of an internal combustion engine is that useful work is performed by the expanding gases acting directly on the piston(s). It is this force that is transferred into actual work being produced (torque).

Internal combustion engines are heat engines. In other words, the heat released by burning (oxidizing) the fuel is converted into mechanical energy (torque and power). To increase the power output of an engine, we can increase the amount of useable heat transferred, improve the efficiency of the transfer process, or reduce losses associated with the system. To increase the amount of useable heat transferred to mechanical energy, we can increase the displacement of the engine. This has the affect of passing a larger amount of fuel and oxygen into the system (engine). Another possibility is to make the engine "think" the displacement is larger by increasing the mass of combustible products which pass through the system by making the ducting (intake and exhaust passages) more efficient.

An engine's ability to make power is directly related to how efficiently it is able to induct fuel and oxygen and rid itself of the by-products of combustion. The magnitude and balance of these fluid flows affect the amount and efficiency of the heat released by the fuel, during the combustion process. For this reason, assuming the engine is operating properly and within reasonable limits, more mass flow equates to more power.

Oxygen is needed to burn fuel. But, during the inlet cycle, air is what the engine inducts. Dry air contains 21 percent O2 (oxygen) and 79 percent inert gases. Typically, for a racing gasoline, the leanest air/fuel ratio for best torque is approximately 13.5:1. Therefore, for every pound of fuel consumed, 13.5 pounds of air must be ingested, although only 2.8 pounds of this supports combustion.

One cubic-foot of air at standard temperature and pressure (STP), assuming average composition, weighs approximately 0.0807 pounds. At STP, 13.5 pounds of air is approximately 167 cubic feet of air. If a Cup engine uses 170 lbs/hr of fuel at peak power (at 13.5:1 air/fuel ratio), that would equate to 2,295 lbs/hr of air. So, at standard temperature and pressure, 2,295 pounds of air would be 28,439 cubic feet per hour. This would be an astounding 474 standard cubic feet per minute. In this particular case, STP would be 60 degrees F and 14.696 psia. If you heated this air to a more reasonable (typical) inlet temperature, the volume would increase by the ratio of the absolute temperatures. In reality, this is an incredible amount of air to flow through a port.

Now let's get to something practical. No matter what you race, from a lawnmower to a Cup engine (for a given fuel type and if you want to make more power from a four-stroke spark-ignited engine and have limited resources), you want to work on improving the mass flow rate of fuel and oxygen (air) through the engine. The three areas to concentrate on are:

* Cylinder heads
* Intake manifolds
* Valve event timing (valvetrain)

From the previous theory section, it should now be apparent why gas exchange is important. It should also be evident that improving the efficiency of that exchange should help increase power.

Essentially, air is moved into the cylinder by the difference between pressure in the cylinder (during the induction stroke) and atmospheric pressure. When compared to the pressure in the cylinder during combustion, the negative pressure created by the piston moving down is not very large. Any losses in the ducts (ports) leading to the combustion space become significant. Because virtually any loss is important, considerable effort should be placed on improving the efficiency of intake ports.