RE: Intake Cycle

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The Intake Cycle Part One

Paul Yaw,

The last two articles explained the importance of an efficient exhaust cycle. As you now know, if the exhaust cycle is less than optimum, there is not much point in making other modifications. If, on the other hand, the exhaust cycle is optimized, improvements in the intake cycle will result in large power increases. Since the power output of the engine is directly proportional to the amount of air and fuel that it can ingest, the goal is to pack as much mixture as possible into the engine during the intake cycle.

  • Volumetric Efficiency and Charge Density

When considering intake efficiency, we normally think in terms of volume, or volumetric efficiency. Volumetric efficiency is stated as a percentage of the engines total airflow potential. (Total airflow potential, or pumping potential = displacement X rpm.) Volumetric efficiency is determind by measuring airflow into the engine while it is running. For instance, a 13B has a displacement of approximately 80 cubic inches. This means that it can potentially displace, or pump 80 cubic inches of air per revolution. At 6000 rpm, this equates to 480,000 cubic inches, or 277.8 cubic feet per minute. If measured airflow into the engine is also 277.8 cubic feet per minute (cfm) the engine is said to have 100% volumetric efficiency. (Volumetric efficiency = measured airflow/pumping potential.)

This is a convenient way to measure intake efficiency, but it is a bit misleading. What we are really concerned with is the mass of air and fuel that the engine can ingest. For instance, an engine operating at 100% volumetric efficiency at sea level will make much more power than an engine operating at 100% volumetric efficency at the top of Pikes Peak. In both instances, the engines are ingesting the same volume of air, but the engine operating at sea level will have taken in a greater mass of air. This is due to the greater air density at sea level. In more simple terms, one cubic foot of air at sea level weighs more than one cubic foot of air at Pikes Peak.

This is easy to understand if you consider the air in your tires. If one tire is inflated to 10 psi., and another tire is inflated to 50 psi., they both contain the same volume of air, but the tire with the higher pressure contains a greater mass of air. Air density, or mass, is directly proportional to pressure. At the higher elevation of Pikes Peak, there is less pressure, and therefore the air is less dense.

  • The Effect of Temperature

Temperature also has an affect on air density. The formula to determine air density in pounds per cubic foot is: Pressure in inches of mercury times 1.326, divided by absolute temperature in degrees fahrenheit. (Absolute temperature = temperature + 459.6) If, for instance your induction system draws intake air from under the hood, the intake air temperature on an 80 degree day can easily exceed 130 degrees. Using a barometric pressure of 29 Hg" for our example, the air density under the hood at 130 degrees is .0652 lbs. per cubic foot. Now if you change to a cold air setup which draws 80 degree intake air from outside of the car, the air density is .0713 lbs. per cubic foot This is an improvement of 9.3%! Using a 200 hp engine as an example, this is an improvement of 18.6 horsepower! In practice the horsepower improvement will be less than theoretical because the incoming air will be heated by the intake manifold, and the engine itself. Further improvement can be made by insulating the intake manifold so that it picks up less heat from the exhaust system, and radiator. It is probably not possible to achieve the theoretical density increase, but it should be clear that there is much to be gained by keeping the intake charge cool.

  • Air Fuel Ratio

With all of this discussion of airflow, and air density, it is important to understand why we need air in the first place. Quite simply, because without air, and the oxygen that it contains, the fuel will not burn. The optimum air fuel ratio varies slightly depending on the chemical makeup of the fuel, but generally, peak power occurs at a ratio of 13lbs. of air, to one pound of gasoline. Notice that this ratio is stated in lbs., and not volume. As you can see, we need much more air than fuel to develop peak horsepower.

Air weighs approximately .076 lbs. per cubic foot at sea level, and gasoline weighs approximately 46.7 lbs. per cubic foot. At an air fuel ratio of 13:1, you will need approximately 8,000 gallons of air for every gallon of fuel that you burn! When you look at it that way, it becomes obvious that the power output of an internal combustion engine is limited by the amount of air that it can ingest. If this doesn't convince you that airflow, more than anything else determines the success of a performance engine, nothing will!

  • Airflow

Total, or mass airflow into the engine is the single biggest factor in determining horsepower output. Put more simply, airflow is everything! The purpose of the intake stroke is to fill the engine with as much air as possible. There are many factors that contribute to the total airflow into the engine. The most important of these is the airflow capability of the induction system which includes everything from the air filter to the intake ports themselves. To optimize the airflow capability of the induction system, we must first be able to measure it. In more exact terms what we are measuring is the induction system's resistance to airflow.

Airflow measurement is done with a flowbench. The flowbench is the engine builders equivalent of a wind tunnel. The difference being that we are measuring flow restriction through a passage, rather than around a stationary object. A flowbench consists of a series of vacuum motors which create a pressure drop on one side of the passage being tested. The pressure drop causes air to flow through the passage, and that flow is then measured. In this way, the resistance to airflow can be determined, and the results of modifications can be quantified.

  • Pressure Differential

Airflow is initiated by a pressure differential. A pressure differential can be most simply described as a pressure difference between two points. Pressure, at least as it applies to the automotive field, is normally stated in psi. (Pounds per square inch), Hg" (Inches of mercury), or H2o" (Inches of water)

The earliest pressure measurement devices were simple manometers. A manometer is made up of a clear tube formed into the shape of a U, and partially filled with liquid. If there is no pressure differential between the two ends of the tube the liquid will fill both sides of the tube by the same amount If there is a pressure differential between the two tubes, the liquid will flow from the high pressure side of the tube to the low pressure side. The difference of the fluid height between the two sides of the tube is the pressure differential, stated in inches, millimeters, or whatever unit of measurement you prefer. If the tube is filled with water, the pressure drop is stated in inches of water, if it is filled with mercury...well you get it.

Atmospheric pressure at sea level, at 59 degrees farenheit is 1bar., 14.7 psi, 29.92 inches of mercury, or 406.9 inches of water. This is referred to as SPST, or standard pressure, standard temperature. From this you can see the realtionship of these different units of measure.

  • Intake Pressure Drop

During the intake stroke the chamber volume increases. Since the mass of the air in the chamber is the same, but its volume has increased, its density has decreased, and so then has its pressure. (Remember, air density is directly proportional to air pressure, and air pressure is directly proportional to air density.) The result is a pressure drop, or a pressure differential between the chamber, and the air at barometric pressure outside of the engine. This initiates air flow into the chamber

  • Airflow Restriction

As I mentioned earlier, when airflow is measured on the flowbench, it is really the restriction to airflow that is being measured. If for instance ther were no restriction to airflow, a pressure drop could not be realized. To be able to compare the airflow of one induction system to another, flowbench testing is usually done at a standard pressure differential. The most commonly used pressure drop for flowtesting is is 25 inches of water.


The pressure drop caused by the engine itself is not steady like that of the flowbench. In fact, the pressure varies a great deal throughout the intake cycle. There are many factors that determine the pressure throughout the cycle, (These factors will be discussed in the second part of this article.) but for the sake of simplicity, let's consider it as being steady.

Flowbench testing shows that the complete induction system of a second generation six port 13B flows approximately 125 cfm. at a pressure drop of 25 H2o" With a little bit of porting, this can be increased by approximately 10%, or 12.5 cfm. This 10% airflow increase will be true at any pressure drop. For instance, at 6 H2o", the stock ports flow 61.2 cfm, and the reshaped ports flow 67.4 cfm, for a gain of 6.2 cfm. In both cases, the result is a 10 percent increase in airflow. So as you can see, regardless of the actual pressure drop throughout the intake cycle, the increase of total airflow into the engine will equal 10%. In practice, the power increase is not always equal to the airflow increase, but it does follow quite closely. A few of the factors that affect the result of increased airflow are; heating of the intake air, pumping losses during the compression and exhaust stroke, and combustion efficiency.

The Intake Cycle Part Two

In this article I intend to discuss the many factors that affect total intake airflow, and horsepower output.

  • Total Intake Airflow

In the previous article I stated that the horsepower output of an engine is directly proportional to the amount of air and fuel that it can ingest. It stands to reason then, that if the goal is increased horsepower, we must increase the airflow potential of the induction system. The total airflow through any passage (Ports, intake manifold, etc.) is affected by three variables. These variables are: 1. The size of the passage. 2. The pressure differential between the inlet and the outlet. 3. The coefficient of discharge of the passage.

  • Coefficient of Discharge

Coefficient of discharge is normally stated as a percentage, and is a measure of how efficiently a passage will allow air to flow through it. A "perfect" venturi, having an inlet angle of 16°, an outlet angle of 7°, and an inlet and outlet area four times larger than the operating cross sectional area (Vena contracta, the smallest point of the venturi.) has a flow efficiency of 100%. A venturi such as this would flow 137.7 cubic feet per minute (CFM), per square inch, at a pressure differential of 25 inches of water. Using this as a baseline, we can determine the efficiency of a port, or intake manifold runner. 100% flow efficiency is not possible in most cases, but knowing the efficiency of a given combination is a large step towards being able to optimize it.

For example, consider two intake manifolds that flow the same amount of air at the same pressure differential, but one of these manifolds has a runner cross sectional area twice that of the other. The manifold with the larger runners will have a coefficient of discharge that is half that of the smaller manifold. In addition to being less efficient in technical terms, the larger manifold will also lower the horsepower output of the engine as compared to the smaller manifold. Why you ask?

  • Velocity

Hopefully all of you remember the previous discussion of velocity, and have a good understanding of its effects on unsteady airflow. If not, go back to part one of the exhaust cycle article for a freshen up. As I stated in that article, flow velocity through the exhaust system is not steady, and in many cases, the flow will reverse at some point in the cycle. This is also true of the intake system.

  • Pressure Wave Tuning

Since most rotary applications utilize a stock, or off the shelf aftermarket manifold, manipulating the pressure waves by changing the length of the induction tract is not as practical as with the exhaust system. For this reason I will not cover this in great detail.

The pressure wave theories that I discussed in the exhaust article apply to the intake system as well, but there are a few differences between the two. 1. The pressure waves will be much weaker, and so their effect will not be as great. 2. Since the intake manifold is typically much shorter than the exhaust system, the pressure waves will be reflected back and forth several times before they arrive at the intake port at the appropriate time in the cycle. Each time they reflect, they will lose some energy which reduces their usefulness. 3. In the case of the induction system, it is the positive, or high pressure waves, rather than the negative, or low pressure waves that are useful for increasing horsepower.

By timing the positive return wave to arrive at the intake port right before it closes, the pressure differential between the port, and the chamber will be increased. This will increase the flow into the chamber at the end of the cycle when it is typically at it lowest.

There are a few basic rules that apply to pressure wave tuning the induction system. A longer manifold will delay the waves for a greater period of time, and so tune the manifold for a lower rpm range, just as with the exhaust system. A longer manifold will also increase the peak torque output of the engine, in addition to the above mentioned effects. This is the result of the manifold containing a greater mass of air. (Remember, energy = mass times velocity squared.) At the end of the intake cycle, when the chamber pressure is increasing, this greater mass (Which is travelling at a high velocity) will better overcome the rising chamber pressure, resulting in greater airflow during that critical period. Additionally, a greater pressure drop will be created at the beginning of the cycle when the chamber begins to expand, because the engine will have to "pull" harder to get this greater mass of air moving. It is this initial low pressure condition which starts the pressure wave cycle, and the result is a pressure wave of greater intensity which if timed correctly, will increase volumetric efficiency.

  • The Peak Horsepower Myth

If you are thinking to yourself that high rpm horsepower is all that matters for your application, consider this. Even with a close ratio racing gear box, you will need to make power over a range of at least 2,000 rpm. If the engine makes a staggering amount of horsepower at redline, but drops off quickly below that, acceleration will suffer. This is relatively common on race engines, and is the result of low velocity, or poor flow efficiency.

Peak horsepower is a measure of the absolute maximum horsepower that the engine can produce. It is a relative indication of an engines performance, but it only tells you what the engine is doing at one particular rpm. It tells very little about the actual performance, unless you will only run the engine at one rpm!

What is important is the average horsepower throughout a specified operating range. This operating range should be specified based on the gear ratios of the transmission.

I have presented quite a bit of information here, and to make all of this easier to visualize, I will once again refer you to the illustration of the rotary engine during its different phases.

1. 45° after TDC. The chamber is slowly expanding, and the air/fuel mixture is just starting to enter the chamber. This is the beginning of the intake cycle for a conventional side port engine, and the intake port has been open for approximately 20°.


A bridge port, or peripheral port engine will have had the intake port open for 150° to 200° at this point. If the exhaust system is working properly, the low pressure wave will have arrived, and initiated intake flow by TDC, or even sooner, replacing the exhaust gasses in the clearance volume with fresh intake charge.

2. 90° after TDC. The rate of expansion is now fairly rapid, and the low pressure in the chamber initiates a negative pressure wave in the induction system.

3. 180° after TDC. The intake port is completely open, and the point of maximum rate of expansion and flow has occured 45° earlier.

4. BDC At this point, the chamber is at its maximum volume, and past this point the chamber volume will decrease as the compression cycle begins.

5. and 6. 45°, and 90° after BDC. It is during this period that the intake port will close, and the effects of inertial supercharging become critical. The period between BDC, and intake port closing has the greatest effect on the volumetric efficiency of the engine. Velocity, velocity, velocity!

Well, I think that just about covers it. For a very thorough understanding of the gas exchange process, review the previous exhaust cycle articles, and consider the intake and exhaust as a complete cycle. Pay careful attention to the entire process, starting with the exhaust cycle, and the pressure condition that is left at TDC, which is where it all starts.

Paul Yaw Yaw Power Products

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