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In this section we will take a look at all intake related information and formulas. This will include air clearners/ stacks, carbs, injectors, manifolds and some of the intake port.

Theoretical CFM
Volumetric Efficiency

Theoretical CFM

I'm sure that you've all heard the analogy likening the internal combustion engine to a pump. And that analogy is true. An engine functions when the flow of mixture (air and fuel) is maximized both on the intake side, then as fully combusted as possible, then exhausted as fully and completely and quickly as possible.

The first step in maximizing this process is to know, at least theoretically, what amount of air your engine needs to flow. This requirement will be different at different at different RPMs as both measurements involve a time element. CFM or Cubic Feet per Minute is how air flow is measured. And RPM is the measurement of the speed of the engine at which that flow occurs. This means that at higher RPMs, you need more air per minute than you do at lower RPMs.

To compute that theoretical flow, here is the formula:

                      RPM x Displacement in cubic inches
          Air flow = --------------------------------------
                                 3456
    

The terms RPM and Displacement need no explanation. The constant 3456 is derived from taking the number of cubic inches in a cubic foot and multiplying by 2.

It should be noted at this point that the CFM figures you will come up with are purely theoretical. In reality, an intake system is much more complex than this formula would lead you to believe. There are positive pulses, negative pulses, restrictions, etc - all of which conspire to make the intake tract one very difficult place.Even if you are lucky enough to have your components tested and set up on a flow bench, that flow bench only simulates what goes on in the intake. And only in a static sense. There are many dynamic forces which are also taking place. Again, the intake tract is subject to many forces, most of which cannot be duplicated in the laboratory.

You should really graph your CFM requirements against your torque and horsepower curves (and maybe even against BMEP). What you want to achieve is optimal (not maximum) airflow at peak torque/peak pressure. Also, the drop off between air flow at peak torque and the airflow at peak horsepower should be minimal.

Compute your CFM at a given RPM

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Volumetric Efficiency

One other measurement of an engine's efficiency is Volumetric Efficiency or VE. This measures how well your engine performs at a given RPM by comparing theoretical CFM versus actual CFM. The formula for this is:

                  actual CFM
         VE = ------------------- x 100
                theoretical CFM
    

Typically a street motor will have a VE of about 75%. A high performance motor will run about 85%. And a full race motor about 95%. As we learned above, CFM is an RPM related measurement. So these VE figures are based on the RPM range where maximum torque occurs. This RPM range is where your engine is operating most efficiently. The VE figure for the RPM range where maximum horesepower occurs will typically be about 5% to 10% less.

An extremely well tuned motor, with well integrated components such as heads, carb, manifold, exhaust, etc, can produce VEs in excess of 100%.

Compute your VE at a given RPM

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OK, one other quick point to remember here. All the terms used so far are based on the volume of the component, specifically air. However, if you are lucky enough to have access to a dyno with an air/fuel ratio test probe, the resultant number (ratio) is acutally based on weight.

For example, a good range to be in on the dyno is 12.5:1 to 13.5:1. This is parts of air to parts of fuel, by weight. However, a cubic foot of air weighs only about .087 pounds (at an ideal temperature of 32 degrees Fahrenheit and with 0% relative humidity). That means that you would need from 145 cubic feet to 157 cubic feet of air for each pound of fuel (or .81 cubic feet of fuel). Man, that's a lot of air!

Hmmmm, we could launch into a discussion of BTU per pound of fuel and how much pressure could be produced per BTU but I think we'll save that for later.

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