Engine building contains a lot of numbers but how can you better understand those numbers and use them to build faster motors without spending any extra money?

After 100 years of development, the internal combustion engine has come a long way but there are a few basic formulae that have been derived over that time which provide a very solid basis for making the maximum power that any given combination can produce. Unfortunately, these formulae are only known to a few engine builders and are used by even fewer. In this article Perth Street Car, with the assistance of Leon Withnell from A1 Hi Performance, will look at the relationship between those mystical head flow figures that come with any decent set of re-worked ports and the black art of camshaft figures. The concepts we will outline in this article relate specifically to engines with higher than 10:1 compression, using conventional flat tappet and roller cam profiles – not the new large base circle designs now available for high-end race engines. This introductory article will lead into a more detailed look at interpreting camshaft figures in our next issue.

When your heads are flow tested on a flow bench (at a given test pressure) the operator will print out a chart for you giving the intake and exhaust port flow in cubic feet per minute (CFM) for each increment of valve lift. For a realistic result, the intake ports should be tested with the intake manifold bolted up and similarly, the exhaust headers should be bolted up also. Otherwise, the test is carried out on only one part of a complex chain. While it is important to know how much your ports flow and the ratio of intake to exhaust flow, this is by no means the whole story. What can we do with the basic flow information?

Using the flow information, in conjunction with other numerical measurements, it is possible to build a mathematical model of the cylinder heads that can be used with camshaft information to maximise torque and power output in an engine.

Average Port Area:

The first such figure is a measure of the average area of the port. To calculate this figure firstly measure the volume of the port in ccs. Then measure the length of the port floor in inches and the length of the portroof in inches. Add these two lengths together and divide by two to find the average port length. Then, divide the cc volume by the average length and finally divide this figure by 16.387 to reach the average port area in square inches.

A1’s unique degree wheel gives the ultimate in accuracy when it comes to degreeing-in a camshaft.	The valve curtain area is highlighted in red.

Port Velocity:

Port velocity is a measure of how fast the air will theoretically travel through a port of a given cross sectional area at a given test pressure. Just like holding your thumb over a running hose, the greater the restriction, the greater the velocity of the fluid (air, water, fuel and oil are all described as fluids in physics). Thus, the smaller the minimum port restriction, or the smallest cross section of the port, the greater the port velocity will be. This velocity is calculated by dividing CFM by average port area and then multiplying the result by 2.4. At a test pressure of 25-inches of water the optimum figure that most engine builders aspire to is 350 ft/sec. To convert this figure to miles per hour simply multiply by 1.466 (and then multiply this by 1.61 to arrive at kmh).

If you are still with us at this point then you are doing better than average. If the going is getting a bit hard then take a breather and see the lovely Michelle with her roller camshaft!

The valve is depressed exactly .100-inch – but that’s not how far the valve head is actually off the seat!

Minimum Port Restriction:

The minimum port restriction, or minimum cross sectional area (MCA) of a port is simply the smallest part of the intake tract. This point may occur at the bowl, the short turn radius, the port opening, inside the intake manifold or even at the throttle body. It is critical to determine exactly were the MCA occurs and its area in square inches because it determines how much power an engine of a given size can potentially produce and at which rpm. Porting shops normally make rubber moulds of a port which can then be cut into sections in order to accurately calculate the minimum cross sectional area.

Valve Curtain Area:

The MCA can even occur in the valve curtain area. This is the area around the valve, created when it is lifted off its seat and is measured by multiplying the diameter of the valve by pi (3.1415927) and then multiplying this by the actual amount of lift at the valve. It is interesting to note that just because the valve has been opened .700-inch for example, it does not open up a gap above the valve of exactly .700-inch! Consider the various angles cut into the seat and the similar angles cut into the face of the valve. A .700-inch vertical movement does not translate into a .700-inch gap around the valve, often this gap is as small as .619-inch. At .100-inch lift the gap can be only .073-inch! This stresses the need to actually measure these openings in your own engine if you are to truly understand how to make it reach its full potential. You are spending the money so why not get the best value for each dollar?

At .100-inch lift a .100-inch drill bit will not fit between the valve and the seat.	The valve curtain area is highlighted in red.

Maximising Your Cylinder Heads:

The trick to producing a great cylinder head is balancing maximum flow with maximum port velocity. Naturally a port must flow enough air at a given lift to support a given amount of horsepower but if it can do so using a smaller port volume with a smaller cross sectional area then the engine will generally produce more mid range torque and accelerate harder. Optimising port velocity ensures that the cylinders are filled to the absolute maximum (often greater than 100% volumetric efficiency) when combined with the correct camshaft and that the highest torque and power levels are attained.

The following examples demonstrate how excellent power can be generated with modest airflow figures if certain basic dimensions are adhered to. In these cases 100% volumetric efficiency is assumed (ie: the cylinders are fully filled) and the bowl diameter is 80% of the valve diameter.

Port velocity is critical to an engine’s behaviour rather than its total power potential. For example, two engines may both produce 450hp at their peak but the first engine has a smaller cross sectional area and a camshaft matched to the air flow characteristics of the cylinder heads. This more efficient port will have higher port velocity, resulting in higher volumetric efficiency which generates more bottom-end and midrange torque and power than its less efficient opposition. The second engine produces the same peak power but less torque to shift the car’s mass, thus it will be significantly slower over the quarter mile.

Various Windsor port moulds.

Heads and Cams:

Camshafts must be matched to the air delivery characteristics of the head and the demand of the engine. Whether the head should be developed first and then matched to a cam or vice versa is a matter of debate. Many leading US engine builders are taking a novel approach to the whole argument by considering the exhaust side before the intake side. That is, they are concerning themselves with the timing and extent of exhaust evacuation from the cylinder – particularly at bottom dead centre (BDC). Leading engine mathematicians, maintain that 80-85% of exhaust flow should occur as the piston passes through BDC. Mathematical theory also shows that the exhaust port should flow around 68% of usable intake flow across BDC (with a conventional camshaft).

This argument is based on the notion that unless sufficient exhaust gases are evacuated from the cylinder by this point then they will be pumped back up the bore, into the intake runner and then drawn back into the cylinder – polluting the fresh incoming charge. An additional benefit of this level of scavenging from the cylinder is that negative torque (pumping losses) are also minimised. There is very little point having a superb intake port if it is prevented from working to its maximum cylinder filling potential by an inefficient or mismatched exhaust port.

If we have a flow chart for the cylinder heads in question then we know how much the exhaust port will flow at any given lift. It is then simply a matter of measuring the exhaust valve lift at BDC and determining exactly how many CFM the exhaust is flowing at that point. If it is 68% of the available intake flow or 80-85% of maximum exhaust flow then the engine will accelerate rapidly. If not then it may be time to carry out some more work on the exhaust port or introduce some more valve lift at that point. The critical point is that you, as an engine owner, will know exactly how your heads and cam are working together and whether there is room for improvement.

While these percentages are not hard and fast, they have been derived from rigorous testing and provide valuable indicators which often set good engines apart from lesser engines (which are invariably more expensive and filled with exotic parts). Using mathematics to test port flow and efficiency, volumetric efficiency, camshaft effectiveness and an engine’s torque and power characteristics is a solid step towards creating optimum output from a given combination. Making the power in real life – on the dyno or the dragstrip – is all a matter of working out what the required numbers are to produce that power and then ensuring that those numbers are achieved by the various components involved.

In our next article, we will examine camshaft intake centrelines and lobe separation angles and their affect on the torque and power characteristics of an engine. All late model engine owners – watch this space!

Re-printed from Volume 12 Number 2 of Perth Street Car Magazine with permission.