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Head flow basics *general information*
Hey all, after some extensive research on the topic i thought it would be a good idea to share some articles and info regarding optimum head performance and how to achieve this. here are just some basics, hopefully there is more to add by others.
the following articles have alot to do with port and polishing
Cylinder head porting refers to the process of modifying the intake and exhaust ports of an internal combustion engine to improve the quality and quantity of the air flow. Cylinder heads, as manufactured, are usually suboptimal due to design and manufacturing constraints. Porting the heads provides the finely detailed attention required to bring the engine to the highest level of efficiency. More than any other single factor, the porting process is responsible for the high power output of modern engines.
This process can be applied to a standard racing engine to optimize its power output as well as to a production engine to turn it into a racing engine, to enhance its power output for daily use or to alter its power output characteristics to suit a particular application.
Daily human experience with air gives the impression that air is light and nearly non-existent as we move slowly through it. However, an engine running at high speed experiences a totally different substance. In that context, air can be thought of as thick, sticky, elastic, gooey and heavy (seeviscosity). Pumping it is a major problem for engines running at speed so head porting helps to alleviate this.
• 1 Port modifications
• 2 Port components
• 3 Areas of importance
• 4 Wave dynamics
• 5 The "Porting and Polishing" myth
• 6 Two-stroke porting
• 7 Methods
• 8 Summary
• 9 References
• 10 External links
When a modification is decided upon through careful flow testing with an air flow bench, the original port wall material can be carefully reshaped by hand with die grinders or by numerically controlled milling machines. For major modifications the ports must be welded up or similarly built up to add material where none existed.
This illustration shows the difference between a poor performing port and an excellent design after porting modification. The difference between the two show the general idea of improving port flow. Higher and straighter is better for peak power.
The modification shown is commonly referred to "increasing the downdraft angle", and is limited by mechanical constraints such as engine bay height, the amount of material in the parent casting, or the relocation of valve gear to accommodate the longer valve stem. Modifications this extreme are rarely done.
An example of a poorly performing port, a mold of the ports of a Ford two-liter head destined for use in Formula 2000 racing. It is shown as manufactured with the intake port on the right.-(Interestingly, this head received extensive flow bench development by Ford at design time but because of engine height restrictions, the use of cast iron and lack of knowledge at that time, this was the best that could be manufactured for road use. Today, manufacturers are able to do much better although they still cannot approach the quality of aftermarket racing designs.)
The Ford two-liter shown above in stock trim was capable of delivering 115 horsepower@5500 rpm for a BMEP of 136 psi. Contrast this with the Pro Stock ports shown below.
This aftermarket racing GM Pro Stock head was capable of 1300 horsepower@8500 rpm with a BMEP of 238psi. Since BMEP is an excellent efficiency measure and closely related to volumetric efficiency, the aftermarket Pro Stock head is vastly better than the stock Ford. In fact a BMEP of 238 puts it near the top of the racing engine world. It is close to the limit for a naturally aspirated gas burning engines. For four-valve/cylinder engines the BMEP limit is about 265 psi.
Of course cam profiles, engine rpm, engine height constraints and other limitations play a role in this difference as well but the difference in port design is a major factor.
This photo is of port molds of a highly developed 500 cubic inch aftermarket racing GM Pro Stock head. Note the height and straightness of the ports, particularly the exhaust port on the left. -(This design is based on a cylinder head casting, which is purpose-built just for Pro Stock racing applications. The head is supplied with small ports with ample material everywhere for individual porting specialists to shape to their requirements without welding on additional metal.)
Parts of the port and their terminology
Areas of importance
Considering the flow through the intake port as a whole, the greatest loss must be downstream of the valve due to the lack of pressure recovery (or diffusion). This loss is unavoidable on intake ports due to the nature of the poppet valve. On the exhaust ports the opposite condition exists and one can control the geometry down stream of the highest speed section, namely the valve seat. This allows the possibility of good pressure recovery and is the reason exhaust ports flow better than intake ports of equal size.
Accepting the expansion into the cylinder loss as unavoidable, the rest of the port becomes that much more important. The critical areas are those that pass the most air at the highest speed for the longest time.
The valve seat configuration on the port and on the valve together form one of the most critical areas. The highest speed within the port is at or near the valve seat for most or all the duration of the cycle. After that, the throat area and short turn radius become critical at higher lifts in the middle of the cycle. The valve seat and valve head angles require careful study in each design.
The bowl area and the rest of the length of the port have important functions in controlling some of the dynamic behavior of the waves that traverse the system as well as setting up the air for a good entry to the throat. Shape, cross section, volume, cylinder swirl or tumble, and surface finish must be considered together with the overall design of the rest of the engine and vehicle to achieve good results.
The port is shaped to allow the maximum use of the available cross sectional area as the flow velocity should be optimized for the conditions the engine is expected to encounter. Well-shaped ports have few dead spots.
Some typical losses and their sources on a small block Chevrolet intake port.
1. Expansion exiting valve to cylinder 31%
2. Expansion, 30° (bowl) 19%
3. Short turn radius bend 17%
4. Expansion, 25° (valve seat entry) 12%
5. Bend at valve guide 11%
6. Expansion behind valve guide 4%
7. Wall friction 4% * (For sand cast surfaces. This falls to 3% for smooth surfaces)
8. Contraction at push rod 2%
Total = 100% 
This highly simplified animation shows how air flows as waves in an intake system. Note the green "valve" opening and closing.
When the valve opens, the air doesn’t flow in, it decompresses into the low-pressure region below it. All the air on the upstream side of the moving disturbance boundary is completely isolated and unaffected by what happens on the downstream side. The air at the runner entrance does not move until the wave reaches all the way to the end. It is only then that the entire runner can begin to flow. Up until that point all that can happen is the higher pressure gas filling the volume of the runner decompresses or expands into the low-pressure region advancing up the runner. (Once the low pressure wave reaches the open end of the runner it reverses sign, the inrushing air forces a high pressure wave down the runner. Not shown in this animation.)
Conversely, the closing of the valve does not immediately stop flow at the runner entrance, which continues completely unaffected until the signal that the valve closed reaches it. The closing valve causes a buildup of pressure that travels up the runner as a positive wave. The runner entrance continues to flow at full speed, forcing the pressure to rise until the signal reaches the entrance. This very considerable pressure rise can be seen on the graph below, it rises far above atmospheric pressure.
It is this phenomenon that enables the so-called “ram tuning” to occur and it is what is being “tuned” by tuned intake and exhaust systems. The principle is the same as in the water hammer effect so well known to plumbers. The speed that the signal can travel is the speed of sound within the runner.
This is why port/runner volumes are so important; the volumes of successive parts of the port/runner control the flow during all transition periods. That is, any time a change occurs in the cylinder - whether positive or negative - such as when the piston reaches maximum speed. This point occurs at different points depending on the length of the connecting rod and the throw of the crank, and varies with the connecting rod ratio (rod/stroke). For normal automotive design this point is almost always between 69 and 79 degrees ATDC, with higher rod ratios favoring the later position. It only occurs at 1/2 stroke (90 degrees) with a connecting rod of infinite length.
The wave/flow activity in a real engine is vastly more complex than this but the principle is the same.
At first glance this wave travel might seem to be blindingly fast and not very significant but a few calculations shows the opposite is true. In an intake runner at room temperature the sonic speed is about 1,100 feet per second (340 m/s) and traverses a 12-inch (300 mm) port/runner in 0.9 milliseconds. The engine using this system, running at 8500 rpm, takes a very considerable 46 crank degrees before any signal from the cylinder can reach the runner end (assuming no movement of the air in the runner). 46 degrees, during which nothing but the volume of the port/runner supplies the demands of the cylinder. This not only applies to the initial signal but to any and every change in the pressure or vacuum developed in the cylinder.
Why couldn’t we just use a shorter runner so the delay is not so great? The answer lies at the end of the cycle when that big long runner now continues to flow at full speed disregarding the rising pressure in the cylinder and providing pressure to the cylinder when it is needed most. The runner length also controls the timing of the returning waves and cannot be altered. A shorter runner would flow earlier but also would die earlier while returning the positive waves much too quickly and those waves would be weaker. The key is to find the optimum balance of all the factors for the engine requirements.
Further complicating the system is the fact that the piston dome, the signal source, continually moves. First moving down the cylinder, thus increasing the distance the signal must travel. Then moving back up at the end of the intake cycle when the valve is still open past BDC. The signals coming from the piston dome, after the initial runner flow has been established, must fight upstream against whatever velocity has been developed at that instant, delaying it further. The signals developed by the piston do not have a clean path up the runner either. Large portions of it bounce off the rest of the combustion chamber and resonate inside the cylinder until an average pressure is reached. Also, temperature variations due to the changing pressures and absorption from hot engine parts cause changes in the local sonic velocity.
When the valve closes, it causes a pile up of gas giving rise to a strong positive wave that must travel up the runner. The wave activity in the port/runner does not stop but continues to reverberate for some time. When the valve next opens, the remaining waves influence the next cycle.
This graph shows the pressure taken from the valve end (blue line) and the runner entrance(red line) of an engine with a 7-inch (180 mm) port/runner and running at 4500 rpm. Highlighted are two waves, suction wave and valve closing wave, seen and the valve end and runner entrance showing the signal delay. A lag of about 85 deg for the peak suction wave versus about 32 deg for the peak pressure wave. A difference of some 53 deg due to the movement of the gas and piston position.
The graph above shows the intake runner pressure over 720 crank degrees of an engine with a 7-inch (180 mm) intake port/runner running at 4500 rpm, which is its torque peak (close to maximum cylinder filling and BMEP for this engine). The two pressure traces are taken from the valve end (blue) and the runner entrance (red). The blue line rises sharply as the intake valve closes. This causes a pile up of air, which becomes a positive wave reflected back up the runner and the red line shows that wave arriving at the runner entrance later. Note how the suction wave during cylinder filling is delayed even more by having to fight upstream against the inrushing air and the fact that the piston is further down the bore, increasing the distance.
The goal of tuning is to arrange the runners and valve timing so that there is a high-pressure wave in the port during the opening of the intake valve to get flow going quickly and then to have a second high pressure wave arrive just before valve closing so the cylinder fills as much as possible. The first wave is what is left in the runner from the previous cycle, while the second is primarily created during the current cycle by the suction wave changing sign at the runner entrance and arriving back at the valve in time for valve closing. The factors involved are often contradictory and requires a careful balancing act to work. When it does work, it is possible to see volumetric efficiencies of 140%, similar to that of a decent supercharger, but it only occurs over a limited RPM range.
The "Porting and Polishing" myth
It is popularly held that enlarging the ports to the maximum possible size and applying a mirror finish is what porting is. However that is not so. Some ports may be enlarged to their maximum possible size (in keeping with the highest level of aerodynamic efficiency) but those engines are highly developed very high speed units where the actual size of the ports has become a restriction. Larger ports flow more fuel/air at higher RPM's but sacrifice torque at lower RPM's due to lower fuel/air velocity. A mirror finish of the port does not provide the increase that intuition suggests. In fact, within intake systems, the surface is usually deliberately textured to a degree of uniform roughness to encourage fuel deposited on the port walls to evaporate quickly. A rough surface on selected areas of the port may also alter flow by energizing the boundary layer, which can alter the flow path noticeably, possibly increasing flow. This is similar to what the dimples on a golf ball do. Flow bench testing shows that the difference between a mirror finished intake port and a rough textured port is typically less than 1%. The difference between a smooth to the touch port and an optically mirrored surface is not measurable by ordinary means. Exhaust ports may be smooth finished because of the dry gas flow and in the interest of minimizing exhaust by-product build-up. A 300 - 400 Grit finish followed by a light buff is generally accepted to be representative of a near optimal finish for exhaust gas ports.
The reason that polished ports are not advantageous from a flow standpoint is that at the interface between the metal wall and the air, the air speed is ZERO (see boundary layer and laminar flow). This is due to the wetting action of the air and indeed all fluids. The first layer of molecules adheres to the wall and does not move significantly. The rest of the flow field must shear past, which develops a velocity profile (or gradient) across the duct. For surface roughness to impact flow appreciably, the high spots must be high enough to protrude into the faster moving air toward the center. Only a very rough surface does this.
A developed velocity profile in a duct that shows why polished surfaces have little effect on flow. The air speed at the wall interface is zero regardless of how smooth it is.
In addition to all the considerations given to a four-stroke engine port, two-stroke engine ports have additional ones:
The ports are responsible for sweeping as much exhaust out of the cylinder as possible and refilling it with as much fresh mixture as possible without a large amount of the fresh mixture also going out the exhaust. This takes careful and subtle timing and aiming of all the transfer ports.
Power band width
Since two-strokes are very dependent on wave dynamics, their power bands tend to be narrow. While struggling to get maximum power, care must always be taken to ensure that the power profile does not get too sharp and hard to control.
Two-stroke port duration is often expressed as a function of time/area. This integrates the continually changing open port area with the duration. Wider ports increase time/area without increasing duration while higher ports increase both.
In addition to time area, the relationship between all the port timings strongly determine the power characteristics of the engine.
Wave Dynamic considerations
Although four-strokes have this problem, two-strokes rely much more heavily on wave action in the intake and exhaust systems. The two-stroke port design has strong effects on the wave timing and strength.
The flow of heat in the engine is heavily dependent on the porting layout. Cooling passages must be routed around ports. Every effort must be made to keep the incoming charge from heating up but at the same time many parts are cooled primarily by that incoming fuel/air mixture. When ports take up too much space on the cylinder wall, the ability of the piston to transfer its heat through the walls to the coolant is hampered. As ports get more radical, some areas of the cylinder get thinner, which can then overheat.
Piston ring durability.
A piston ring must ride on the cylinder wall smoothly with good contact to avoid mechanical stress and assist in piston cooling. In radical port designs, the ring has minimal contact in the lower stroke area, which can suffer extra wear. The mechanical shocks induced during the transition from partial to full cylinder contact can shorten the life of the ring considerably. Very wide ports allow the ring to bulge out into the port, exacerbating the problem.
Piston skirt durability
The piston must also contact the wall for cooling purposes but also must transfer the side thrust of the power stroke. Ports must be designed so that the piston can transfer these forces and heat to the cylinder wall while minimizing flex and shock to the piston.
Engine configuration can be influenced by port design. This is primarily a factor in multi-cylinder engines. Engine width can be excessive for even two cylinder engines of certain designs. Rotary disk valve engines with wide sweeping transfers can be so wide as to be impractical as a parallel twin. The V-twin and fore-and-aft engine designs are used to control overall width.
Engine sealing ability, cylinder, piston and piston ring life all depend on reliable contact between cylinder and piston/piston ring so any cylinder distortion reduces power and engine life. This distortion can be caused by uneven heating, local cylinder weakness, or mechanical stresses. Exhaust ports that have long passages in the cylinder casting conduct large amounts of heat to one side of the cylinder while on the other side the cool intake may be cooling the opposite side. The thermal distortion resulting from the uneven expansion reduces both power and durability although careful design can minimize the problem.
The turbulence remaining in the cylinder after transfer persists into the combustion phase to help burning speed. Unfortunately good scavenging flow is slower and less turbulent.
The die grinder is the stock in trade of the head porter and are used with a variety of carbide cutters, grinding wheels and abrasive cartridges. The complex and sensitive shapes required in porting necessitate a good degree of artistic skill with a hand tool.
Until recently, CNC machining was used only to provide the basic shape of the port but hand finishing was usually still required because some areas of the port were not accessible to a CNC tool. New developments in CNC machining now allow this process to be fully automated with the assistance of CAD/CAM software. 5-Axis CNC controls using specialized fixtures like tilting rotary tables allow the cutting tool full access to the entire port. The combination of CNC and CAM software give the porter full control over the port shape and surface finish.
Measurement of the interior of the ports is difficult but must be done accurately. Sheet metal templates are made up, taking the shape from an experimental port, for both cross-sectional and lengthwise shape. Inserted in the port these templates are then used as a guide for shaping the final port. Even a slight error might cause a loss in flow so measurement must be as accurate as possible. Confirmation of the final port shape and automated replication of the port is now done using digitizing. Digitizing is where a probe scans the entire shape of the port collecting data that can then be used by CNC machine tools and CAD/CAM software programs to model and cut the desired port shape. This replication process usually produces ports that flow within 1% of each other. This kind of accuracy, repeatability, time has never before been possible. What used to take 18hrs. or more now takes less than 3hrs.
Valves and valve seats are ground with special equipment designed for this purpose.
The internal aerodynamics involved in porting is counter-intuitive and complex. Successfully optimizing ports requires an air flow bench, a thorough knowledge of the principles involved, and engine simulation software.
Although a large portion of porting knowledge has been accumulated by individuals using "cut and try" methods over time, the tools and knowledge now exist to develop a porting design with a measure of certainty. Porting by inexperienced individuals without a full understanding of the fluid dynamics of the process still continues but the results are spotty and the process is expensive and time consuming with many more failures than successes.
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more to come
---------- Post added 21-10-12 at 21:18 ----------
It has been said many times that an internal combustion engine is nothing more than an air pump, and the more air it can pump, the more power it will make. As with any simplification, it is not all true, but it's a heck of a good place to start. Here, we are going to take a serious look at not only what it takes to get air through an engine, but also how to use it as effectively as possible on the way through.
The number-one flow restriction in any engine is within the cylinder head. Get air to effectively pass these restrictions and you are good to go. It's a big subject to cover, so to chop it down to more manageable bites, we are going to apply five basic rules:
1.Locate the point of greatest restriction, and work on that first.
2. Try to let the air move the way it wants to, not the way you think it should.
3. Air is heavier than you think. Keep the port velocity up and avoid redundant cross-sectional areas.
4. Mixture motion (swirl or tumble, or a combination of both) is important--do not ignore it.
5. Shape is all-important--a shiny finish is not.
Locate the Point of Greatest Restriction
Like it or not, the valves are part of the ports, and when they are closed their ability to flow is exactly zero. This means until they have opened quite a way, the valve is the main restriction to the engine's airflow. Even when the valve is at a decent lift, it still presents a tortuous path for the air to travel on its way into or out of the cylinder. Making a comparison of the valve's ability to flow compared to the rest of the port on a stock production small-block Chevy head demonstrates just how much of an impediment the valve is even when it is wide open.
First the main body, Section A. This is a highly flow efficient, rectangular cross-section straight tube and flows 300 cfm. Section B, the turn into the seat area, flows some 200 cfm. Now let's consider section C. This is the only part of a port that has a moving component--the valve. When the valve is seated, flow is zero. If the valve is lifted high enough, it moves beyond the seat's influence, but to do so takes a lot of lift. In practice, half the valve's typical full lift is about representative of its average flow potential. In our example, this is about 140 cfm. From this we can see that at some small amount of valve lift, flow restriction is 99 percent at the valve seat and one percent in the port. At very high valve lift (.75 inch or more) this situation is pretty much reversed.
Our number-one priority then is to make the valve capable of passing as much air as possible--whatever the lift involved. To do this we need to address both size and efficiency. What we are searching for is the biggest effective valve.
Having identified the most restrictive point along the engine's gas flow path, let's see what can be done to improve it. Although it may be the last operation during a porting exercise, the valve seat design is the first priority toward effectively filling a cylinder. So let's look specifically at valve seat design.
Forms for Flow
Let's consider the valve seat isolated from the rest of the port. Fig 2 shows a progression from a very basic form "A" where the throat of the port is exactly the same size as the valve, to a relatively well-developed form at D. At first sight, it would seem the hole under the valve head needs to be as large as possible so as to flow the most air. Before flow benches became popular, it was often common practice (and often written up as such) to make the valve seat as thin as possible so as to achieve the maximum throat diameter. The flow bench shows this to be a bad move. In reality, maximum flow is always a combination of size and form around the valve before and after the seat. As you can see from Fig 2, the form for the most effective valve/seat/throat combination progresses toward that shown in Fig 3.
Applying the basics outlined for seats will get good results, but to get the max requires a lot of time on a flow bench, as subtle changes can make measurable differences. Often some changes in seat angles are used by top pros to enhance some particular feature. For instance, most Cup car heads have 50- or even 55-degree valve seats. These give up a little low-lift flow, but start to pay back above .5-inch lift. Also, the steeper valve angle acts as an impact damper, reducing the valve's tendency to bounce off the seat at closure time. For big-inch under-valved engines, 30-degree seats can be advantageous, as they make the valve appear bigger than it really is and flow more air during the initial opening phase.
Seats and Port Approach Angles
Air has mass and does not like to hug a port wall around a short-side turn. That is why purpose-built race heads have steeply down-drafted ports. But when heads have to fit under low hoods, port angles have to come down. With low-angle ports, the air (at mid and high valve lifts) does not make it around the short-side turn very well. As a result, most of the air goes out of the long-side turn. This is a situation that becomes more exaggerated the higher the lift becomes. As a result, the streamlining of the port on the long side needs to cater for low, medium and high lifts, while the seat approach on the short side needs only to deal with the requirements of low-lift flow. Fig 4 illustrates what is going on with a low angle of attack port (i.e. less than about 30 degrees).
Just so you know, we are still working on rule number one, and the subject is now valve shrouding. Remember, the goal was to have as large and efficient valves as possible. For a typical parallel- or nearly parallel-valve head, valve shrouding is about the biggest impediment to achieving that goal. Worse yet, the bigger the valve gets, the bigger the valve shrouding problem becomes. Valve shrouding seriously impedes flow and starts having a negative impact at about 75 thousandths lift. If we are talking two virtually parallel valves, then the negative effect of shrouding increases until the valve gets to a lift value equal to about a quarter of its diameter, or as it is more often know, 0.25D (Fig 5) If nothing impedes the flow and the air exits uniformly all the way around its circumference, then the valve can be said to be un-shrouded. Unfortunately, few cylinder heads have totally un-shrouded valves. Fig 6 shows the situation for most parallel-valve, two-valve engines. As you can see, the presence of the cylinder wall, and (in this case) the proximity of part of the combustion chamber wall, cuts the area that the air can use to escape from around the valve's circumference. The circles around each valve show how far the cylinder and chamber walls would have to be clear of the valve's periphery to produce an un-shrouded situation. In essence, this would be like having a valve at the bottom of a cone with the walls 38 degrees off being flat (Fig 7).
Be it the intake or exhaust, the worst part of the port for air to negotiate is the short-side turn. If the air fails to make it around the short-side turn, there obviously won't be much air exiting the valve in that area. Since the valve is not being fully utilized around the short side, there is no point in de-shrouding the valve to the extent needed on the long side. A modern high-performance head has little shrouding from the combustion chamber, but be aware, it is possible to cut the chamber wall too much while un-shrouding the valve, especially the exhaust, and pay a performance penalty. This is just one reason why a flow bench is a great asset.
If we were to plot out the flow efficiency of either valve for a parallel two-valve engine, we would find that above 0.25D of valve lift the flow efficiency starts to climb back up from a low. The increase in efficiency starts at a point when the flow stops trying to exit all around the valve and instead "windows" predominantly out one side of the valve (Fig 8). This is why two-valve engines with well-developed ports thrive on ultra-high valve lift. Since I have brought in the subject of "windowing," let's see what the implications are.
Mods to Minimize Shrouding Effects
At this point, we are easing into the application of rule number two. If shrouding is to be minimized, we must take steps to establish what alternative route the air could have to find its way into the chamber in a relatively unimpeded fashion.
A study of the flow into and out of the cylinder of a two-valve engine shows that the principle direction of entry or exit is either toward (for the intake), or away (for the exhaust) from the center of the cylinder. This being the case (and applying rule number two) we need to find a way to let the air flow more nearly along its preferred route. This is done by "port biasing" in the direction the flow wants to "window." This helps reduce the negative effect of shrouding. Applying this biasing technique can make substantial improvements in high-lift flow. Although port bias pays off from mid-lift values, it really comes into its own when lift exceeds 0.25D.
---------- Post added 21-10-12 at 21:21 ----------
FIG. 1-Here's a small-block Chevy head (lower left) divided into three sections. From the flow figures of each section, it can be seen that the worst impediment to flow occurs at the valve, not in the main body of the port
FIG.2-With its sharp edge, A has the worst efficiency of this group. The single-cut 45-degree seat (B) is smaller under the valve but flows much better. With a three-angle cut (C) the seat area is better yet, and a forth cut (D) at the bottom (blue) can usually add a little more
FIG. 3-Three- and four-angle valve jobs are approaching the form seen on the port side of this seat. The same idea (i.e. curving the shape away from the seat) is also functional on the chamber-side of the seat.
---------- Post added 21-10-12 at 21:22 ----------
FIG 4-At high valve lift, most of the air fails to make it around the short-side turn and instead exits section A on the long side. This biases the direction of the flow and contributes toward a strong swirl action, which is desirable for low- and mid-speed torque.
---------- Post added 21-10-12 at 21:25 ----------
FIG. 5This shows what is meant by valve curtain area. At a lift equal to .25 of the valve's diameter, the curtain area is equal to the valve area
FIG. 6-For the valves of this small-block Chevy head to be totally un-shrouded, there would need to be a clear space out to the circles around each valve. As can be seen here, the bore causes shrouding (red) as well as the combustion chamber (green).
FIG. 7-If the combustion chamber wall progresses away from the valve seat at a 38-degree angle as shown here, the valve will be just at the point of being totally un-shrouded.
---------- Post added 21-10-12 at 21:28 ----------
FIG. 8-Biasing the port so that the flow "windows" toward the center of the cylinder reduces the mid- to high-lift shrouding seen by the valve. The port bias can also help generate swirl.
FIG. 9-This theoretical ideal port form progressively accelerates the air into the high-speed section of the port. The velocity it builds can then be converted back to pressure energy at the expansion section. This can help increase the pressure difference across the valve, thus pushing a greater weight of charge into the cylinder
FIG. 11-By applying the simple techniques shown to a basic round cross-section intake port, the high-lift flow efficiency can climb from a typical 45 percent to as high as 65 percent. With extensive flow testing it is possible to get well into the 80 percent range.
---------- Post added 21-10-12 at 21:33 ----------
The ports of this Nextel Cup head demonstrate how large a short-side turn needs to be for the air to make it around the turn and still have some semblance of attached flow.
This view of a very effective intake port shows how the port expands as it goes around the guide boss. The same technique is applied to the exhaust, but it is not quite as evident in this view.
FIG. 11-These tests, done on a 350 Chevy, show that though the bigger ports flowed about 5 cfm more at .5-inch lift, the smaller, higher-velocity ports produced a much better power curve. At 2,250 rpm, the smaller ports were up by 30 lb-ft. An engine equipped with the bigger ports would need about 25 cubes more to match the low-speed output of the smaller-port heads.
FIG. 12-The steeper the port angle, the larger the optimum size port is for the valve size used. As the angle is lowered and the short-side turn becomes tighter, so the optimum port diameter drops.
---------- Post added 21-10-12 at 21:38 ----------
FIG. 13-A good exhaust port strongly resembles a nozzle. This is one of the reasons why the exhaust flow efficiency numbers are typically higher than the intake.
Here is the difference between a typical aftermarket high-performance street head and an all-out race head. At typical street cam lift figures, the advantage of the race head is not very significant in most instances.
And there you have it people, im still extensivly researching the topic and more than likely there is more to add, stay tuned for other topics ill be looking into if this sort of stuff interests you.
kind regards sle3per
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Last edited by sle3per; 21-10-12 at 20:33.
Join Date: 02-06-10
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awesome read really gave me a great insight into the way the head really works and the detail on the porting is awesome i now have a much much better understanding on how porting works and how to improve/hinder performance .
thanks a million
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Having read most of this and having had a crack at my own 30 head a little while back...I have reached the conclusion that the exhaust port in particular on the 30 head is sh1te? Correct? There is almost no short turn??? I took the floor out in proportion to the walls and started to see the seat : /
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You almost need to add material to the exhaust port floor on a 30 head. then open up the roof a little. With a bit of time these heads could really flow hard with raised ports
3D CAD/Production Drawing - PM
Sleeper build thread - http://www.calaisturbo.com.au/showthread.php?t=235527
Join Date: 01-03-05
Location: International, In the shadows
Car: 8th Gen Civic...VLT BT1 mock up uleh : /
Trader Rating: (23)
Anyone else had a crack at their own 30 head? Post up some pics of the results!
9/11...the new Pearl Harbour- http://www.youtube.com/watch?v=PwqLu...layer_embedded