Parts of the Port

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 we are able to 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 do.

Accepting the expansion into the cylinder loss as unavoidable, the rest of the port becomes that much more important. The areas which pass the most air at the highest speed for the longest time are the areas that are most important.

The valve seat configuration on the port and on the valve together form one of the most critical areas in the port. The highest speed seen in the port will be at or near the valve seat for most if not the entire 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 should be studied carefully in each design.

Sometimes in the pursuit of airflow, greed can get the best of any porter, and the tendency is to go too big in some places. Nowhere is the price to pay higher than going too big in the port throat, the point of constriction just below the valve seat. Make the throat too big, and the venturi effect is ruined, and usually the flow will be too. Keep the intake port throat no larger than 90percent of the valve diameter, and the exhaust throat down around 85percent.

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 are factors which must be considered in concert with the overall design of the rest of the engine and vehicle to achieve good results.

Wave Dynamics

When the valve opens, the air doesn’t flow in, it decompresses into the low-pressure region. All the air on the upstream side of the moving disturbance boundary is completely isolated and unaffected by what happens on the downstream side of it. 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.)

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 has closed reaches it. The closing valve causes a buildup of pressure which will travel 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. At the closing of the intake valve, pressure rises far above atmospheric.

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 principal 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 in the gas inside the runner. The boundary between the wave affected gas and unaffected gas could be compared to the event horizon of a black hole.

This is why port/runner volumes are so important. The volumes of successive parts of the port/runner control the flow during all transient periods. That is any time a change occurs in the cylinder whether positive or negative. Such as when the piston reaches maxumum speed half way down the stroke.

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 1100 feet per second and will traverse a 12 inch 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. 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, which is the source of the signal, 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, further delaying the signal. The signals developed by the piston do not have a clean path up the runner either. Large portions of it will bounce off the rest of the combustion chamber and resonate inside the cylinder until an average pressure is reached. Then there are temperature variations due to the changing pressures and absorption from hot engine parts. These variations 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 which 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 7inch 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.

The graph above shows the intake runner pressure over 720 crank degrees of an engine with a 7-inch intake port/runner running at 4500 RPM, which is it's 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 and 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 in order to fill the cylinder as much as possible. The first wave will be what is left in the runner from the previous cycle while the second will primarily be one 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.

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. Often the size of the port is reduced to increase power. A mirror finish of the port does not provide the increase that intuition would suggest. 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 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 but an optical finish is wasted effort and money.

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. 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. In order 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. Surface roughness (Reynolds Number) does have an affect on the velocity profile. Smoother walls produce long spikes in velocity and rough finished tend to keep the profile more compact.

If you have one, get out your high school physics book and study up on Bernoulli's equation. It describes the relationship between pressure and velocity in a fluid as it flows through a pipe, which changes in cross sectional area along its length. Bernoulli's equation translated, says that as you increase the velocity of the fluid, the pressure of the fluid at that point decreases, and if you slow the fluid down, the pressure of the fluid increases, and how much it increases or decreases. How do you change the speed of the air in a port? Simply by making the port bigger (slower) or smaller (faster). Also, no fluid, including air, likes to change direction, because doing so causes it to lose velocity and energy which is hard to recover. To better understand your mission in porting heads, you should spend some time thinking about how all this works in an engine.

The cylinder head is the part of an engine that is most responsible for its performance characteristics. Once the basic geometry of an engine is established, there is no other part that has as much influence on the amount of power developed, and the shape of the power curve. All the other parts are merely supporting cast.

So, what determines the worth of one head over another? First, you must understand that any design is a compromise between what is desirable and what is possible. Engineers who initially design an engine rarely have free rein to make it the most powerful piece possible - and they may not want to either. Even in Formula 1 racing, where engines are designed from scratch to make as much power as possible, there are compromises that are determined by the rules of the sanctioning body and the necessity to install the engine in the car. Like other vehicles, aerodynamics and handling requirements require compromises in size, shape and weight of the engine.

The vast majority of today's popular aftermarket cylinder heads are compromised because they adhere to standard OEM port geometry. This is done so the supporting components designed to that geometry can be used on the new head. As engine builders, most of us have to work with parts that already exist. They may be production parts or aftermarket parts, but they all have compromises, and it's up to us as porters, to minimize the compromise.

How Airflow is Measured
Most people interested in performance know that a flow bench is used to measure airflow, but lacking hands-on experience, don't understand how it works, how it is used to measure flow in a cylinder head, or what the flow numbers actually mean.

You should know that a fluid flows from high pressure to low pressure. Air, being a fluid, follows that rule. If you turn on your vacuum cleaner, the motor creates a low pressure inside the cleaner, and atmospheric pressure, now being higher, pushes air in to fill the void. The rate of flow of the fluid is proportional to the difference in pressure. Seal the vacuum hose to the combustion chamber of a cylinder head, open the intake valve and turn the motor on. Air will flow through the port and into the cleaner. Add a valve in the hose to regulate how much pressure or suction you are using, and a means of measuring the suction and the amount of flow - usually done with manometers - and you have a flow bench.

All this flow bench does is move air through a port by creating a predetermined pressure differential, and then measure the quantity of air being moved. Tests can be done at any pressure you choose, up to the limit of the bench's capability. Most are done at 10", 25", or 28" of water, but the trend is to higher pressures, like 60".

A cylinder head adapter is commonly used to mount the head to the bench (as opposed to the vacuum hose previously mentioned) so the effect of cylinder wall shrouding can be simulated. Either a radiused inlet guide or an intake manifold can be attached to the head to eliminate turbulence at the manifold flange, and if testing the exhaust side, a short length of appropriately sized exhaust tubing is mounted on the header flange. A rigid fixture that will open the valve in .001" increments is needed too. Mount the head on the head adapter, open the intake valve to the first increment of lift, say, .100". Turn on the motor, and set the control valve at a test pressure such as 25", and record the amount of flow. Open the valve to the next increment of lift, such as .200", and repeat the test, again at 25" of water. A similar test is done at each increment of lift you wish to test. You have now flow tested one intake port, and have some data telling you how much flow your port has at 25" of water, at each increment of lift you tested. To be meaningful, all tests should be done at the same pressure, and use the same inlet or outlet configuration, and the same test procedure. Simply using a different inlet radius, valve shape, or cylinder diameter can change the flow. In other words, sweat the small stuff and pay attention to the details to ensure repeatability and make comparisons from one test to another valid.

To this, you can add tests for tumble and swirl in the cylinder, do localized testing within the port with test probes to determine velocity distribution and turbulent areas, try different valve and seat shapes, and even do wet flow testing if you have the capability. You can also reverse the head on the flow bench and check the flow characteristics around the valves in the combustion chamber (blow through the intake port and suck through the exhaust port).

A standard for maximum flow through a valve is 146 CFM per square inch of valve opening. This is used to rate the efficiency of a port. I use the valve curtain area - the circumference of the valve head (3.1416 x diameter), x valve lift, x 146 CFM, and then divide the result into the flow. This gives a percentage of the standard at each valve lift. If the port in the head can be made to flow up to the standard then it would in effect be achieving 100 percent efficiency. If it only flows half as much, then it would be 50 percent efficient. Another way to rate flow is to relate it to the area of the head of the valve. So now you have a means to test a port and a means to rate that port at each valve lift, relative to a standard. From that you can evaluate the efficiency of different heads based on valve size and lift.

Remember, a flow bench, like a dynamometer, is just a tool. It will give you data, but it will not tell you how to interpret that data, nor tell you what decisions you need to make regarding the suitability of the port you tested, nor will it tell you how to change the port to make it better. From this point, you must evaluate your data and make those decisions - and therein lies a large portion of the skill in modifying cylinder heads.

How Airflow Influences Engine Performance
Volumetric efficiency (VE) is the measure of how well the cylinder is being filled with air, as a percentage of what it would be if it were filled to the same pressure as the atmosphere outside the engine.

As the piston moves away from TDC, it creates a vacuum in the cylinder, which draws in the fresh charge of air and fuel. As the piston accelerates from TDC toward its point of maximum velocity at around 75 degrees after TDC, flow lags behind demand, creating a higher and higher vacuum in the cylinder. Then the piston slows down until at BDC it parks for a brief period before starting back up the cylinder. As the piston nears BDC, the inertia of the air from its velocity causes flow to catch up with piston demand and then finally exceed it.

After bottom dead center, the piston is going the wrong direction to pull in air, so even though we don't get 100 percent VE on the down stroke of the piston, we can achieve high overall volumetric efficiency by holding the intake valve open long after bottom dead center, to take advantage of the momentum of the intake charge (from its velocity) and the resonant tuning of the intake port, to keep filling the cylinder after the piston changes direction. This packing of the cylinder continues as the piston continues back up the cylinder until the valve closes. In a properly "tuned" intake system, a pressure wave will also arrive at the valve shortly before it closes, packing even more air in the cylinder. If it were not for the inertia of the incoming air, and resonant tuning of the port, it would be impossible to achieve even 100 percent cylinder filling. Typical low performance production engines operate in the 60 percent VE range.

The demand on the intake port is partially a function of the piston speed, which is zero at top and bottom dead centers, and can reach 8,000 feet per second or more (about 5,400 mph) somewhere between 70 and 80 degrees after top dead center. For a 4" bore by 3.48" stroke 350 Chevy to achieve 100 percent volumetric efficiency at this point, the head would have to flow over 500 CFM. It is unlikely that we will find a head for a small block Chevy that will flow this much, and if we did size the port for the 500 CFM demand at this point in the cycle, it would be too big during the remainder of the cycle and have insufficient velocity to continue filling the cylinder after bottom dead center. Ports and valves that are too big don't generate sufficient velocity in this segment of the intake cycle, and therefore don't make as much power as a port that is properly sized. Bigger is not always better. With resonant tuning and high velocities, typical performance engines can operate close to 100 percent, while more highly refined engines can achieve 120 percent or more, especially engines with multiple intake valves per cylinder.

On the exhaust side, velocity is important too. The velocity of the exhaust pulse in the port and header creates a vacuum behind it, creating a pressure drop in the cylinder as the piston approaches TDC on the exhaust stroke. This pressure drop from the exhaust during valve overlap, gets the intake system started before the piston even starts down on the intake stroke.

The kinetic energy in the air moving in and out of the engine is a function of the square of the velocity, so small changes in velocity make large changes in the energy of the flow. Even though the conditions in a running engine are constantly changing throughout each intake and exhaust cycle, steady flow on a flow bench can give a good representation of the power available from the engine by approximating the average conditions in the engine. Tests done at 25" of water-test pressure seem to closely approximate the average conditions that exist in an engine.

As I mentioned earlier, there is a trend to testing at much higher pressures. Peak velocities in a port can be over 600 feet per second, but testing an intake port sized for high rpm power at 25" of water may only have 200 feet per second on the flow bench. It may be very efficient at that velocity, but have high levels of turbulence at twice that velocity. The only way to determine the worth of a port at the higher velocities is to test it at those velocities, requiring higher test pressures.

When you increase the efficiency of a port in the normal direction, you also increase the efficiency of the port in the wrong direction. This makes the engine more sensitive to camshaft changes, and to intake and exhaust system tuning. An intake manifold or exhaust system that worked just fine before, may not work very well after the heads are done, not because it's a bad piece, but because it's the wrong piece.

So, what is the process to determine the correct cylinder head or head modifications needed? Whether you are dealing with a street rod or a serious race effort, the first thing to do is determine how the engine will be used, the rpm range it will see, the engine specs, and a power goal. I regularly deal with customers that have never answered those questions, they just want their heads ported because they believe it will make more power, but until they are answered, you have no way to determine the optimum port and valve sizes, and flow requirements. If you want a pair of heads for a rock crawler that mostly runs just above idle, but you ported them the same as you would for a 7,000 rpm bracket racer, you would probably not be happy with the results. Once these things are known, you can proceed.

There are mathematical relationships that can predict the airflow requirements to reach a specific power - or predict the power potential of a known airflow - and the rpm at which peak power will be developed. This includes the intake manifold and carb or throttle body. These equations can be used to see if you are in the "neighborhood" of what you want to do. If the rest of the engine combination is complimentary, they will be pretty accurate. If you have the airflow desired, but the engine does not make the power it should, then you have another problem that needs fixing.

Horsepower per cylinder = .43 x airflow @ 10" of water, .275 x airflow @ 25" of water, or .26 x airflow at 28" of water. To find required airflow for a given horsepower, divide the horsepower per cylinder by .43, .275 or .26 respectively. 300/8/.26=144cfm 350/8/.26=168cfm 400/8/.26=192cfm

RPM at peak horsepower will be 2,000 divided by the displacement of one cylinder x airflow @ 10? of water. Use 1,267 @ 25" of water, or 1,196 @ 28" of water. The more airflow available to the cylinder, the higher the rpm required to reach peak horsepower.

The maximum practical intake valve size in a two valve, wedge chamber head is about .52 x the diameter of the bore. In a hemi type chamber, .57 is about the maximum.

How to Improve Airflow
The vast majority of porting work can be described as merely cleaning up an existing port and doing a valve job for improved performance for a street rod, boat, or for sportsman level racing. No matter what the application, an understanding of what is needed and a methodical approach to the modifications will usually result in a satisfactory level of performance. A flow bench is essential, because it's really easy to make things worse. It can be a commercial unit or home built, as long as it is repeatable.  That being said, a flow bench should be used as tool and a tool only!  Flow benches can be used to blow a lot of smoke up your shop coat when you're looking for horsepower. You can always make air flow numbers rise by increasing valve head diameter, or by enlarging the passages leading from the atmosphere. But higher air flow numbers do not necessarily translate into more power, as many in the engine development field have discovered.

Ford's 1960's four-cam V-8 had huge intake ports, and while it turned more revs than the Offy four-banger engines then dominant at Indianapolis, it was no better than a match for them. When given an early peek at the Indy Ford's cylinder-head castings, I expressed the thought that its ports might be too big. Ford's engineers were too polite to tell me how absurd they considered my remark to be, but their expressions made it plain. I was too polite to send them an "I told you so" note after Dan Gurney sent one of the engines to Weslake Engineering in England, where it's intake ports were made smaller and its output got bigger.

Ford's engineers were then vastly ignorant of the world beyond Michigan's borders. They had no idea Harry Weslake and Wally Hassan (who created the very successful Coventry-Climax racing engines) had learned years before not to take too literally what the flow bench said. They were narrowing intake ports to provide nominal gas speeds in the range of 350 to 400 feet-second, making good use of the fact that kinetic energy packing air into the cylinders increases with the square of it's velocity.

To estimate runner air speed, take flow cfm @ 28” divide it by the limiting cross section area and multiply by 2.4, i.e.:

272cfm/2.1sqin*2.4 = 311 ft/sec theoretical. Actual is often higher.

FPS = ( CFM / CA ) * 2.4   i.e. FPS = (210/2.1 - Cobra intake) * 2.4 = 240 fps

In practice rules of thumb have developed saying that at peak power, the ft/sec figure should be somewhere within 280-380 exh and 240-355 intake.  Also see the tech article "Determining Port Velocity and Volume".

CFM = FPS * CA * .41666667

CA = ( CFM / FPS ) * 2.4

RPM = ( FPS * CA ) / ( Bore * Bore * Stroke * .00353 ) 
       
FPS = ( Bore * Bore * Stroke * RPM * .00353 ) / CA
        = ( 4.03 * 4.03 * 3.47 * 6,700 * .00353 ) / 2.2
        = 606 fps

CA = ( Bore * Bore * Stroke * RPM * .00353 ) / FPS

Mach number = FPS / 1116  (.627 Mach = 127.5 % Volumetric Efficiency potential,.55 MACH = 121.1 % VE)

where;
RPM = point of desired Peak 
FPS = Feet per Second
CA = Cross-Sectional Area in Square Inches (smallest measured)
614 fps = ~ .55 Mach. This is a rate that should not be exceeded due to flow difficulties.
============================================
i.e. if your 347 engine chokes @ 6600 rpm with a 2.2 CA use:
RPM = ( FPS * CA ) / ( Bore * Bore * Stroke * .00353 ) 
        = ( 606 * 2.2 ) / ( 4.030 * 4.030 * 3.470 * .00353 ) = 6701 rpm

pretty close to the 6600 RPM "Choke" you are experiencing

then solve the other way:
CA = ( Bore * Bore * Stroke * RPM * .00353 ) / FPS

2.145 = ( 4.030 * 4.030 * 3.480 * 6600 * .00353 ) / 614

the 2.145 rounds off to 2.2 ..pretty close to your 2.2

So, what are the flow rates that we should shoot for – in general the following can be used as guide:

240 ft/sec - intake - ram effect faint (.21 Mach)
- exhaust- scavenge faint

260 ft/sec - intake - ram effect moderate (.23 Mach)
- exhaust- scavenge weak to moderate

280 ft/sec - intake - substantial ram (.25 Mach)
- exhaust - scavenge moderate

300 ft/sec - intake - * ideal ram (.269 Mach)
- exhaust - substantial scavenge

320 ft/sec - intake - possible loss (.287 Mach)
- exhaust - * ideal scavenge

340 ft/sec - intake - likely loss (.305 Mach)
- exhaust - possible loss

In practice rules of thumb have developed saying that at peak power, the ft/sec figure should be somewhere within 280-380 exh and 240-355 intake. Velocity over 600fps (.55 Mach) often cause inertia blocks and/or flow separation and take a very well designed port to work.

you can use this with Air Velocity FPS to solve for what is the required Intake Valve diameter needed for a certain "Peak HP RPM"

Intake Valve = (( RPM * CID ) / ( Cylinders * 314.5 * 282.743)) ^.5

1. 528 = ((5,500*302) /(8*314.5*282.743)) ^.5

1.724 = ((7,000*302) / (8*314.5*282.743)) ^.5

1. 60 = ((5,500*331) / (8*314.5*282.743)) ^.5

1. 80 = ((7,000*331) / (8*314.5*282.743)) ^.5

where;
RPM = the point you want Peak HP to occur
CID = total engine size in Cubic Inches
Cylinders= the number of engine cylinders
314.5 = Air velocity in Feet per Second
282.743 = Units Constant
^ .5 = Square Root of a Number

The level of porting you wish to do will determine the approach taken. You can start with a good valve seat shape and blend it into the bowl under the valve and to the chamber and call it good, or you can get into a complete ongoing cylinder head development project, or somewhere in between.

The area from around the valve guide across the seat and into the chamber responds more to changes (both good and bad) than any other part of the port, especially the short turn, and the shape of the valve and valve seat is the single most critical part. The largest flow loss is in the expansion of the air as it exits the valve/valve seat into the chamber. Different valve shapes which sometimes include rolling or radiusing the margin area of the valve, different widths and angles on the back side of the valve, and different angles approaching and leaving the seat are some of the "tunable" things that make an otherwise ordinary job come to life. Many ports can be improved substantially by merely blending the seat cuts into the bowl under the valve and into the surrounding combustion chamber, frequently referred to as "pocket porting."

My approach to head work is to first determine a power goal as explained above, which determines the required flow and port and valve sizes.

Next, I flow the head to get a baseline flow curve, and decide what modifications are most likely needed to meet the performance goal. Depending on how much material has to come out, I may sonic test the ports for thickness to make sure I don't have a problem there.

If I'm to do a full porting job, I make a rubber mold of the port and slice the mold into segments 1/2" to 3/4" wide. I lay each segment on graph paper, draw around the circumference, and count the squares in the outline of each segment to obtain its cross sectional area. This gives me a diagram of the shape of the port. The silicone rubber I use is Dow Corning Silastic V base and curing agent. I have tried the Silastic M, but it is too hard, which makes it difficult to get out of the port once it cures. Expect to pay close to $150 for a gallon of it.

To the general automotive community, CNC porting is the hot item these days. CNC merely means that a computer controls the tool paths of a milling machine to take material out of a head. The main advantage to CNC porting heads is time and repeatability. In most cases, the port being CNC'd is a digitization of a port that was developed by hand porting using the methods described above, and the success of the port being CNC'd depends on how good the port is that was used to develop the program in the first place. If the original port wasn't too good, then all you have is a whole lot more ports that are also not too good. Even if it's an outstanding port, it may be wrong for your application. The key here is to find a good port that fits the application, and the only sure way I know to do that is to get your hands on one, make a mold to evaluate the shape, and flow test it, then decide if it's suitable.

Expert Advice from Joe Mondello:

Joe Mondello, who’s name has long been synonymous with high-performance cylinder heads, said a lot of people who don’t really know what they’re doing jump into head porting and make big mistakes.

"They take out metal where they shouldn’t be taking out metal and end up with ports that are too big and don’t flow as well as they should. The shape of the port is far more critical than the overall size of the port," stated Mondello.

Mondello, who teaches the secrets of building, porting and flow testing high-performance cylinder heads at his Mondello Technical School in Paso Robles, CA, said he also sells special porting tools that are designed for every part of the cylinder head.

"When you’re doing the short-side radius of a port, you don’t want to take out too much metal. You just want it to be nice and smooth," instructed Mondello. "Trying to get around the short-side radius bend is difficult unless you use a cutter that’s designed for that purpose.

"When cleaning up the bowl area, blending alone won’t improve flow unless you also remove some metal to increase volume. Many people don’t do valve bowls properly. You have to blend everything from the base of the valve guide to the base of the primary valve seat, and then do a 3-angle valve job. Otherwise you’re just scratching the valve bowl and ports, and aren’t really gaining anything."

As for matching ports, Mondello said not to use gaskets as a guide because there’s too much variation in gaskets and most aftermarket gaskets have openings that are up to 1/8" larger than the port runners. If the port is enlarged to match the gasket, it can reduce air velocity and hurt performance.

"We teach port matching, not gasket matching. I pick the largest port, match all the others to it, then do all the work inside the port to maximize air flow around the pushrode tube turn because that’s where the biggest restriction is in the port," said Mondello.

"The largest gains in horsepower are found on the intake side by raising the roof of the port (the side closest to the valve cover) by .100" to .175". The amount of metal in the top of the intake manifold runner will determine how high you can raise the roof.

"On late-model Chevy Vortec heads, you don’t want to change the shape of the port much. The best advice here is to clean up and equalize the ports so they have the same height and width. On small-block heads, there’s a large pocket right below the rocker arm stud in the roof of the port. This should be filled in with epoxy to improve air flow. Doing that will give you an extra 15 cfm.

"On exhaust ports, if you tried to match the port to a header gasket you’d probably destroy the port. The secret of exhaust porting today is not how big the port is, but the shape of the port and the velocity of the exhaust flowing through it. We don’t even flow test exhaust ports anymore because most heads have plenty of flow capacity as is. All we care about is velocity and pressure.

"Nearly every single exhaust port today, except for Ford 302, 5.0L and 351 heads, are big enough. The only thing we do to enhance air flow is raise the roof of the port about 0.100", depending on the headers used. We don’t touch the floor of the exhaust port or the sides unless we have to get rid of a hook, seam or rough area in the casting," said Mondello. "Any time you start making the ports bigger on the exhaust side, you usually end up killing air flow in the head. I’m talking a reduction of 25 to 30 cfm. All you need to do is clean up the valve bowl, blend the short-side radius, and raise the roof slightly. Don’t touch the floor or walls."

Mondello explained that CNC machining and hand grinding are two different techniques for porting heads. "Everybody says CNC is the way to go. But you first need someone who can take a raw casting and rework it so it has good air velocity and flows well. Then you can digitize it and reproduce it with CNC tooling on other heads. There are a lot of CNC profiles being sold today, but I think most have some room for improvement. Additional hand grinding can usually pick up another 10 to 12 or more cfm."

As for polishing, Mondello said a smooth finish is great for exhaust ports, but a rougher finish flows better on the intake side. He recommends using 300- or 400-grit paper followed by a Cross Buff for polishing exhaust ports, and 50- or 60-grit paper for the intake ports. A slightly rough surface texture in the intake ports and intake manifold runners creates a boundary layer of air that keeps the rest of the air column flowing smoothly and quickly through the port.

Here is another outstanding Popular Hotrodding article on head porting:

http://www.popularhotrodding.com/tech/0610phr_cylinder_head_porting/index.html