The AMC Pages
Head and port tuning on muscle-car era heads
Valves - Face
The area that mates with the seat in the head to form a seal is called the face. Valve faces are ground at two angles (30° and 45°). The angle is measured from the top of the valve, Fig. 10-3. At one time the 30° face was preferred because of its flow characteristics, but present-day technology has proven the 45° angle has more overall advantages. Two of the biggest advantages of the 45° face is that (1) it will wedge tighter as it closes,] Fig. 10-4, and (2) it will flow more air at valve lifts greater than 1/4". While the valve is closed, heat is transferred from the valve head to the seat. Heat transfer will occur at a greater rate as the pressure of contact increases. This means that the 45° face will allow the valve face contact to transfer additional heat while also providing a tighter seal. To further encourage sealing, valve face angles are ground with one degree less than the seat angle. For instance, if the seat is ground 45°, the valve face is ground at 44°. This one-degree difference is called the interference angle, Fig. 10-5. Interference grinding will raise the surface loading at the seat contact point due to the reduced seating surface area. This additional seating pressure will give greater sealing, especially during the break-in period. The interference angle is usually worn and pounded out by 100 miles of operation.
1Giles; "Fundamentals of Valve Design and Material Selection,"
SAE No. 660471 part 14.
ACKNOWLEDGMENTS
Air Flow Research Company, Warren Brownfield (Van Nuys, Calif.)
Dana Corporation (Perfect Circle)
Ford Motor Company
Hot Rod Magazine, Specialty Publications Division of Peterson Publishing Co.
TRW and Norman Musil
Valves – seat shapes
The angle that the seat makes will be either 300 or 45°. In some engines, the 30° angle is used on intake seats. 30° seats are used because it is felt that air flow over this angle is greater than with the 45° seat. It has been proven with air flow testing equipment that air flow is greater over a 30° seat only at valve open positions under 1/4". For conventional use, the 45° seat will provide more overall benefits than the 30° seat. The 45° seat wedges tighter than the 30° seat, thereby producing better sealing and heat transfer important in preventing leaking and burned valves/seats.
[pic]
Fig. 11-3. A properly ground seat should have three angles: (1) top, (2) seat, and (3) throat.
Breathing Technology
Any port and seat work other than that already covered would be to accomplish an increase in volumetric efficiency. Engineers and scientists have spent years trying to understand what happens to air and gasoline mixture flow inside a port and manifold. To give some idea as to the complexity of mixture flow, here are a few of the most obvious factors that affect flow and a brief explanation of each.
1. Pressure Differential. As the pressure difference between atmospheric pressure and cylinder vacuum increases, a larger volume of flow will enter or leave the cylinder. If the path that the flow must take retards or hinders the flow, some of the pressure differential will be used to overcome the flow restriction. A loss of pressure to the restriction will result in less cylinder filling or evacuating.
2. Piston Speed. Piston speed is usually given in feet per minute of travel. Piston speed affects the pressure differential for a given position of crank rotation.
3. Air-Fuel Ratio. Air-fuel ratio affects flow because of its density. A rich mixture is more dense due to the extra fuel A heavier mixture will be slower to react.
4. Cam Design. The opening and closing points of the valve relative to crank rotation, the rate of valve lift, and the maximum valve lift, all affect air flow into the engine.
5. Port Length. Port length includes the intake and the exhaust manifolds, the head ports, and any pipes such as the exhaust pipe or velocity tubes. The overall length of these passages will affect gas flow.
6. Reflected Waves. Sound waves travel up and down the port and pipe length as the mixture flows. These sound waves travel at around 800 mph. The maximum mixture flow speed at wide open throttle is around 200 mph. Since the waves travel four times the speed of the mixture, the waves will have to travel back and forth in the port length. With proper design, it is possible to get the waves going with the mixture flow at the time the valve opens. These waves would then aid flow during exhausting and intaking.
7. Ratio or Port-to-Valve Size. The port has to have a cross-sectional size that is slightly larger than the total open area under the valve head when the valve is fully off its seat. If the ratio is not correct, mixture flow will be retarded at wide open throttle.
8. Piston Dome Shape. During the time that the piston approaches and leaves TDC of the exhaust stroke, both the intake and the exhaust valves are slightly off their seats. While both valves are open, cool fresh mixture leaves the intake port, crosses the combustion chamber, and leaves through the exhaust port. Cross flowing mixture like this serves three purposes:
a. Establishes intake flow. Starting flow early means a good flow will be going about the time the piston starts down.
b. Cools the combustion chamber surfaces and the exhaust valve. Cooling these areas elim9inates abnormal combustion potential; and allows high compression ratios.
c. Purges (pulls out) the exhaust gases that collect in the combustion chamber above the piston at TDC. If these burnt gases are not removed, they will dilute the next intake mixture.
This important flow can be disturbed by piston dome shapes that block the cross flow, Fig. 11-37.
Filg. l I-37. Both pistons can he modified to increase cross flow
while the piston is at TDC of the exhaust stroke.
9. Port Bends. The radius and shape of the port bend can aid or retard mixture flow.
10. Surface Friction. Surface friction refers to the port surface texture. A rough texture causes the laminar boundary layer to be turbulent. The rougher the surface, the thicker the turbulent boundary layer will be.
To further complicate things, all of these factors interact on each other. This ultimately means that any increase in volumetric efficiency is a product of experimentation. In recent years, a greater understanding of air flow has occurred with the use of machines developed to simulate engine air flow conditions, Fig. 11-38. Laboratory experimenting with these air flow machines greatly reduces the time needed to acquire knowledge. Much of the recently acquired knowledge has proven many of the longtime established concepts of port and seat work to be of little or no value. The following explains the methods used by high-performance people (and today, factories) to increase volumetric efficiency or improve on factors 7, 8, 9, and 10 listed above.
LAMINAR AIR FLOW
An understanding of what happens in the port and across the seat requires an understanding of air flow. Air can flow through a pipe in two ways, (1) laminar and (2) turbulent. With laminar flow, the air flows smoothly past a surface. The center of the flow travels faster than the flow at the edges. This is easily observed by watching water flow in a river. Throw a block of wood into the center of the river and one next to the bank. The block in the center travels much faster because the water is traveling faster there. The surface of the ports acts like the bank did to the water flow. The port surface produces a drag or resistance to air flow and causes that layer flowing next to it to travel slower. This layer is known as the boundary layer, Fig. 11-39.
TURBULENT FLOW
As long as laminar flow exists, the gas flow tends to follow the surface it is flowing over. If air velocity becomes quite high or a port bend becomes too sharp, the gas flow will lift off the surface and form small eddys or whirlpools in the boundary layer (boundary agitation), Fig. 11-40. If the air has to flow around an object or make a sudden bend, the smooth laminar flow will be disrupted, causing turbulent flow. Turbulence, in turn, causes the main air stream to deflect as it flows over or around the agitated area. This deflection of flow reduces the effective port flow area.
FUEL SEPARATION
Up to this point we have considered that only air is flowing in the port. In actual engine operation, the air is mixed with small particles of fuel. These fuel particles are considerably heavier than air and, therefore, are more severely affected by inertia. If the port flow through a straight pipe is fast, the heavy fuel particles are held in suspension. However, if the port flow is slow, these heavy fuel particles may tend to settle out of the stream and wet the port walls. Port flow speed for a particular desired rpm is controlled by the port cross-sectional size.
As the mixture flows in a constant diameter straight pipe, the flow will use the entire cross-section of the pipe. The center flow in the pipe will be faster than the boundary layer, and the speed of flow will diminish due to friction loss. If the pipe makes a turn, the air and fuel mixture will behave differently. As the mixture makes the turn, the gases on the outer wall will have to travel much faster to make the turn. Since there is no way for them to gain energy to increase speed, they will take the path of least resistance and try to "short cut" the turn - Fig. 11-41. As the air compacts against the inner radius of the turn, it presses against the port wall harder, thereby increasing the port wall surface friction. The heavy fuel particles, on the other hand, have established inertia in the straight-ahead direction. If the mixture speed is fast enough as it approaches the turn or if the turn is short enough, the inertia of the fuel particles will cause the fuel to try and travel straight ahead. If the fuel does travel straight ahead, it will separate from the air flow and splatter against the far side of the turn; thus the incoming mixture will no longer be uniformly mixed. The far side of the turn will be rich due to the fuel separation, whereas the flow across the inner radius (near side) will be lean. If the turn happens to be the port turn just before the valve seat, the fuel particles would leave the far side of the valve while the air would leave the near side, Fig. 11-42.
This rich and lean flow into the cylinder produces rich and lean areas in the combustion chamber.
Proper cylinder and combustion chamber turbulence will help remix the mixture. In general, the air flow in ports will be laminar while fuel flow tends to separate. Separation from flow will severely affect horsepower. To correct a separation problem, the ports will have to be reshaped.
PORTING - ONLY AN EXPERT'S JOB
Porting can be defined as enlarging or reshaping of the cross-sectional area of the port by removing metal. Porting can only benefit an engine that is designed for "all out" operation. Speeds encountered in everyday driving do not require mixture flow deliveries and velocities that necessitate port work. Proper port work may change mixture flow so drastically that other parts or factors such as carburetors, cams, exhaust systems,
compression ratio, etc, will also need to be changed to take advantage of the port work. While porting, try to keep the cross-sectional area of the port as uniform as possible along the entire length of the port, Fig. 11-43. As port cross-sectional area decreases, the mixture velocity has to speed up to obtain the same amount of mixture flow in a given time period. The new flow velocity may be high enough to cause separation.
As mixture flows at high speeds along a port, it may encounter bumps or restrictions large enough to disturb the flow. The greater the speed, the greater the disturbance will be. It is possible to have a disturbance at high flow speed and no disturbance at low flow speed. If the mixture flow speed is high enough, these restrictions may form a high pressure area at the bump as air compresses and a low-pressure area behind the bump as the air spreads, Fig. 11-44. Using the air flow machine, the technician can measure and find these high- and low-pressure areas in the port. This means that man, for the first time, can trace the actual path of flow in the port and locate flow restrictions. In some instances, bumps or foils are placed into a flow stream to slow or provide more control of the flow. If undesirable port restrictions exist, the technician can reshape the port and eliminate the restriction. Air flow machines are capable of simulating air flow for any amount of valve opening. As the valve opens further, greater air will flow in the port possibly producing new flow problems.
Air flow technicians have found that most factory-developed ports of the 70’s on are capable of flowing 40% more air than the valve and seat opening can handle. Since the port has more flow capabilities than the seat, minor boundary layer agitation or restrictions may not be of concern. If they are of no concern, then in most cases porting work has little or no value. These same technicians also feel that the cast texture of the port wall is a desired finish and cannot be duplicated once the texture is removed. If the wall is very slick, the heavy fuel particles that are suspended in the mixture flow can stick to the smooth surface as the mixture flows against the wall and will not reenter the mixture flow, causing a momentary leanness in the mixture. If the surface is rough, the fuel particles will bounce from peak to peak of the textured surface and eventually reenter the mixture flow.
Another factor against indiscriminate grinding of the ports is the port wall thickness. Manufacturers use precision casting techniques in the production of cylinder heads and blocks. This means that they control the final wall thickness very closely. With the thin-wall casting technique, engines were made lighter in the 70’s and manufacturing costs were reduced as less metal is used. In addition to thinner walls, the wall thickness is not uniform throughout its length. Unless you know where the thin areas are, you may grind through the port wall leaving a hole that may or may not be repairable. Some accidental port holes cannot be repaired due to their location. In the 70’s and later, port technicians normally would often saw another production head into many pieces so that wall thickness could be checked and studied prior to grinding a good head.
As has been implied, the grinding of ports should be left to the experts. Many times heads have been ruined or horsepower decreased by indiscriminate grinding. Port grinding without the knowledge acquired on a flow bench will usually destroy low- and mid-range power. Top end may benefit. If you must grind the ports, limit the grinding to making the port uniform in cross-sectional area and try to eliminate abrupt changes in wall direction.
GUIDES AND PORTING
Shortening of the valve guides in the port should be avoided. Any shortening of the guide will reduce the valve stem bearing surface and cause guide wear to occur at a faster rate. Once guide wear occurs, the valve will spend some of its open time going sideways to remove guide clearances. Once this occurs, the valve will not perform according to design specifications.
The exhaust guide should be left as long as possible to provide bearing and to eliminate the hot exhaust gases vaporizing the oil off of the valve stem. The exhaust guide also serves to remove heat from the valve head. The further the guide is shortened from the valve head, the hotter the valve head will become.
An advantage can be obtained by shaping the guide to either slip or direct air, To slip air, grind the guide in the shape of an air foil or wing. This will enhance laminar flow and reduce turbulence. Special guide shapes can be designed using an air flow machine. These special shapes will either redirect air or "grab" air that is not flowing properly. Remember, the intake and exhaust gases flow in opposite directions in the port, so the foil will have to be reversed when grinding the guides.
PRESSURE TESTING
After grinding, have the heads pressure tested to check for small cracks or paper-thin areas. The pressure for testing should take into account the pressure differential between the water jacket and the port vacuum during deceleration. Regardless of engine type, deceleration is the point of greatest pressure differential. Water jacket pressures can be as high as 18 lbs. and the intake manifold can reach a vacuum near 30" during deceleration. The pressure differential between the two is 33 lbs. A test pressure of 50 lbs. should be more than enough. If cracks are detected, they can be repaired by brazing or silver-soldering. Be careful when applying heat; the paper-thin areas will burn through very easily. Once a large hole forms, it becomes very difficult to fill.
PORT SHAPE
Round ports will flow more volume if the round port is straight and the cross-sectional area is constant. The minute that a turn occurs in a round port, the air will ''glue in'' harder against the bottom of the port turn and increase surface friction or drag, Fig. 11-45. If this area is flattened out, the air will spread out, allowing more of the air to short cut the turn, thereby reducing boundary layer friction and increasing air flow through the turn.
PORT MATCHING
Aligning the port surfaces between the intake manifold and the head is desirable if the mismatch exceeds 1/16" and can be accomplished by the amateur. Metal removal should be limited to port matching. Figure 11-47 shows how the intake or exhaust manifold should match and the effects of a mismatch.
1. Paint the port opening with machinist dye. Use an old intake manifold gasket as a pattern, and scribe the gasket openings onto the dyed surface. Use bolt holes to align the gasket. Another method is to bolt the parts up and scribe the port outline on the dyed surface. The surface can be scribed by passing a sharp pointed wire down the port. If the scribing is done from the valve seat a 180° bend must be placed at the point so that the area to be scribed can be reached.
2. Remove metal using a die grinder.
SEAT APPROACH AREA
The area of the port from the guide down to, the seat is the most critical portion of the port. Proper shaping requires the use of an air flow bench. The sharpness of the inner radius of the port bend in the seat approach area is critical because if the turn is too tight fuel will separate causing rich and lean mixtures to exist in the cylinder.
In most cases, air flow testing shows that the mixture does not flow around all edges of the valve. The amount of valve edge used to flow mixture varies with (1) the mixture flow speed. (2) the distance the valve is off its seat (lift, and (3) the shape of the approach area. Here is a comparison of the amount of valve actually used with the throttle wide open:
This study shows that proper seat approach (throat area) is very beneficial to increased flow past the valve and seat. Proper seat approach requires that the throat angle and the width of the throat surface be correct. The proper seat approach width and angle are best determined with an air flow machine. Correct approach may require that the seat approach be modified in such a way that the approach area will not be the same all the way around.
VALVE AND SEAT FLOW
Once the air makes it to the seat, the seat should be of sufficient width to control the air and further direct it towards the open cylinder. If the seat is too narrow, there will be insufficient flat surface to control the mixture flow, and poor flow direction and/or separation will occur. Some seat grinders feel that a radiused seat and valve face will increase air flow. The narrow contact area of the radiused seat reduces heat transfer and could cause exhaust valve temperatures to rise. This can lead to preignition or valve burning. On the other hand, the intake requires enough seat-and-valve face surface to control the gas flow. It has been found by air-flow testing that good mixture flow will result if the seat contour and approach area are shaped as shown in Fig. 11-48.
High-velocity air flow benches have proven that the seat approach area is the most critical area of the port. With proper grinding and shaping in some instances, the air flow through the valve opening on muscle-car era heads can be almost doubled.
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Fig. 10-5 Valves are ground with an interference angle, which means that the valve is actually ground at 29o or 44o
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