Porting Principals
Parts of the Port
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 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.
You do not want the throat too big in relation to the rest of the bowl. Bowl hogs usually do this. You want the same or slightly larger cross sectional area at the pushrod restriction as the throat area. Over the short side will be even larger. Low lift cams (.550 and below) will not want the runner ground with equal cross sections at the runner throat whereas cams with high lift will. Smaller lift cams will want to be smaller in section to keep velocity up since the lift is short and the valve is not moving as much air. Basically, with high valve lift, the pushrod area can become a choke point whereas with low lift it usually won’t, unless it is extremely small.
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.
Zeroing Out Geometric Shrouding.
When addressing valve shrouding with the intent of minimizing it we need to make a start somewhere and ascertaining what the form of a chamber may be, if it was geometrically un-shrouded, is as good a place to start as any.
The breathing area presented to the chamber by a valve moving through its lift envelop is not quite as simple a geometry problem as it may first appear. The reality is that as the valve lifts it moves through three distinct regimes, each of which requires its own particular set of math formulas to produce an answer as to what the through-flow area is. We are not going to deal with this now as it is more advanced stuff. However, even if we ignore that we can still come up with a very good approximation of what it takes in the way of chamber form to produce a geometrically un-shrouded chamber. What we find is that at low lift the angle of the chamber wall as it leaves the valve seat needs to be very close to 45 degrees and as the lift progresses up to the critical 0.25 D lift point the angle needs to increase to about 52 degrees from horizontal.
The drawing below gives us a good guide to the form that needs to exist around a valve as it progresses through its lift envelope to ensure that the flow area around it is at least equal to the effective curtain area beneath the valve head.
Look closely at this drawing. The green line represents the angle of the chamber wall as it comes off the seat. For all practical purposes this is right around 45 degrees. As the valve lift progress the point of zero shrouding of the edge of the valve in relation to the chamber wall gets slightly steeper until at 0.25D the wall angle is close to 38 degrees off the vertical (52 from horizontal) as represented by the blue line. Although not totally accurate we can say, within close limits, that when the valve is at 0.25D lift the gap between it and any possible obstruction should be equal to a minimum of 0.20D. Above 0.25 D valve lift the chamber wall can be vertical for zero geometric shrouding as the valve has reached the limit of the area it will present to the cylinder.
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.
The graph 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.