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Parts of the Port
Parts of the port and their terminology
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.