The first obvious thing you see looking into the port from the manifold end, is a "hump" in the floor. And right here let's take a moment to thank the Lord that we are dealing with an overhead valve motor right now so that there is no confusion as to which part of the port is the floor (hint - if you were short enough you could walk on it). Just imagine the confusion if we were forced to look at a Flathead port like we did to explain the concept of curtain area in our last post. Even I'm not sure which would properly be termed the floor and roof on a Flathead. Would one have to turn the Flathead cylinder upside down in order to keep the "short side" radius as the floor and the " long side" as the roof? Or do you leave the Flathead cylinder in the orientation that God created it in, and rename the short side the roof and the long side the floor? The potential for confusion is immense. Whew. Good thing were talking OHV right now! By the way, the short side radius is just what the words mean. The length from the port opening to the valve will be considerably shorter on the floor than it is on the roof (again from an OHV point of view).
As I said, the hump in the floor of the port is pretty obvious and changes the port shape from round to a sort of "D" shape (tipped 90 degrees counter clockwise). Obvious as it is in an Evo, the hump is even more pronounced in a Twin Cam. Though it's going to be very difficult to measure the Cross Sectional Area once we get into the port where it is no longer round, we can certainly get some sense of it from a few measurements.
I keep a suitably modified inside caliper on my porting bench. You may notice from the picture that there I have two sets of notches (filled with a bit of red paint for clarity) as reference points. These arbitrary reference points are a handy way to locate a fixed point for measuring since the port walls are tapered. By putting the caliper into the port so that the matching reference points align with the face of the port opening, one can easily take multiple measurements at a set depth. My calipers are marked at .750" and 1.500".
On this particular Evo head, the port opening measures a nominal 1.610" (it not quite round). But at a depth of .750" in from the port opening it measures 1.745" side to side and 1.375" top to bottom. So the port has gotten .135" wider, but at the same time it has become .235" shorter. That makes it pretty self evident that is a smaller Cross Sectional Area than at the port opening. When we make the same measurements on the Twin Cam head, the results are no better. The port opening of this particular head measures 1.650", but 3/4 of an inch in, the measurements are 1.845" side to side and 1.235" top to bottom. The question you must ask is this: did the increase in the width of the port make up for the decrease in height?
Remember what I mentioned about the preferable place for the minimum Cross Sectional Area being the throat under the valve seat? Since it is quite obvious that is not the case with these heads in their stock configuration, we can be pretty sure that to get the full flow potential from these heads, something needs to be done to enlarge this "pinch point" so that the minimum Cross Sectional Area is in fact under the seat where it belongs.
Big Hint: don't cut down the "hump" to get the port closer to round. The hump is there for a good reason - and that reason is twofold, though related and codependent. The primary reason is form over function. Fatbob tanks have been a signature feature of Big Twin Harleys since the 1930s, and in order to keep the air cleaner from interfering with those tanks, the ports (both Evo and Twin Cam) had to be kept as low on the heads as possible. A port entry with a higher angle of approach to the valve would have made far more sense from a performance standpoint, but the factory knows the audience they are playing to. But in an admirable attempt to have their cake and eat it too, the Motor Company added the "hump" after the low entry point in order to help gently turn the air around that short side radius.
Don't get me wrong though. Just because the minimum Cross Sectional Area is in the wrong place, it doesn't mean the significant flow gains cannot be realized by porting without addressing that fact. In other words, if you (or the person porting your heads) never noticed this point of restriction, it doesn't mean the heads can't have considerably better than stock flow, it just means the flow will not reach the optimum. Now I should mention that there are probably some practitioners of the porting arts out there who will plea that some "pinch' of the port CSA is a good thing to have at the 'hump" and that it helps turn the air. I am not sure that I am smart enough to agree or disagree with that theory, but to attempt to prove it, I think one would have to be able to accurately measure the CSA of that pinch point and then by varying its size, empirically show how much constriction yields the best results. Whatever method you use to get there, you will know that you arrived at a near optimum port flow when it matches the CSA of the throat under the seat multiplied by 133.
Now keep in mind that this "secondary" choke point in the vicinity of the hump is pretty difficult to measure (as previously mentioned) due to its irregular shape. But adding to that irregularity is the valve guide boss. That right, by the time you are 1/2" into the port, the cross sectional area is not only being reduced on the bottom by the hump, but on the top by the valve guide boss. By now that cross section looks more like the drawing below:
Anyone want to take a stab at giving an accurate Cross Sectional Area for that shape?
(it is possible, but quite time consuming)
Okay, once you have the port flowing close to that magic 133 x the minimum CSA of the throat, you will know that you have the rest of the port to the point where it is no longer the limiting factor. And since this post is intended to informational in nature rather than a full blown "how to" we will leave the "hump" section of the port.
Keeping in mind the ideal of the minimum CSA being at the throat just beneath the valve seat, what then happens when you add a larger intake valve? Well, if you don't open up the diameter of the throat, then the minimum CSA stays the same and thus the theoretical maximum air flow remains the same. The larger valve has not helped at all except that it gives you more room to turn the air more gradually if you shape the seat correctly, but that only gets you closer to the maximum possible flow through that same minimum CSA.
But here we probably need to add another number to the mix. That is the relationship between the valve diameter and that of the throat below the seat. This is expressed as a percentage of the throat diameter verses the valve diameter. In other words if you have a stock Evo/Twin Cam intake valve that measures 1.843" with a throat of 1.625", the percentage would be 88.2% (rounded off). Today it is commonly accepted in the porting world that the range for this percentage is from 88 to 91% for best performance, though early Superflow literature suggested 85%. The exact number within that range may be tailored by application, with the lower RPM engines with low lift cams benefiting from the lower percentages. 90% is generally a good safe figure to use; 91% can be too big; while 92% is definitely too large and will likely hurt performance (though not necessarily flow). With that in mind, what happens when you put in a 1.900" intake valve (as is most common when porting Evo and Twin Cam heads)? If you open up the throat to 90% of that 1.900" valve, it will be 1.710". That has obviously moved our minimum CSA from the throat to the port opening (or to the "hump" as it may be). In other words, the 90% valve throat has a flow potential of 305 cfm, but your port opening will only handle 275.
Again, I would like to stress that just because your port opening is only of stock size, that does not mean that a 1.900" intake valve will not help your flow. We are dealing with theoretical maximum potential here, not hard and fast rules on how your heads must be configured to improve performance.
One thing that I might mention here relates back to part one of this series, having to do with curtain area. If you were to put 1.900" intake valves in your heads, keeping the stock port opening diameter the lift required for the curtain area to equal your minimum CSA will only go up an insignificant amount (.002") because that minimum CSA has not changed much, it is merely located in a different place in the intake tract.
since we are looking for the lift at which CSA equals the curtain area, insert the
CSA figure in the formula in place of Curtain AreaRemember the minimum CSA with the stock valve was 1.999 (throat CSA minus stem CSA). With the larger valve and matching larger throat under the seat, the minimum CSA is now at the port opening which is 2.074 square inches. If however, the port opening was not the place of minimum CSA, and the throat was, then the lift required for the curtain area to equal that point would be .372" (that is arrived by calculating the CSA of the throat [2.297], subtracting the CSA of the stem [.075], and then dividing that by the product of 3.1416 multiplied by the 1.900 valve diameter [5.969] ).
To get some perspective of how this lift = curtain area point changes when taken to an even more high performance application, let's consider the Screamin' Eagle 2.175" intake valve used in their "Hurricane" heads. IF they used a 90% throat, it would work out to 1.958" I.D. or a throat CSA of 2.935 square inches when corrected for the valve stem diameter. Plugging that into our formula we find that the valve curtain area will not equal the throat area until a lift of .430". That might change your perspective a bit on what might be considered low lift flow.
Just one more warning at this point. This calculated curtain area does not give you the lift needed for a given valve size or port size. It only gives you the lift at which the curtain area is no longer a primary limiting factor to flow. It is not the lift at which the your port will reach maximum flow (unfortunately not even close) but it does give you some insight as to how important the valve size and seat shape is at lifts below the point where the curtain area equals the minimum CSA.
At the end of part 1 I said that I would eventually get to how much air flow your engine really wants. Looks as though that means there will be a part 3.