By Alvin Calhoun
Porting is something that I have always been curious about. Back in the mid-1970s, I used to read Hot Rod Magazine. One of its writers, Jim McFarland, wrote a lot about head porting, and I could hardly wait until the next issue came out.
It was actually in the mid- to late-’80s before I first put a grinder-borrowed from a friend-to a head. These were big-block Ford heads and I got good results. Then the next set came, and the next. I had a full-time job at the time, and did porting on the side until the early ’90s.
Then I got into an accident that put me in a wheelchair. At that point, I was not able to do a lot of the other things that I did before. But, I could still port heads. So I moved into it full time.
What this work requires is the hands of an artist and the mind of an engineer.
These two characteristics don’t usually exist in the same body. I am a little bit different. I studied engineering many years ago when I was in college. I have actually gotten my hands on a lot of stuff, taking it apart to see what works and what doesn’t.
I am also an artist. I paint, draw and sculpt. I have been into that since I was a little kid. Whenever I approach a head, it feels like I am doing a sculpture from the inside.
The other thing this work requires is patience. Not many people can sit in front of the same head for three days. I have had people walk into my shop the first day and say, “OK, you’re working on those heads” and then come back the next day and say, “Are you still working on that same head?”
I say, “Yes I am, and I will be working on them again tomorrow.”
When you are porting by hand, and get it to flow just the way you want it to, you have accomplished something that is nothing short of a miracle. Then you have to replicate your work seven more times. That’s a tough thing to do.
Keeping At It
My customers are drag and circle track racers, mainly from South Louisiana and Mississippi. They are weekend racers and they take it very seriously.
It is amazing how, when you start out porting heads, nobody wants to trust you. Once you begin producing results and build more confidence in what you are doing, then you will probably find more work than you can handle.
I was porting heads by hand using a homemade flow bench for the first few years. Then I moved up to a SuperFlow model, which I still have. I continued porting heads by hand up until about a year-and-a-half ago, when it reached a point where I could no longer keep up with the amount of work people were bringing me.
That is when I decided to get a CNC machine. I went up to Centroid to take a look at its 5-axis machine and Mastercam with Port Expert software (from CNC Software Inc., Tolland, Conn.) and decided that would be the way to go.
Before my son Davin and I took the training, we didn’t have any prior machining knowledge. Neither of us had ever operated a manual mill.
In fact, Centroid’s sales guy, John Cowher, laughed at me when I told him, “I don’t want to be a master machinist” and “I don’t want to be a computer programmer. I just want to port heads.” He thought it was the funniest thing he had ever heard.
The complete package, machine with the software, was a little over $170,000. That is quite a bit of change to drop on something that you have no idea how to use.
There were self-proclaimed experts who came by shortly after we got the machine and predicted that we were going to crash it. It hasn’t happened yet.
Once we get the port and everything worked out, and have generated the tool path in Mastercam, we run the simulation to see exactly what is going to happen. The simulation makes sure you are coming in with the shortest possible tool at an angle that keeps it rigid and the cut smooth. It also shows you how to avoid all the surfaces that your tool and holder might crash into.
Hand Porting Art
The goal of porting is to maximize engine combustion efficiency by creating internal shapes that optimize the flow of a multiphase liquid, air and fuel into the cylinder. The shape of the port must create a certain amount of turbulence to keep the fuel in suspension, but not so much that it becomes detrimental to the rate of flow.
That is the goal, but how exactly does the head porter go about achieving it?
One of the best hand porters I know told me that he could not describe how to shape the short turn of a port. As he was speaking, I was thinking that he must not be very articulate. Later, when I had thought about it some more, I agreed with him.
I don’t believe that I could explain exactly how it should be shaped, either. It’s done by intuition and feel, working back and forth between the grinder and the flow bench until everything is the way it should be.
There are a lot of things that go on inside of a port that are not fully understood even today. The real “experts” will not acknowledge this, but they are still arguing over what is happening.
Whenever the experts don’t agree, I conclude that they really don’t know. I am probably one of the few people foolish enough to admit that I don’t know everything that goes on inside of a port.
I will say, however, that once I discover what works, I now have the ability to catch it and reproduce it. That is one variable, at least,that is no longer a variable.
Digital Porting Science
My son digitizes the ports I create using a touch probe to capture the data that is brought into Mastercam. We use the data to create a wireframe and then put a skin on it, giving us a realistic virtual representation of the head.
After porting heads for 30 years, grinding by hand, peeping in with one eye, using my finger to feel it, using inside calipers to measure, pulling port molds and all that-none of it compares to having a CAD model of the head. It is like being able to pick the port out of the head, to look at it from all angles, and quickly find any little flaws and revise them.
Inside the engine of an automobile, the runners, heads and intake manifold are probably some of the most unique sculptured shapes you can imagine. I would call them organic.
You have all kinds of compound curves and decreasing radii. There is not one flat surface in there. You may cut two ports by hand and think they are the same.
Then you digitize them, and make models in Mastercam. When you overlay these models on the screen, you will see that you have a lot of variation. It is just incredible. However, once you make some fixes to your design in the model, Mastercam will create 5-axis tool paths that will drive the 5-axis machine to cut exact replicas of your ports every time.
When you are hand-porting, you have no idea how thick the remaining wall is until you cut through. In the CAD model, you see everything.
For example, there is a bolt hole on the other side of the wall that most hand porters just cut through, leaving the bolt exposed for combustion gasses to corrode. But we can tell the machine to stay within 0.030 inches of that opening and it will move around it and make nice little contours, and not break through.
This makes for a nice, smooth transition around something that might otherwise cause an unwanted obstruction to the flow. Mastercam allows you to slice the port into sections and you can measure the cross-sectional area.
This makes it easy to find the smallest area in the port, which is a potential choke point. There are all sorts of tools like that in the software. We are just scratching the surface.
Weeks into Hours
Before we got our 5-axis machine and software, it used to take almost a day to grind the first port, then another 32 to 40 hours to make the complete set.
Today, with our 5-axis machine and Mastercam, we can create and digitize a port within a day, and then in a couple hours machine a complete set of heads with each port being a perfect replica of the initial design.
When we first got into porting on the CNC machine, I had the mistaken notion that after about five months we would have ported just about every head that could come through the door. At that point, I wouldn’t have to do very much hand porting.
What I found is that there are a lot of different kinds of heads. If you look in a catalog, you can see that there are about 25 or 30 small-block Chevy heads alone. Each one requires its own program. So we are still doing a lot of hand porting and digitizing.
When people walk in the door with a head, it is usually still something that I have to work up a port design for. We don’t charge them for all the design work that is involved. Our pricing is based on what hobby racers might reasonably be expected to pay.
We will recover our investment of time once we are doing a greater volume of heads that have already been ported, digitized and modeled. Word has gotten out and a lot more business is coming in. We are well on our way!
Alvin Calhoun is the owner of Calhoun Custom Porting in Baton Rouge, La. Learn more at www.facebook.com/CalhounCustomPorting.