With almost any internal combustion engine, one of the easiest ways to develop horsepower is by increasing the amount of air and fuel the heads can pump in and out of the cylinders.
We wanted to see the gains that are available from a set of Zipper's billet heads by checking the combustion chamber's volume, porting the heads, and calculating the optimal cfm capacity.
Fortunately, we didn't have to look for a shop to have the procedure performed because Kendall Johnson, of Killer Clown Performance Products in Germanton, North Carolina, offered to show us how it's done the right way.
Follow along as one of Kendall's associates, Jesse Derrenbacher, puts our heads through the paces.
 First, with the old valves...  First, with the old valves and springs still in place, Jesse flips the Zipper's billet head upside down and places a liberal coating of all-purpose grease on the area where the gasket would normally reside. Then, a special acrylic plate with a small hole is placed on top of the grease to create a watertight seal. A graduated cylinder with a petcock and nipple filled with water is placed into the small hole in the acrylic. |
 With a new set of standard...  With a new set of standard valves, we'll get more accurate flow numbers because the flow will not be hindered or obstructed by the carbon buildup on the old, used valves. This is a comparison shot of the old valves and spring retainers. |
 Before any real machining...  Before any real machining is performed, Jesse checks all the threaded holes on the head with the proper-sized bolt to make sure they're not damaged and do not bind. |
 Here, we see Jesse using a...  Here, we see Jesse using a die grinder to remove material from the intake port on the rear head. By cleaning up casting flash (especially common in stock heads), built-up carbon deposits, and contouring the port for a more gradual passage, the intake gasses will be able to enter the head faster; therefore developing more horse power and quicker revs. |
 This photo illustrates the...  This photo illustrates the difference between the polished intake port (top) and the untouched exhaust port (bottom). Notice how smooth the intake looks, allowing more air to enter the cylinder each time the valve opens. |
 Jesse uses this machine to...  Jesse uses this machine to clean up the valve seats and give the proper angle so the valves will fully close, creating an airtight seal with the head. The next step is to install the new valves and retainers with weak springs -- strictly for testing so we can flow the heads. |
 The surface of the flow bench...  The surface of the flow bench must be cleaned with a razorblade to ensure that it mates to the cylinder without any air gaps. |
 Then he places the cylinder...  Then he places the cylinder on the bench and lays a bead of clay around the cylinder and bench so there is no possibility of un-metered air entering the cylinder. |
 Before the head is bolted...  Before the head is bolted on, Jesse covers the gasket surface of the cylinder with a generous coating of grease to act as a gasket in order to get the most accurate reading on the bench. Depending on a gasket's thickness, it can change the compression of your engine. The grease completely conforms to the gasket surface of the head and cylinder and does not take away from compression. |
 Clay balls, the size of marbles,...  Clay balls, the size of marbles, are used to clog other passages on the head that might leak air during testing. |
 Once the head is bolted onto...  Once the head is bolted onto the cylinder, an intake manifold is installed with every hole plugged up except for the rear cylinder runner. Jesse rolls up some clay like a snake and places it on the surface where the carburetor would normally go. He then smoothes the clay to match the inner portion of the manifold. The clay actually acts like a radiused edge on the manifold, eliminating the turbulence that a flat edge would cause and improving the speed of the air entering the engine. |
 The stubby exhaust pipe is...  The stubby exhaust pipe is installed for basically the same reason. A length of pipe is required to efficiently usher the exhaust gasses out of the engine. |
 Each valve is tested separately...  Each valve is tested separately -- we'll show you all the steps for the intake valve. A Trock valve compressor is used to keep the valve open to a specific amount. That way, the opening can be measured where the head is most efficient. |
 By screwing the valve open...  By screwing the valve open with a hex key and measuring the amount with a dial indicator, Jesse can see exactly where the intake and exhaust valves should be open to achieve optimal performance. |
 There are plugs at the top...  There are plugs at the top of the Super Flow that allow a certain amount of air in percentage stages. By allowing more air into the cylinder, Jesse is able to tell how much air and vacuum is necessary for any head/cylinder setup. |
 Here, we can see Jesse adjusting...  Here, we can see Jesse adjusting the intake flow to achieve the desired 60 to 100 percent. (Note: 100-percent flow is almost impossible to reach. Most well-built engines usually flow between 60 to 80 percent.) |
 It's hard to see here, but...  It's hard to see here, but the intake achieved a 72-percent flow, right where we wanted it. The same procedure is followed for the exhaust valve, and each flow value for the rear head is matched to the front head. After flowing, the heads were torn down again, reassembled with the proper springs, and they're ready to be matched up to a cam and add a big boost of power. |