The ball detent locks around...
The ball detent locks around the valve guide and clicks into the vertical grooves cut into the stem of the P-D Valve. This keeps the P-D Valve from spinning and allows you to know precisely where the pressure tap is pointing inside the combustion chamber. The ball detent stays in place if you are grinding the head or making other adjustments, so when you return the head to the flow bench and do more testing, you can locate the P-D Valve in the very same positions as before to get accurate comparisons.
"I began to experiment with ways to construct Pitot tubes that didn't obstruct the airflow and tried different designs to see how well you could use them to map pressure differentials within the port. From that, I developed the P-D Valve which, I believe, will aid people doing research and development on cylinder heads and port designs because, unlike a standard Pitot tube, it can give you a true map of how the air flows around the valve."
Along with the valve itself, Touchette's invention (which is so new the patent is still pending) includes the ability to control the depth of the valve in the combustion chamber and software that helps map your results so you can interpret your findings through graphics instead of staring at a bunch of numbers. Just about everything else needed is standard equipment on almost any flow bench.
The first step in using a P-D Valve is to determine exactly what cylinder head you will be testing. In order to get the best-possible numbers, you will need to use a P-D Valve sized exactly like the valve that will be running in the completed engine. Touchette says he plans to keep many of the most popular sizes in stock, and custom P-D Valves can be made for specific applications. If you like, you can even cut your own valve seat angles in the P-D Valve.
Here, the P-D Valve is set...
Here, the P-D Valve is set up and ready for use. The clear tube is connected to a pressure gauge. Also notice the height standard between the ball detent and the locking collar. The locking collar is put in place with the height standard in place, then the standard is removed and the collar pushed down until it contacts the top of the ball detent. Now you know the valve is open exactly 0.400-inch (the height of the standard).
Once you have the correct valve, testing can begin. The P-D Valve has a hole (the correct term is a "pressure tap") in the seat area of the valve that extends all the way through the tip of the valve stem. To get your readings, all you have to do is connect a piece of tubing from the top of the P-D Valve's stem to a pressure gauge. In most instances, the flow bench will have provisions for connecting a Pitot tube to a built-in pressure gauge, and hooking up the P-D Valve is no different.
In order to get repeatable results, Touchette includes a slip collar with a set screw and a set of height standards. The standards allow you to set your valve height off of the seat in increments of 0.050-inch all the way up to 1 inch. Once your height is set, use the set screw to lock the valve in the slip collar and its height will be consistent during your test.
Because the P-D Valve has one pressure tap, it measures one specific area of the port. In order to get a complete look at the airflow all the way around the valve, you need to do a sweep at a particular valve height. To enable this, the P-D Valve's stem is cut with eight splines equally spaced along the length of the stem. These connect with a ball pin detent in the slip collar so you can click to the same eight spots time after time. The slip collar connects to the top of the cylinder head and stays in place during porting or other work to the head, so after you have made changes, you can use the P-D Valve and test in the same locations as before. It's this repeatability that allows a good cylinder-head man to pinpoint exactly where a port and combustion chamber need to be improved and make his changes.