Cylinder Head Testing - The Science Of Flow

The art of creating great cylinder heads includes many different things, but a large portion of it comes from lots of time spent with a grinder in hand and in front of a flow bench, determining just how effective your changes are.

Yes, the most critical component is how the engine responds to the cylinder heads you either purchased or ported yourself and how the racecar performs on the track, but the last thing you want to do is race a set of cylinder heads if you have no idea how they will perform. And dyno testing every cylinder head, or port design, is prohibitively expensive.

That's why a flow bench is so important to the cylinder-head specialist. So far, it's the best way to quantify just how well a port in a cylinder head can flow air and fuel. Typically, cylinder-head specialists use a flow bench to measure how much air (measured in cubic feet per minute) will move through an area when a specific amount of vacuum is created. The strength of a vacuum is measured by how far it will pull water through a tube, and the common value for measuring cylinder heads is 28 inches of water. It isn't important to understand exactly how much vacuum that is; just that it's the standard. So if you see an ad or someone throwing out cylinder-head flow numbers that just don't seem right, check to see at how many inches of water they did their testing.

A flow bench will measure the overall flow through a port, but often, cylinder-head specialists want to know more than just a general flow number. For example, is the short turn of the intake port too sharp, killing flow? Or can the roof of the port be raised without killing velocity? When cylinder-head specialists want to know how air flows through a specific section of a port, they typically use a Pitot tube.

A Pitot tube is a small tube connected to a pressure measuring device that can tell you how much air is flowing past a specific point. This is a very useful tool that has been used for years by head porters because it allows them to look at very small areas of the intake and exhaust ports and determine where the strengths and weaknesses of the port are. After all, all areas of an intake and exhaust port do not flow air and fuel evenly, so a head porter is always looking to determine exactly where the weakest areas of his port designs are and make changes to strengthen them.

Although it's a valuable tool, the weakness of a Pitot tube is that in order to measure airflow, it has to be inserted into the airstream. And even if it is very small, it still displaces some area in the port. So no matter how you use it or try to minimize the problem, a Pitot tube is always interrupting the airflow in the very area you are trying to measure! For cylinder-head specialists, that drawback has been considered a necessary evil that you have to account for. Until now, that is.

Richard Touchette, a longtime engine builder and cylinder-head specialist, has invented a new device that allows anyone to precisely measure flow around either the intake or exhaust valve without impeding the path of that air.

Touchette, who also founded RTST to handle the manufacturing, calls his invention the "Pressure-Differential Valve," or P-D Valve for short, and its genius is in its simplicity. Touchette actually embeds his Pitot tube inside a mock valve so that it's completely hidden from the airflow. The readings with this setup are more "pure," because there's nothing in the way to obstruct airflow.

"Around 1995, I was doing a lot of research and development on race-engine cylinder heads with flow-bench test equipment," Touchette explains. "I was also using a Pitot tube to do port mapping to see how the pressure differential changed though different ports. But a Pitot tube placed in the flow path of a cylinder head produces flow obstructions which makes it hard to get the true numbers when you are trying to produce a pressure differential map of a port. It can be a bit frustrating.