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We also wondered how he actually could see what the fuel/air mixture was doing using the wet flow bench. He told us the bench doesn’t use actual fuel for safety reasons, but rather a solvent with the same specific gravity as gasoline with a dye mixed in it that illuminates under ultraviolet light so that they can see how and where the 13:1 air/liquid ratio mixture flows in the intake and exhaust runners, around the valve and valve pockets, and in the combustion chamber.
Visual observation is one component of the process. A computer is used to monitor flow rates and air/fuel ratios and then record the data. When asked what he had observed by watching the fuel/air mixture flowing under UV light McAfee said, “As the fuel/ air mixture flows through [the head] vortices develop which can cause the fuel to fall out of suspension. There are portions of the mixture that are rich and other portions that are running real lean. There are areas of the combustion chamber that tend to collect fuel. The object is to get the air/fuel mixture as uniform as you can. Relocating the valve seat and getting it into the air stream is one solution.”
Anytime you pull a motor apart and it looks greasy inside the intake or in other areas, that's not oil, it's unburned fuel. If you could make the perfect fuel mixture where it could combust perfectly in the chamber you could drop your air to fuel ratios and you'd cut exhaust emissions in half in a heartbeat because in most engines you're dumping a lot of raw fuel out the exhaust pipe. In most cases you're not seriously burning more than 70 percent of the fuel that runs through a combustion chamber.
McAfee attributes many of the changes in their newest series of heads to what he has learned using the wet flow bench. He says that he has learned a lot about head and port design, especially when it comes to heads with big intake valves and exhaust ports (not much of which he was willing to share, however). Like many of the other cylinder head experts we talked to, McAfee said that a wet flow bench is not the magic bullet, but it is another tool to help understand what air flow and fuel/air mixture do. The manufacturer can then use that knowledge to
improve the performance of its cylinder heads.
McAfee told us that his wet flow bench had helped Dart improve the performance of every cylinder head manufacturer and that it has been especially helpful in the “one-off” heads they build for Maskin’s Pro Stock engines. “We can see horsepower gains after just one test,” he said. He did have one world of caution, though.
“You have to be careful with a dry flow bench and what you're telling yourself, but if you are really serious about optimum cylinder head performance then you would have one of these [wet flow] benches because using a dry flow bench alone will not allow you to do what you need to do if you are going to develop the very optimum head.”