Tech Talk #40 – Wet Flow Revelations: The Monsoon Inside Your Motor

By Darin Morgan, Reher-Morrison Racing Engines
, author of this issue’s Technically Speaking column, is in charge of cylinder head research and development at Reher-Morrison Racing Engines.

“We felt like blind men who had been given the gift of sight.”

If you expected to find David Reher’s column on this page, I hope you won’t be too disappointed to find my words here instead. I’m in charge of cylinder head development at Reher-Morrison Racing Engines, and David asked me to write about the extraordinary impact of wet flow testing on our R&D program. I’ve given presentations on our wet flow development at the Performance Racing Industry (PRI) trade show and the Advanced Engine Technology Conference. Since most National DRAGSTER readers can’t attend these events, David thought that Technically Speaking would be a good way to spread the news. He also promised that he’d be back on this page in a few weeks.

Every racer is familiar with dry flow testing; it’s become commonplace in every form of motorsports. In wet flow testing, we add a liquid to the airstream to simulate the behavior of atomized fuel in the engine. Many professional engine builders have built there own wet flow benches. They have told me that after hundreds of hours on there benches, they learned nothing. That does not mean that wet flow testing is useless. It simply means there testing procedures where useless. That all changed last summer when Joe Mondello and Lloyd Creek came up with the fabulous idea. Design a wet flow bench that everyone could use, Make it affordable and make it useful. They have accomplished there task in a BIG way! I was the lucky benefactor of one of the third machine they built. Joe Mondello called and asked me to work with there new bench using our current Pro Stock heads. Since then we have learned so many new and wonder full things about wet flow dynamics and continue to learn more every day.

The mechanics of wet flow testing are straightforward: the intake port is pressurized and liquid is introduced into the air stream with an atomizer. A clear plastic cylinder sleeve allows the operator to observe and record the behavior of the fuel droplets in the valve bowl and combustion chamber. The liquid is then separated from the air and captured in a recovery canister.

Watching wet flow in a cylinder is like trying to see individual rain drops in a hurricane. Adding dye to the mixture helps, but only slightly – the dye reveals gross trends, but not the important details. The breakthrough came when Mondello and Creek came up with the idea to add fluorescent dye to the mixture and observe the motion with an ultraviolet light. What had been invisible suddenly became perceptible – and what they saw was a revelation!

The fluorescent dye allowed us to see the behavior of the air/fuel mixture in fine detail. We could watch as a vortex would form, grow, and move around the chamber like a miniature tornado as we adjusted the valve lift. We could spot areas where the vortices joined to form a cyclone of fuel and air. We could see where flow was turbulent, and where it was stagnant. We felt like blind men who had been given the gift of sight.

The first thing that came to light was the myth of fuel wash. Like most racers, I believed that clean areas on the chamber walls and piston dome indicated where fuel had fallen out of suspension and cleaned off the carbon. Wet flow testing revealed the truth: The shiny areas are where there is the least amount of fuel. This lean mixture burns quickly and completely. In fact, the most fuel fallout occurs where there is a dark, sooty burn pattern on the chamber and piston. That is where the fuel falls out of suspension, creating a vortex. In this condition, the fuel burns, but it burns too slowly and too late in the cycle to create usable cylinder pressure.

Usually the burn pattern that we see on the chambers and pistons mimics what we see on the wet flow bench at the upper ranges of valve lift. For example, our Pro Stock engines use a cam with 1-inch valve lift, and the burn signature in the chamber is almost identical to what we observe on the wet flow bench at .800-inch valve lift.

What had appeared to be chaos in the cylinder now became a fluid and predictable motion, thanks to the fluorescent dye and ultraviolet light. We saw how the vortices spin in a clockwise or counterclockwise direction depending on the port orientation and the direction of the airflow. We watched as they started small, gained size and speed, and rotated clockwise or counterclockwise around the chamber.

One of the variables that significantly affects power is spark plug wetting. If there is a vortex of raw fuel droplets near the plug gap, it is very difficult to ignite the mixture. The wet flow bench allowed us to see whether a vortex formed near the plug gap and then take steps to move it away.

We also discovered that there is a strong tendency for vortex generation around the exhaust valve. The fuel converges in a big rotating ball, and its energy is wasted because it burns too late and too slowly to create useful cylinder pressure on the power stroke. In an inefficient engine, this fuel is expelled into the exhaust port while it is still burning, raising the exhaust gas temperature and superheating the valve guides. This isn’t a simple problem to solve, but now we know where to focus our efforts.

Wet-flow technology is already migrating to sportsman engines. The shape, cross-sectional area and velocity characteristics of the ports in our Raptor big-block sportsman cylinder head came directly from our Pro Stock research. There is a small “wing” in the Raptor’s intake runner that takes the wet flow off the floor and literally launches it back into the airstream. The port has to be designed around this wing; you can’t just put a wing in a port and gain power. That’s one example of how we have applied wet flow technology to sportsman engines.

In a perfect world, the air/fuel mixture would be 100 percent homogenous. The atomized fuel droplets would all be the same size and surrounded by an adequate number of oxygen molecules for complete combustion. Unfortunately, we don’t live in a perfect world. Thanks to Joe Mondello and Lloyd Creek the wet flow bench is starting to show us how imperfect the world inside a racing engine really is.