Wet-flow testing with gasoline would present an unacceptable hazard, so the nonflammable liquid that’s injected into the intake air stream has the same specific gravity as racing gas. The liquid also has a fluorescent dye that glows under ultraviolet light, allowing the flow bench operator to see the movement of the fuel as it moves through the port, valve and chamber.

It may look like a scene from a dream, or Darren Aronofsky's 'The Fountain,' but fluorescent dye injected into the intake airstream shows the swirls and eddies that form as the fuel mixture enters the cylinder. The dye glows under ultraviolet light, making the movement of the droplets visible.

“Injecting liquid with the same specific gravity as racing gas and testing at a much higher depression than standard dry-flow benches gives us a better simulation of how the air and fuel behave under actual operating conditions,” McAfee explained. “With the fluorescent dye, we can also see the effects of the changes we make.”

Gasoline-burning engines typically produce maximum power at an air/fuel ratio of approximately 13:1. That ratio is measured by weight, not volume, so it takes 13 pounds of air to react with one pound of gasoline. Gasoline is much denser than air; at 70 degrees F, a cubic foot of air weighs only about 1.2 ounces, while a cubic foot of gasoline weighs more than 40 pounds. In a running engine, the lightweight air molecules change direction much more quickly than the heavy gasoline, which moves through the intake tract as vapor, atomized droplets or globs of liquid fuel. In a perfect world, varporized gasoline is evenly dispersed and suspended in the air stream; in the real world, gasoline droplets often fall out of suspension en route to the cylinder. The gasoline stains left on intake manifold runners and cylinder head ports offer mute testimony to the odd behavior of these mixtures.

A clear plastic cylinder sleeve allows the wet-flow bench operator to observe the behavior of the simulated fuel as it moves through the valves and combustion chamber. The fluorescent dye reveals where the flow is turbulent and where it is calm. The air and fuel may form eddies and whirlpools that resemble miniature tornadoes as they migrate around the combustion chambers. The effects of changes in the shape or cross-sectional area of the ports and valve bowls become instantly apparent on a wet-flow bench.