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Wet Flow Testing

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Racers and engine builders are familiar with flow testing, used to measure the airflow and efficiency of their cylinder heads. In theory, there are two basic aerodynamic factors that are intuitive in the human mind. Number one, a bigger hole will flow more air. Number two, a smoother hole will flow even more air. To many engine builders and cylinder head designers, both of these notions make perfect sense.

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And according to Darin Morgan, both of them are wrong.
“In pure aerodynamic terms, they’re correct, but they don’t work inside a running engine because people tend to forget about two factors,” says the induction research and development expert at Reher-Morrison Racing Engines, Arlington, TX. “First, the induction system itself creates dynamic effects on the flow, and second, the working fluid inside an engine is an air/fuel mixture, not just air alone.”

If the engine ran on air alone, Morgan acknowledges, the best solution would be to have all surfaces mirror smooth, with all transitions, seats and valves perfectly radiused to create perfect flow paths. However, introduce fuel into such a system and port walls would be so smooth that the fuel could never maintain adequate atomization in the airstream.

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If the intake tract is rough enough, it will help keep the air/fuel mixture suspended in the airstream. If it’s smooth, the fuel wants to go to the wall and sheet, falling out of suspension, explains Morgan. Then you have the worst-case scenario. You have the worst wet flow you can have because all of the air/fuel mixture is deatomized and puddling everywhere. You’ve got lean spots and rich spots in the combustion chamber and it’s a preignition nightmare.

The seat contours and the sharpness of the seats don’t relate exactly back to the dry flow bench. If you can smooth them out and radius all the edges, you’ll definitely flow more air. You can make a sharp-edged seat flow ALMOST as much air, but the difference between the full-radius seat and the sharp-edged seat can be about 30 hp.

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“Dry flow testing is great for certain things,” explains cylinder head design legend Joe Mondello. “It gets you to the point of good cylinder head design, good chamber shapes, good valve angles, good valve shapes and it can tell you a lot about air flow. What it doesn’t tell you is what happens when liquid is induced into the runner.”

Mondello says the concept of wet flow testing is hardly new; in fact, engine builders and cylinder head designers have known for at least 20 years that a wet flow bench was important. Many people have built their own wet flow benches in an effort to determine what really goes on inside the combustion chamber.

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The problem, say Mondello and Morgan, is that even wet flow testing has given engine builders precious little information to go by. To make the results more useful, Mondello and Lloyd Creek introduced and patented a wet flow bench that allows users to see inside the combustion chamber. This relatively simple idea has helped to change some long-accepted flow concepts.

Morgan, who received the third Mondello/Creek-designed wet flow bench, explains that the new machine has been a great research tool.

“The way we used to do flow testing was to get an idea of what was happening in the combustion chamber. We’d spray dye into the air and then let the intake manifold runner take it and we’d see where the dye would fall out,” Morgan says. “It was extremely hit or miss and it was very difficult to determine what was going on within the chamber.


“I wasn’t too impressed when Joe Mondello said he had built a wet flow bench, because other people had been telling me the same thing for 10 years and they hadn’t learned anything. But, I figured I’d give it a shot,” Morgan recalls.

In wet flow testing, a liquid is introduced into the airstream to simulate the atomized fuel. The liquid is either distilled water or a solvent with the same specific gravity as gasoline. It includes a staining dye that leaves a trail where the air/fuel mix enters the combustion chamber when the intake valve opens at given lifts and test pressures.

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A clear plastic cylinder sleeve lets the operator watch and record the behavior of the liquid in the valve bowl and combustion chamber. The liquid then separates from the air and is captured in a recovery canister.

“Watching the wet flow in a cylinder is like trying to see individual rain drops in a hurricane,” says Morgan. “Adding the 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.”

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Morgan says the dye allows him to see the behavior of the air/fuel mixture in fine detail. “We would 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 the flow was turbulent and where it was stagnant.”

It’s a very dynamic situation, says Mondello. As the valves cycle open and closed, the vortices are moving back and forth. As the valve opens, the vortices that are generated at various lifts move around the chamber. At lower valve lifts and lower velocities, both of them are up in the corner. As they climb, the vortices move toward the center of the chamber.

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So if a smooth surface is undesirable, would something like a dimpled surface – say, similar to a golf ball – help? “Morgan says no. “People have tried it to help mix up the airflow. Unfortunately, anywhere you have a dimple, there’s a vortex in the center of it. And it doesn’t shrink the original vortex, it just creates more of them.”

And just eliminating the existing vortices can be a huge problem. “I’ve done some wet flow testing on a hemi Pro Stock engine that runs a 2.530? intake valve and a 1.850? exhaust valve. It had really good dry flow numbers, but they couldn’t fire all 8 cylinders evenly.”

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Mondello says he put the cylinder head on his wet flow bench and discovered “it was a disaster. It was so wet, the owner of the car (an electrical engineer) had built his own ignition system that would fire each spark plug at a different voltage, just to make sure they would fire. The plugs were soaking wet, there were heavy vortices in the back of the exhaust valve, wet fuel was trailing out the exit to the recovery tank. The minute he saw the wet flow testing results, he said, ‘I know now why this thing doesn’t run, and it absolutely confirms what I thought but couldn’t see.'”

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Morgan says in a perfect world, the air/fuel mixture would be completely atomized and there would be no vortices at all. Of course, perfection isn’t possible, so he tries to make the best out of what he learns.

For example, one of the hindrances to 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 fuel.

“We’ve built engines in the past that just wouldn’t run right,” says Morgan. “It would just run too lean. Now we know that that is the number one indicator that there is a vortex generated on or near the spark plug. If you can get rid of that vortex – or at least move it off the plug – you’re halfway home,” Morgan says.

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Both Mondello and Morgan say they were surprised by one of the first things they learned after wet flow testing. “Fuel wash” in the combustion chamber is not what many people believe it to be.

“Like most racers, I believed that clean areas on the chamber walls and the piston dome indicated where fuel had fallen out of suspension and cleaned off the carbon,” Morgan says.” In fact, the shiny areas are where there is the least amount of fuel. This lean mixture burns quickly and completely. The dark parts are actually a sooty, burned pattern on the chamber walls and piston dome. This is where the most fuel falls out of suspension, due to a vortex.

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“There is a strong tendency for vortex generation around the exhaust valve,” Morgan says. “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.

Mondello asks, “How do we make this more effective? How can we dry it up? Chamber shapes, valve shapes, valve angles and real sharp angles in the chamber can all help wet flow dynamics. While many people think too sharp an angle is a hot spot and can cause detonation, the real fact is if you keep the edges sharp in the chamber, the wet flow dynamics are a lot better.”

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The biggest key to wet flow testing, says the expert, is to keep records of each change. “Take photos of each change you make. Document each one, then look at them all. You’ll be surprised by how little change it takes in some cases – and in some cases it’s a monumental amount of work. Sometimes you’ll have to weld. Sometimes you’ll have to change the casting. Sometimes the only thing you can do is move the spark plug.”

Morgan agrees, “One leading cylinder head maker came out with a new 12° head that moved monstrous amounts of air, had low chamber volume had great quench/squish ratios and lots of fuel mix motion. But the things just wouldn’t run,” says Morgan. “The problem was, the plug was right in the middle of the worst vortex I’d ever seen. Once they moved the spark plug over just .380? they got the heads to run great.”

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Many aftermarket heads have inherent design flaws, says Morgan. “I’ve seen vortices, usually 2, sometimes 3. The worst case scenario is 4, which is what almost all 23° heads have, two can be found over the exhaust valve, one on the opposite quench pad and one right near the spark plug. The one near the plug can decrease from about 3/4? down to about the size of a pinky nail – IF you sharpen the edges of the seat.”

Learning from Testing

“We gained 28 hp one year,” explains Morgan. “That was a huge number because our average yearly gain is 10-15hp. If we can gain 10-15 hp, we feel we can keep our engines in Top 5 form on the Pro Stock circuit.”

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About 8 of that horsepower Morgan says can be attributed directly to what he learned on the wet flow bench – because it involved plug wetting. “The single biggest thing we learned is to get the vortex away from the plug. We did it by building up the intake side of the chamber or laying back the exhaust side.

“You might think it would be best to lay back the intake side, but every time we laid the intake side back and kept the exhaust steep, that simply moved the vortex directly over the plug. It was the opposite of what we expected to happen. We took the area directly behind the vortex and laid it back more – it moved the vortex just .200? but it was enough to clean up the plugs.”

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Again, though, testing won’t solve all the problems. “Will wet flow testing tell me how to design the port? No, first comes the design of the induction system,” Morgan says. “I feel the induction system needs to be taken from the starting point back – which is the valve. Everything in the induction system will either be a ratio or proportional to the valve. Everything has to pass the valve.”


Morgan says airspeed in the induction system will be proportional to the engine cubic inches, rpm range and valve area as presented to the cylinders. “After that, when you’ve put your airspeed in and the induction system is designed, then you put it on the wet flow bench and see how you’ve screwed up,” he says.

Crystal Ball?

“Right now, we’re at about the same place we were when the dry flow bench came out,” says Morgan. “It’s still pretty much in its infancy because so far only the high-end race teams are using wet flow benches. Everybody is stumbling around in the dark because we’re not totally certain what affects what without affecting the bottom line.”

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Will the average engine builder need this technology or will it just be too much information overload? “The guy who’s just buying cylinder heads doesn’t need it. Neither does the guy just doing porting. But if you’re designing ports, manifolds and heads you’ll need it.

It’ll probably take many more years before people have it all figured out, even to where they’re advertising the heads have been wet-flow designed,” Morgan predicts.

Of course, many people thought the regular flow bench was unnecessary technology for anyone other than the highest level of performance engine builder. But as technology progresses, Morgan says he thinks more engine builders will take advantage of it. “There are retrofit kits available for dry flow benches now,” he says. But I think it’ll be 10-15 years before everybody finally gets the picture.”

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