You've likely read accounts of the benefits derived using wet-flow benches. Other proponents suggest proper pressure profiling of air movement and patterns is the preferred method. Swirl and tumble meters can be helpful. Those with access to CFD and related computer software maintain their approach is superior. And then again, you'll find experienced engine builders simply making changes to inlet paths and combustion spaces (piston crowns included) and using brake specific fuel consumption (BSFC) data as the yardstick. Frankly, I've been fortunate to have, at one time or another, used all these methods and found each to provide its own benefits. Sometimes, end-goals will dictate the best method of analysis.
Cut through it all and you'll probably find that the more experience a person has with connecting the dots between the pre-dyno analysis and specific dyno results, the better each method seems to become. As a sage old engine builder I once knew often said, "It ain't what you do, but the way how you do it that counts." Fortunately, all of these techniques can produce worthwhile data that addresses concerns about mixture conditioning.
The region in a flow field nearest surfaces over which a working fluid is passing that is predominantly "stagnant" and dominated by fluid viscosity is the so-clled "boundary layer" and can create significant resistance to net flow. It is also a region where fuel tends to accumnate in air/fuel, wet flow environment. Roughened or dimpled surfaces in the boundary layer region can add energy and help restore separated fuel to the mainstream working fluid, thereby improving the possiblity of iits combusion.
Theoretical pressure distribution in a circular flow passage, steady-state, constant pressure drop across passage. Highest dynamic pressure tends to be at or near the geometric center of passage. The highest point of pressure can vary as a functon of passage and section area shape and section location.
Location and Definition of Boundary Layer in a Flow Passage
Now let's get practical. Inlet air can be likened to a fluid. In fact, Smokey often called it the "working fluid." On many occasions, he talked about studying water movement in creeks just to see "what affected what" during fluid flow.
When air moves across a surface, there's a dynamic interaction between the moving fluid and static surface, amounting to a form of frictional resistance. The smoother the surface, the more random this interaction becomes. Introduce a fluid (in this case a fuel) into the air stream and this randomness can translate into a wide range in particle sizes from the upstream atomization process.
The result? An effective variation in droplet size and, therefore, downstream air/fuel ratios. Power-wise, none of this is good. So while "smooth" flow surfaces may help promote net airflow, a wet-flow environment doesn't always benefit from a fundamental increase in flow. In such situations, creating an increased level of energy at the interface between static surface and wet-flow (at the boundary layer) can be helpful. Best case, you'd like to create a measure of control in the way the boundary layer aids either fuel atomization or fuel suspension (or both). That's where the so-called "rough surface" notion comes into play.
Essentially, without delving too far into an aerodynamic analysis of the interaction between an object moving through (or being passed by) a working fluid, here're a couple of practical thoughts.
A "roughened" surface (ranging all the way from a simply textured to fully dimpled pattern) will create a network of eddies, downstream of the point(s) where the working fluid contacts the surfaces. Even though the time periods of flow during a typical inlet cycle can be very brief and bi-directional, there is sufficient time for the working fluid to become more active along the passage walls.
It's this activity that can aid otherwise separated air and fuel to re-combine and an opportunity to reduce particle size. Regardless of how insignificant this may seem, results (depending upon how badly fuel is not being conditioned prior to these type modifications) can be dramatic. Some seasoned engine builders have been introduced to this concept and have become believers in its merits. Just ask Dennis Wells of Wells Racing Engines
So whether you choose to simply "roughen up" intake passages, selected spots in the combustion space (including areas of piston crowns) or take the time to identify where these locations are the most sensitive, chances are better than good you'll see gains at the flywheel. Smokey even took the idea to the point of developing a combustion chamber and piston crown that created rotational motion of the air/fuel charge, during the compression stroke, but that's an entirely different story about mixture conditioning.