As you work your way through this month's column, be aware there are successful engine and parts manufacturers who do not subscribe to what is being shared—and that's OK. There are also numerous of both these groups of people who do and have benefitted from the information. We will let you draw your own conclusions. Wet vs. dry flow benches notwithstanding, and neither of these will replicate the results of unsteady-state, pulsing airflow of a running engine, you may find value in some of the following.
First, we know that air and fuel require mixing before combustion. The manner and extent to which this is accomplished foretells the level of combustion efficiency achieved.
At the risk of oversimplification, you could liken the so-called "burn" process to touching a burning match to one corner of a piece of paper. Generally, a uniform flame will consume the material. But if there are small spots of oil or gasoline on the paper, flame travel will not be uniform. Ideally, in a running engine, we'd like for all atomized fuel droplets to be the same in size (no oil or gasoline spots), but this isn't the case.
The process also requires more time to combust large droplets than smaller ones, so flame travel is correspondingly variable. Given the very brief time available to mix and burn air/fuel charges, conditions that affect droplet size uniformity tend to reduce combustion efficiency and power.
Combustion efficiency can vary for numerous reasons, in particular as a function of load and engine speed. In an environment of continually varying mixture speed and interrupted flow, air/fuel charge ratios will not be uniform in the combustion space that notably includes the chamber, piston crown, and inlet path. So because of these inequalities in mixture ratios, rich conditions tend to leave darker combustion residue than leaner ones (please refer to the accompanying pictures).
Of course, there are other elements in play that can "color" the surfaces of these parts (intrusive oil, coolant, and so on) but it's not too difficult to distinguish between residue from excessive oil and rich air/fuel ratios.
Turbulence is important in the combustion process. Whether it's deliberate as in "tumble" and "swirl," or coincident with trying to achieve higher levels of volumetric efficiency (or all of these), turbulence can aid keeping fuel properly mixed and suspended.
Just because fuel may pass from its point of delivery into the inlet airstream in a well-mixed fashion, it doesn't necessarily follow that the results of efficient mixing will remain that way in the combustion space. Virtually any disruption in what we'll call "mixture quality" can diminish combustion efficiency and decrease power.
Hopefully, as you try to visualize these various events, envision them in ultra-slow motion, because we need to remember that combustion is a process, not an event.
Probably one of the most common problems result from flame movement over, or past, sharp edges in the combustion space, and this includes piston crowns. These consist of valve notches or clearance "eyebrows" in the crown, sharp corners on piston domes, or other changes in surface direction or finish.
Not to be confused with intentional surface conditions (like dimpling intended to improve mixture quality), sharp edges can create unwanted eddies or vortices which, in turn, can cause mechanical separation of air and fuel. Evidence of this condition, in terms of combustion residue patterns, is the absence of color or "clean" areas on piston crowns and in chambers not associated with the quench area.
One area where "clean" spots occur is on line-of-sight paths from the intake port to combustion chamber wall. An example of this can be found in most small-block Chevrolet V-8 cylinder heads (OEM or aftermarket) where the "back wall" of the chamber is clean, as viewed through the intake port with the valve removed. In fact, based on experience, here is an area where benefits often result from placing small "dimples" on this portion of the chamber.
It's also worthwhile to consider combustion chambers as the "roof" of the space that needs to work in dynamic compatibility with the piston crown as the two come in proximity with each other early in the burn (nearing TDC).
Smokey did some experimental work in this area by configuring the chamber and crown to create a circumferential rotation of the burning mixture just prior to TDC, and it worked. He called it his "yin and yang" concept. For as long as I knew him, I never asked the reason for this name. Probably better that I didn't.
Here's another point about the back wall of combustion chambers. It's possible to gain airflow (with some chambers) by "laying back" the back wall. In fact, it's entirely possible to modify combustion chambers to increase raw airflow and net a loss in power and a result of decreased combustion efficiency through damaged mixture quality. Such cases are frequently caused by separated air and fuel, increasing the range of charge ratios within the combustion space, or both.
Other evidence of undesirable combustion patterns is dark (rich) areas on crowns and chamber walls, just past the point of air/fuel mixture separation. Examples of such areas include valve clearance notches, protruded spark plug tips (or those recessed too far into their threaded holes, sharp-edged chamber walls, and improperly shaped piston crowns.
It's also possible to discover evidence of widespread air/fuel ratios (in the combustion space) resulting from poor airflow quality as a result of flow patterns in or around the inlet valve pocket…particularly when using very high valve lifts. Interestingly, a small shot of dye sprayed into an entry-radiused intake port (no manifold installed) can reveal pattern tendencies (not precise, but helpful) in a running engine. You might give it a try.
Not to be excluded from "pattern reading" are signs developed along the intake port path. Frequently, dark or sooty areas along this route signal excessive exhaust gas residing in the combustion space when the intake valve begins to open. If the condition is sufficiently severe, the underside of carburetors can also exhibit the same residue. The influence of backpressure is not confined to an engine's exhaust system. It can clearly upset combustion efficiency (and power) if the problem is of sufficient magnitude.
Learning to "read" post-combustion patterns might be likened to following someone's footprints in moist sand. One interpreter might simply gain knowledge about a person having walked in a certain direction. Another might only gather this information but be able to approximate the person's weight, gender, and type of shoes worn. Determining causes for certain combustion patterns and being able to identify and apply the means for addressing them requires experience, but it also means you must make the attempt.
Now, back to the issue of whether or not all this might work. Well, several years ago during a visit to the shop of Dennis Wells (Wells Racing Engines), we talked about much of what you have just read. In the span of just a few months after we visited, he poured through numerous dyno sheets and inspected used parts, particularly heads and pistons, becoming convinced that some of the engines he'd built were experiencing combustion efficiency problems.
His first attempt applying what had been discussed was directed to the combustion chambers and pistons he'd been using in his house Sprint Car engine (asphalt Silver Crown series). And even though this was an engine he has previously "optimized" for an entire racing season, the changes netted an increase in power and improved combustion efficiency by producing lower brake specific fuel numbers. He was convinced.
So although conventional wisdom may suggest that reading combustion patterns is not a viable tool, experience has pointed to the benefits from what the patterns can reveal. Time spent matching specific engine characteristics with the residue left on combustion surfaces can be a valuable addition to just about any engine builder's skill set.