Swirl is considered a recent development in cylinder head design. OEMs have placed a greater emphasis on swirl because of some desirable effects reaped from achieving intake ports with good swirl characteristics. The first question is, "What is swirl?" As you might expect, swirl is obtained when the air/fuel mixture enters the cylinder in a definite rotational pattern (Helical pattern down the bore). The next question would have to be, "So what?" Research has found that in typical internal combustion engines, there is a lot of variation in the timing of peak cylinder pressure from one combustion cycle to the next. The key to power production is to manage the peak cylinder pressure in relation to the piston position during combustion. Most of an engine's power production occurs in the early stages of combustion, while combustion pressures are the highest and the piston can take maximum mechanical advantage of the expanding gasses. Unfortunately, the variation in the pressure curve from one burn cycle to the next is the most erratic during the critical early stages of combustion. This is not good, if maximum efficiency and power are the goal.
Swirl has been found to reduce the combustion pressure variation, due--in theory--to a more consistent gas mix in the vicinity of the plug when firing. This promotes more efficient combustion. It was also discovered that the action of the swirling gasses during ignition helps speed flame propagation, providing a higher resistance to detonation, and a quicker pressure rise at the critical early stages of the combustion process. Less erratic combustion boils down to the poten-tial to produce greater torque, and the quicker burn promoted by swirl increases the detonation limit. This allows for an increase in compression for even more power. Chrysler first took advantage of swirl design in the 1985 318 two-barrel, and true to that theory, increased the compression ratio. OK, swirl can be a good thing, but my 915s were cast 20 years before all the hubbub about swirl. Whether it was by design or coincidence, the production Mopar big-block heads exhibit excellent swirl characteristics. In fact, the swirl numbers for our heads in stock form compare favorably with many of the latest high-swirl head designs. Creating high swirl numbers alone is not difficult to achieve (shrouding half the valve is one quick way to do it). However, providing high-flow efficiency with adequate swirl is another matter. In the case of production B/RB heads, the trick is not to kill the swirl when modifying these heads for improved airflow.
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Along with the rage in swirl-promoting designs, and after a long romance with open-chamber designs, OEs have returned to embrace the closed-chamber configuration. Again, the reasons stem from greater combustion efficiencies. A faster burn and a more turbulent mixture are obtained when the compression height of the piston is set to take the maximum advantage of the flat quench face of the closed-chamber head. In other words, if the area between the flat face of the head and the piston top is sufficiently close, as the piston reaches TDC on the compression stroke, the mixture that occupied the area over the piston is forced at high speed into the direction of the advancing flame front. The net effects are faster burn and greater turbulence at the time of combustion--advantages for efficient combustion. This is the squish effect. A second benefit is the mechanical resistance to detonation is improved when a closed-chamber head is used with a close piston-to-head clearance. Detonation begins at an area other than the main flame front. With a flat-top piston, the area opposite the spark plug in the quench area is prime detonation territory. If this area is sufficiently tight, the ability for it to detonate reduces since the combustion at the desired flame front occurs quicker (squish effect), and lessens the time for heat rise in the mixture at the far end of the chamber. Furthermore, the thin section at the squish area exposes only the small volume of mixture to an equally high surface area, diminishing the heat rise in this part of the mixture.
This explanation is where the term quench originated. Soaking this heat from the area most likely to detonate means the tendency toward detonation is reduced, and the compression tolerance is increased. Of the 1967-and-up B/RB heads, only the 1967 915s had closed chambers with the improved ports (the 516 heads of earlier B/RBs and the 1967 B-engines were also closed chambers). If we throw those 915s on a low-compression 400 B-engine, with the stock-type low-compression pistons in the bore, 0.100-inch at TDC, we pick the compression up, but the quench and squish effects are lost. That 0.100-inch (plus head gasket thickness) minimum clearance between the piston and head is prime detonation territory. The same result applies to the 906, 346, 902, 975, and 452 heads, all of which come through with a 0.100-inch recess open chamber. Filling the quench area (welding or metal spraying), or using special quench-dome pistons, has been the solution for engine builders who seek the quench and squish advantages with these open-chamber heads. The former method is definitely the better alternative.
