After its Quadrajet carburetor...
After its Quadrajet carburetor was removed, the effects of reversion can be clearly seen on this intake manifold. Note the "cleanest" area of the plenum is in the one of highest exit velocity of the carb; on the primary side and nearest the upper plane's plenum floor. The slower-flowing secondaries and lower plenum floor for the other primary throttle opening all create less flow resistance to reversion. That allows for deeper re-entry into the intake manifold during periods of reversion.
Depending upon who you ask about it, reversion is a condition not always clearly understood, even though its effects are pretty widely recognized. So, at the risk of possibly oversimplifying the issue, let's begin by describing the condition itself.
The Principle Cause for Reversion
At the beginning of an engine's exhaust cycle, cylinder pressure is higher than atmospheric pressure and this enables combustion residue (essentially unburnable byproducts of the previous burn cycle) to flow into the exhaust system. At some point in the exhaust cycle, cylinder pressure approaches atmospheric pressure. For purposes of this discussion, we're deliberately bypassing some of the far more complicated elements of "wave motion" tuning or any other references to ways that may enhance exhaust flow efficiency. Even under those conditions, reversion still exists.
As our example exhaust system approaches this condition, and depending upon specific valve timing, the intake valve cracks open. We've already said that to this point in the process, cylinder pressure is higher than atmospheric pressure, so combustion residue can now begin flowing back up into the intake path serving our example cylinder. (Note the accompanying sketches that detail the various pressure relationships during the so-called "reversion period.") Certainly at part-throttle and possibly wide open throttle (WOT), the induction system is operating at a vacuum. Compared to atmospheric pressure, this makes the intake path more suited to combustion residue back-flow than the normal exhaust path. Because the effects of reversion (reverse flow of exhaust gas back into the intake track) include being a function of rpm, it's possible for some other cylinder in the engine to have its next fresh air/fuel charge diluted with exhaust gas from the previously-described cylinder during its exhaust cycle. So, the effects of reversion can, and are, transferred randomly from cylinder to cylinder based on firing order. And the process becomes repeated over and over again throughout the firing order.
An interesting part of the entire reversion process is the fact, again, that its effects are time sensitive. At lower engine speeds, what we'll call the "reverse flow" situation has time to dissipate more of its energy before the next reverse pulse occurs. But as engine speed increases, the amount of energy in this reverse flow process increases proportionately, tending to penetrate further (and quicker) into the induction system. In some instances, especially with the use of carburetors that will meter fuel whether flowing in the correct or reverse direction, fuel can be seen "standing" above the carburetor (when on an engine dyno) or having "wetted" the underside of an air cleaner with fuel. Then, with further increase in rpm, there is insufficient time for the reversion condition to reach as far into the intake system as it was at lower engine speed, becoming more of a combustion contamination issue again as it was at less rpm.
We'll discuss this a bit further later in the story, but it's a good time to understand that if you were to make a graphical trace of a "reversion curve," it would appear much like a bell curve, peaked at some rpm and diminished on either side of that peak. Especially with carburetors experiencing reversion problems, an engine dyno power curve would show a notable depression in the data measured at the rpm where reversion peaked.
How Reversion Affects Combustion Efficiency and Power
When all is said and done, left unaddressed, reversion dilutes fresh air/fuel charges and decreases combustion efficiency. Whether or not the effects of the condition include contamination all the way back into the induction system, any efforts to lower cylinder pressure at the exhaust cycle onset can help dampen reversion pulses and reduce lost power. Just keep in mind the process tends to blend exhaust gas with fresh air/fuel mixtures. Plus, it also represents a portion of the combustion ingredients that take up space in the place where combustion occurs. By displacing some of the space that could be occupied by combustible material, it further reduces power as evidenced by lower exhaust gas temperature. It's like a built-in EGR (exhaust gas recirculation) system.
It can also be detected by comparing brake-specific fuel consumption (BSFC) data, particularly if you discover that BSFC numbers are skewed higher through the mid-rpm. In fact, if BSFC isn't at or near its lowest numerical value around peak torque, reversion is one of the likely culprits.
As rule, when testing on an engine dynamometer, it is generally desirable to work toward obtaining a relatively flat BSFC curve. Numerical values of the data tend to be higher on either side of BSFC at or near peak torque, even though you're attempting to flatten the curve as much as possible.
We previously mentioned that any deviation from this pattern usually suggests a problem with air/fuel charge quality as well as quantity. It's the quality side of that equation that often becomes affected by contamination derived from reversion, and that condition trails right back to fresh mixtures becoming diluted, either somewhere in the induction system or from inefficient cylinder evacuation during exhaust cycles.