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.
The problem is also somewhat random among an engine's cylinders. It doesn't matter whether a single- or dual-plane intake manifold (for V-type engines) is in use; the randomness by which reversion affects different cylinders is still there, including cycle to cycle in the same cylinder. In some cases, how cylinders are placed in the firing order vs. intake manifold design will contribute to the condition, but the problem can persist. So, your choice of manifold design can be critical to minimizing reversion.
There are also times that reversion plays into ignition spark timing requirements. For example, if an inordinate amount of initial timing is required to optimize power, that's a clue that contamination from reversion is present. Reversion is also a possibility if an engine appears to be relatively insensitive to increases in spark timing or winds up in detonation with what might otherwise be considered safe and necessary spark lead.
Some Signs That Reversion is Significant
We spoke earlier about finding traces of fuel beneath carburetor air cleaner lids. I recall the first time Benny Parsons brought one of his race engines to Edelbrock to evaluate what he needed for an intake manifold. It was a routine practice that one of the benefits racers got from working with the company included having an intake manifold "tailored" to specific engine combinations. This was shortly before Benny won the Daytona 500.
While reviewing the initial data from one pull on the dyno, it appeared the BSFC curve was out of whack. Not only did the engine seem to be laboring around peak torque rpm, the BSFC numbers were unusually high above 6,500. Thinking that his carburetor might have a calibration problem and deciding to make this determination by using one of known value (from Edelbrock's dyno inventory), we removed the one on Benny's engine.
The entire plenum area and bottom of his carburetor was coated with exhaust gas residue, the reversion condition was so severe. We then learned the camshaft was advanced six degrees (further aggravating the situation) and the exhaust timing event was particularly short on the closing side (all gremlins to proper cylinder evacuation). And this was an engine combination with which he'd been winning short-track races!
Of course, there are other signs that an engine is experiencing a combustion contamination problem but it may be that the so-called reversion condition is the most prevalent. Earlier, we suggested it can materially affect how power is produced through a range of rpm.
Classic power curve shapes become changed to show "dips" or depressions where they should not occur. As indicated, this is often most pronounced in carbureted engines when reversion pulses cause the delivery of excessive fuel, thereby creating abnormally high air/fuel ratios. You can determine this more specifically by plotting fuel flow vs. BSFC data, noting spans of rpm where the two curves tend to diverge. It can also manifest itself in fuel-injected engines, including EFI, showing up in the latter more as contamination than fuel delivery control.
In cases where the problem seems more confined to the combustion space, engines tend to lose sensitivity to not only ignition spark timing, but also have difficulty getting good plug reading color. Even plug heat range requirements can become strange. Especially in these instances where the problem points to contamination in the combustion space, exhaust gas temperatures tend to decrease below the norm. Don't confuse the lower e.g.t.'s that are characteristic of rapid-burn combustion with contamination. Reduced power goes with the second of these, and not the first.
Reversion Effects from Different Types of Intake Manifolds
There are certain expectations you can have about ways intake manifold design affects reversion. Since we are dealing with a "reverse flow" and energy dissipation condition, so-called plenum-runner manifolds (particularly of the single 4V V-type engine design) are more absorptive than individual runner manifolds. Even so, reversion material (combustion byproducts) is essentially non-combustible, as stated, so any encroachment into an intake manifold can potentially dilute fresh air/fuel charges. It's just that in plenum-runner manifolds (all else being equal), the effects of reversion typically occur at higher rpm than for individual runner designs.
Manifold runner length and passage section area also play a role. Basically, the higher the runner velocity the earlier in the rpm range reversion pressure can be diminished. As runner length is decreased, section size increased or both occur, lower rpm reversion conditions are made worse. Also, as plenum volume is increased, there is improved reversion influence in terms of upsetting carburetor calibration in the lower engine speeds.
Reversion Effects From a Restrictive Exhaust System
This is likely what Benny...
This is likely what Benny Parsons and the team at Edelbrock saw back in the '70s when they removed a carb from one of his engines, a heavy coating of exhaust gas residue.
Generally speaking, as backpressure is allowed to increase (largely based on header primary pipe size), cylinder pressure at the point of intake valve opening will correspondingly increase the energy level of reversion pulses delivered back into the inlet track. There is a balance that needs to be struck between primary pipe size to effectively improve the exhaust "blow down" period and proper cylinder evacuation. This becomes particularly critical during mid-rpm operation where over-sized headers provide insufficient flow rates for good cylinder cleansing of exhaust gas. Think about the time when smaller primary pipe size improved mid-rpm power with little or no attending loss at higher engine speeds. Reports of such observations among engine dyno operators are quite common.
Reversion Effects From Improper Valve Timing
This is probably a good place to reference the accompanying sketch (Figures A and B) showing a simplified intake path pressure trace from intake opening to intake closing as a function of rpm (time or crank position). Note that there is an initial spike that represents a reversion pulse entering the inlet track.
Until the point (rpm, time, or crank angle) is reached when this pressure pulse decreases to equal that of atmospheric pressure, there is no flow toward the cylinder. But from then until intake closing, pressure in the intake path is less than atmospheric, allowing the cylinder to fill in a normal fashion. But you'll also note there is a second "bump" in the pressure track at the point of intake closing. This so-called "hammer effect" is similar to what you hear when quickly turning off a water faucet. Sometimes, this pulse is considered to be the source for reversion, but it is not, although the wave it creates in the intake path will oscillate back and forth in that passage until the next intake opening. To further pursue this would lead us into discussion involving Wave Motion tuning and not the purpose of this story.
However, using this same pressure trace example, you can begin to see how the intake opening point could affect reversion pressure. For example, the earlier the timing (either from cam phasing in the engine or specific valve events), the higher the potential cylinder pressure at intake opening. Designing or modifying intake valve seats and heads to decrease reverse flow at low valve lifts is an effective way to help dampen reversion pressure and allow for advanced cams or early lift points to be less problematic. Connecting rod length and overall rod/stroke ratios can play a role as well. It's usually a good idea to have some of these types of discussions with your cam supplier of choice. The more knowledgeable ones will have some specific comments to share on the subject.
How Reversion Becomes Contamination As a Function of RPM
We'll leave you with a few thoughts about this issue, but it's at the core of how reversion should be viewed and handled. Normally-aspirated engines are particular culprits for the conditions of reversion. Even though there is a proportionate increase in the number of reversion pulses as rpm is increased, there is less time for these pulses to re-enter the intake track. The dominate variable in this set of conditions is time. Stated another way, the degree of encroachment to which reversion pulses can go back in a counter-flow direction is shortened with increased rpm.
In some early reversion studies in which I participated, we were using an inverted, transparent, bowl-shaped cover above an intake manifold's plenum area. With the engine idling and camshaft somewhat advanced, you could actually see little vapor "puffs" blowing back into the plenum area, obviously following the engine's firing order. As rpm was slowly increased, the extent of plenum penetration by these puffs grew less and less until they were no longer visible. Further increase in engine speed caused these reversion pulses to diminish into merely becoming contained as contaminants within the combustion space where they remained at higher rpm. The conclusion we drew was that the effects of reversion never completely disappeared, becoming contamination for fresh air/fuel charges throughout the rpm span.
So, reversion is a problem. It consists of combustion byproducts that can reduce net power. It needs to be identified, recognized, and dealt with by whatever means that will cause it to be the least effective. It has specific and tell-tale signs, some of which were pointed out in this story. But the overriding point is that it is a subject not to be ignored in the building and tuning of a race engine. Its effects can be minimized, once they are understood.