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.