On Reading Combustion Patterns
Since cylinder heads were on the discussion list this month, it seems appropriate to devote some space to the value of reading combustion patterns. In reality, it is from these tell-tale signs that you can determine much about an engine's combustion efficiency-or a lack thereof.

It has previously been suggested that besides participating in the combustion process, air is the means by which fuel enters the combustion space. Poorly conditioned, airflow quality can lead to a wide range in air/fuel ratios when the burn begins. Aside from the fact an advancing combustion flame can alter ratios ahead of its progress, variations often occur prior to ignition. It is these that an engine builder (or parts designer) can address to improve net power. But first, you need to identify where the problems lie, and then take corrective steps.

Two fundamental surfaces can provide clues to such problems. One is the combustion chamber, the other on piston crowns. With respect to the former and more than 60 years ago, Sir Harry Ricardo advanced the importance of maintaining "turbulence set up by gases during their entry before combustion." While this concept is important in a high-speed racing engine, the value of keeping fuel properly atomized and suspended, prior to combustion, can be overdone by allowing excessive velocity or turbulence to mechanically separate fuel from air. This leads to unwanted extremes in air/fuel ratios and lost power. The evidence of such separation can be "read" on both combustion chamber walls and piston tops. You know of these as "washed" or clean areas which suggest combustion was not present on or around these surfaces. One quick look at the backside of a small-block Chevy combustion chamber (the intake side of the spark plug location) will verify what we're suggesting.

Sooty or heavily-carboned surfaces, particularly in gasoline-fueled engines, is an indication of fuel-rich combustion, often associated with locations of lesser airflow activity prior to combustion. You may also find this type of combustion residue located on the down-side of valve notches or protrusions on piston crowns and often linked to sharp edges that create vortices (low pressure points) and collected fuel.

When you consider peak instantaneous inlet flow velocity in a 5.7L engine turning 7,000 rpm can exceed 400 ft/sec (when flow directional changes occur), there's ample opportunity for fuel to separate from its "carrier" air. As a result, any steps you can take to help ensure fuel is maintained in suspension clearly points to the potential for increased power.

Admitting the approach is more subjective than scientific, you can actually gain insight into an engine's efficiency by studying combustion patterns. Look for extremes in residue color. These can indicate a range of combustion activity from overly-rich mixture "zones" (dark) to no appreciable combustion at all (clean or light in color). When accompanied by an engine's apparent need to require an inordinate amount of spark timing (to optimize power) or what appears to be a requirement for excessive fuel, you can also look for air/fuel mixture quality problems.

Further adding to this problem, data suggests an engine's air/fuel mixtures will often vary in ratio from the point of fuel admission all the way into the burn space, whether carbureted or fuel injected. Both these methods of delivery require that attention be given to the ability of air to transport fuel in its most efficiently combustible form, up to and during the burn. At no point in the process can you assume that initially creating the proper air/fuel ratio will ensure it'll remain at that proportion until combusted.

In the end, it's a matter of the amount and distribution of kinetic energy present and characterized in the inlet stream and combustion space. Making certain that lows and highs in that energy content are minimized, abrupt changes in flow direction are eliminated or reduced, wet-flow surfaces are not made too smooth ("dead" boundary layers can lead to separated air and fuel), working toward increased power from reduced spark timing, exploring best power from minimal fuel flow and spending time examining the "evidence" from combustion can all lead to improved on-track performance. Sometimes, it's proper reading of the evidence that leads to successful completion of all the other objectives mentioned.