The major focus of most cylinder head discussion is flow into and out of the cylinder. While this is certainly an important function, consider that all the tuning efforts directed toward the induction and exhaust system is all aimed at getting air and fuel in or combustion residue out of the cylinder. Ultimately, all this effort is really focused on making cylinder pressure, which becomes torque and horsepower. Since the combustion process is what creates this cylinder pressure, it makes sense that we should pay particular attention to this process. What we’re talking about is the combustion space, which consists of the top of the piston and the cylinder head combustion chamber. It is the shape of this entire combustion space, including the top of the piston that helps shape the combustion process.
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In the early days of the smallblock Chevy and for all engines up to perhaps the mid-1970s, the shape of the combustion chamber was merely a tub or vessel in which to contain the valves. Little attention was given to the early small-block chambers other than the chamber volume’s affect on compression ratio. Generally, most performance small-block heads used a 64-cc combustion chamber roughly in the shape of a tub. The first major change to chambers came when emissions requirements led to increasing the size of the chamber to reduce compression and attempt to reduce the hydrocarbon count in the exhaust residue. This led to open chambers designed to reduce the quench area of the head, which also reduced oxides of nitrogen (NOx) emissions. This approach did not improve performance, however, especially given the accompanying drop in compression ratio.
Into the ’80s and early ’90s, combustion chambers began to receive a bit more attention with an attempt to push or move the combustion process toward the exhaust side of the chamber. Heart or kidney-shaped chambers began to appear that not only improved flow out of the intake valve, but also attempted to encourage mixture movement toward the exhaust side of the chamber. Angled spark plugs also began to appear with some regularity accompanied by information that suggested that the spark plug could improve combustion by beginning the event from a more advantageous position.
It’s only been since the 1990s that serious OEM work has begun on attempting to improve power by evaluating the combustion event using in-cylinder pressure analysis and more accurate dyno testing to evaluate what really occurs in the cylinder during combustion. A big revelation was that the shape of the combustion chamber does have an immediate and worthwhile affect on combustion efficiency. This is one reason why late-model GEN III and GEN IV engines are capable of running 11:1 static compression ratios with relatively short intake duration numbers on 91-octane fuel and still survive a 100,000-mile durability test. These engines also enjoy the benefit of electronic spark control that pulls timing out of the engine as soon as detonation is detected, but there are still many things that we can learn from these combinations.
Combustion chamber shape is actually dictated by several conditions. First is the valve angle, which for a production-based GEN I smallblock Chevy is 23 degrees. As we saw in Chapter 9, a shallower angle is beneficial from a flow standpoint, and this also creates a shallower combustion chamber, which aids in creating compression. The standard small-block 23-degree angle dictates a somewhat deeper chamber. You may have also heard of angle-milling heads. This “roll-over” technique changes the deck angle relative to the valves, but at best this is worth only a degree or so. The main reason for angle-milling heads is to remove more material from the deep side of the chamber in order to reduce chamber volume to increase compression. The chamber must also accommodate the valve sizes. With the small-block now capable of 454 ci or more of displacement with talldeck versions, valve sizes have had to grow as well—well surpassing the 2.02/1.60-inch standard. In fact, it’s possible now to get as big as a 2.250- inch intake valve for larger smallblock race heads.
Squeezing in the spark plug location is another critical component of combustion chamber design. Ideally, the spark plug should be in the geometric center of the combustion chamber where the flame front is required to travel the shortest distance in any direction to complete combustion. With a 4-valve per cylinder chamber or a Hemi, this is very easy to achieve. With a wedge-type chamber, the valves occupy this real estate, requiring the designer to move the spark plug over to the exhaust side of the chamber. In the 1970s, one of the first racers to discover that an angled spark plug was worth a little power was the late Smokey Yunick. Angling the spark plug toward the exhaust valve not only places the plug closer to the center of the cylinder, but also directs more of the combustion process toward the exhaust side of the chamber. Most newer aftermarket heads also use long-reach plugs not only to increase thread engagement in aluminum heads, but also use this longer reach to place the business end of the spark plug closer to the center of the cylinder.
Chamber Shape and Wet Flow
While it may seem that the chamber shape is dictated by some mystical attempt to direct the combustion event, most cylinder head designers will tell you that the chamber design is actually dictated by the ports. Let’s take a look at what occurs here, which should give you a better understanding of combustion chamber design and their functional requirements. As mentioned earlier, early chambers were mere deep tubs. As the chambers became shallower, this unshrouded the valves, improving flow. Taking this one step further is the concept of thinking of the chamber wall that continues the long side radius of the port as a continuation of the valve angle. As we mentioned in Chapter 9, with flatter valve angles, this becomes even more relevant. Laying the chamber wall back in an attempt to continue the flow out of the valve only improves flow. This is why even slight chamber modifications on production heads can improve flow even after the air has moved past the intake valve into the chamber. If the chamber wall does not pinch off or impede flow coming out of the valve, airflow generally improves at all valve lifts, but especially at the higher valve lifts.
Chamber shape is also critical as it relates to the wet flow aspects of chamber dynamics. Most of the discussions about intake ports and chamber shape tend to be concerned only with the effect of changes on dry airflow because that’s been the predominate method of testing until now. But with the advent of wet flow testing, the effect of the intake port and chamber design on the wet flow aspects can no longer be ignored. Chamber shape and the exit of the air and fuel out of the port and into the cylinder have a big impact on power and efficiency. We’ve included photos of this effect from Dart’s wet flow bench on how the liquid fuel enters the cylinder. Currently this evaluation, by design, does not take into account the effect of the piston moving up or down in the cylinder, but these rudimentary first steps still reveal a startling picture of what happens to the fuel as it enters the cylinder.
We’ve stated in this book that high-end racers and cylinder head developers often acknowledge and have experienced multiple examples of where their dry flow test bench has “lied” to them. Some use their flow bench only occasionally to help in development. In talking with the guys at Dart, and specifically to cylinder head specialist Tony McAfee, it’s very possible that changes to the port that contribute to dry airflow improvements actually make the dynamic wet flow picture worse. Then, when the cylinder heads are actually tested on the engine, the results are disappointing, or as one wag put it: “We’re suffering from improvement, again.” The Dart guys were less than forthcoming in terms of specifics as to where or how this happens, perhaps because we were knocking on a door they have just recently opened themselves. Clearly, this is an area that offers tremendous opportunities that are only now beginning to be explored.
There are other cylinder head designers and port specialists who are still professionally skeptical and prefer to design their ports based only on what they see on the dry flow bench along with the results they see on the dyno. While the wet flow controversy will continue to flourish in the coming years, so will evidence that proves to be either a colossal waste of effort, time, and resources, or the next new frontier in which numerous revelations in newfound power will emerge.
Clues to Combustion
We have included some photos of combustion chambers that have been colored by use and there is a time-honored tradition of attempting to evaluate combustion efficiency by “reading” the tracks in the snow, if you will, left over from the combustion process. This usually takes extensive experience and more than a little bit of art combined with a small amount of science. The first and easiest areas to identify are the clean portions of the chamber. Generally, these appear most frequently adjacent to the intake valve against the steep chamber wall. This is most often the result of fuel wash, where liquid or semi-liquid fuel merely washes clean any combustion residue. One attendant theory about clean areas of a chamber (or areas with the lightest deposits) is that this is where combustion efficiency is highest. There is certainly evidence to support this, often found in the simplest of places. If you look closely into a wood-burning fireplace, notice that the bricks nearest the flame tend to be clean and without residue, while farther away from the hottest pat of the flame, black combustion residue begins to form on the brick surfaces. The theory here is that the intense heat of the combustion process burns away the carbon while father away from the flame, this carbon has a chance to deposit.
The same kind of evaluation can be made in the combustion chamber. What we’re looking for is a chamber with an even pattern of combustion across the entire chamber: with an even coating on the piston top, chamber floor, and walls as we get closer to the exhaust valve area of the chamber. Ideally, the combustion space will be evenly colored, but in reality that rarely occurs. Each cylinder exhibits its only special “footprint” that, if evaluated closely, can offer clues as to the combustion space’s efficiency. Large areas of fuel wash are not good, neither are centralized areas of heavy, sooty-black carbon deposits. These are signs of poor combustion activity where the flame front either fails to travel or offers very little heat to eliminate the carbon deposits. Small, clean spots along one side of the spark plug boss are also commonplace, but this also indicates fuel movement across the spark plug. A well thought-out chamber will attempt to push the fuel in a circular motion exiting the intake port, moving the air/fuel mixture across the spark plug and toward the exhaust side of the chamber.
It’s also common to see the exhaust valve a slightly lighter color than the rest of the combustion chamber. This can be attributed to the exhaust valve’s higher operation temperature. An exhaust valve with carbon deposits is clearly not running at its peak temperature, which indicates the cylinder is not running anywhere near its peak potential. All of this information can also be used to help evaluate the piston top as well since it constitutes the “floor” of the combustion space. Because the small-block is a wedge engine, the quench area is generally clean since little combustion occurs in that area, but the rest of the piston should reveal evidence of equal heat if the chamber space is efficient.
We have included a couple photos of a small-block dyno test where the cylinder head modifications were not positive, resulting in an engine that did not respond well to changes (oftentimes not responding at all), while also requiring more ignition timing and much more fuel than a comparable engine package where the combustion space was far more efficient. Once we removed the cylinder heads, the residual burn patterns on the top of each piston clearly indicated that the engine was not performing the combustion process efficiently. There were large clean spaces on the pistons tops where there was no combustion activity present, matched by a center area of the piston evidenced with irregularly shaped areas of greasy black, sooty combustion residue. It was obvious very quickly that this engine was just not happy and the chamber modifications we had tried did not work.
The point of this is that during disassembly, there are many clues to how well the engine is running, far beyond just the power numbers displayed on the dyno curve. So the next time you tear that small-block apart, take some time to look at what the piston tops and combustion chambers are telling you. It may even be worth it to take some digital photos that you can later evaluate on your computer in more detail.
There are few areas in the internal combustion engine that are more shrouded in mystery, yet offer the most potential for gain than the combustion space. We are learning more every day about not only how air and fuel enter the cylinder, but also what happens to that mixture after it has been oxidized and the clues that the combustion process leaves behind. It’s up to the detail-oriented hot rodder and engine builder to decipher these hieroglyphics using a little bit of common sense and a keen eye for the smallest of combustion clues. It’s fun. You ought to try it.
Written by Chris Petris and Posted with Permission of CarTechBooks