If you were to read this book without realizing that in almost every instance the goal I’m trying to achieve is more horsepower, you’d think—and rightly so—that I’m obsessed with airflow. Throughout the chapter on ram charging, air filters, carburetors, induction system, cylinder heads, and exhaust, the airflow potential of the various parts are discussed at length. I assure you this, as you will see when we delve into the details of engine performance, it is no obsession. Airflow is the prime ingredient for maximum horsepower.
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Flow Bench Relevance
Let’s start off with the basic question: Why do engines have carburetors? The simple answer is that they’re there to mix fuel and air. Why is it necessary to mix fuel and air? Again, the simple answer is that fuel will not burn on its own; it needs oxygen. The oxygen is acquired from the air drawn into the engine. For a given amount of induced air the engine can effectively burn a given amount of fuel. It’s the action of burning the fuel that causes the gases to heat up and expand. This increases the pressure in the cylinder, thus pushing the piston down the bore on the power stroke. The greater the amount of air drawn into the cylinder, the greater the amount of available oxygen there is to burn with fuel. The more fuel that can be burned, the greater the amount of heat generated and, therefore, the higher the pressures generated in the cylinders.
Greater pressures mean higher horsepower by virtue of higher torque. The bottom line is that the more air the engine can inhale during each induction cycle, the better off we are for extracting power. Breathing efficiency boils down to using components that allow air to flow as freely as possible into the engine, hence the apparent obsession with airflow capability. Any component that doesn’t flow air will effectively rob the engine of some of its potential as a power producer.
An internal combustion engine is so named because it burns fuel and air internally. It does this within the working cylinders. An example of an external combustion engine might help you to see why we call a gasoline engine an internalcombustion engine. The most obvious example of an external combustion engine is a steam engine. Combustion of the fuel takes place outside the cylinder in the furnace that heats the water in the boiler. Since there are few steam-driven small-block Chevy’s around, I’ll skip anything further on that subject.
Going back to the internal combustion engine, we find that heat produced by burning fuel causes the gases in the cylinder to expand. Incidentally, the gases don’t explode. The process is far too slow for an explosion; it is a burn. It is the heat-induced expansion of the gases creating pressure on the piston and pushing it down the cylinder that develops power.
The fuel has to be mixed in welldefined proportions in order to burn efficiently. Although there are various valid reasons for working above and below the chemically correct ratio of fuel and air mixture, we can say that burning the fuel at the chemically correct mixture will provide good results.
Fig 2-1 shows the approximate proportional volumes of fuel and air consumed in one minute by an engine developing around 400 hp. Drawn to the same scale are the eight intake valves the mixture must pass through in the time they’re open. Assuming a 300-degree race cam, the intake valves are only open for 25 seconds of that one minute.
The valves look small compared to the volume of air that must pass through them. The implication is that having them flow efficiently is of prime importance.
We know that the heat in the cylinder causes the gases to expand and this in turn pushes the piston down the bore. Unfortunately, we can’t use the whole potential of the heat energy produced during the burning cycle. This is due to the nature of the engine’s design and the fact that the cylinders lose heat in many areas. The engine has to vent the cylinder to the atmosphere by way of the exhaust valve long before the cylinder pressure drops to atmospheric pressure, which means a big loss of energy. If you refer to the Fig 2-2 on page 16, you’ll see where most of the heat losses are occurring.
It’s essential at this point to realize that heat energy is directly related to horsepower. Assuming a 100-percent conversion efficiency, it takes 778 British Thermal Units (BTUs) of heat energy to develop 1 hp. Alternatively, it takes 1 hp of mechanical energy to produce 778 BTUs of heat energy. It’s more practical, though, to convert mechanical energy into heat energy than the other way around.
For instance, when we dump a certain amount of a vehicle’s kinetic energy into its brakes, they turn all of the absorbed kinetic energy into heat at a 100-percent efficient conversion rate. At the end of the day we find that, out of the potential energy from the fuel burned, the amount of energy actually extracted in terms of power at the flywheel is limited.
In fact, for every 100-hp worth of fuel burned in the cylinder, a good engine will derive only about 25 hp at the flywheel. This rate of energy conversion of the fuel’s potential energy into flywheel horsepower is known as the engine’s thermal efficiency. A figure of around 25 percent is typical for a good engine. A normal road engine is often around 18-percent thermally efficient.
Examining the cylinder pressures that occur within the engine, we find that the power produced from the cylinder pressure is more than that seen at the flywheel. The difference between these two numbers is a measure of the engine’s mechanical efficiency—that is, its loss of power from friction and pumping losses.
Unfortunately, horsepower lost to friction is turned directly back into the heat that’s carried away by the cooling system.
When an engine is modified, attempts are made to minimize all of these losses on one hand, and to improve the rate at which the engine consumes air on the other. As has already been demonstrated, the more air and fuel mixture that can be passed through the engine at a given time, the more the power output will be. This of course only holds true so long as none of the other inefficiencies are unduly increased. If this is achieved, then the engine will show more power, and we determine whether power has been increased on the dynamometer. A dynamometer, or dyno as it is more commonly called, is a device for measuring horsepower. There’s a lot of confusion about horsepower and rating numbers, but we’ll get to that later. At this point let’s look at what it takes to make horsepower.
Although airflow has figured strongly in our discussion so far, it would be wrong to think that airflow is the sole key to horsepower. It just happens to be one of the most important ones. Like any complex device, if we fail to produce results in one area then the overall results will be less than hoped for. There are factors other than airflow to consider.
To more easily understand what and how these factors affect the overall scheme of things when discussing various topics, refer to the power production flow chart on page 17.
With the aid of that chart, we’ll go through major factors that must be considered in any plan to develop high output, not just from a small-block Chevy, but from any engine.
At the top of the chart we start with atmosphere, which is the prime ingredient that must be moved through the engine in as great a quantity and as efficiently as possible. The atmosphere supplies the oxygen that allows the burning of fuel. This in turn generates the heat that expands the air that—since it’s contained in a closed cylinder—rises in pressure and pushes the piston down the bore.
Obviously, the greater weight of charge the engine inhales, the greater it’s potential for power. This brings us to step two, which involves maximizing the air density passing into the engine. In our example, it’s difficult to cool the air below that of the prevailing (ambient) temperature of the surrounding atmosphere. It’s important to understand that it’s not cubic feet per minute (CFM) that generates high output but pounds per minute. Hot air expands and weighs less, and consequently contains less oxygen, than cool air. As a result, it’s worth the effort to see that the induced air isn’t heated more than necessary, thereby maximizing its density.
The next step involves minimizing intake flow restrictions. Here’s where flow bench work on the induction system, intake port, and (to a certain extent) combustion chamber wields considerable influence. If ever a factor needs emphasizing, it’s the total induction system’s flow capability. Getting a charge to effectively fill the cylinder is the single most difficult function to achieve in the design and development of an engine. Never lose sight of this while spacing and building your engine.
In terms of induction effectiveness, we have to maximize pressure wave tuning. Although the effects of this cannot be quantified on a conventional flow bench, measurement of airflow on running engines does demonstrate its importance to power output. If taken to the limits of current technology, then, with a well-developed induction and exhaust system, volumetric efficiency (breathing efficiency) figures well over 100 percent can be achieved. These results are from the combined effect of both intake and exhaust pressure wave tuning. At this moment we’re considering only the intake, which in practice operates over a relatively narrow power band. But when combined with the stronger and more effective exhaust tuning, the entire system becomes far more effective.
Now we come to the engine, or to use a more common term, the “long block.” Our overall concern with the long-block assembly is friction. Everything within the long-block assembly needs our attention in terms of friction reduction.
By paying attention to friction reduction, not only do we allow the engine to make more horsepower, but it also lasts longer. Just for the record, the effects of friction become increasingly detrimental as RPM increases. Just 10 ft-lbs of additional friction within the engine (an easy amount to incur) will cost 4.8 hp at 2,500 rpm, 9.6 at 5,000, and 14.4 at 7,500.
Contained within the long block and the friction box in the chart above is the valve/cam event box. This refers to when and how high the valves open. It’s an important and critical factor that must address both the intake and exhaust requirements simultaneously. To be successful at intake and exhaust, both have to happen at the appropriate time—in relation to the crankshaft rotation. This makes cam events more of an overall factor rather than something pertinent to the intake event alone.
It’s worth mentioning that the subject of optimizing cam events is one of the least understood areas of high-performance engine building. Fortunately I have some pertinent information in that area because it’s a specialty of mine. If you take the time to absorb—even in its simplified form— what’s covered in the cam selection chapter, you’ll be better than one step ahead of the opposition. Guaranteed!
Now we’re getting down to the real core of the process of the heat engine: combustion efficiency. In the combustion efficiency box there are five factors that we need to deal with. Failure to optimize any one of these factors means reduced torque, which in turn means less power. To avoid failure, be sure to heed (to the letter) what I have to say on the selection of production heads. For a small-block Chevy, heads are a major issue, but in other areas the output may only suffer minimally if a component is a little off optimal.
The first combustion efficiency factor, the mixture ratio, has to be closely controlled within narrow limits and the mixture quality has to be what the engine wants. Here the mixture quality refers to fuel/air mix with the fuel sufficiently atomized for the engine’s combustion requirements, but not over-atomized so as to cut volumetric efficiency. Establishing the best mixture quality for power or economy is a major issue for engines requiring a wide operational band, as all good street motors do. Because of its importance, we will deal with it in detail in the carburetion, induction and cylinder head chapters.
The second factor is mixture swirl and motion. Fortunately, the typical production small-block Chevy intake port has reasonable swirl characteristics.
This is a good start, but the type of combustion chamber a production head may have can make or break any assets the intake port may have. Combustion effectiveness will also suffer if the block assembly is built incorrectly, because the piston-to-cylinder-head-induced squish action hasn’t been optimized. Failure here causes a drop in torque, horsepower, and fuel efficiency. On top of this, your engine will be more prone to detonation.
Third are the compression and expansion ratios. I’m sure most of you are familiar with compression ratio, but probably few are familiar with the term “expansion ratio.” Expansion ratio is simply the reverse side of compression ratio. When we raise the compression ratio, we’re more interested in what it does to the expansion ratio, because it has a significant effect on our optimal valve event timing and the ratio of the cylinder heads’ intake and exhaust valve sizes. We’ll go into sufficient detail in the relevant chapter to arm you with the working knowledge needed.
The fourth factor in our chain of events is maximizing ignition performance. Here we have to ensure the engine has more than just an adequate spark.
Once a more-than-adequate spark is established, any extra spark output will not return a further cost-effective output. High-performance ignition systems will boost output, but by only a relatively small amount. However, anything short of adequate will cost substantial power.
The fifth and last factor in the combustion efficiency box is the minimization of heat loss. This is an area that’s been given too little attention in the past, mostly because the materials to improve it were not really available. They are now, so we’ll look at minimizing the combustion heat loss from the chamber so that more heat is available to expand the air and thus deliver more horsepower.
We have now passed through the friction and combustion efficiency boxes and into the exhaust system. Our first goal, like that of the intake, is to minimize exhaust flow restriction while maintaining cross-sectional areas and lengths appropriate for effective pressure-wave scavenging of the combustion chamber. This may seem like an easy aspect to deal with but, judging by the number of incorrectly spec’d exhaust systems I’ve seen, this must not be the case. With currently available parts, it’s entirely practical to build a “no-loss” muffled exhaust system that meets even the most stringent street standards. On top of that, it also need not cost you a fortune. It’s little more than buying the right parts and then building the system correctly.
The exhaust system actually starts at the exhaust valve, not the exhaust manifold. Restrictions can be reduced in a number of ways; the obvious one is to flow the system, but another is to make sure that the valve is open for as long as possible to get the job done without compromising the effectiveness of the power stroke. This is an important point to appreciate because it’s greatly influenced by the compression and expansion ratios. Understanding the implications can make the difference between 1.5- and 2.5-hp per cubic inch. Granted we won’t make the 2.5 end of the scale with the hardware allowed by our budget, but the statement still holds true in terms of what it takes to achieve such an output from a naturally aspirated engine.
Last we come to maximizing pressure-wave tuning of the exhaust. This is an important factor. It’s not commonly realized that exhaust pressure wave tuning has a far greater effect on engine output than the intake. A normal intake system with one port per cylinder is difficult to tune over a bandwidth of more than about 400- rpm wide. However, if the exhaust system is done correctly, it can be made to effectively scavenge the combustion chamber of spent charge and pull in a greater charge over a power band as wide as 4,000 rpm and sometimes more. In the following pages, we’re going to deal with all of these things for a better understanding of how they interact and how they fit together to make what an experienced engine builder calls “the right engine combination” to make horsepower.
Throughout the pages of this book you’ll see tests of an engine’s output. These figures were developed mostly on my dynamometer. This is a device for measuring power. There are generally two types of dynos in common use: an engine dyno, and a chassis dyno. The engine dyno tests the engine while it’s out of the vehicle. The output at the engine’s flywheel is transmitted directly to an absorption unit.
A chassis dyno measures the horsepower at the driving wheels of the vehicle. This is done by simply driving the car onto a set of rollers that are coupled to an absorption unit. Whether it’s an engine dyno or a chassis dyno, a dynamometer is merely a device for applying a braking load to the engine. Hence, the term: brake horsepower (bhp).
When looking through car catalog information sheets, you no doubt checked the quoted horsepower figures. Survey enough specifications and you’ll see “DIN” figures. Others are marked as SAE “gross” or “net,” and yet another as “standard” horsepower figures. Through the years, manufacturers have obtained horsepower figures from blueprinted engines or with open exhaust, unrestricted air intake, and no accessories hooked up. The results are unrealistic horsepower and torque figures.
As for the power figures quoted in this book, they’re known as “standard” corrected numbers. These numbers are a little more flattering (around 4 percent) than SAE figures, but not without at least some justification. They end up producing a higher corrected number because they correct to a higher air density and a cooler intake charge temperature than the SAE rating uses. However, though these numbers may look a little optimistic compared to SAE numbers, you’ll find the temperature and pressure corrections more in line with those found in a race car. This means if you pay attention to the engine’s operating environment, it will be producing much the same numbers as the standard corrected numbers indicate. You’ll have to pay attention to the car’s induction system and the engine’s coolant and charge temperature management. Do that and the numbers quoted within these pages will be yours. Fail to do this and your motor could be easily losing 10 percent of its torque throughout the RPM range.
Although the dyno is the best place to get power figures, it’s not the only place. You can get a good idea of how much horsepower your engine makes by testing at the drag strip. Dyno testing and setting up, either on the dyno or at the strip, will be discussed in several following chapters. We now have looked at the fundamental principles of developing horsepower from an engine and what horsepower is. Let’s see how these fundamentals can be successfully applied to the small-block Chevrolet.
Written by David Vizard and Posted with Permission of CarTechBooks