So far, I have addressed ways and means of maximizing the air density delivered to the engine’s induction system. However, all this is to no avail unless the fuel is mixed with the air in a well-defined manner and to equally well-defined proportions.
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From Chart 3-1, it might look like we are out of step with the sequence of events that lead to power. We are at the “Optimize Mixture” step and have not yet looked at the factors that apparently precede this. The reality here is that as long as the mixture is of the right proportions and is suitably finely atomized in a proportion to aid combustion initiation, it is of little consequence how it got that way prior to arrival in the cylinder. In practice we find that, however good the mixture of fuel droplets and air may be as it is discharged from the carb booster venturi or injector, things almost always go downhill from there. This means that not only do we have to do a good job of atomizing the fuel at the point it enters the inlet tract, but we also have to employ measures that help maintain good mixture qualities right through the inlet tract and on into the cylinder. At this point, it is clear we have to deal with three issues: mixture ratio, exhaust pollutants, and mixture quality. Let’s start with the mixture ratio.
Getting the mixture right is important; it can cost a lot of power if it is not. The margins within which we must work are relatively small, as can be seen in Illustration 3-2.
In this illustration, we are holding the fuel volume constant and changing the air volume to show what is physically being dealt with. The fuel represents a relatively small volume compared with the volume of the air involved. For a rich mixture, there is more fuel than can be burned completely by the oxygen available. This means that after combustion there is unburned fuel contained in the exhaust (hydrocarbon emissions). When there is 10- to 15-percent excess fuel, maximum power is developed. For a stoichiometric, or chemically correct, mixture there is exactly the right amount of fuel to completely utilize the air; so, theoretically, 100 percent of the fuel is burned and 100 percent of the oxygen contained in the air is used to do so. When the mixture is lean, there is surplus oxygen remaining in the charge after all the fuel has been burned. It is under lean-burn conditions that the best fuel economy is developed.
There is a somewhat famous statement made by someone whose name no one seems to remember that goes like this: “A carburetor is a wonderfully ingenious device that delivers the incorrect mixture at all engine speeds.” Well, that statement is sure to catch attention, but it is totally wrong. By the middle of the last century a good carb, properly calibrated, could deliver mixture accuracy as well as any modern electronic fuel injection setup.
Now let us consider the effects that a change in mixture has on wide-open throttle (WOT) output. Chart 3-3 shows just what can be expected in the way of a change in power in relation to a change in mixture. The width of the green column is our target and, as can be seen, this is not a very wide target to be shooting for. That is why it is so important to get the mixture right.
As the chart shows, output drops off far faster on the lean side of the curve than on the rich side. Therefore, the calibration should err slightly on the rich side because any power loss is smaller than the same error on the lean side. The target zone (green column) for a gasoline-fueled engine is typically between 13.2:1 and 12.5:1. Anywhere between these two ratios, the change in power is not usually measurable. However, if the engine is to be used for long-distance racing, fuel economy (to cut the number of pit stops) also becomes a requirement. Under these conditions, running the engine slightly on the lean side of the peak power point (13.5:1) is a worthwhile strategy. If the engine concerned has good fuel distribution between cylinders, the optimum power ratio is right on 13:1.
The blue zone of the graph represents the air-to-fuel ratios we should target for cruising with fuel economy. Here, we are obviously looking to go the farthest distance possible on the least amount of fuel. To do this we run the engine lean; that is, less fuel than the air is capable of burning.
To a point, the leaner we can run the engine, the more fuel efficient it is; but as it is leaned out, the harder it becomes to ignite the charge. At this stage, spark energy and temperature play a more important role. A good ignition system (and most modern ones are) effectively lights off a mixture as lean as 17:1, but leaning beyond 17:1 does cause a lean-mix misfire.
With suitable design of all the pertinent factors and a very high intensity spark (large and of high temperature), I have successfully run mixtures as lean as 22:1, and some research establishments are reporting 30:1. Running mixtures as lean as this produces some really good fuel efficiency numbers, but getting there is not a simple job by any means.
Although it is not nearly as relevant to performance as overall calibration, we should, in this day and age, consider the effects of our tuning on exhaust pollutants. If you are intent on running an engine without a catalytic converter, then you sort of owe it to the rest of the world to do your best to save the planet. Alone, your effort won’t be of much consequence. But if we look at everybody who is likely to read this book, making sure the exhaust is as clean as possible is relevant. Chart 3-4 shows what we are dealing with here.
What this chart shows is that the exhaust is essentially cleaner as the mixture becomes leaner. This means the time spent setting up that idle, so that it runs as lean as possible, is a worthwhile step toward not only saving fuel but cutting pollution. The same goes for the cruise calibration. Keeping things on the lean side not only helps cut pollution and save on the fuel bill, but your engine also lasts considerably longer in the bore and ring department. Running mixtures from 14.7:1 on up compared to, say, 13:1, can increase bore life by as much as 400 percent.
When that throttle goes wide open and maximum power mixture hits 13:1 we are going to see quite an amount of unburned hydrocarbons, along with 6 percent of the exhaust consisting of carbon monoxide. CO, as you should be aware, is poisonous. Since the duty cycle of most performance engines is 99-percent idle and cruise, fuel delivery calibrations at these operating positions are far more important than what happens at WOT.
I don’t address the important subject of mixture quality in too much detail here, but I do bring it up where relevant in subsequent chapters. Suffice it to say that fuel will not burn in a liquid state. That means fuel rivulets within the induction system are a power detraction. The mixture needs to be finely atomized so that the bulk of the fuel arrives in the cylinder as suitably small droplets. Just how small they need to be depends on the temperatures involved. For the ignition system to effectively light off the induced charge, there needs to be at least 15 percent of the fuel vaporized. However, if too much is vaporized within the intake manifold, the greater volume taken up cuts the amount of air drawn in. This means that too much vaporization reduces the engine’s volumetric efficiency (breathing efficiency), and that is not good for power.
Getting the right balance of droplet size at a given temperature is not so much an issue with a fuel-injected engine because the typical injection pressure of 45 psi and the (usually) close proximity of the injectors to the intake valve help ensure positive results. That situation does not necessarily exist for a carbureted engine. Getting the mixture quality to be as near optimal as possible involves paying a lot of attention to the fuel delivery at the carb’s booster venturi (auxiliary venturi). After this, you need to pay attention to port runner shape and finish to minimize the negative effects of fuel drop out.
Carb Function Basics
Virtually all carbs function by creating a reduced air pressure over some kind of metering jet connected to a fuel bowl. There is a type of carb known as a constant-vacuum carb. The most popular and well known of this type is the SU carb, which has been used on British cars for nearly half a century. Although they could be made to meter fuel extremely accurately (as good as any fuel injection), they were not popular beyond the shores of England. That being the case, I focus more on the type of carb known as a fixed-jet carb. This employs a venturi to create the depression that draws the fuel into the intake system.
Illustration 3-6 clearly shows how the increase in velocity within the venturi causes a drop in air pressure at that point, which draws the fluid from the reservoir up the tube. With a little increase in speed, this example would start discharging the fluid from the reservoir. Of course in real life there is only one connection between the reservoir and the venturi: the one at the minor diameter.
Now, if you are observant, you may have spotted a problem here. Because the fuel needs to be a certain distance below the discharge hole (known as the spill height) quite a lot of air must flow before any fuel is drawn into the intake tract. So this system won’t work for an engine at idle. In addition to this, the system won’t deliver twice as much fuel when the airflow increases.
The fix for this is an ingenious little jet known as an air-correction jet. Just to avoid any confusion, here it is (in photo 3-5 and on page 27) on a Holley carb. It is known as a highspeed bleed, when referring to the main jet circuit. The air-correction jet has little effect on the mixture at low engine speeds, but as RPM increases it has an increasing effect. By suitable sizing (in relation to the main jet), a fuel curve much more closely approaching a straight line can be produced from low to high RPM.
With the addition of an aircorrection jet, it might look like our calibration characteristics are pretty much sorted out. Unfortunately, that is still far from the case. An engine does not draw in air like a vacuum cleaner. We find that as the intake and exhaust go through periods of breathing effectively, and then not effectively by virtue of system resonance, the pull on the jets produced by the intake airflow vary in a seemingly near-random manner. What our simple carb needs at this point is a means of recognizing this and compensating accordingly.
To manage the mixture calibration under what may be rapidly changing volumetric efficiency conditions (as RPM increases), a fixed-jet carb utilizes what is known as an emulsion tube. Although this may take on many different forms, a quick study of any example shows they all have a common mode of function. However, deciphering just how an emulsion tube and jet system may function just by looking at it is not so easy.
In Illustration 3-7 we have an emulsion tube in the well it resides in. The well receives fuel from the carb float bowl via the main jet located at the base of the well. This fuel surrounds the emulsion tube. At the top of the emulsion tube, we have the air-corrector jet. The hole pattern in the emulsion tube modifies the air drawn into the air corrector jet. If there were no holes in the emulsion tube, it would remove the effect of the air corrector from the system. However, what happens here is the air drawn in from the air-corrector jet pushes the fuel level down in the emulsion tube. The air then bubbles out of the emulsion tube into the fuel in the emulsion well. Now, instead of there being neat fuel in the emulsion well, the fuel surrounding the emulsion tube holes is a mixture of fuel and air. In other words, it is an emulsion; and it is delivered to the venturi as part fuel and part air, rather than straight fuel. By modifying the ratio of fuel and air composition of the emulsion, we can modify the overall fuel/air ratio delivered by the carb.
To calibrate the emulsion tube and well operation, the first step is to know where in the RPM range the mixture goes rich or lean. The next step is to understand that the top of the emulsion tube affects the bottom of the RPM range, and the bottom of the emulsion tube affects the top of the RPM range. Looking at the emulsion tube and relating its length to the RPM range, we can now say: Where there are holes in the tube, the mixture is leaned out; where there are no holes, it will be enriched.
An example should make all this clear: Let us say we have arbitrarily selected an emulsion tube pattern and find it delivers the right curve, except at the lowest of engine speeds where it is too rich. Remembering the top end of the emulsion tube affects the low speed, we fix this problem by selecting an emulsion tube with more holes at the top. This bleeds more of the air-corrector jet’s air into the emulsion tube at this point, and thus leans-out the mixture at the low end of the RPM range while having minimal effect on the mid and top ends. If the mixture is correct at low and high speeds but rich in the middle, then we add holes in the middle of the emulsion tube. Armed with this information, you should now be able to rectify any fuel-curve issues a fixed-jet carb may be having.
With most fixed-jet carbs, the mixture calibration is moved up or down the scale by the sizing of the main jet. Install a bigger main jet and the mixture becomes richer throughout the RPM range. Install a smaller one and the opposite happens. Sounds simple enough, but the main jet calibration still must deal with the mixture requirements that, though fairly wide open, still fall short of WOT. With an independent runner (IR) system that employs a Weber or similar carb, we find that the undamped induction pulses at WOT cause a richening of the mixture. As the throttle is closed part way, the pulses are damped and this conveniently leans-out the mixture to something in the order of what is required for lean (or leaner) cruise.
When a carb such as a Holley is used on a plenum-style intake, induction pulse enrichment is almost nonexistent. This means enrichment from a cruise mixture of, say, 15:1 to a full power mixture of 13:1 has to be handled differently. This is typically done by means of a so-called power valve circuit. A diaphragm in the power valve senses when the intake manifold vacuum drops below a certain value and, at that point, opens up an additional jet, which then supplies more fuel to the booster venturi. Illustration 3-8 shows the interaction between all the principle components in a Holley carb when going to WOT operation.
We have worked our way through the system and now it is time to take a look at the critical job done by the boosters. First: Why the term booster? This is so called because its function is to amplify the pressure drop or signal generated by the airflow through the main venturi. Depending on its design, a booster can amplify the main venturi signal from 80 percent to as much as 480 percent. By amplifying this signal, you attain the means to achieve both finer atomization and more accurate calibration.
Without the aid of a functional booster, the carb would have to utilize a much smaller main venturi, so that sufficient signal would be generated at the low end of the RPM range. But a small main venturi would then limit the top-end output due to insufficient total airflow. As you can see, the importance of a booster being compatible with the operating needs and RPM range of the engine is starting to shape up as an important issue.
The first step toward knowing what to expect of a booster is to understand how it works. Illustration 3-9 shows the essentials. The principle is simple. Air is drawn through the main venturi due to a pressure drop at the exit. The velocity of the airflow at the minor diameter of the main venturi produces the characteristic drop in pressure at that point. Because the end of the booster is located at the main venturi minor diameter, the booster sees a greater suction drawing air through it. This results in a higher velocity at the minor diameter of the booster, which produces a greater depression (signal) than seen at the minor diameter of the main venturi.
As with most aspects of engine performance, more is not always better. For best performance at WOT, a carb booster needs to help produce the ideally sized fuel droplets for combustion. If the droplets are too large, combustion efficiency suffers. If they are too small, the fuel evaporates in the intake system too soon and reduces volumetric efficiency. For part throttle, where the best fuel efficiency is sought, it is best to have the fuel completely vaporized by the time the piston is close to the top of the compression stroke and the spark is about to fire.
Idle and Transition Circuits
Up to this point, we have talked about the production of maximum power from our engines and what may be needed to cruise fuel efficiently at relatively high speed. As important as that is, a high-performance street engine spends as much time at idle and transitioning to low-speed cruise (just off idle) as it does at, say, 60 to 75 mph. The bottom line is that the term drivability is almost all about getting the idle and transition circuits calibrated right on.
Let’s talk fuel vaporization for a moment. When an engine is at idle or cruise, the intake manifold is at its highest vacuum situation. At typical temperatures under these conditions, almost all of the fuel vaporizes. If the throttle is now opened, the absolute manifold intake pressure quickly rises or, in other words, the manifold vacuum decreases. Whatever fuel was vaporized in the air now rapidly condenses on the walls of the manifold. This means the mixture being pulled in at that moment is really too lean to burn. The result is a total engine cut-out.
To have the engine perform as intended under the transient conditions of rapid throttle opening, we need to force-feed the system with extra fuel. With an injection accelerator and pump, the fuel delivery system has a means of adding extra fuel as the throttle blades are opened. With a carbureted system, this function is achieved by means of an accelerator pump. The idea is to have the accelerator pump (or the equivalent action for a fuel-injection system) rapidly pump in just enough additional fuel so as to cover the hole in the fuel curve during the opening of the throttle.
A quick means of determining this is to have too much fuel as a starting point. This starting point can be observed as black smoke in the exhaust as the throttle is rapidly opened from a closed position. From here, it is just a question of cutting the amount of fuel pumped in until the engine just stumbles, and then taking a step or two back.
As for calibration methods, we find that the amount of pump action is set by a cam or a spring-loaded rod on the throttle arm. This usually determines the rate and length of time for which additional fuel is injected. Other means of calibration involve sizing the pump and pump jet at the discharge point and a calibrated spill-back valve, which dumps a portion of the pump discharge back to the float bowl.
With a fixed-jet carb, if the venturis presented to an engine are too big, we find that the air speeds through the main and booster venturis are too low to effectively atomize the fuel. What this tells us is that we need to size a carb so it functions perfectly at the lower RPM, while delivering the airflow requirements for the sought-after top-end output. There have been many methods employed to produce as wide an operating span as possible here. The Quadra Jet and the Edelbrock carb employ a small fixed-jet design on the two primary barrels of the four, with an air valve that acts like a constant vacuum carb on the secondary side. This mode of operation can be likened to “size-on-demand” and as such can be made to work well. For a Holley-style carb, the answer for good all-around street performance is the vacuum secondary carb.
The first step toward installing an optimally sized carb is to make a preliminary selection based on the engine’s displacement. The next step is to modify this result by factoring in relevant engine spec details, such as the heads and cam used. For the initial calculation step, we need to determine the amount of CFM the engine is likely to inhale if it is able to breathe at 100-percent efficiency. To do this, we first multiply the displacement (CID) by the anticipated RPM the engine is likely to turn to. It is important to be realistic when making the RPM estimation. Start by estimating where peak power is likely to occur and then add 500 rpm to allow for over-speed. Because we’re dealing with a four-cycle engine, which has an induction stroke every other revolution, we need to divide the CID x RPM result by 2. The resulting number tells us the cubic inches of air the engine displaces per minute. To change that to cubic feet, divide by 1,728.
Our calculation so far assumes the engine has a 100-percent breathing efficiency. For a race engine, where the exhaust scavenging is a factor, the volumetric efficiency can exceed 100 percent by quite a big margin. For instance, a well-built race 350, with no regulatory race restrictions placed on it, can reach about 115-percent volumetric efficiency. This means that the engine, as far as the carb is concerned, looks as if it is 400 ci. At the other end of the range, we find that an absolutely stock street engine may have a volumetric efficiency of only about 80 percent at best. This means that an engine of the same 350-inch displacement appears, from the carb’s point of reference, to be only about 300 inches. Our carburetor selection needs to take this into account. The airflow required by an engine depends primarily on the cam, and the breathing capability of the heads.
Assuming that the compression ratio and exhaust system are appropriate for the engine, the heads and cam are left as the most influential components on carb size selection. As cams get longer, so the engine’s volumetric efficiency improves. The volumetric efficiency also improves as the cylinder head’s flow capability improves. Chart 3-10 gives a correction factor (CF) to take into account cam duration and cylinder head flow. Using this correction factor, we’re now in a position to come up with a simple formula that gives a good prediction for required carb CFM:
The carb sizing example we have just gone through is a little on the conservative side, as it makes no allowance for the fact that a tricked-out carb with high-gain boosters can successfully use greater CFM. This allows a little more power to be developed without sacrifice in the lower-RPM range. Going this route does mean you have to know your carbs or work with a carb specialist such as AED or the Carb Shop (to mention but two of the many out there).
Carbs Versus Fuel Injection
Since about 1985, people have commented on the fact that I do not do enough with fuel injection, and that maybe I should get up to speed in that department. Just for the record, I have been building fuel-injected engines since the late 1960s. But the situation is such that due to my skills at sorting out carburetion problems both in basic design of such and calibration, I keep getting called back into the carburetion camp.
This is not all bad. When the use of electronic fuel injection began to take hold in the early 1980s, it was popularly predicted that carburetors would be dead in just a few years. Well, here we are and the carb business is still strong, and carbs for use on hot-rodded V-8s still dominate the scene. Why? Because they are cost effective, easy to understand (relatively speaking), and they flatout work when correctly sized and calibrated. But does this mean that I am solely focused on carbureted induction systems? No, I actually prefer fuel injection, and the good news here is that systems are becoming more user friendly as time goes by. Also the price is coming down!
To date I have run systems from (and can recommend) Holley, Fast, Mass-Flo, Accel, and a few others. In addition, I have also used factory systems that have been re-programmed. Basically, the difference between calibrating carburetors and fuel injection is: Carbs need to be persuaded to deliver what you think the engine wants, and with fuel injection you tell the computer what you think the engine wants. It is much easier to know where things are at in the second instance!
Written by David Vizard and Posted with Permission of CarTechBooks