Regardless of the type of supercharger you use, there are certain operating parameters that must be considered for your setup to work properly. Among them are particulars for each type of supercharger used – those will be detailed in individual chapters of this book dedicated to each ’charger type. However, the more general factors for a successful installation will be discussed here. These general considerations include:
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- Increased intake airflow requirement
- Fuel system flow capability
- Ignition alterations, including timing, spark intensity, spark plug and wire selection
- Charge cooling (when used)
- Increased exhaust airflow and temperature
- Under-hood temperature considerations
- Cooling system requirements and upgrades
The basic idea behind supercharging is to make the engine think it’s bigger than it really is. If we agree to dispense with the engineering math and finicky stuff for a moment, let’s assume we’re talking about a 302-ci engine, and an unnamed, general supercharger setup that can build as much as 15 pounds of positive manifold pressure, or boost. Also assume the engine will take that boost and make good use of it without turning into a warhead.
On a theoretical average day, the static air pressure is 14.7 psi – against every surface that isn’t in motion, and above an equally theoretical “zero” point. In reality, the static air pressure around us varies considerably with altitude and the amount of moisture; an increase in either (or both) will cause a reduction in the atmospheric pressure.
For our example, though, let’s presume we have a day that fits this average description. If anything causes pressure to fall significantly below the ambient pressure, it causes a vacuum. Think about how it feels to put your hand over the end of an idling engine’s intake tube – as the engine runs, it will leave a formidable hickey on your palm before the engine stalls from lack of air. You’re creating a pressure difference between the ambient air and the lower pressure inside the inlet pipe, which is why it sucks your hand in. The ambient air is merely attempting to regain its equal influence against every surface, everywhere.
Incidentally, an idling V-8 only creates a couple of pounds of negative differential pressure at its inlet. If it were more (let’s say 20 psi of vacuum), it would easily break your hand into soft mushy stuff. That’s enough meteorology for now.
When you’re talking about boost, a static air pressure of 14.7 psi is usually called an atmosphere. You’ll hear that term used when the conversation turns to boost pressure. For example, if a supercharged engine’s manifold pressure rises to about 7.3 psi above ambient pressure, the engine is said to be running “half an atmosphere of boost.” If the engine and supercharger can operate together well enough to bring the manifold pressure up to 14.7 psi, it’s making one atmosphere of boost. That figure is rarely seen on the street, but it’s completely reasonable for most race engines designed for supercharged operation. The difference there is primarily in the materials used to build the engine, and its architecture.
Now we get to the airflow part of our 302-ci scenario. A fresh small-block Ford consumes roughly 80 percent of its displacement volume for every two revolutions (720 degrees) of its crankshaft. It takes two revolutions to cycle all of the cylinders in a four-stroke, whether it’s a five-horse Briggs & Stratton or one of John Force’s fuel-burning Mustang funny cars.
The 80 percent factor is the volumetric efficiency (VE) of the engine, or the amount of air that is actually moved through the engine versus its total displacement. That’s a fairly reasonable VE figure for a modern engine, but it’s about 10 percent high for older engines with less computer time invested in optimizing their airflow characteristics. A naturally aspirated engine relies on the ambient air pressure to refill the cylinders with a fresh intake charge. The more efficient an engine is, the more it will take in while the intake valve is open, the higher the VE will be. With the addition of a supercharger, the VE will reach and exceed 100 percent as the manifold pressure goes positive (boost), regardless of engine RPM. Basically, with boost, you can stuff 10 lbs of crap in a 5 lb bag.
There can be occasional exceptions to this statement. If the intake tract is tragically wrong in its engineering, it won’t matter how high the manifold pressure gets, you won’t be able to get enough airflow to the cylinder heads. Amazingly enough, this has happened more than you might imagine, thanks to improperly matched airflow upgrades with an over-ambitious supercharger selection. In every case, the owner will have a look of disbelief on his face as the boost continues to rise, but the power output, well – doesn’t.
In our example, the un-blown 302 manages to ingest, mix, burn, and get rid of about 400 cubic feet of air per minute (cfm) at 5,500 rpm, presuming its operator is not pussy-footing around and has the throttle wide open.
At that same 5,500 rpm, but with a supercharger installed and providing half an atmosphere (about 7 psi) of boost, the engine will be processing air at the rate of about 600 cfm. In effect, we’ve now tricked the engine into thinking it displaces 450 ci. It’s dealing with the airflow of an engine that big, and if we did everything right, it’ll make power like an engine of that size.
If we crank the boost up to a full atmosphere (about 15 psi), the engine will behave very much like it displaces 604 ci by processing more than 810 cfm of air at 5,500 rpm. That would be a formidable 302, indeed.
Those airflow figures are conservative because of another, less obvious, engine characteristic. A supercharger not only supplies the amount of air an engine uses under normal (non-supercharged) conditions, it also fills the internal volumes that can’t be displaced by piston movement. Those areas include the combustion chamber volume above the pistons, which can be considerable, depending on the engine’s cylinder head design, and also the entire intake manifold.
Fords have relatively compact combustion chambers in the interest of keeping the flame travel distance short, which help control emissions. Luckily, that compact volume provides a benefit when supercharging, because it reduces the distance the flame-front must travel after ignition, contributing to faster combustion and better control of the chamber temperature – which definitely goes up under boost, regardless of the chamber shape.
This all comes together when you decide how much boost you’ll be using with your engine. The intake tract includes everything – each inch of tubing and all the bends, meanders, metering devices, sensor tips, screens, filters, and duct surfaces – between the combustion chamber and the air surrounding the car.
The intake tract must be capable of easily handling the airflow increase after a supercharger installation. If you plan to run more than a pound or two of boost, you’ll need to loosen things up a bit along that twisted path, or the engine will never give you the results you want.
Horsepower is simply a figure that suggests how many times per time-unit an engine can produce a given amount of torque. It’s calculated by multiplying torque by engine RPM, and then dividing it by a peculiarly interesting number: 5,252. Not exactly by coincidence, in the case of 4-cycle engines, 5,252 is the point on a horsepower/torque plot where the two lines converge – every time. This universal truth may have never jumped off a page at you before, but if the lines on your dyno chart don’t meet at 5,252 rpm, someone made a mistake in the math.
This relationship is important because superchargers bring about a major change in torque curves versus a naturally aspirated engine. Once a blown engine establishes and begins maintaining a constant boost level in the manifold, the engine’s torque curve flattens out and stays at that point for any higher RPM, unless the engine’s airflow cannot keep up for some reason.
That’s amazing news, especially if the blower can make good boost down low in the RPM range and keep up the good work as the engine speed climbs. As the RPM increases, the amount of air it processes will climb accordingly, so it’s logical that the blower must keep pace with the demand.
This function is easily seen at work if the needs of the entire system are not taken seriously. If the manifold pressure rises and stays put, but the torque curve then begins to sag, especially as it approaches 5,252 rpm, it means the airflow is being restricted somewhere between the air filter and the exhaust tips. Usually, the problem is in one of five areas listed below, beginning with the most important:
More blower speed than the engine can tolerate
Restrictive exhaust system
Inadequate intake manifold volume or flow rate
Very poor cam choice
Gasket mismatch at cylinder head ports
The accompanying plots reveal how torque and horsepower numbers relate to one another, and the difference a properly tuned supercharger will bring about in useful work from the engine. They also show how the area “under the curve” changes for the better – that’s the improvement that should make you want to start saving for a blower kit.
Model-for-model intake tract modifications are detailed in later chapters because they’re usually specific to the type of supercharger used, its inlet configuration, intake manifold requirements, and packaging considerations.
If there’s more air, you need more fuel. The stock fuel system may not have the ability to keep up with the demand of your supercharged 302. If your engine happens to be carbureted with a blowthrough (supercharger blows through the carb) supercharger installation, the stock carb will usually be inadequate in both airflow and fuel-handling capabilities, even with very modest boost.
On the other hand, Fords with electronic fuel injection (EFI) of any kind – even TBI – can be made to work in most cases. If your engine is equipped with SEFI (Sequential Electronic Fuel Injection), you can really optimize the setup and reap the benefits of a fine-running supercharged engine.
Let’s cover the carburetor situation first. When Shelby Cars decided on a Paxton blow-through setup for the (very few) supercharged GT-350s, they did so for many good reasons. The most important of these had to do with simplicity – an irresistible option for a veteran racer like Mr. Shelby, and most of the guys in Inglewood at the time.
Putting the carb in a box and pressurizing the box with the blower is pretty basic compared to the other strategy, which involved putting the carburetor on the inlet side of the blower (draw through). This second approach is far more complex mechanically and from a tuning standpoint. There’s a plumbing challenge, for one thing. From the outlet side of the blower, you need to move the atomized mixture through the rest of the intake tract – without the fuel dropping off onto the walls of the ductwork along the way. If that isn’t handled properly, the engine runs lean until one of those puddles happens to blob its way near an intake port. Then the engine will run rich for an instant, and then resume its lean-running condition.
When you enclose the carburetor in the pressurized box, it just presumes it’s running normally, except for the fact that the barometric pressure inside the box is a tad high. Since the entire carb is inside the box, all the air bleeds in the carb work normally. As long as the fuel pressure is regulated to maintain a reasonable margin to keep up with the pressure in the box, the fuel metering for boost conditions should be pretty close as well.
Tuning problems with a drawthrough system usually involve fuel separation, which isn’t usually a problem with a blow-through system. However, there is one fuel-system problem common to both setups: the sharp change in fuel required when the system transitions between boost and no boost.
The transition from vacuum to boost happens quickly with any of the ’charger types discussed in this book. The change is so fast, in fact, that many fuel systems can’t react quickly enough to prevent a momentary lean condition. Over the years, there have been a lot of talented people studying this phenomenon and working toward resolving it without resorting to the most obvious solution, which is to simply throw fuel at the engine even when it isn’t running under boost.
Many of us are old enough to remember when computers were a bad thing (actually, this was just a nasty rumor circulated by the carburetor companies, and we all knew better). But we had no concept of how fast modern EFI systems would react to changing conditions. In a modern closed-loop system, the slowest part of the entire setup is the time it takes for red-hot exhaust gas to travel from the exhaust port to the oxygen sensor, and to have the sensor report to the engine’s computer. At high RPM, that takes about 30 to 50 milliseconds.
Everything else in the system (meaning the timing and instructions to the fuel system) is being accomplished at much shorter intervals. In fact, that’s why people are still working on the transition problem, because it’s difficult to make the hydraulic aspects of a fuel system react as quickly as the electronic circuitry.
Throughout this book, there are many details on specific fuel-system upgrades. While most aim for a similar effect, they are specific to a supercharger type and vehicle installation, so the details should be looked at relative to the particular project.
There’s a special challenge created when the working pressure in the combustion chambers is bumped up significantly – the stuff in there gets harder to light.
Although you’d think that squeezing the mixture harder would make it easier to buzz an arc across a spark-plug gap, the reverse is true. The reason for this has to do with electron travel from tip to core across the plug gap. The familiar bluish arc that occurs in a plug gap is only the visible part of what is really happening there – trillions of electrons are migrating from one surface to the other, more or less, all at once. There are plenty more electrons where those came from, but the ignition system’s job is to move about the same quantity of electrons each time, regardless of the effort it takes to move them.
That’s the key to this situation – no matter how difficult it is to make the spark, the ignition system sends the same approximate number of electrons per cycle; only the length of time changes.
Here’s an extreme example: Consider what would happen if you placed a sliver of wood into the plug gap. In order to make the plug fire, you’d first need to overcome the insulating qualities of wood. Compare that to when there’s absolutely nothing between the core and tip of the plug. In both situations, there were the same amounts of electrons waiting to make the trip. As long as the tip and core are still attached, the electrons will be there, because they are literally contained in the atoms of the metal parts. While the wood prevented their travel across the gap, in the vacuum, they made the trip much quicker than usual. A spark plug will fire just fine in outer space – in fact, it will fire much easier than when installed in any engine. The process has nothing to do with air, except for the convenience and fun of viewing it with the human eye. Without air to ionize along the way, the migration isn’t visible, but it still occurs.
In a supercharged engine, the mixture’s molecules are more densely arranged than in a naturally aspirated engine, which makes it more difficult for the electrons to travel across the plug gap. Since there’s more matter (air in this case) between the tip and core of the plug, the ignition system must do everything it can just to fire the plug. When that happens, the fast-moving electrons will whack a few of the carbon and oxygen molecules together, and combustion will begin.
Various ignition systems have different capacities. Some are able to move more electrons in one cycle than others. Modern cars have very powerful ignition systems because the manufacturers understand that a strong spark and a complete burn are necessary to keep emissions down. Even with a supercharger, you probably won’t need much in the way of ignition upgrades, except for the correct plugs and wires, but the supercharger manufacturer’s recommendations should be followed in any case.
Older ignition systems might require substantial upgrades to deliver a clean, reliable spark. Earlier ignition systems are often, but not always, inadequate for use with blowers, and must be upgraded to more contemporary electronics. Follow your supercharger manufacturer’s recommendations if there’s any doubt.
Spark-plug heat range is another consideration. Essentially, a plug’s range refers to how quickly it can dissipate heat into the surrounding cylinder head material. A plug with a short nose has a shorter heat path between the tip and the surrounding shell, so it’s considered a colder plug. A plug with an extended nose will tend to run hotter because the path is longer between its tip and the threaded shell area.
Plugs don’t have an easy life. Under normal circumstances, they aren’t allowed to settle on a nice, stable operating temperature. Instead, they are subjected to sharp increases in temperature whenever you open the throttle. Even when the vehicle is cruising at a steady speed, the plugs see small variations in temperature. When selecting a proper heat range, you want a plug that stays hot enough to keep itself clean, but has the heat transfer capability to shed heat quickly when you stick your foot into the throttle.
Choosing the right plug is often a trial-and-error process, because your driving style and the terrain in the area will both contribute heavily to the decision. Generally, when you add a supercharger, you’ll need a plug at least one heat range colder than stock. With the blower, your plugs will have to deal with higher combustion temperatures. What’s happened is that the engine is making more power, so there’s more heat in the engine, and everywhere else under the hood.
Any effort to compress air will introduce heat, and that works against us when we want to keep the intake charge nice and dense. When air is warmed, it becomes less dense because the molecules are more active, so they move apart in giving each other a little “comfort space.” When that happens, there are fewer molecules packed into a given space (less dense), making them less available for burning fuel. So, hot air works against your supercharger.
What we need here is something to keep a lid on the rising temperature of the air as it passes through the blower. We need a charge cooler. Some people call them intercoolers, some call them aftercoolers, but the idea is the same – to reduce the temperature of the intake air by pulling out some of the heat. For our purposes, let’s agree to call them intercoolers.
An intercooler is very similar to a radiator. There are two types; one works exactly like a radiator – but in reverse –using a cooler liquid to pull heat from the hotter air. This is called an air-to-liquid or air-to-water intercooler. The other type of intercooler uses relatively cool ambient air passing over the fins and core of the intercooler to pull the heat out of the warmer air inside. This is called an air-to-air intercooler. In either case, the heat comes out – coolness is not added in.
A heat exchanger (a more general term that engineers use for an intercooler) works by bringing hot stuff into such close proximity to cooler stuff that the heat is happy to jump toward it. In a typical radiator, the heat in the coolant is contained in a thin-walled, finned core that has a large amount of relatively cool ambient air flowing through it. The speed of the heat transfer is greatly affected by the density (actually, the specific gravity) of the media being heated or cooled. A radiator for the cooling system requires a very large airflow to be effective, because the temperature of the water is much slower to change than the temperature of air – that’s why cars have fans to keep the airflow up when the car isn’t moving. If there were an endless supply of ice-cold water flowing around and through the core, a fan would be completely unnecessary, because the heat in the coolant would happily and immediately jump ship, leaving the coolant free to go back through the engine again.
With an intercooler, we know there will be hot air inside the gadget. It’s the speed of the actual heat transfer that makes one of the two intercooler designs preferable. Heat will be sucked out of the hot intake charge much more quickly if there is liquid flowing across the outside of the core, provided that the liquid is cooler than the air inside. Using that speed premise alone, an air-to-liquid heat exchanger can have a smaller exchange (surface) area than the other air-to-air type.
Regardless of the cooler type, what’s important is the temperature differential. The greater the difference in temperature between the air inside and the liquid (or air) outside will determine the efficiency of the intercooler to a large degree.
After a period of time, an air-to-liquid intercooler will heat the water or coolant until it’s just as hot as the intake air charge, unless some provision is made to control the temperature of the liquid. When the two temps are equal, the intercooler is nothing but a bottleneck in the intake tract.
An air-to-air intercooler keeps working as long as there’s adequate airflow around and through the core’s fins. There is very little chance of the inside and outside temperatures equalizing unless the vehicle is involved in an equatorial African rally, and those cars use air-to-liquid coolers.
Given those distinctions, the big difference between the two types of intercoolers is the space required for the intercoolers themselves and their requisite plumbing. As stated above, the air-to-liquid intercooler is more compact, but it requires quite a bit of plumbing. An air-to-air type is larger and must be located where it has a good supply of air flowing through it. That real estate is sometimes hard to find, especially in Mustangs.
Exhaust Airflow and Temperature
With a supercharger, the engine will be burning more fuel and air. The exhaust system will probably need to be improved accordingly so it doesn’t become a bottleneck in the system.
The exhaust gases will not only be more voluminous, they’ll also be a bit hotter with a supercharged engine. This is the result of the larger amount of heat energy available from the increase in air/fuel being burnt. Engines create power by burning fuel inside the enclosed cylinders. The heat liberated from the fuel burnt in that process causes a pressure increase above the piston at Top Dead Center (TDC). Since there’s nowhere for the heated gases to expand, the piston is pushed down the bore.
The particulars for various exhaust systems are included throughout this book. Be sure to check with the existing state-mandated laws regarding emission controls and the exhaust system.
Under-Hood Temperature Considerations
By now you understand that an engine making more power will also be generating more heat than stock. Not all of that heat is applied as rear-wheel horsepower, though. Much of it is lost through heat simply radiating from the engine. We’ve all opened the hood of a car that has been run hard, and then touched a radiator or header. Everything is hot under the hood, which should serve as an indication of just how much heat is not leaving through the exhaust pipe. The primary heat loss areas (after power production) in order are:
Radiated, under-hood losses
Convection (to the transmission)
With radiated losses ranking a solid third place, it’s apparent why some attention should be paid to the items under the hood that can melt, especially those that are important for safety and reliable engine operation. Items of particular importance include the brake lines, for example.
Most supercharger installations would not require any rerouting of brake lines at all, but let’s suppose one required you to alter the line coming from the master cylinder – a dangerous premise, agreed, but a good example of how to deal with a potentially bad situation. In our hypothetical scenario, we could create a barrier of some kind to protect the new brake line from as much engine heat as possible. The most effective heat barrier has always been distance, preferably filled with free moving air. Next on the list would be a sheet metal barrier that allowed a good amount of relatively cool air to circulate past the line, while keeping the hotter, engine-side air away. This is very often completely feasible in an under-hood environment.
Barriers should be made of thin stainless-steel sheet. Aluminum is easier to work, but may actually form a heat sink, drawing heat from the engine and actually warming the brake line (or other part) instead of protecting it. A thin sheet will also have a lower tendency to retain heat. Third, stainless won’t rust and break the way a carbon steel barrier might if situated in a hot environment.
Most importantly, a barrier should be long enough and positioned so that it induces air movement between itself and the item you want to protect. Giving this aspect some concentrated thought will reward you in a part that might just be both pretty and effective.
When the car is moving, the general airflow under the hood enters through the radiator shroud/core support, passes across the sides of the engine, and exits under the firewall. When the car is moving more slowly, however, things get more complicated. Under-hood temperatures can literally triple if a car is caught in stop-and-go traffic. That’s when you’ll discover how effective your stainless barrier design will be.
Under those circumstances, fans are your only hope for air circulation, but there are some things you can use to your advantage there. When the fans are operating, airflow tends to move in a manner that is very useful to our purposes, just above what is called a boundary layer. Every surface under the hood has a relatively static layer of air between itself and any more animated air that may surround it. The useful aspect of this phenomenon is that as you close the distance to the boundary layer, the density of the air increases significantly from that of the circulating air. That means if we can somehow direct a bit of that dense air behind our barrier shroud it will be made much more effective. This wouldn’t work as well as if we sent a torrent of cold air behind it, but it would help. As an example of this thinking, feed it air from the surface of the inner fender rather than from the firewall.
Cooling System Upgrades
The rule-of-thumb for cooling system upgrades is this: Proper maintenance is everything. That may sound like an oversimplification, but the truth is that today’s cooling systems are designed with a considerable margin for error built in. They’re engineered to stay functional even if they might become compromised by inadequate maintenance or from the natural effects of vehicle use and aging. This margin for error can compensate for the extra heat produced by your supercharger.
Keeping the cooling system healthy becomes much more critical when supercharging. Your best insurance is to follow Ford’s recommendations precisely for the coolant specification and mixture percentages. If there are variations for regional weather situations, use the desert/high-temperature recommendations in every case. Those specifications typically suggest the best coolant mixture percentages for optimizing heat transfer through the radiator, which you’ll need on a daily basis.
You should also shorten the periodic maintenance recommendations, possibly even cutting the intervals in half if you run it very hard on a regular basis. Coolant loses a certain amount of its effectiveness as it ages. This applies to its ability to aid heat transfer and fight corrosion and sediment in the cooling passages. Shorter service intervals will help keep the coolant behaving, as it should to prevent problems from the new heat load from the blower.
Although it might seem logical to use coolant without any dilution with water, in most cases, at least some mixture is recommended by Ford. A little water actually speeds the heat transfer ability of the coolant. Many gallon jugs of coolant on the market contain at least some water for this purpose. Make sure you don’t leave any splashes of coolant on your garage floor or driveway after working on the system. This is especially true if you have a crawling baby or a family pet that might want to taste the stuff. It’s extremely poisonous, even in small amounts, and can lead to death in some situations.
This plot shows a comparison between the torque theoretically available from a fresh and capable 302 with and without a supercharger. Torque is purely a function of cylinder pressure. The most important aspect here for the supercharged configuration is the area under the curve. As long as the supercharger can supply and maintain an elevated manifold pressure, the engine will make approximately the same torque over a very broad RPM range.
These plots show the calculated airflow for the various Ford small-block V-8s. In each chart, the lowest plot indicates the airflow for a naturally aspirated engine with an 80 percent volumetric efficiency (VE), which is what we get from a well-tuned high-performance street engine. The higher plots indicate the minimum airflow that should be used when considering upgrades to the intake tract when a supercharger is installed and setup to provide increased boost. Those numbers are based on a 100 percent VE. It is important to note that these plots do not account for faults in airflow through the engine from improper manifolding, poor camshaft choices, or the myriad of other snags that may exist in the total engine setup.
This plot shows what happens when there’s a bottleneck in airflow through the engine. Notice how the boost pressure takes a turn upward at about the same point the torque starts to flag and drop off. The implication is that the blower is working fine, but at that point the engine cannot handle any additional airflow. The intake manifold starts filling up with air that never makes it into the cylinders. The airflow impediment might be anywhere from the intake ports to the exhaust pipe tips. Adding to the problem is the fact that the supercharger is forced to work very hard against the higher manifold pressure with no increase in power. It’s a bad situation from every aspect.
Written by Bob McClurg and Posted with Permission of CarTechBooks
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