Let’s focus on the word “supercharge” for a moment. In the context of this book, it means to fill the cylinder above and beyond the displacement that it ordinarily has. For instance, if a cylinder has a 50-ci displacement and, by one means or another we cram 75 ci into it, then we can say it’s not just “charged” but “supercharged.”
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Chart 5-1, shows that supercharging falls into the “Maximize Air Density” category. The so-called normal situation is to not lose any air density that exists naturally. This is the non-supercharged situation and is commonly called naturally aspirated, or NA for short (the F1 guys often called them Atmo Motors).
A supercharger is a device that has the ability to overfill the engine’s cylinders. But why use a supercharger? Why not just design the engine with greater displacement in the first place? That’s a good question, with a less-than-obvious answer. The answer is very much a question of component size and geometry. An analysis shows that a piston/rod/crank mechanism is a very effective means in all respects at taking pressure energy and converting it efficiently into rotating motion. It is not only mechanically efficient but also relatively effective in terms of size and component weight necessary to do so.
What a piston/rod/crank assembly is not so good at is moving air at an atmospheric pressure of typically 14.5 psi absolute (pressure above a total vacuum) from outside of the engine into the cylinder. For this job a piston/rod/crank assembly is a heavy and cumbersome mechanism in terms of component size for the mass of air induced. For a supercharger to justify its existence, it has to be (size for size) far more effective at moving significantly greater quantities of air from point A to point B than a piston/rod/crank mechanism.
As it happens, there are plenty of ways to do just that. A good example is to compare the size of a turbo compared to the engine it’s feeding. What we find is that if a turbo is about one-twentieth of the size of the engine it is on, it can pass as much as four to five times the volume of air.
What this all means is we can let the crank/rod/piston assembly do the job it does so well—converting pressure energy from the combustion process and turning it into usable power—while the supercharger takes over the function of inhaling air more effectively. By adding a supercharger to an engine, we are making it act as if it had far more cubes. This is because the supercharger, not the basic displacement of the long-block assembly itself, determines the magnitude of the induction process.
Superchargers fall into two fundamental categories: the positive displacement type, and all the rest. Almost certainly the best-known positive displacement supercharger is the Roots type. This unit, developed in England during the mid 1800s by the Roots brothers, was intended to pump air into deep mine shafts. However, during World War I, it saw use on aircraft engines as a means to boost power at altitude. Success here subsequently led to its use between the wars on Mercedes and Auto Union F1 cars as well as the highly successful British ERA F2 cars.
The Roots blower really came to the fore when the likes of Don Garlits, and a few other leading Top Fuel drag racers of the late 1950s and early 1960s, figured they could go faster with a big Roots blower atop the motor—and they did. Back then, because there was not much else to choose from, the blower of choice was the GMC supercharger used on GM’s two-cycle diesels. The sizes most commonly used were the 6-71 and the 8-71. These big blowers could puff about 20 to 25 psi or so into a Chrysler Hemi and bump the output to what was then an incredible 2,500 or so hp and about 3,200 ft-lbs of torque.
Today, we see many superchargers that owe their heritage to the GMC range of blowers. A few examples are those from Holley/Weiand, Edelbrock, Magnuson, BDS, and Eaton. When taking a casual look at this type of supercharger, it is easy to assume it draws the air into the middle of the rotors and passes it down into the manifold. In reality, this does not happen. The best way to see how it works is to look at a smallblock Chevy oil pump, because this is, in fact, a mini Roots-type pump. From its inception to possibly as late as the 1980s, the big drawback with the Roots supercharger was its relatively inefficient (about 55 percent) pumping characteristics.
In this quarter, real credit can be given to Jerry Magnuson, one of the world’s leading supercharger designers. His extensive work over a number of decades has brought about dramatic improvements to the Roots style positive displacement blower. This has been achieved over the entire operating range and duty cycle, to the extent that it is closely comparable to a turbine- or centrifugal-style supercharger. To put that into prospective, he has, in the last 30 years, done what engineers in the previous 120 years largely failed to do. Today, 98 percent or more of superchargers use Magnuson technology. In other words, your modern Roots-style supercharger is not the one your father knew.
Although the most common the Roots-type supercharger is far from the only positive displacement type available. Among the many others in this category are the Zoller vane-type supercharger, the novel (and supposedly very efficient) VW G-Lader, and the Whipple-style screw compressor. Of these, only the Whipple unit, also a high-efficiency unit, is in any kind of aftermarket volume production.
Turbos and Centrifugal Turbine Superchargers
Barring leakage, a positive displacement supercharger moves a certain defined amount of air per revolution. A turbine, or centrifugal supercharger (as they are more usually called), develops boost by imparting motion into the air. When the rapidly moving air meets resistance, the slowing of the air converts the air’s kinetic energy into pressure energy. For that reason, the boost and airflow throughput of a turbine supercharger is closely linked to the characteristics of the engine it is feeding.
Here is an important point concerning boost: It is all too easy to suppose that a supercharger’s whole existence is to develop boost but, as obvious as that may seem to be, it’s not quite true. If the goal was the biggest boost numbers possible, then welding the intake valves closed would do just that, but then the engine would not produce much power. The real job of a supercharger is to move the greatest mass of air through an engine, which then must subsequently use it effectively. A turbine supercharger can move a great deal of air very effectively. Also, since centrifugal superchargers’ mode of operation does not involve so much beating around of the air, it can do so with high efficiency when optimally designed.
But there is a downside; a turbine-style supercharger is speed sensitive. As the turbine speed increases, the boost possible also goes up with the square of the RPM. If you double the turbine RPM you get, in round terms, four times the boost. As far as the engine’s torque curve is concerned, at low speed there’s minimal boost, which means low torque. Assuming a supercharger is sized right for the amount of boost required for the mid-range, we find that the boost goes out of sight at the top end. Although the trend is somewhat set in concrete, there are partial remedies to this low-speed (low boost) situation. In short, a lot of work in the past 50 years has brought about a number of improvements that minimize this characteristic.
Boost Curve Shapes
We can best see how the “too little low-down and too much highup” situation has been addressed by looking at a typical turbocharger installation. In essence we have an intake turbine driven by a second turbine in a housing through which the engine’s exhaust is fed. Simply put, the exhaust energy is driving the intake compressor turbine.
The general principle here is to size the intake turbine so that it is a little oversize—without any limiting device, it produces more high-speed boost than needed. This over-sizing means boost to the engine starts sooner but, if left to its own devices, provides too much at high speed. To prevent this, the exhaust side is equipped with an exhaust wastegate. When the boost reaches a certain predetermined level, the wastegate opens and bypasses any exhaust excess for the job of spinning up the intake turbine. This, along with impellor design characteristics and inlet sizing, helps spread a turbo’s boost range down into the lower-RPM band.
For a mechanically driven turbine supercharger (ProCharger, Vortec, Paxton, etc.) of the type originally pioneered by Paxton 60 years ago, we find the situation a little different. Because the drive ratio between the supercharger and the engine is fixed (Paxton actually developed a variable-drive unit used on Ford Thunderbirds in the late 1950s), limiting top-end boost while trying to enhance low-speed boost comes down mostly to impellor design and overall unit sizing. In that area, significant strides have been made since the mid 1990s.
At your local library, in the reference section, you can find books on performance going back as far as the 1930s. A look through these might just give you the impression that the heyday of superchargers was in the 1930s through to maybe the mid 1950s. The reality is the heyday is from now into the foreseeable future.
If you are concerned about fuel economy, and the possibilities it has to negatively impact building big power, let me put your mind to rest. From the auto manufacturers’ viewpoint, there are numerous avenues toward employing a supercharger to enhance both performance and mileage. That alone more or less guarantees their continued use for a long time to come. If your intent is to get additional power in the form of a bolt-on supercharger system, then your only real problem is making a decision as to which of many excellent systems you should use.
If It’s Low Speed You Want
As of 2010, I own a 4.8 GMC Sierra truck and, like many truckowning performance enthusiasts, I want it to perform like a sports car for day-to-day driving. But I also want it to carry out its work functions—towing a loaded trailer and such—better than stock. When I am towing the racer to the track, that 4.8-liter V-8 can be hauling a payload of as much as 10,000 pounds. To do this effectively, the engine needs real low-speed torque along with a good fuel economy capability. About the time I was ready to start modifying this truck’s engine, I had a fair amount of experience with Holley/Weiand, Magnuson, Edelbrock, and Whipple kits, but mostly with the Holley/Weiand and Magnuson stuff. My simple blower build done in the mid 1990s produced, on a mild-cammed 350 with pocket-ported aluminum heads, had some 541 hp and 545 ft-lbs output using a Holley/Weiand blower. This was a pretty good showing for what is essentially one of the lower-cost installations on the market.
As with most trucks used for towing race cars, the distances involved can be quite substantial, so fuel mileage was important. This factor alone caused me to re-visit Magnuson supercharger installations available at the time. Since my last go-around with a Magnuson supercharger, the design had been modified with a cruise-mode bypass valve. This valve serves to reduce parasitic losses to barely more than that taken to spin a couple of sets of roller bearings.
So with serious heavy-duty towing and mileage in mind, I selected a bypass-valve-equipped, positive displacement, intercooled Magnuson kit. Because the Magnuson kit had charge intercooling via a water-to-air intercooler between blower and block, it could manage 8 psi on a 9:1 engine without being detonation prone or octane sensitive. To make the most of this supercharger package, the stock 4.8 heads were ported and a Gale Banks exhaust system was installed.
Results were very much what I expected. The quarter-mile performance turned in a result just short, by a truck’s length, of that produced by a near-stock (K&N cold-air kit only) 2004 five-speed Mustang GT. Freeway mileage was just shy of the 21 mark while a consistent 17-plus was seen about town. That was almost unchanged from stock. When towing sensibly, mileage was surprisingly good. Most of my tows involve 90 percent or more freeway travel. If speed was kept to 60 to 65 mph or so, and with a 6,000-pound trailer load, I saw low 14s for MPG.
The results of these tests strongly suggest that for a good balance between performance and mileage, a smaller engine with an efficient supercharger is the way to go. In 2008, Magnuson introduced yet another significant step forward in rotor design. This newer design in photos on page 48 is essentially a hybrid Roots/screw compressor. The efficiency figures rival those of a turbine-style supercharger but have the advantage of boost right off idle. This in itself allows an auto manufacturer to use a smaller engine because the low-speed boost totally compensates for any torque reduction due to the smaller displacement.
At this point we can say that (in the main) off-idle and low-speed torque, be it for a truck or a true street car, means looking first at a positive displacement blower. If low-speed torque below about the 2,000- to 2,500-rpm mark is not an issue, then a whole slew of supercharger types become a viable bolt-on option. That is what we next consider.
In most instances, the simplest installation is a belt-driven centrifugal supercharger rather than a turbo. Although they may lack the lowspeed capability of a positive displacement unit, they can usually (but not always) deliver a cooler boosted charge, thus allowing more power to be developed before an intercooler or water injection becomes necessary to eliminate detonation.
Regardless of the cooler charge advantage, most installations still cool the boost with an intercooler. When combined with intercooling, the results can be little short of spectacular. Here’s an example to illustrate the substantial difference that can be made: Bolting a 10-psi boost ProCharger unit on an otherwisestock 4.6-liter, three-valve 2007 Mustang produced a rear-wheel-hp increase from 260 to just over 460! Here, we have an engine boosted to 68 percent above atmospheric pressure but making 75 percent more power.
How does this work out? There are two factors at work here: keeping the charge temperature down by means of an efficient supercharger, and intercooling it. In addition, we now have a situation where the boosted intake charge tends to blow much of the residual exhaust in the combustion chamber out through the exhaust valve. This leaves us with an intake charge equal (more or less) to the cylinder displacement, plus that of the combustion chamber.
When it comes to big mid- and top-end numbers, a centrifugal supercharger is the way to go. Now it becomes a question of opting for a mechanical drive (belt driven) or an exhaust-driven turbo setup.
Just as in making a decision between positive displacement and centrifugal superchargers, there are pros and cons for each type of drive. If the supercharger is mechanically driven, there is no boost lag as the throttle is opened. This implies that a turbo will have boost lag, but that is not necessarily so (which I discuss on page 54).
In addition to possible lag, there are other issues that need to be considered. The first of these is that, unlike turbo-driven systems, a mechanically driven supercharger produces much less engine compartment heat. That said, a turbo setup, which recovers some of the normally lost exhaust energy, still ends up to be the top contender for total output. Another advantage of the turbo is that it can be sized to come on sooner, with the top-end boost limited by the wastegate. At the end of the day, both types produce dazzling performance from most otherwise-stock engines.
The performance is so sufficient that the problem becomes one of getting all the power to the ground. When purpose-built from the start with the intent to supercharge, the power potential is little short of staggering. As of 2010, I have helped on the build of turbo engines making about 10 hp per cube (just over 600 per liter), and that is without a budget of any real consequence. With a much bigger budget to work with, I have seen some factory development projects that have pushed the turbo envelope on a four-valve engine to 13.5 hp per cube (835 hp per liter).
But let’s drop down the budget ladder a little here. A couple of more-affordable projects I have worked on put a realistic perspective on what can be achieved. A ProCharger-equipped, 351 Windsor-powered Mustang turned up the dyno rollers to 850 hp and at that point just smoked the tires. I estimate it was still more than 1,000 rpm from a peak power number that could well have been in the four-figure range. With a turbocharged small-block Chevy with a Performance Techniques installation—same deal. Both of these vehicles most certainly exceeded 1,000 rear-wheel hp. Even on the grippiest road tires available, neither of these cars hooked up at less than 120 mph! That’s on a dry road; on a wet road, you would need all of Michael Schumacher’s driving skills just to survive.
If we are dealing with a well-engineered supercharger installation, cost, ease of installation, and personal preferences need to influence the system you choose. But let’s get our feet back on the ground here. A more bolt-on turbo kit for an otherwise-stock engine, such as an LS-series GM engine or Ford’s three-valve 4.6/5.4 unit, responds to the tune of a 200- to 300-hp increase with a boost of 8 to 10 psi. A good example here is the kit offered by Turbonetics for the 2005–2008 Mustang. This boosts rearwheel hp from the stock 250/260 to about 500 and all done with less than 10-psi boost.
Here it is worth addressing the subject of so-called turbo lag. About 1977, I was helping my friend Jim Flynn with his turbo 2-liter Pinto. It was a stick-shift setup, so the engine could not be loaded up against the torque converter prior to a launch. This meant I had to come up with an alternative way to create boost before the car was launched from the line. The idea I came up with (one that at least two other people have subsequently claimed to be their own) is very simple. If, at the launch RPM, the engine’s ignition timing is drastically retarded or even has a random spark cut, the RPM will not climb; but the throughput of air and fuel, and consequently the production of exhaust, continues. This keeps the exhaust turbine spinning at high RPM, producing boost even when the car is in neutral. The technique at the start line is to set the launch RPM, and then floor the throttle against this limiting RPM. At this point, the boost comes up while the car is still staged and ready to go. When the clutch is released, the ignition timing comes right back to its proper setting, and suddenly you have more power than you know what to do with.
This system worked very well to the extent that we had to actually tame it down a little. I wrote about it in detail in a book I did many years ago on 2-liter Pinto engines. Subsequently, virtually all of the turbo Formula 1 cars of the 1970s and 1980s used this system. These cars were virtually fly-by-wire systems, in which the throttle blades were not actually connected to the throttle pedal. Instead, the throttle pedal was a torque-demand pedal. When the driver lifted off the pedal, the throttle blades only partially closed; but at the same time, the ignition retarded by a large amount. The charge throughput did not make for any flywheel horsepower but it did keep the turbo spooled up.
Superchargers and Built Motors
So far, we have mainly looked at supercharging as a bolt-on for an otherwise-near-stock powerplant. In such a situation, the kit manufacturer pretty much takes care of issues that may arise by limiting the power increase supplied, limiting boost, and including an intercooler. Now let’s move on to consider what issues you may need to address if you are looking for competition-killing numbers for opposition annihilation at the track. There are four primary issues we need to address: detonation avoidance, control of excess heat, maximizing mass air through the system, and the effective and reliable conversion of the mass air throughput into torque and horsepower.
Combating the onset of detonation is the most important factor to deal with. The bottom line here is: If the engine is strong enough to withstand all other loads and stresses, then detonation ultimately limits power production. First, higher boost means lower compression ratios (CR), but there is a balance to be struck here. For good part-throttle fuel economy, the balance needs to be more in favor of the CR. If ultimate power is sought, the balance is toward maximizing boost. The engine’s combustion chamber, exhaust valve temperatures, and the fuel’s octane rating determine just how high the CR can go under such circumstances. Charge cooling by means of an intercooler and higher fuel octane are the two routes to success here.
Control of Excess Heat
As a means to combat excess temperatures and/or inadequate fuel octane, water injection has no equal. First, the octane rating of water is virtually infinite and, while boosting the octane, it won’t even come close to putting out the fire. Spraying even a huge amount of water as a fine mist into a burning air/gasoline mix serves only to reduce peak temperatures and make a lot of steam! The cooling action is effective to the extent that melting anything is totally countered. Here’s an example to give you an idea of just how effective water injection is: During a class project, we successfully ran a 17:1-compression tractor engine on kerosene (less than 50 octane) and water injection. More in line with what we are doing here, I helped build a 1,100-hp, 350 small-block Chevy that ran 35 psi and 87-octane fuel. Water injection absolutely works. And if you are in the market for a system, check out Snow Performance.
Controlling excess heat with water injection is a good start, but when very high output is called for, this needs to be complemented with a big radiator and piston-oil squirters. The big radiator is self explanatory but to those who may be new to supercharging’s little nuances, an explanation of squirters is in order. Here, engine oil is squirted through jets onto the underside of the pistons, thereby oil cooling them. I have used this as a procedure on all my serious nitrous and supercharged engines since the late 1980s. Many of the newer, small four-valve factory turbo engines are now adopting this method of increasing piston life.
Maximizing Mass Air
There are many misconceptions associated with this subject. The most notable concerns the cylinder heads. Do not fall for the myth that heads don’t need to be good because the supercharger forces the mixture into the cylinder. Ask yourself where the power to generate the extra force necessary to push the charge into the cylinder came from. Just as normally aspirated engines respond to better flowing heads, so do supercharged engines—to an even greater extent. Also note that for higher boost figures it is better to trade off some intake valve diameter for a larger exhaust diameter. At this time, I am building a 20-psi-boosted, 331 smallblock Ford. The valve combination being used is 1.94/1.7 instead of the more normal 2.02/1.6 combo. Be aware that the lower the CR used, the greater the bias toward exhaust valve size needs to be.
Getting the Cam Right
Let us set aside the cam requirements for a turbo motor for the moment. We can then set ground rules that apply to all supercharged engines where the exhaust flow is uninhibited. The most important factor here is the lobe centerline angle (LCA). This is very dependent on the boost/cubes combination in relation to the circumferential length of the intake valve and the intake/exhaust flow ratio. When boost is applied progressively we find the optimal LCA needs to get progressively wider. Also, as is so often the case, the exhaust valve is too small for a supercharged application. It needs to be opened earlier, thus widening the LCA itself. Although not so good for the bottom-end output, the early exhaust opening can help the top end appreciably. What we are trying to avoid here is the boosted intake charge passing right through the combustion chamber during the valve overlap period and on out through the open exhaust.
Another factor that needs to be taken into account for a street setup is that the cam for the job can be shorter. This effectively boosts the low-speed output from an engine equipped with a centrifugal-type supercharger. In this situation, if the heads are good, then the supercharger takes care of the top end.
If you understand what I am about to explain, you probably have a 90-percent chance of saving yourself from wasting money. What you need to understand is that a cam company cannot sell you an appropriate cam for a turbo motor until they know the pressure differential across the motor.
When a turbo goes into boost on the intake side, it is driven there by pressure built up between the engine and the turbo. Other than in a pulsedriven turbo system, it is this pressure that drives the exhaust turbine. Most turbo kits have an install design priority. This means they may give up power-enhancing moves, which allows for a simpler and cheaper installation. As a result of necessary compromises (or just a not-so-good design) a typical turbo kit can have an intake-to-exhaust pressure ratio of about 2:1, but that can vary from 1:1 for a really well-designed installation right through to a mediocre 3:1.
Let’s assume that your turbo setup is going to be about 2:1. If that is so, then, when there is 15-psi boost in the intake, there is 30 psi of pressure in the exhaust manifold. Let’s say a cam with a typical street overlap is installed and the intake valve opens. Then, rather than the mixture entering the cylinder, exhaust passes out through the intake valve into the intake manifold. This heats the charge considerably and often increases the likelihood of detonation.
Working with one of the country’s leading turbo shops, we investigated cams with negative overlap where a high-negative-pressure ratio existed. For about a 2:1 pressure ratio, we calculated that about minus 25 degrees of overlap should work, and it did—big torque right off idle, big power, and a glass-smooth 600- rpm idle. Also the engine was far less sensitive to fuel octane.
When building my own turbo motors, I work hard to target a 1:1 pressure ratio across the engine. If this is achieved, the cam selection is a whole lot easier. If this 1:1 ratio is achieved (as was the case with our turbo Ford Cosworth Sierras, which won the road race championship in England), any cam that produces the best events for a normally aspirated engine also works best for the turbo setup.
There is one more valvetrain factor to take care of concerning the valvesprings. Because boost pressure tries to open the valve, a stronger spring is required on the intake of a regular supercharged engine, and on the intake and exhaust of a turbo motor.
Now that we have dealt with pushing air through the engine, it is time to consider just how the appropriate amount of fuel is added. Basically we have three options: a suck-through carb, a blow-through carb, or fuel injection. This choice dictates much when planning a supercharged installation. If we go back to just the 1980s, the fuel injection option was probably little or no better than the carbureted option. But in this day and age of precision electronic fuel injection, and budget permitting, fuel injection is the way to go with any blower setup where total output is the number-one priority. However, don’t infer from that statement that a carbureted supercharged setup is a distant second best, because it most certainly is not.
The advantage of spending $3,000 on an injection setup is that there is zero fiddling and zero doubt about the tune when you are done. Getting the fuel curve right-on is just a matter of reading the oxygen sensor output and adjusting accordingly—that’s if the injection system itself did not actually do it for you. Going the carbureted route can save between $1,000 and $2,000, but you almost certainly need to do at least a little fine tuning.
If the blower is a Roots-style unit, then the most obvious and convenient place to put carburetion is on top of the blower itself. This is the classic draw-through mode of operation. Apart from referencing the power valve to the intake manifold boost, the carb pretty much operates as normal. But with some installations, building a draw-through system is not quite as easy; centrifugal superchargers sort of fall into this category. Here, it is more convenient to have a blow-through system. Unless you have a carb built by a company knowledgeable in this mode of operation, you could encounter many problems. On a scale of 0 to 10, your carb calibration problems could easily be 5 or more— but you need zero problems. Although I am sure there are others out there, I have successfully used Holley-style, blow-through carbs, from Holley, the Carb Shop, and AED. As for what you may give away in terms of power, you should realize the potentially large number of ft-lbs of torque that the engine inevitably makes, if it’s done right.
Written by David Vizard and Posted with Permission of CarTechBooks