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Supercharging


The centrifugal supercharger is the only type of after market supercharger that has been fitted to a 4th Gen F-body at the time of this writing. As they are not mounted on top of the intake manifold (as any of the positive displacement type blowers are) they are easier to retrofit to vehicles that started their lives NA. It would not be economically feasible to adapt an M90 roots supercharger from a GTP to a 4th Gen F-body. If you are not satisfied by that statement, do a search or build the kit yourself.

By the way, a turbo (aka turbosupercharger, aka turbocharger) is a form of supercharger that is driven by exhaust gases. It is technically a subtype of superchargers because the defining feature of a supercharger is that it compresses air. The method of driving the compressor is irrelevant to the definition.

Turbocharging


Turbochargers are simply exhaust driven centrifugal superchargers. The compressor is directly linked to a turbine that is placed within the exhaust system. The compressor section of a turbo is generally smaller than the compressor of its cousin the crank driven centrifugal supercharger because it is going to spin at 10 times the RPM. The turbine looks like the compressor section of the turbo, except that the flow path is reversed and energy is taken out of the high kinetic energy exhaust gases in order that energy may be put into the intake charge (via compression). It is the turbine (in conjunction with the wastegate) that allows a turbo to function at many different RPMs at a single of engine speed. This allows greater tunability as compared to a crank driven supercharger. Ideally, the turbine is placed as close to or on the exhaust manifolds. This is so that the exhaust gases will lose the least amount of energy by the time they interact with the turbine.

Given that English as a language is somewhat imprecise, and that terms in science may not have exactly the same meaning as they do in everyday usage, there are many claims made about what exactly drives the turbine. Heat is the term that the layman uses when trying to describe the driving force of the turbine. However, whereas in everyday usage heat may be synonymous with temperature, in science it strictly means the flow of energy between systems when no work is done. If the interaction of the exhaust gas with the turbo was limited to heat exchange, then we would have placed an expensive mechanical device in our exhaust path that would sit around and do nothing. A more scientifically precise term for what drives the turbine is kinetic energy. Temperature, which everyone is familiar with, is the measure of the average kinetic energy of a system.

I have also seen it claimed that it is not just kinetic energy that drives a turbo, but also pressure. As with many other areas in physics, it is possible to describe or calculate effects using different terms. Saying that pressure gradients drive a turbine would be an accurate description, however saying that pressure and kinetic energy drives a turbine is redundant. Pressure is commonly known to most as Force per Area (for example, the SI unit of pressure is the Pascal, which is a Newton per square meter). However, it is also possible to express pressure of a fluid as Energy per Volume. Thus, pressure can be thought of as an energy density. The change in energy density (pressure) before and after the turbine tells you the same thing as the change in energy of the gas before and after the turbine.

Note: In a way you can think of two different types of kinetic energy in the exhaust path. That is to say, the two types are the movement of the gas as a whole from the manifold out to end of the tailpipe, and the random movement of each individual gas particle (which is measured as temperature). When I stated that the turbine is powered by the “kinetic energy” of the exhaust gas, it could be a combination of either of the two, but is principally the temperature component.

Twincharging


Twincharging is using two compressors in series to compress the air. This is different than sequential setups which use one compressor stage at low RPMs and switch to the next at higher RPMS. Such setups usually take the form of a turbo feeding a supercharger, although technically you could feed the turbo from the supercharger or use two superchargers or two turbos (dual stage superchargers or two turbos in series were commonly used on WWII fighter aircraft). Obviously a system with two compressors will inherently be more complex and expensive than a system with one. Why bother? There are some real benefits to twincharging at high boost levels, and a car like the supercharged Wbody already has a roots blower so a twincharged setup is nearly 50% complete.

Roots blowers, like on the Wbody, are positive displacement units. Although they can provide good low RPM boost, they are the most inefficient of all compressors. Eaton’s twisted rotor design makes up for some of the limitations, but adiabatic efficiency is still limited to around the 60% range. This mean that increases in boost on the stock unit disproportionately increase intake temperatures as compared with a centrifugal compressor (such as in a turbo, which can operate in the 70 to 75% efficiency range).

Imagine that you want to run 20psi of boost on your Wbody. Theoretically, you could spin the Eaton until it produces that amount of boost. However, given low efficiency, you will have some pretty hot air. How hot you ask? Let’s assume we are at sea level, the air temperature is 80F, and the compressor efficiency is 0.6 (60%). At 20psi of boost, the inlet of the supercharger will see about 14.7psi absolute pressure, and the outlet will be at 34.7psi absolute. This is a pressure ratio of about 2.36 (34.7/14.7). With these conditions, the air fed into the cylinders will be 327.5F. You could almost bake a cake.

What would happen if we fed the roots blower with a turbo? We will use the turbo to add 10psi of boost and the roots to add 10psi to get the total of 20psi of boost. We will keep the above assumptions and add that the turbo is at 70% efficiency. The turbo itself will be operating at a pressure ratio of 1.68 (24.7/14.7). The turbo inlet temperature is 80F and the outlet temperature will be 201.9F.

The air from the turbo then goes to the roots supercharger. The roots supercharger compresses the already compressed air. The supercharger’s inlet temperature is 201.9F, and the inlet pressure is 24.7psi. Thus to add another 10psi of boost, it will operate at a pressure ratio of 1.4 (34.7/24.7). The outlet temperature will thus be 313.3F. We have reduced the inlet temperature to the motor by 14F, which isn’t surprising since we added a more efficient compressor to do half the work. However, we haven’t quite justified the twincharger setup.

Now, what would happen if we added an intercooler between the turbo and the supercharger? (The intercooler received its name because it was originally used between compressor stages on aircraft). Let’s assume the intercooler is 80% efficient and will have a pressure drop of 1psi. To make up for the drop, we will use the turbo to produce an extra 1psi so that in the end the boost at the motor is still 20psi.

Adhering to our original assumptions, the turbo is now operating at a pressure ratio of 1.75 (25.7/14.7) with an inlet temperature of 80F and an outlet temperature of 211.6F. So far, we have an extra 10F of temperature in the air.

The intercooler will have an inlet temperature of 211.6F, but an outlet temperature of only 106.3F with a “pressure ratio” of 0.96 (24.7/25.7).

The roots supercharger in this case will be starting with the same inlet pressure, but a much lower inlet temperature. With the same pressure ratio, 1.4 (34.7/24.7), the outlet temperature of the supercharger is now only 201.4F. That is over 125F less than if we had used the supercharger by itself to compress the air.

The obvious question is then why not rip off the supercharger and just use a big turbo and intercooler to make the boost? You may be able to save another 70F of air temperature, but it will be difficult to find a turbo that will spool to a 21psi boost as quickly as one that only has to get to 11psi. Bottom line is that if you can tune it, which is difficult, a twincharged setup can be very effective.

Equations:

T2 = T1 + [T1*(P2/P1)^0.283 – T1] / CE

Where T1 is the ambient temperature, T2 is the outlet temperature, P1 is the inlet absolute pressure, and P2 is the outlet absolute pressure. The temperatures must be in units of Kelvin or Rankine.

And

Tout = Tin – IE*(Tin – Tamb)

Where Tin is the inlet temperature (K or R), Tamb is the ambient temperature, Tout is the outlet temperature and IE is the intercooler efficiency.

And finally

K = [(F – 32)*0.5555] +273.15

To convert Fahrenheit to Kelvin.

Electric Turbochargers


These come in various names, but the tech is usually the same. Various individuals market electrically driven fans that they pawn off as "electric turbos" or "electric superchargers." Most of them claim that these can create 2-3psi. However, typically any pressure that they might create is at low airflow. For example, a 3.8L motor with a volumetric efficiency of 75% will need more than 300CFM at 6000RPM.
It is unlikely that these systems will do anything more than cause a restriction in the intake.

That said, the concept of an electrically driven supercharger is nothing new and is theoretically possible. There are experimental setups that are in development; however, there is a need for greater electrical storage in the form of auxiliary batteries and capacitors, high output electric motors, and larger alternators. These systems currently can only be used in small bursts (15 seconds or so) before the electric storage media needs to be recharged. While an interesting experiment, at present technology they present little advantage over conventional superchargers.

Intercoolers


All this air compression will cause the temperature of intake charge to increase (i.e. can’t beat thermodynamics). Intercoolers are an attempt to bring the temperature of the air closer to ambient. Lower temperature air decreases the chance of detonation and also results in a higher flow rate through the engine. Higher flow rate is proportional to power output. Intercoolers received their name because some piston engine era warplanes utilized twin stage superchargers in order to maintain engine power at high altitudes. Even though many of those aircraft ran on 160 octane leaded fuel, heating of the intake air was a concern (see Rolls Royce Merlin engine design). A device, essentially a radiator, was placed between the first and second supercharger stages, and the “intercooler” was born. A similar device could also be placed after the second stage and was called the “aftercooler.” Although technically what we see on the automobiles today are more directly related to aftercoolers (some supercharger kits refer to them in this way) apparently intercooler sounds ‘cooler’ (pardon the pun) and that is what description we commonly use for these little radiators.

The two main types of intercoolers are the air-to-air and air-to-water types; the main difference is which medium accepts heat from the intake charge.

Air-to-air intercoolers exchange heat between the intake charge and the ambient air. Efficiency is commonly in the neighborhood of 80%. Air-to-air intercoolers must be placed in a location with sufficient airflow or they will not be able to effectively exchange heat. Two subclasses of air-to-air intercoolers are the cheaper tube and fin design, and the more robust and efficient bar and plate design. The main advantage of an air-to-air intercooler is simplicity of design.

Air-to-water intercoolers may operate at efficiencies greater than 100% if the water is at a temperature below ambient. These systems do not need to be placed in the path of airflow, so there is some freedom in choosing a location for it within the vehicle. The actual intercooler portion of the system is generally smaller than a comparable air-to-air intercooler. Unfortunately, air-to-water systems are more complex in that they need a coolant reservoir and some method to extract heat from the coolant.

Water, Alcohol, and Nitrous Injection


First off, this is not a section about nitrous oxide in general. The section is to elaborate on a several methods in which to cool the intake charge of a forced induction motor.

As stated in the previous section, high intake temperatures are something that should ideally be avoided. Injection of water, alcohol and nitrous oxide into the intake path are all additional strategies that may be employed. Some people use water or alcohol injection in addition to an intercooler, whereas other people may use it by itself.

Water is has many unique properties, but one of them is not that it will burn in your cylinder. So why would you want to inject it? It turns out one of the properties it has is a relatively high specific heat, especially in liquid form. Specific heat is a term that describes how much energy it takes to raise a given amount of a substance a given temperature. Water is injected into the intake tract at some point before the intake manifold. Generally the farthest away without injecting before the supercharger, turbocharger, or intercooler is considered ideal. As water is injected in the liquid form, energy is taken out of the intake air and placed into the water molecules. This energy transfer causes the intake air to decrease its temperature, and causes the water to heat, evaporate, and then most likely to heat some more in the vapor phase. The amount of temperature reduction of the air and the amount of increase in water temperature depends on the original temperatures of both mediums, and the amount of water injected. The sole function of water injection, therefore, is to lower intake temperatures and hopefully reduce the risk of detonation. Any power increase from the use of water injection is due to more boost or more aggressive timing.

Alcohol injection works in a similar fashion, except for a few important differences. The specific heats of methanol, ethanol, and isopropyl alcohol are all less than the specific heat of water. The also are more volatile. But unlike water, all three alcohols can participate in combustion. Additionally all three have an octane rating of slightly more than 100. Therefore alcohols have the potential to richen the air/fuel mixture and increase the octane rating (if you are running less than 100 that is). A mixture of water and alcohol can be used if additional cooling is required.

Nitrous oxide injection cools the intake air in a completely different manner. It is generally not used as a simple intake cooling method, but since it does have that effect I have decided to include a small discussion here. In the tank it is stored at high pressure. Injection into the intake causes rapid expansion into the gas phase. This expansion results in rapid cooling of the nitrous oxide and air mixture. A note of caution, however, is that whereas alcohol injection richens the air fuel mixture, nitrous oxide will tend to lean the mixture. Therefore additional fuel must be provided to the motor.

Turbocharger vs. Supercharger


In simplistic terms, the engine functions as an air pump. The more air and fuel that is pumped through, the more power you can make. In order to pump the air, pressure on the intake side must be higher relative to pressure going out the exhaust. In a naturally aspirated engine, valve timing events are used to create a pressure. Since you are reading this guide, you are probably not interested in naturally aspirated engines, so we can leave it at that. That said, we can all agree that it makes no sense to build a naturally aspirated performance engine. From a performance standpoint, it would generally make sense to use some means to pressurize the intake, while using some means to decrease the pressure in the exhaust path. The second part is easy; almost everyone and their brother has some type of exhaust work. The first job is a little trickier. Fortunately we have superchargers (and turbos) to save the day.

A crank driven supercharger will most definitely increase the pressure on the intake side of the engine. Since it is limited to the intake track, it will not adversely affect the pressure in the exhaust. The pressure on the intake side should always be greater than the pressure in the exhaust. However, power doesn’t come free, and you must use some of that newfound torque to spin the supercharger. How much that takes is calculable, but is purely academic because significant power is netted. In the case of positive displacement superchargers, boost can be had at very low RPMs, and in the case of the centrifugal and screw supercharger, good efficiency can be had. Other reasons to choose a supercharger are that the retrofit to an NA car should be smoother because there are no changes to be made to the exhaust path. The power curve is predictable because boost is largely dependent on RPM of the motor and not some less tangible factor such as engine load.

Now why would anyone want a turbo? Turbo systems are more complex because they require revision to the intake and exhaust sides of the motor. From the air pump standpoint, at first glance they seem to be inferior to a supercharger as you are placing a restriction in the exhaust flow path (i.e. the turbine). And given what we know of centrifugal compressor efficiency at low RPMs, there may be a significant portion of the rev range before the turbo will reach threshold and begin to create boost (this is what “lag” is). However the relative independence from engine RPM is the turbo’s greatest advantage over any other supercharger type. Boost can be reset with ease, and therefore tunability is also greatly increased as compared to a crank driven unit. While the adiabatic efficiency of the compressor may not be as great as that of a screw type supercharger, the drive mechanism is much more efficient, as a turbo relies on utilization of largely wasted kinetic energy in the exhaust gases. All of this combines to form a versatile, tunable unit that has the potential to make more power than a crank driven supercharger.

So a turbo must be superior to a crank driven supercharger, right? If that was the case the crank driven supercharger would have died out long ago. For all out power the turbo reigns supreme, but life unfortunately is full of compromises. Packaging is a huge concern during a retrofit of forced induction onto an NA motor, and in that instance the crank driven supercharger has the turbo beat handily. The user must decide on his or her priorities and decide from there.

A Practical Guide to Forced Induction for V6 F-Bodies

A guide written outlining the basics of Forced Induction for a 4th gen F-Body.

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