For Supercharging and Turbocharging
By Mike Mavrigian
This article will touch on forced induction and directly-related exhaust applications. Forced induction, through the use of either a turbocharger or a supercharger, forces additional air to the intake charge. This forced air is referred to as boost. This creates higher cylinder pressure, allowing the engine to produce more power. By contrast, a naturally-aspirated engine uses available (ambient) air to enter the engine, mix with fuel and ignite in the combustion chamber.
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SUPERCHARGING
While turbocharging packs additional air into the cylinders with the use of an exhaust-gas-driven compressor, a supercharger accomplishes the same task by being mechanically driven by the engine’s crankshaft.
In many cases, depending on the specific engine design, a supercharged engine’s crankshaft may feature a longer snout as compared to a naturally aspirated version of the same type of engine. Additional crankshaft snout/nose length may be required in order to accept the supercharger’s belt drive. If you’re building an engine intended for supercharger use, talk to the crankshaft maker to determine if their crankshaft will accept a supercharger drive, based on the type of engine, as some engine designs may offer a short or long snout.
TURBOCHARGING
A turbocharger contains a turbine and a compressor. In essence, the turbo is fed by the engine’s exhaust gas, which spins the turbine. As exhaust increases, the turbine spins faster. The compressor side of the turbo, which serves as a centrifugal pump, is driven by the exhaust-driven turbine and packs pressurized air into the intake manifold. This “forced induction” packs more air, and subsequently a greater air/fuel charge, into the cylinders than would be available on a naturally aspirated engine. The greater the air/fuel charge, the more power the engine can produce.
Turbocharging does not provide “free” horsepower. Because placing the turbocharger in the exhaust stream creates added restriction in the exhaust, the engine is required to fight against this added backpressure during its exhaust stroke. For this reason, a turbocharged system will benefit from a larger diameter exhaust.
Engine upgrades need to be considered for any build where the use of one (or two) turbochargers will be installed. Due to higher cylinder pressure and resulting higher temperatures, strongly consider the use of forged or billet pistons, which will withstand these increased pressures better than cast or hypereutectic pistons. Also consider taking advantage of specialty thermal barrier coatings for the piston domes, exhaust valves, cylinder head combustion chambers and cylinder head exhaust ports. We’ll discuss coatings later in this chapter.
When a turbo is “under boost,” and forces a more dense charge into the cylinders, the air is pressurized. When air is pressurized, the air temperature rises. If air temperature becomes too hot, this can ignite the charge before the spark plug fires. This is, of course, a case of preignition, which produces engine knock. This is why turbocharged engines typically feature a lower compression ratio, to help compensate for potential preignition. Turbocharged engines typically also require a higher octane fuel rating for the same reason…to avoid preignition.
A turbocharger needs rotational time to generate boost. A smaller size turbo will spool faster than a large turbo, reducing the chance for “lag.” Using a smaller turbo along with a wastegate valve prevents the turbo from spinning too quickly at higher engine speeds.
The purpose of either forced induction system is to pack more air into the engine than is available under normal atmospheric pressure. Atmospheric pressure is 14.7 psi (at sea level). If a turbocharger or supercharge creates, say, 7 psi of boost, we’re packing about 21 psi into the engine (nearly a 50% increase). However, since neither system operates at 100% efficiency, we likely won’t see a 50% increase in horsepower. More than likely, power would increase by around 40%. One of the ways to help maximize efficiency is to tailor the exhaust system for maximum exhaust gas scavenging.
Regarding superchargers, there are three types to consider, including the twin-screw type, Roots type and centrifugal type.
Twin-screw type superchargers feature a pair of inter-meshing male/female helical rotors, very similar in nature to a twin-screw shop air compressor. The mated pair of high-precision matched rotors progressively compress the air between the screws as they spin in opposing directions. The screws don’t touch each other. A tight clearance between the screws compresses the air as it is drawn from one end of the screws to the opposite end. A twin-screw type supercharger “squeezes” the air more efficiently as compared to a Roots type supercharger.
The Roots type supercharger features opposing “male” rotor lobes inside a housing. As the lobes rotate, air is pushed between the outside of the lobes and the housing case, “blowing” additional air into the intake system. Twin-screw superchargers are said to produce less heat as compared to the Roots type for a cooler air charge, but with intercooler aid, this may be a moot point. The twin-screw type of supercharger is theoretically more efficient.
Both a twin screw and Roots type supercharger mounts to the intake manifold location on the engine.
A centrifugal type supercharger, at first glance, looks and functions similar to a turbocharger, in that the unit features an internal impeller. Instead of mounting in place of the intake manifold, a centrifugal style unit is mounted anywhere at the front of the engine as long as its pulley aligns with the engine’s belt drive. This makes the centrifugal style more versatile in terms of the installed location.
The impeller operates at a high speed, drawing air into the volute (compressor housing). The high air velocity features a relatively low pressure. The air travels through a diffuser that converts the air into a low-speed, high-pressure charge that is forced into the engine’s air intake. An internal geared “transmission” provides the increase in impeller speed.
Depending on the kit design for a given vehicle, the air discharge plumbing from the centrifugal supercharger may be located between the engine’s throttle body and the intake manifold, although some centrifugal kits are designed to plumb the air charge into the throttle body, eliminating the need to relocate the throttle body.
All three types of superchargers produce the same end result of packing more air into the engine, but accomplish this by different mechanical designs. In theory, a screw type of roots type supercharger will produce better low-RPM power, as compared to a centrifugal type which should produce better high-RPM power.
The boost produced by a supercharger is directly affected by the size/displacement of the engine as well as the speed at which the supercharger is driven. For instance, if the supercharger is driven at the same ratio as the crankshaft, a supercharger with a larger-displacement will produce more boost than a smaller blower on the same engine.
A rule of thumb is to select a smaller blower size for smaller displacement engines and a larger blower size for engines with bigger displacement. The boost can be tuned by altering the diameter of the drive pulley in order to make the blower run slower or faster. A larger supercharger on a smallblock engine can be tuned by driving the supercharger at a slower speed in order to reduce boost, in order to prevent detonation. However, operating the supercharger too slow can reduce boost, resulting in inadequate air compression/boost. If the supercharger is too small, this would require speeding it up to a point where the intake charge temperature is too high, resulting in poor air density.
PRESSURE RELEASE VALVES
A turbocharged system requires two adjustable pressure-release valves in order to avoid over-boosting and management of pressure limits. This includes both a wastegate (WG) and a blow-off valve (BOV).
A wastegate valve is installed in the exhaust stream before the gas enters the turbocharger, which slowly opens if/when exhaust pressure exceeds a pre-set limit. This controls boost to the turbo.
A blow-off valve is installed in the air charge path that runs from the turbocharger to the engine’s intake, regulating the limit of boost pressure that is being delivered to the intake manifold. This is located in the air charge pipe between the turbo and the intercooler. When throttle is lifted, the blow-off valve prevents forced air from being packed into the engine as engine speed decreases, protecting the engine from excess pressure when the throttle is released.
A supercharger requires the use of a bypass valve, which regulates the limit of air pressure being delivered to the engine.
COMPRESSION RATIO
When air intake and combustion pressure are increased by a forced induction system, this effectively increases the engine’s compression ratio. An engine that features a specific compression ratio in naturally aspirated form will experience an increase when forced induction is used. When building an engine that will feature a forced induction system we need to consider the initial compression ratio in order to handle the increase that will be provided by a turbo or supercharger, to avoid preignition or “knock.” In the case of an engine specifically built for forced induction, lowering the compression ratio allows more boost with the same octane.
When considering the engine’s compression ratio, we first consider its static compression ratio (CR). This refers to the compression ratio of the engine without forced induction. When we consider the use of a forced induction system, we then consider the engine’s final compression ratio (FCR). This is the compression ratio that the engine will experience during forced induction operation under the available full boost level.
The formula for calculating final compression ratio, or FCR, is as follows:
(Boost divided by 14.7) + 1 X Static CR = Final Compression Ratio (FCR)
Variables of each engine build need to be considered, such as the strength of the crankshaft, rods and pistons, but in general terms, the FCR of any forced induction engine should not exceed about 12:1 if the engine is to be operated on pump gas. Higher FCRs will demand the use of higher octane race fuel. For most street applications, the static compression ratio, or CR, should be limited to about 10:1. It’s important to understand that the amount of boost that you plan to use will directly affect the final compression ratio.
Following is an example of how the final compression ratio corresponds to both static compression ratio and the boost psi. The static compression ratio (CR) is listed in the far left column. The boost pressures are listed at the top, from left to right. The intersecting numbers indicate the theoretical final compression ratios.
TURBO TEMPERATURE MANAGEMENT
The turbocharger is exposed to intense heat, as are the pistons, exhaust valves, and exhaust ports. Thermal barrier coatings, available from aftermarket coating specialists, are definitely worth considering. Specifically, the turbocharger housing, the exhaust manifold, piston domes, combustion chambers, exhaust valve faces and cylinder head exhaust ports.
With the application of a true thermal barrier coating, this retains more heat inside these components, resulting is less heat transfer. This means that the exhaust component surfaces will be less hot to the touch and will retain the heat inside. Keeping the exhaust gas hotter, with less transfer to the outside, helps to speed up the exhaust gas flow and improves exhaust scavenging. And, specifically when dealing with a turbo setup, greater exhaust gas velocity makes the turbo spool quicker. In other words, the application of effective heat retention/thermal barrier coatings in the combustion chamber, pistons domes, turbo housing and exhaust piping can directly aid in terms of turbo performance.
When we consider thermal barrier coatings for the exhaust stream, you need to recognize that there are different levels of thermal barrier coatings out there. Some are more cosmetic and designed to enhance appearance, and may be as thin as 0.002-inch, while others are designed for function, with little regard to appearance, and can be as thick as 0.015-inch or more. By using applied layers of real ceramic which is applied in a molten state to the components, and by using a thicker application, heat retention can be improved by as much as 40-50%. Thin coatings that emphasize appearance as opposed to function may only improve heat retention by as little as 5%. To cite one coating service in particular, Swain Tech Coatings designs, manufactures and applies all of their coatings in-house. Their thermal barrier coatings feature a pure ceramic content and is applied in layers to a substantial thickness. The only negative, if you want to use that term at all, involves appearance. Their “White Lightning” thermal barrier coating provides a somewhat rough, white appearance that may not be acceptable for “show car” folks. However, you need to ask yourself one question up-front: Do I want to look good without regard to performance, or am I more concerned with function and increased performance? The White Lightning features a melting point of 3000 degrees F, which will certainly withstand extreme turbo temperatures. If you’re serious about performance, you shouldn’t care about color or surface texture. I’ve had many components coated by Swain and I can bear witness to the fact that their stuff works. This is not to say that other coating services can’t provide the same level of performance. Examples of additional skilled coating services include Cerakote, Polydyn and Calico. Just do your research before deciding on which coating house to use.
When we consider a thermal barrier coating for piston domes, ceramic thermal barrier coatings are generally applied at a maximum thickness of about 0.002-inch. Typically, piston dome thermal barrier coatings feature a TBC-ceramic (thermal barrier coating ceramic). This is effective for any naturally aspirated application. Forced induction, including turbocharging, supercharging and nitrous applications often call for a higher resistance to heat. Many specialty coaters offer a dedicated ceramic coating for turbo pistons, with some generically calling this a “gold coat.” This is applied a bit thicker, at round 0.003-inch. The extreme temperature ceramic coating is specifically designed for the extreme temperatures that will be generated in a forced induction engine.
Another option to retain heat inside the turbocharger is to consider a thermal “wrap” that installs over the housing, capturing heat. This lowers underhood temperature as well, in addition to retaining heat in the exhaust for improved exhaust gas scavenging. DEI, as an example, offers thermal barrier wraps and covers for a multitude of exhaust components including turbochargers, downpipes, headers and exhaust manifolds. Forced induction raises the exhaust gas temperature. Managing this heat affects both component durability and exhaust scavenging. The more efficiently you can pull the exhaust out of the combustion process, the quicker more air can be packed in. Since a forced induction engine is creating this heat anyway, it just makes sense to take advantage of it by keeping it where it belongs…in the exhaust stream, instead of being soaked into the cylinder heads and pistons.
Granted, a forced induction system allows the engine of a given displacement to generate more power, but with that power comes a concern for the durability of various engine components. When “up on boost,” and with the engine cranking out its maximum power, the reciprocating and rotting assembly is subjected to additional abuse. This includes the pistons, piston rings, piston pins, connecting rods, connecting rod bolts, connecting rod bearings, crankshaft and main bearings, and the engine block’s main caps. Generating more power creates a domino effect that exposes these components to added stress. In a street driven engine that features minimal boost of 6 – 8 lbs or so, the concern is relatively minor, But when we begin to use boost above around 18 lbs, and start to really push the engine to generate maximum power, mechanical stresses can reach a point where piston domes fail, bearings take a beating, connecting rods have a greater potential for bending or cracking, crankshafts are more prone to failure and main caps are subjected to potential distortion or failure. For this reason, any serious forced induction build should consider forged or billet pistons, more durable rod and main bearings, forged connecting rods, forged crankshafts and steel billet main caps, along with high tensile strength connecting rod bolts, and higher-strength cylinder head and main cap fasteners.
HEADERS VS CAST IRON MANIFOLDS
In some cases, the choice between the use of cast iron exhaust manifolds and tubular headers may be dictated by the available space. The benefits of using cast manifolds include durability, since they’re stronger and should be less prone to cracking, and the fact that cast iron exhaust manifolds retain more heat, even without a thermal barrier coating, which allows the turbocharger to spool more quickly.
However, OE cast iron manifolds commonly offer a more restrictive exhaust gas path, which can be detrimental to horsepower. If you’re limiting the power boost with the use of a turbocharger by planning to generate no more than an additional 50 to 100 horsepower, the use of cast manifolds will likely be acceptable. If you plan to add more than 100 horsepower by virtue of your turbo setup, you really need to use tubular exhaust headers. On a V8 engine, a 2-inch diameter primary pipe size will likely be the best choice. In either case, whether you opt for cast manifolds or tubular headers, it’s advisable to have them coated with a ceramic thermal barrier coating for heat retention, which adds the benefit of lowering underhood temperatures.
EXHAUST PIPE DIAMETERS FOR FORCED INDUCTION
The installation of a supercharger does not require a larger-diameter exhaust pipe system, but horsepower and torque will benefit from increasing the pipe diameter, generally to the 3-inch-plus range on V8 engines. Ideally, the exhaust headers should feature 2-inch primary tubes, which can add around 10 horsepower and approximately 30 lb-ft of torque. Granted, free-flowing headers may slightly lower the boost, but that can be remedied by slightly raising the supercharger boost through a management, such as decreasing the pulley diameter to make the supercharger spin faster.
A turbocharged system, however, requires larger diameter exhaust piping, which is critical for both engine durability and the production of horsepower and torque. In general terms, a V8 engine that runs a single turbo in the range of 6 to 15 lbs of boost requires a minimum 3-inch exhaust, while a twin-turbo setup exhaust pipe requires only a 2.5-inch exhaust. As you increase boost, the exhaust pipe diameter needs to increase proportionately. Running an exhaust pipe that is larger than needed won’t hurt, but running a too-small exhaust will be detrimental to both power and the engine’s maximum RPM. A safe rule of thumb is to run turbo exhaust piping in as large a diameter as will practically fit in the available space along the chassis.
Also, for a turbocharged system, it’s best to run the same diameter exhaust pipe for the entire length, with no need to step up or down along the path.
TURBOCHARGER LOCATIONS
Since a supercharger is mechanically driven by the crankshaft, it’s mounting location necessitates that it is mounted to accommodate the engine’s front belt drive. The installation of a turbocharger, however, offers a degree of latitude. Since the turbo is driven by the engine’s exhaust gas, the turbocharger unit may be located wherever space permits, as long as exhaust piping and air charge piping can be accommodated. As a result, a turbo may be installed within the confines of the engine bay, or at the rear of the vehicle. A rear-mount turbo setup is often favored when dealing with a crowded and cramped engine bay. A typical rear-mount system will feature the exhaust pipes running to a point behind the rear axle, with the turbo(s) mounted in a convenient location (for example, at the location of the original rear muffler(s). A fresh air intake with an air filter is plumbed to the fresh air intake side of the turbo. The air charge plumbing, which delivers the compressed air charge to the engine, is then routed from the turbo up to the intercooler, continuing from the intercooler to the intake system.
A typical V8 rear-mount turbo system should feature 2-inch header primary tubes, 3-inch exhaust piping to the turbo, a larger 3.5-inch fresh air intake to the turbo, and 3-inch charge pipe exiting the turbo up to the intercooler.
Advantages of a rear-mount turbo include reduced underhood temperatures, as compared to an underhood installation, lower oil temperature through the turbo, and theoretically a superior pressure differential across the turbine, as the extended exhaust lowers the outlet pressure and in turn raises the turbo inlet pressure. Pressure differential is the relationship between inlet and outlet pressure at the turbo.
The exhaust gas inlet to the turbocharger is important, but not as critical as the air charge outlet from the turbo. If the air charge pipe is too small in diameter, this can result in excessive back pressure, which can lead to oil leakage, blowby, oil smoke exiting the exhaust when off-throttle, and will reduce both horsepower and maximum engine RPM.
INTERCOOLERS
A turbocharger or a supercharger generates heat to the pressurized air charge that’s being packed into the engine. An intercooler is simply a heat exchanger (radiator) that’s plumbed between the turbo or supercharger air outlet and the engine’s intake system. This heat exchanger lowers the air temperature entering the engine to create a more dense air charge. Lowering the air intake temperature (IAT) produces more power. How large should the intercooler be? A rule of thumb is to select as large an intercooler as will fit into the available space. This is one component decision that follows the “bigger is better” approach.
By applying a heat emitter coating to the intercooler and its air plumbing, we can improve heat transfer by approximately 5 – 10%. For instance, if the air running through the intercooler normally runs at around 200 degrees F, a properly applied heat emitter coating can pull an additional 20 degrees from the air charge to 180 degrees F.
PLUMBING A TURBO AIR CHARGE PIPE
The air charge piping that runs from the turbo outlet to the intercooler, and from the intercooler to the engine, will be plumbed in sections in order to facilitate assembly. At each connection of pipe-to-pipe, or pipe-to-intercooler, or pipe to intake, the connections must be secure to avoid accidental disconnection/leakage caused by separation of the connections. Commonly a high-temperature-resistant silicone hose will be used at each connection, secured by a band clamp or a wide T-bolt clamp. These silicone hose connectors are available in a variety of diameters and shapes, including straight, 45-degree and 90-degree bends. If a slipping or disconnection is possible, this will most likely occur at a straight connection. In order to provide additional insurance against accidental pop-off of the connection, a “boost brace” may be added. This is simply a steel, aluminum or stainless steel brace that connects mating pipes. This involves tack-welding a post to each pipe on each side of the hose connection. The posts are then secured together with a bolt-on or clip-on brace bar. Vibratech’s boost brace is a good example. Each post features a ball end, and each end of the brace bar features a spring-loaded female coupler that snaps onto each post ball. This is easily removed for pipe servicing.
Basically, any locking system that would prevent the air charge pipe connections from slipping loose is worthwhile. For instance, you could simply weld a tab onto each mating pipe, behind the silicone rubber sleeve locations, and then secure the two tabs together with a bolt and a locking nut (or by connecting the two tabs with a heavy spring), in order to provide a fail-safe connection that will prevent the tubes from walking out of the connection sleeves when the tube is under charge pressure.
The air charge pipe should be routed to avoid close contact with either potential rub locations and high heat locations. If the piping is close to a heat source, a thermal wrap may be added to the pipe in that location.
Read this article with all images in the digital issue of Engine Professional magazine https://engineprofessional.com/2020EPQ4/#p=14