Feed the Beast: An Indispensable Introduction to Forced Induction

Does driving up even the mildest inclines make your truck work overtime? Do you avoid steep trails for fear of not making it up all the way? Do even subcompacts pass you by on the road with impunity? Don’t you wish your truck had more power? Well, look no further, there’s a fix for that. Adding a supercharger or turbocharger to your ride could be just the answer to your truck’s power-output woes.

The advantages of a forced-induction engine over a naturally aspirated one are no secret. There’s an unquestionably better power-to-weight ratio, it’s more efficient, and forced induction significantly improves performance at high altitudes.

An internal-combustion engine is basically a big air pump that takes in air at one end, and blows exhaust out of the other—with a byproduct of work, of course. An engine is considered “naturally-aspirated” if it operates using incoming air at normal atmospheric pressure (14.7 psi at sea level, or around one bar) and no external compressor. Therefore, there is generally a vacuum condition within the intake manifold of a naturally-aspirated engine. You can greatly improve the performance of a naturally-aspirated engine by adding forced induction to it.

Forced Induction

A forced-induction system, creates a denser air charge to push into the engine. This creates a positive pressure condition inside the manifold, making more oxygen available for combustion within the same volume of air. At that point, more fuel can be added to mix with the extra oxygen, creating more power but also doing so more efficiently.

A forced-induction compressor can be driven mechanically or by expelled exhaust gases. If it’s mechanically driven (usually by the crankshaft), it’s a supercharger. If it’s driven by exiting exhaust gases, then it’s a turbocharger. Whether we’re discussing petroleum or diesel-fed engines, the theory of forced induction remains the same: Air is driven into the engine, creating a positive pressure scenario (under boost) within the intake manifold, and greatly improving the volumetric efficiency of the engine.

 

Feel the Turbo Boost

A turbocharger is a compressor unit that is driven by hot expanding exhaust gases to create a denser air charge. It’s made up of two basic parts: A turbine and a compressor.

Turbocharging

The housings of the turbine and compressor are mated together, and the turbo unit is usually mounted directly on the end of an exhaust manifold or on a bracket fastened to the engine. The turbine and compressor wheel are spun by the exiting spent gases; converting gas pressure into mechanical energy. This gives turbochargers the ability to compensate for higher altitudes exceptionally well by spinning the turbine faster to account for thinner air.

Turbocharging systems use wastegates and blow-off valves to ensure safe operation for both the engine and the turbo. A wastegate regulates the exhaust gas flow that enters the turbine; this controls how much boosting is done (and how overboosting is inhibited). An in-cab boost controller can allow you to manually adjust boost levels on the fly, for more or less power.

A blow-off valve between the compressor and the engine is used to dump high volumes of excess compressed air when the throttle is closed. This venting of excess air pressure prevents potential turbo damage. Turbos require an external oil feed from the engine oil supply, but require no special maintenance, and will typically last as long as the engine does.

Because a turbocharger is driven by expelled exhaust gases, the boost will not kick in until the exhaust gases are sped up enough to start creating positive pressure with the compressor side of the turbo (this is known as turbo lag). When the system is not under boost conditions, there is no need to add extra fuel, so they are naturally more fuel efficient than superchargers.

Since there is no direct mechanical connection to the engine, there is also minute parasitic loss with a turbocharger; however, there is an increase in exhaust backpressure, which adds to pumping losses (far exceeded by the increase in power). Even though turbochargers are driven by hot exhaust gases, they generally have better adiabatic efficiency (the ability to compress air without adding excess heat to the air) than superchargers.

 

Oh, that Supercharger Whine

A supercharger is an air compressor or air blower that is driven mechanically by the crankshaft of the engine. This can be done via belt, shaft, gears or a chain. The most common method in automotive applications is a belt or a gear drive. Because the supercharger is directly connected to the engine, it is responsible for more parasitic power loss than a turbocharger. There are two ways a supercharger creates boost: through external compression or internal compression.

While trying not to overlook the wide array of supercharger design styles, nor dismiss any particular design as a copy of another, we’re going to say that there are three basic variations of superchargers available on the market: The roots (blower), the screw-type variant of a roots supercharger and the centrifugal supercharger.

A roots-type supercharger was the first of its kind. It is a positive-displacement unit that delivers almost the same volume of air per revolution at any and all speeds. Usually, manifold-mounted on top of the engine, it has a poorer adiabatic efficiency; plus, it suffers from heat soak and the impossibility of adding an intercooler.

A roots blower creates boost using a backflow that compresses incoming gases in the intake manifold. Although it’s less efficient than compressing the air inside the housing, the roots blower has an unmistakable look and sound, which will always be sought after.

There are a number of screw-type superchargers available, and though they may resemble a roots supercharger, there is one very clear difference: These internal compression units create pressure within their own cases, resulting in a large pressure difference between their inlets and outlets.

Centrifugal Force

Instead of lobed rotors, positive-displacement screw-type compressors use two meshing helical screws to compress the air and push it into the engine. Due to the mounted location of a screw-type compressor, no air-to-air intercooler is really possible—though some kits utilize a liquid-to-air intercooling system. Screw-type superchargers are more efficient that roots blowers, but less so than centrifugal compressors.

Lastly, a centrifugal supercharger is one that creates boost using centrifugal force to compress air and push it into the engine. It is usually mounted to the front of the engine with a bracket, and is driven by a belt spun by the crankshaft. Most often, it’s incorporated into the accessory drive-belt system.

Centrifugal units have a geared drive inside of them to spin the compressor at a much higher RPM than the crankshaft rotates. The velocity of the air is traded for pressure by using a diffuser to slow down air speeds, and give a highly pressurized charge of air out towards the engine.

This dynamic compressor is not limited by a fixed volume output, and pressure can increase as it is turned faster. This makes it a more efficient unit than a positive-displacement compressor or blower. Another advantage of the centrifugal supercharger is the ability to easily package it with an air-to-air intercooler.

 

All About that Power

Both superchargers and turbochargers can add significant amounts of power by increasing the volumetric efficiency of an engine. Volumetric efficiency is a measurement of how well an engine can move air and fuel in and out of the cylinders. The higher the volumetric efficiency, the higher the output power. This efficiency can be tweaked, too, depending on the amount of boost pressure applied. Both superchargers and turbochargers can be adjusted for more or less boost, increasing or decreasing the power advantage of the forced-induction addition.

But if you want to adjust boost levels on a supercharger, you’ll need to change the pulley size to adjust the supercharger’s ratio. They have a large power advantage over turbos at lower RPMs, though, at which point a supercharger’s boost comes on much sooner. A turbocharger can take a second to spool up, but it can make a more significant power increase while using less fuel. On top of that, turbo boost can be easily adjusted via an in-cab controller.

Lag Time

Of course, there is no contest when it comes to on-the-spot, throttle-punching power. A supercharger is king at making boost at low RPMs. There are a variety of turbo sizes and types of compound systems and variable geometry designs to try to minimize turbo lag, but there will always be a short duration before the engine produces enough exhaust gases to get the turbo spinning and boosting. This is known as turbo lag (there’s a reason no one calls it supercharger lag).

If you’re using your vehicle for some type of short-course competition, or are constantly on and off the throttle when you go out and play, then a supercharger is likely your best bet.

 

Ease of Installation

Generally, an aftermarket supercharger kit is going to be easier to install than any turbo kit, because it does not require the exhaust re-routing that an aftermarket turbo kit would. That being said, some supercharger kits that are manifold mounted instead of remotely mounted can require disassembly and reassembly of the top of the engine. The ease of installation and time taken to do so is always going to vary from kit to kit, but a centrifugal supercharger mounted on a provided engine bracket is usually going to be the easiest system to install.

 

Reliability & Longevity

Generally, we’d have to say that adding an aftermarket turbo or supercharger system to any engine intended to be naturally aspirated is going to lessen engine life. If the vehicle was built from the factory with forced induction and an engine equipped to handle it, then we don’t see any reason it wouldn’t live as long as a stock, naturally-aspirated engine.

As for the aftermarket kits themselves, you should expect a turbo kit to match the life of the engine. A supercharger kit will last a long time, but may eventually require some maintenance, due to its mechanically-driven design. Belts may wear out, or pulley bearings may need to be replaced, but for the most part, it should be reliable and self-contained.

 

Legality

Since we’re discussing street-driven vehicles for practical purposes, legality can certainly be an issue. Unless you’re living in some no-emission-check county, or dealing with a classic vehicle, you’re going to want an emissions-legal aftermarket system (usually tagged with a “50-state legal” label).

This is where superchargers really shine over turbochargers: It’s much easier to get a government-issued EO number (exemption order number) for a modification that goes inline before the intake of the engine, as opposed to one that goes inline between the engine and catalytic converter(s). That’s one big reason you see more street-legal supercharger kits for sale than turbocharger kits.

 

Looking to Have More Power?

Hit Up These Guys:

• Armageddon Turbo Systems
• Banks Power
• Edelbrock
• ProCharger
• Off Road Power Products
• Weiand

 

Pros & Cons of Compressor Kits

Supercharger	Turbocharger
PROS	PROS
Lots of legal kits	Longer service life
No lag time	In-cab adjustable boost
Makes boost at low RPM	More fuel efficient
No external oiling needed	Noise reduction (exhaust)
	More power potential
	Compensates for higher altitudes
CONS	CONS
Somewhat-fixed boost	Turbo lag
More parasitic loss than turbo	Harder to package (takes up more room)
Less adiabatic efficiency	Tougher to pass emissions legality
Less fuel-efficient	Boosted threshold
Constant boosting at higher RPMs	External oiling requirements

 

Keep It Cool

An Intercooler, also known as a charge air cooler, is a heat exchanger used to improve the thermal efficiency in a forced-induction system. They are often put inline between the engine and compressor unit (turbo or supercharger) to cool incoming air before it reaches the engine.

While they aren’t necessarily required in forced-induction setups, Intercoolers are a great idea if they can be added into the system. A byproduct of compressing air is heat, and an Intercooler helps dissipate that heat throughout a number of fins.

A cooler air charge is a denser air charge, packed with more oxygen molecules and allowing for a more efficient burn. Consequently, an Intercooler helps deliver a cooler, denser air charge and usually allows for an extra pound or two of positive boost pressure to be added over a non-Intercooled forced-induction system.

There are two basic types of Intercoolers: an air-to-air cooler (left) and an air-to-liquid cooler (right). Generally, an air-to-air Intercooler is ideal for street use, and this is why: While water has excellent (better than air) heat-exchanging capabilities, the coolant will eventually heat up as the engine reaches normal operating temperatures. So, the liquid you’re trying to use to cool the air charge is close to engine temperature in a closed-loop system.

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Editor’s Note: A version of this article first appeared in the Winter 2016 print issue of Tread Magazine.