Muscle Mustangs & Fast Fords
Forced Induction - Why Boost It?
Everything you've ever wanted to know about forced induction on your fast Ford.
In the simplest of terms, an engine is nothing more than an air pump. Air from the atmosphere is drawn through the throttle body (or carb), into the intake manifold, where it is mixed with fuel (this is known as the air/fuel ratio), before entering the combustion chamber. When the mixture is compressed and ignited, the energy released from the air/fuel mixture burning (and expanding rapidly) causes force to be applied to the pistons. The pistons apply force to the crankshaft, and torque is produced.
When relying on natural aspiration, air is drawn into the engine due to the pressure differential between the low pressure in the cylinder (created by the rapid down sweep of the pistons) and the higher (atmospheric) pressure in the manifold. Our atmosphere is roughly 14.7 psi, give or take, depending on the actual barometric pressure and the altitude where you happen to be standing.
So, basic theory tells us the more air an engine can consume, the more fuel it can burn (of course you have to maintain the efficient a/f ratio), and the more horsepower and torque it will ultimately produce. More air equals more fuel equals more power.
Because we rely on air pressure we are at the mercy of our location and Mother Nature to supply air, but in contrast, we can totally control fuel supply as deemed necessary. So, the tricky part is adding additional air.
Any time we modify an engine, adding air (improving airflow) is part of the goal. A new cold-air intake, for example, will generally flow more air and, in turn, pick up power. A bigger intake manifold? More air. Those new 325-cfm heads? Well, cfm is cubic feet per minute of airflow, which is exactly what we're after. Those headers you just installed? They help scavenge spent gasses from the exhaust ports and suck a little bit more air into the chamber through the intake valve, which creates more power!
But what happens if we've already optimized an engine with all of these parts? What happens when your all-motor combination is maxed out and you're flowing as much air as possible? Or you want a simple bolt-on worth big power? The answer? Force more air in with a power adder.
But how do we do that? Well, you add a supercharger or a turbocharger. As these devices acquire air at atmospheric pressure, compress it, and transfer that additional air into the engine, where it can be combined with extra fuel, and more power is made.
For starters, additional air forced into an engine is usually referred to as boost. Actually though, boost is the measure of additional air pressure (normally measured in pounds per square inch or psi) found within the intake tract (before it makes it to the combustion process). You can do this with a centrifugal supercharger, a positive displacement blower, a turbocharger, a pair of turbochargers, or any combination of the three. But to find the right one for you, it's important to first clearly define your goals (and budget) and then understand how each particular system works to benefit you and your engine.
Let's take a look.
Typically mounted on front of the engine in-line with the front accessory drive, these blowers are driven by a belt attached to the crankshaft. As the crankshaft rotates, the belt turns a pulley on the supercharger, which turns a set of internal "step-up" gears. These gears multiply the speed at which the impeller rotates (often to well over 100,000 rpm). Once the impeller is moving, it draws air in from the atmosphere and sends it into the supercharger's volute, where it's compressed before it's passed over a diffuser and delivered to the engine for consumption. Simply put, a centrifugal supercharger pulls air into itself and compresses it through centrifugal force before sending it through a series of pipes (and often an intercooler) to the throttle body (or carb). Centrifugal superchargers rely on centrifugal force to create high-pressure air within the intake tract.
Thermal efficiency: Centrifugal superchargers are inherently simple and efficient in both power consumption (how much power they take from the engine in order to operate) and thermal output (how hot the air is after it is compressed). The less heat they generate during compression, the lower the air intake temperatures, which results in increased power production and increased resistance to detonation. Plus, because the blower is not directly bolted to the intake, less heat is transferred to the engine from the blower itself.
Compact design: Modern centrifugal superchargers can be incredibly compact for their given output, and can fit within any modern engine bay rather easily. Because they mount in line with the front accessory drive instead of atop the engine, there is no need for an aftermarket hood, and many systems don't require the relocation or replacement of any OEM equipment.
Linear boost production: Driven by the engine's crankshaft, centrifugal superchargers often produce very linear boost curves, which makes them easy to drive on the street and simple to manage on the track. At low rpm, boost is minimal with big gains typically occurring above 3,000-3,500 rpm on a modular Mustang motor. This makes traction off the line easier to manage, with a big charge of power up top for large mph gains and a thrilling run to redline.
Linear boost production: Driven by the engine's crankshaft, centrifugal superchargers often produce very linear boost curves with very little airflow production at low- to mid-engine speeds. This results in a minimal amount of additional torque production during low and mid-rpm operation, which can feel "laggy" to drivers looking for a large dose of torque right off the line.
Belt slip: On high-horsepower applications where small-diameter supercharger pulleys are used, it is possible to run into belt-slip issues, in which the supercharger drive belt slips on the upper pulley, causing a drop in impeller rpm and a loss of boost. Modern belts, as well as sturdier brackets, increased tension, and thicker pulleys, have helped combat belt slip.
The rpm of the impeller is ultimately determined by engine rpm, the internal step-up gearing of the supercharger, and by the pulleys used on both the crankshaft and the supercharger itself. Adjustments are made by swapping pulleys of varying diameters, either on the crankshaft or on the supercharger itself, until maximum desired boost is reached. It's important to note that it's possible to exceed a centrifugal superchargers maximum rpm and doing so can potentially damage the unit.
Positive Displacement Superchargers
Most commonly mounted atop the engine in place of the factory intake manifold, positive displacement (PD) superchargers are driven by a belt connected to the crankshaft and generally proved a huge increase of tip-in throttle response and horsepower. Unlike a centrifugal supercharger, which compresses air by diffusing it, a positive displacement supercharger simply collects and delivers air by either a set of screws (twin-screw) or a pair of lobes (Roots- or TVS-type).
It's important to note that screw blowers compress air in the housing (using the actual screw lobes), whereas a Roots or TVS blower compresses air in the manifold. Either positive-displacement design delivers a fixed amount of air for every revolution, typically measured in liters, and because air delivery isn't rpm dependent, can produce significant increases in airflow at very low engine rpm.
For example, a 2.3-liter supercharger can deliver—you guessed it—2.3 liters of air for every revolution. This is fixed and—although some leakage between the rotors does occur—doesn't change with speed. PD blowers can create such instant torque as soon as you hit the gas because they provide excellent cylinder-filling capabilities at low rpm.
Low-rpm torque production: Both Roots-type and twin-screw PD superchargers work by moving a fixed volume of air per revolution. At low rpm, this airflow is significant, and forcing it into the combustion chambers results in a major increase in power production. If you're looking for stump-pulling, get-off-the-line power, a PD blower will do it.
Packaging: A positive displacement supercharger replaces the factory intake manifold on practically every late-model Ford, and doing so makes packaging very straightforward and simple. Thanks to the built-in air-to-water intercooler found on most OE and aftermarket systems, almost everything is contained in one area and is easy to install or remove for service.
Heat and efficiency: Positive displacement supercharging generates significant heat, both under the hood and within the compressed air being delivered to the combustion chambers, and this heat can be difficult to handle. Air-to-water intercooling is essential, but care must be taken to cool the unit properly between runs to maintain consistent power delivery. Some companies have devised cooling systems that will reduce inlet air temp, and we're seeing more technology in this area lately.
Clearance: The larger PD superchargers require the addition of an aftermarket cowl hood to clear the tall supercharger assembly. This adds cost to some systems and can take away from the desired look of your project. Or, it could be awesome if you've always wanted one.
Traction: Good luck.
The speed at which the internal rotors rotate is determined by the pulley ratio set by the upper and lower pulleys. Driven by a belt attached to the crankshaft, both the crank pulley (the lower) and the supercharger pulley (the upper) work together to set overall boost.
A turbocharger is technically a type of supercharger—engine exhaust gasses, instead of the engine's crankshaft, drive the impeller. Connected to an engine's exhaust manifolds through a series of pipes, a turbocharger features two wheels connected by a shaft, which spin at the same rpm. The turbine wheel is powered by the expansion of exhaust gasses across its blades, while the impeller wheel is tasked with grabbing fresh air and compressing it within the volute. By using exhaust gas instead of a set of engine-driven pulleys, turbochargers offer increased efficiency and unmatched adjustability, although they are significantly more complicated to plumb and install.
Efficiency: Turbochargers are both internally and externally efficient. Modern impeller wheel and compressor housing designs create significant airflow at moderate temperatures, which results in a cool, reliable air charge within the intake tract. And, since they are driven by spent exhaust gas instead of a crankshaft, turbochargers do not require as much power to be driven (there are some losses), which can result in a net addition of several hundred horsepower compared to a similarly sized supercharger.
Power delivery: Turbochargers are extremely controllable—power delivery, boost, and impeller speed can be controlled down to the tenth of a psi. This allows drivers, racers, and tuners to dial in exactly the power they need when they need it to deal with traction issues or driveability on the street.
Lag: Turbochargers are not driven by engine rpm and their output is not linear, which means an improperly sized turbocharger may take several thousand rpm to come up into boost. This results in a sluggish feeling at take-off. Properly sizing a set of turbochargers to an engine combination can easily solve the lag issue, although it requires a user to set realistic goals and chose a turbocharger wisely.
Packaging: A turbo system is complicated as there is a lot of piping—and the rerouting of the exhaust manifolds to the turbos—plus the intercooler and the inlet. This requires a significant amount of room under the hood of a modern Mustang. Plumbing can be difficult, and routing pipes—sometimes up to 5 inches in diameter—through a Mustang bay usually requires custom fabrication or removal of factory parts.
Cost: Turbocharged systems generally cost more than supercharged systems, if for no other reason than the amount of parts involved. You're essentially buying an exhaust system, a power adder, a boost control system, and an intercooler all at once, which can be tough on the wallet.
Adjusting turbocharger boost levels is done by opening or closing the wastegate. Plumbed inline with the turbocharger, the wastegate works by diverting exhaust gasses away from the turbocharger turbine wheel, which slows it down (or speeds it up) and increases or decreases the boost level. With aftermarket electronic boost controllers, drivers can adjust boost levels on the fly, which is great for a street/strip project, or one you want to turn it up on the weekends and back down during the week.
In typical aftermarket and/or stock forced-induction applications, there are two types of intercoolers used to pull heat away from the compressed air. Air-to-air intercoolers feature a front-mounted aluminum unit (typically behind the front grille, in front of the radiator) that flows compressed air through a set of tubes, which are chilled by a set of fins that are exposed to ambient air pressure.
Air-to-air intercooling is simple and great for street cars that have ample airflow over the front while driving at speed. Air-to-water intercoolers, on the other hand, transfer heat by flowing a coolant over a set of tubes that contain the compressed air. Many OEM applications ship from the factory with air-to-water units that utilize a front-mounted heat exchanger (think radiator) to keep the coolant at a normalized level. In race applications, the heat exchanger is typically removed and an ice/water mix is used to cool everything. Of course, once the ice melts and the water heats, the system can experience heat soak, which is not desirable.
On the big end, it was making like 15 pounds, and then the gate stuck wide open and it overspun the impeller. The turbo is shot and I think a piece of turbine is stuck in the front mount now." If these statements make sense to you, welcome to the boost issue of MM&FF. If it doesn't, fear not, we're here to get you up to speed (literally), and fill you in on all of the lingo and product that encompasses the boosted aftermarket game. In lay terms, we're going to explain everything boost. Once you get the hang of it, it's easy, and nothing says "a fun time" like comparing kPA logs over a cold one at the local hangout, or arguing with your buddies over the theoretical advantages of chemically intercooling your heat exchanger in the lanes…
Boost is the name of the game and, in the simplest of terms, represents any additional pressure present within the intake manifold. Typically measured in pounds per square inch (psi), boost pressure tells us how much additional pressure a supercharger or turbocharger is introducing to an engine at any given time. For example, 14.7 pounds of boost (or 14.7 psi) represents an additional atmosphere (since Mother Nature provides us with roughly 14.7 psi of pressure naturally at sea level) worth of air inside the intake manifold, which drastically increases the engine's volumetric efficiency and potential power output. Other methods of measuring boost include kPa (Kilopascal), Bar, or inHg, which are all different units of pressure that relate to the same thing.
14.5 psi = 100kPa = 1 bar = 29.53 inHg
A supercharger is simply an air compressor that is attached to an engine in order to increase the amount of air the engine is fed. There are two major types found in the aftermarket: centrifugal and positive-displacement superchargers. Centrifugal units are typically mounted to the front of the engine with a bracket, while positive displacement superchargers are usually attached directly to the cylinder heads, in place of the intake manifold.
Typically driven by a belt (although gear-driven units are becoming more frequent on race cars), a supercharger features an impeller wheel or rotors that draw in ambient air, compress it, and then transfers it to the intake manifold. Roots-, TVS-, or twin-screw superchargers rely on pulleys to drive them, whereas centrifugal blowers use pulleys combined with an internal transmission with a designated step-up ratio to drive the impeller. Adjusting the pulley ratios sets the rotor or impeller speed, and thusly the boost level.
A turbocharger is also an air compressor that delivers increased airflow to an engine, although instead of an engine's crankshaft, exhaust gasses drive it. Featuring two wheels connected by a central shaft (a compressor and a turbine wheel), a turbocharger relies on the expansion of exhaust gasses over its turbine to spin the compressor, which compresses fresh air and sends it to the intake manifold. By using exhaust gas instead of pulleys and a belt, turbochargers are very efficient. They rely on wastegates to control overall impeller speed and boost, which allows users to adjust target boost levels on the fly, without having to swap pulleys.
A blow-off valve is a pressure-relief valve found in the intake tract, which is mounted between the compressor (supercharged or turbocharged) and the throttle body (or carb). Fitted between the compressor and the throttle body (or carb), the blow-off valve's job, literally, is to blow off (or release) excess boost pressure trapped in the system when the throttle blade closes. Under normal driving conditions, the blow-off valve is closed and compressed air is contained in the charge piping, sending it through the open throttle blade and into the intake manifold. When the throttle is closed rapidly after a wide-open throttle run, the engine will still make boost; since the throttle is shut, the excess boost causes reversion in the intake, which can damage the turbine wheel or cause driveability problems. The blow-off valve senses a change in pressure (from above atmospheric to vacuum) and the valve is opened, releasing the compressed air out of the charge pipe and into the atmosphere. It also makes cool noises.
A wastegate is a device that diverts exhaust gas before it reaches the inlet of the turbocharger turbine housing. To fully understand the concept, let's review a turbo system without a wastegate.
As exhaust fills the manifold, it is directed toward the turbocharger and enters the turbine housing before exiting through the downpipe. In a closed system, the turbine would see all of the exhaust pressure/gasses throughout the engine's rpm range, and boost would continue to rise uncontrollably until either the throttle is shut or the turbine wheel reaches its choke point. For practically any engine, this creates an excessive amount of boost, and can ultimately destroy parts. To control boost and overall engine airflow, turbocharger systems rely on wastegates, which are mounted before the turbine housing (or inside of it in the case of an internally gated turbo) and act as a controlled bypass for a percentage of exhaust gas to regulate turbine speed and, thus, overall boost. Many wastegates are tunable so you can dial in the desired boost/performance level.
The hot side of a turbocharged system refers to any piping or component that circulates exhaust gas. Typically, the hot side of a system contains the exhaust manifolds, the manifold-to-turbocharger piping, and the turbocharger's downpipe. Because the hot side deals with extremely hot exhaust, it is typically built from stainless steel tubing (mild steel is also used in some budget-friendly systems) and should either be coated, wrapped, or routed in a way that reduces underhood temperatures. The hot side is hot—hence the name.
A downpipe connects to the turbochargers turbine outlet and transfers spent exhaust gas from the turbocharger to the atmosphere. On a typical street car, the downpipe will connect to a traditional exhaust system, and route spent gasses alongside the bottom of a Mustang and out the tailpipes. In race cars, the downpipe usually just dumps exhaust directly out of the engine bay or through a hole in the front fender. This looks cool and sounds awesome, but isn't always legal, so check the rulebook before cutting a 5-inch hole in your front bumper. Or don't and send us pictures! Either way, downpipes are normally constructed out of stainless steel (it's on the hot side!), although racers looking for the ultimate weight savings have experimented with aluminum downpipes.
As you can probably guess, the cold side (inlet) of a boosted system deals with any piping or component that circulates compressed air through the system. This includes any piping that feeds the inlet of the turbocharger or supercharger, any piping from the compressor side of the turbocharger or supercharger, the intercooler (if one is present), and any piping connecting to the throttle body or carburetor. Because temperatures on the cold side are relatively low (compared to the exhaust), the tubing is generally aluminum, which saves weight and efficiently transfers heat. The cold side of a system is usually held together with silicone couplers and clamps. If you start "missing some boost," check the couplers first.
Intercoolers provide a way to reduce the temperature of the inlet air after it has been compressed, but before it reaches the combustion chamber. Compressing air heats it (sometimes a lot)—and hot air is bad air, as any racer will tell you. The hotter the inlet air, the less dense it becomes (which is bad for performance since less oxygen is present), and the more prone the engine will be to detonation. This is an unfortunate part of the Ideal Gas Law (we'll save you the science lesson), but the heat created from compressing air must be dealt with if you're looking to optimize your boosted Ford. To cool compressed air, many forced-induction systems rely on an intercooler, which is simply a device similar to a radiator that transfers heat.
Air-to-air intercoolers are the most popular for turbocharged street cars, and rely on the transfer of cool air over the small aluminum fins to pull heat away from the compressed air within the tubes. Air-to-water intercoolers rely on a similar principle in that they pull heat from compressed air, although an air-to-water system uses water (or coolant) to enable the transfer of heat. Stock Ford systems found on SVT vehicles use an intercooler with a heat exchanger to reduce the heat in the intercooler cooling system.
Low Boost, Pump Gas, No Timing
This common phrase can be heard around any racer, tuner, dyno queen, or forum jockey, and it is meant to downplay the current performance of the car in question. "Yeah, and that's only on low boost, pump gas, and no timing—it's got a lot more in it!" This is usually followed by the owner/driver/tuner either actually turning it up and breaking something or leaving it exactly as is for the rest of time. Never trust this statement.