Muscle Mustangs & Fast Fords
Forced Induction - Why Boost It?
Everything you've ever wanted to know about forced induction on your fast Ford.
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.