Here we cover the definitions of some of the words used to describe boost behaviour and boost systems. If you don't know how to describe it, it's a lot harder to get someone to help you fix it.
Boost Level
When people state that their car is running (say) 15 psi of boost, they're referring to the maximum constant value that occurs at full throttle. Depending on the boost control system, the car might not achieve this level in some gears (eg first) and/or at some rpm (eg third gear at peak revs). However, if the boost pressure (measured in the plenum chamber, not at the turbo compressor or anywhere else!) reaches a peak of 15 psi and then holds it at that level for at least some time, the car is said to be running this much boost.
Wastegate
The wastegate is the valve that allows exhaust gases to bypass the exhaust turbine of the turbo. Bypass more gas (open the wastegate further) and the turbo will slow, reducing the amount of boost being developed. Close the wastegate, and more exhaust gas is forced through the turbine, increasing turbo speed and thus the boost level being developed. The easy rule to remember is that closing the wastegate increases boost. Wastegates are typically built into the turbo, but some cars (like the one pictured) use external wastegates.
Wastegate Actuator
The opening degree of the wastegate is controlled by a wastegate actuator. This device comprises a diaphragm backed by a spring. The spring holds the wastegate shut, while boost pressure deflects the diaphragm against the spring, so opening the wastegate. If a 7 psi spring is fitted to the wastegate, and the other side of the wastegate is connected straight to a source of boost pressure, the turbo will develop 7 psi boost. Lowering the pressure that the wastegate actuator sees (eg by a bleed) will mean that the wastegate doesn't open as far, causing in this example boost to be higher than 7 psi. Often the term 'wastegate' is used when the person really means 'wastegate actuator'.
Boost Control System
A boost control system is used to vary boost pressure. Factory turbo cars usually use an electronic system to do this, with boost level varied depending on engine coolant temp, the occurrence of detonation, throttle position and so on. When applied to a modified car, a boost control system is invariably designed to increase boost above the level set by the manufacturer.
Pneumatic Boost Control System
This type of system used passive valves through which air flows. The three DIY boost control systems covered in AutoSpeed are all of this type, though one does interface with the standard factory electronic control. A 'bleed' is an example of a pneumatic boost control system.
Electronic Boost Control System
This type of system still uses pneumatic air pressure to push against the wastegate diaphragm, but the amount of air allowed to flow out from the wastegate actuator pressure line is determined by an electronically-controlled valve. Good electronic systems use feedback, where the actual boost pressure is constantly measured.
Bleed
A bleed allows air to flow from the wastegate line, reducing the pressure seen by the wastegate actuator diaphragm, and so closing the wastegate over its normal opening. As a result, boost increases. While people happily place a lot of pneumatic boost control systems under the generic name 'bleed', there are huge and important operating differences between various pneumatic systems.
Wastegate Creep
This is the term given to the slow opening of the wastegate prior to the peak boost level occurring. For example, if the wastegate is at its maximum opening at 7 psi boost, in many cars at 3.5 psi boost it will already have started opening ('creeping'). Nearly all factory boost control systems allow this to occur so that boost does not come on in such a rush; wastegate creep allows much better driveability and gives a flatter torque curve. However, it also reduces acceleration over that which is possible without any creep. As the graph shows, it's the 'coming onto boost' part of the full-throttle curve where reduced creep makes a performance difference. However, with good boost control systems, the part-throttle behaviour can also be very much changed if wastegate creep is reduced or eliminated.
Boost Spike
A boost spike is a sudden overshoot from the designated boost pressure. For example, if boost is set to 15 psi but during hard acceleration rises for 1 second to 20 psi before again dropping back again to 15 psi, a spike has occurred. Boost spikes that occur when the engine is coming on boost are common in poorly set-up bleed systems.
OverBoost
An overboost is a short-term increased boost level that rises above the constant level. It is the same as a spike except for two aspects - it occurs for a longer period (eg 3 seconds) and it is a desired feature of the control system! Boost control systems (both electronic and pneumatic) can be set up to allow short-term overboosting in some driving situations. An uncontrolled and unwanted excessive boost level should not be called an overboost. This graph shows the boost behaviour in a car where overboost is triggered only by full throttle. Some aftermarket systems call overboost 'scramble'. (No, I don't know why... eggs maybe?)
Boost Fall-Off
This occurs when the boost level reaches a peak (eg at max torque) and then gradually falls in level through the rest of the rev range. This can occur in pneumatic systems (which don't have any feedback loop) as compressor efficiency drops off or pressure inadvertently builds again in the wastegate line because of control problems.
Surging Boost (full throttle)
If the boost at full throttle constantly varies up and down (eg by a few psi) the car is said to have a full-boost surge problem. This is common in poorly set-up or badly designed electronic boost control systems featuring feedback. It is easily felt on the road.
Surging Boost (part throttle)
While similar in name, this is a quite different characteristic, where maximum boost is available on small throttle openings. It is a characteristic of most aftermarket electronic boost controllers, a few factory electronic boost controllers, and just a very few aftermarket pneumatic boost controls. It is achieved by reducing wastegate creep, and so this characteristic is a trade-off between excellent part throttle torque and a resulting lack of fine throttle control.
Boost Cut
A Boost Cut is actually either a fuel or ignition cut, triggered as a result of exceeding the factory boost limit.
A device (either electronic or pneumatic) which overcomes the Boost Cut. Both types work by reducing the boost or airflow levels seen by the ECU.
Blow Off Valve
A valve designed to relieve any pressure build-up between the compressor and the throttle body that occurs when the throttle is sharply closed. All original equipment valves vent this air back into the intake system, while most aftermarket valves vent it into the atmosphere with a pssshst! It is only relevant to boost control if the valve is designed to leak under heightened boost pressure - then it becomes part of the boost regulation system.
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