Terminology

[Sluttypatton]

Hopefully this will help save time answering simple questions about terminology. Don't expect anything too complicated or in depth here, the purpose of this list is to briefly explain some of the more common terminology regarding turbocharging. Much of this terminology also applies to superchargers and other aspects of tuning, particularly centrifugal superchargers. Hopefully this helps answer some questions. I am open to any suggestions and will add on to this if anyone has anything else to add. If there are any mistakes, please point them out to me.

Terminology

Turbocharger – An exhaust driven centrifugal supercharger. A turbocharger is technically a type of supercharger.

Boost - Pressure inside the intake tract, above the ambient atmospheric pressure.

Vacuum – Pressure inside the intact tract, below the ambient atmospheric pressure.

PSI - This is a term meaning "Pounds per square inch", and is a measure of pressure. When someone quotes pressure in PSI, they are likely referring to PSIG.

PSIG – A pressure measurement in PSI, indicating a gauge measurement, and therefore boost.

PSIA – A pressure measurement in PSI taking into account ambient atmospheric pressure. PSIA is an absolute pressure scale.

Pressure ratio – (Boost + atmospheric pressure / atmospheric pressure). A pressure ratio of 1.2 is 0.2 times higher than atmospheric pressure. PR is quite often used when reading compressor maps. To find PSIA from PR multiply atmospheric pressure by the PR.

Compressor map - A chart detailing the characteristics of a particular compressor. It includes such information as surge point, choke point, compressor efficiency, compressor speed in relation to PR. This chart is extremely valuable when sizing a turbocharger for a particular vehicle.

Blow Off Valve (BOV) - A device used to vent the pressure that builds in the intake piping between the compressor and the throttle plate when the throttle plate closes. The reason for this is to avoid "compressor surge", a nasty condition where the air flow reverses direction through the compressor and rapidly oscillates due to air pressure being greater at the outlet side of the compressor, this condition can cause damage to the turbocharger. The BOV's distinguishing feature that sets it apart from it's cousin, the "Bypass valve", is the fact that it vents the pressure build up to the atmosphere, rather than recirculating it.

Bypass Valve - Similar in purpose to the BOV, this device recirculates boost buildup to before the compressor, so that the work done by the compressor to compress that air in the first place is not wasted.

Boost controller - A general term for a wide variety of mechanical and electronic devices which allow you to modify the original boost level by fooling the wastegate actuator into seeing less boost than originally intended so that it opens later. These do not allow you to lower the boost level past the actuator spring load, and therefore are only useful for raising the boost level, not lowering it.

Turbo timer - A device that lets the vehicle idle for a preset time after the key is removed from the ignition, in order to allow the turbocharger to cool down and more importantly, the oil to cool, so coked bearings can be avoided.

Coked bearings - Less common today than yesterday as a result of advances, this happens when oil breaks down in the bearings that support the spindle, leaving a residue than builds up and can be catastrophic to the life of your turbocharger. Water-cooled turbochargers greatly reduce the chances of coked bearings.

Pop off valve - A poor method of boost control, using a sort of radiator cap to relieve pressure from the intake tract when it reaches the point where it can overcome the spring load. Useful as a failsafe in case of wastegate failure, but has no place as your primary boost control.

Wastegate - The wastegate assembly is a multi part device, used to control boost levels by limiting the exhaust energy to the turbine. The wastegate actuator recieves pressure from a signal line that is hooked up to the intake plumbing somewhere after the compressor. When the signal pressure is great enough to overcome the spring preload in the actuator, it opens the wastegate flapper door in the turbine housing, which allows exhaust to bypass the turbine, thus controlling turbine speed and boost levels. This is referring to the internal type of wastegate.

Spindle - This is the shaft that connects the compressor wheel to the turbine wheel.

Compressor - The compressor wheel is the bladed wheel on the compressor side of the turbocharger which catches the incoming air in it's cells and due to the spinning motion transmitted by the spindle, throws the air towards the compressor housing walls. This centrifugal force is what is responsible for the compression of the air. This is the reason turbochargers are called "centrifugal compressors".

Turbine - A bladed wheel that converts the exhaust energy to motion, which is then transmitted through the spindle to the compressor wheel.

Compressor housing - The snail shaped housing on the compressor side of the turbocharger.

Turbine housing - The other, usually rusted, snail shaped housing on the turbine side of the turbocharger.

A/R ratio – (Cross sectional area / distance from the center of that cross section to the spindle). This ratio affects the characteristics of the turbocharger profoundly and pertains to the turbine housing design.

Intercooler - A heat sink that removes heat from the charge air, increasing its density and reducing the likelihood of detonation as a result of excess air temperatures. There are two types of intercooler; air to air, and air to water. This has to do with the cooling media, in the case of the air to air, heat is transferred to ambient air, in the case of the air to water, it is transferred to water (or some type of cooling liquid). Think of the intercooler as a radiator for your intake air. Regardless of what some people might tell you, an intercooler and an aftercooler are the same thing, just with different names.

Detonation – Abnormal combustion, also known as knock or ping. Avoid at all costs.

Lean – A condition where not enough fuel is present for the normal operating conditions of the motor. A lean mixture promotes detonation.

Rich – Opposite of lean.

Blow-through – A setup where the turbocharger is placed before the throttle. The throttle is on the outlet side of the compressor.

Draw-through – A setup where the turbocharger is placed after the throttle. The throttle is on the inlet side of the compressor.

[Polygon]

Very good post. I believe it deserves a sticky.

[pro_am]

Thanks for getting that straightened out. I have no idea about much, or any of that stuff. Not much into forced induction, but more knowledge is always good to have.
Jake

[SaabJohan]

Impeller - the compressor wheel, sometimes it means the rear part of the compressor wheel where the flow is radial.

COMPRESSOR
Inducer - the small end (inlet) of the compressor wheel
Exducer - the large end (outlet) of the compressor wheel
Trim - a ratio between inducer and exducer, inducer^2/exducer^2*100

TURBINE
Inducer - the large end (inlet) of the turbine
Exducer - the small end (outlet) of the turbine
Trim - a ratio between inducer and exducer, exducer^2/inducer^2*100

Larger trim ratios are suitable for higher expansion ratios (aka turbine pressure ratios) and higher compression ratios.

[Sluttypatton]

Good additions SJ, I completely forgot those.

[Mojo_stevo]

Slutty, I need little more help with that A/R thing. What do you mean by "cross section". Do you mean like from the edge of the blade to the spindle? I read through the 'turbo tech', but I am missing something with the A/R formula. Oh and off topic: what's a "sticky"?

[tom turbo]

Well the A/R definition is not accurate. If you really want to know, the most complete definition of A/R ever published is in a new book titled "TURBO", by a Jay K. Miller. A friend of mine just got it and it rocks!

[wade623]

isnt another reason for a blow off valve to keep the presure from forcing the throttle plate open thus keeping the engine from slowing down?

[tom turbo]

No not really. If the throttle plate blows open from boost you've got a poorly designed throttle valve. The piviot point on a throttle valve will typically be levered such that it is practically "pressure nutral". Any off-center should be well overcome by the throttle spring. The Blow-off valve serves two primary purposes, the first being the off-loading of the pressurized column of air when the throttle valve is immediately closed thus preventing the compressor from entering surge, the second is that it helps to off-load the compressor and actually keep the rotor assembly speed at a higher RPM for a lower cycle speed once you're back into the throttle such as coming out of a curve. If the compressor goes into surge your turbo has gone way into a positive differential situation and thrust bearing failure will occur soon if the condition persists. Whe the compressor surges, the rotor assembly instantaneously stops and restarts again as the column of air losses its adheasion to the pressure side of the blades. Once the pressure column is relieved the turbine energy spools the rotor group back up to speed and the process happems all over again as the rotor speed quickly drops and hammers the thrust bearing in the process.