Generic Definitions

A/R
· A/R describes a geometric property of all compressor and turbine housings. Increasing compressor A/R optimizes the performance for low boost applications. Changing turbine A/R has many effects. By going to a larger turbine A/R, the turbo comes up on boost at a higher engine speed, the flow capacity of the turbine is increased and less flow is wastegated, there is less engine backpressure, and engine volumetric efficiency is increased resulting in more overall power

Choke Line
· The choke line is on the right hand side of a compressor map and represents the flow limit. When a turbocharger is run deep into choke, turbo speeds will increase dramatically while compressor efficiency will plunge (very high compressor outlet temps), and turbo durability will be compromised.

CHRA
· Center housing rotating assembly - The CHRA includes a complete turbocharger minus the compressor, turbine housing, and actuator.

Clipper Turbine Wheels
· When an angle is machined on the turbine wheel exducer (outlet side), the wheel is said to be 'clipped'. Clipping causes a minor increase in the wheel's flow capability, however, it dramatically lowers the turbo efficiency. This reduction causes the turbo to come up on boost at a later engine speed (increased turbo lag). High performance applications should never use a clipped turbine wheel. All Garrett GT turbos use modern unclipped wheels.

Corrected Airflow
· Represents the corrected mass flow rate of air, taking into account air density (ambient temperature and pressure)

Example:
Air Temperature (Air Temp) - 60°F
Barometric Pressure (Baro) - 14.7 psi
Engine air consumption (Actual Flow) = 50 lb/min

Corrected Flow= Actual Flow Ö([Air Temp+460]/545)
                                      Baro/13.95

Corrected Flow= 50*Ö([60+460]/545) = 46.3 lb/min
                               14.7/13.95

Efficiency Contours
· The efficiency contours depict the regional efficiency of the compressor set. This efficiency is simply the percentage of turbo shaft power that converts to actual air compression. When sizing a turbo, it is important to maintain the proposed lugline with a high efficiency range on the map.

Free-Float
· A free floating turbocharger has no wastegate device. This turbocharger can't control its own boost levels. For performance applications, the user must install an external wastegate.

GT
· The GT designation refers to Garrett's state-of-the-art turbocharger line. All GT turbos use modern compressor and turbine aerodynamics which represent huge efficiency improvements over the old T2, T3, T3/T4, T04 products. The net result is increased durability, higher boost, and more engine power over the old product line.

On-Center Turbine Housings
· On-center turbine housings refer to an outdated style of turbine housing with a centered turbine inlet pad. The inlet pad is centered on the turbo's axis of rotation instead of being tangentially located. Using an on-center housing will significantly lower the turbine's efficiency. This results in increased turbo lag, more backpressure, lower engine volumetric efficiency, and less overall engine power. No Garrett OEM's use on-center housings.

Pressure Ratio
· Ratio of absolute outlet pressure divided by absolute inlet pressure

Example:
Intake manifold pressure (Boost) = 12 psi
Pressure drop, intercooler (DPIntercooler) = 2 psi
Pressure drop, air filter (DPAir Filter) = 0.5 psi
Atmosphere (Atmos) = 14.7 psi at sea level

PR= Boost +DPIntercooler+ Atmos
                Atmos-DPAir Filter

PR= 12+2+14.7 = 2.02
          14.7-.5

Surge Line
· The surge region, located on the left hand side of the compressor map, is an area of flow instability typically caused by compressor inducer stall. The turbo should be sized so that the engine does not operate in the surge range. When turbochargers operate in surge for long periods of time, bearing failures may occur.
Trim
· Trim is an area ratio used to describe both turbine and compressor wheels. Trim is calculated using the inducer and exducer diameters. As trim is increased, the wheel can support more air/gas flow.

Wastegate
· A wastegated turbocharger includes an integral device to limit turbo boost. This consists of a pneumatic actuator connected to a valve assembly mounted inside the turbine housing. By connecting the pneumatic actuator to boost pressure, the turbo is able to limit its maximum boost output. The net result is increased durability, quicker time to boost, and adjustability of boost.

How to Read a Compressor Map

Parts of the Compressor Map: The compressor map is a graph that describes a particular compressor’s performance characteristics, including efficiency, mass flow range, boost pressure capability, and turbo speed. Shown below is a figure that identifies aspects of a typical compressor map:

Pressure Ratio

Pressure Ratio ( ) is defined as the Absolute outlet pressure divided by the Absolute inlet pressure.

Where:
- = Pressure Ratio
- P2c = Compressor Discharge Pressure
- P1c = Compressor Inlet Pressure
It is important to use units of Absolute Pressure for both P1c and P2c. Remember that Absolute Pressure at sea level is 14.7 psia (in units of psia, the a refers to “absolute”). This is referred to as standard atmospheric pressure at standard conditions.
Gauge Pressure (in units of psig, the g refers to “gauge”) measures the pressure above atmospheric, so a gauge pressure reading at atmospheric conditions will read zero. Boost gauges measure the manifold pressure relative to atmospheric pressure, and thus are measuring Gauge Pressure. This is important when determining P2c. For example, a reading of 12 psig on a boost gauge means that the air pressure in the manifold is 12 psi above atmospheric pressure. For a day at standard atmospheric conditions,

12 psig + 14.7 psia = 26.7 psi absolute pressure in the manifold

The pressure ratio at this condition can now be calculated:

26.7 psia / 14.7 psia = 1.82
However, this assumes there is no adverse impact of the air filter assembly at the compressor inlet.
In determining pressure ratio, the absolute pressure at the compressor inlet (P2c) is often LESS than the ambient pressure, especially at high load. Why is this? Any restriction (caused by the air filter or restrictive ducting) will result in a “depression,” or pressure loss, upstream of the compressor that needs to be accounted for when determining pressure ratio. This depression can be 1 psig or more on some intake systems. In this case P1c on a standard day is:

14.7psia – 1 psig = 13.7 psia at compressor inlet
Taking into account the 1 psig intake depression, the pressure ratio is now:

(12 psig + 14.7 psia) / 13.7 psia = 1.95.
That’s great, but what if you’re not at sea level? In this case, simply substitute the actual atmospheric pressure in place of the 14.7 psi in the equations above to give a more accurate calculation. At higher elevations, this can have a significant effect on pressure ratio.

As you can see in the above examples, pressure ratio depends on a lot more than just boost.