Nowadays, more people use simple millimeter measurement to discuss about turbos. For example, for certain drag racing allowance limits the maximum turbo size mostly the dimension of the inducer of the compressor with this millimeter rating. Today, inducer of compressor wheel of most popular turbo is limited for certain competition such as for Hotrod-class platforms and Sport FWD is 72mm and 67mm for True Street competition is 67mm. In the other hand, the exducer of compressor wheel are also limit in some case, but generally, when referring to a millimeter racing is only for the inducer diameter of compressor wheel.

When considering the turbo size, Trim is another method. “Trim” expresses the relationship of the diameters between the exducer and inducer from turbine or compressor wheel, and also determines how much air can make one to move. It's derived by squaring the diameter of the wheel's inducer, dividing that value by the squared diameter of the exducer, multiplying by 100, and rounding up.

To understand the meaning of A/R is that the more housing will flow, the larger the A/R. Take the turbine housing of GTX3071R of Garrett for example, there are three different A/Rs: 0.63, 0.82 and 1.06.

The 1.06 A/R means better performance on a larger displacement engines or when using nitrous oxide in conjunction with turbo charging. at higher rpm by offering more area for exhaust gasses to collect inside the turbine housing, which mean better performance.

In contrast, the A/R 0.63 will funnel the exhaust gasses faster and tighter more effectively through/ via the turbine wheel which allow the compressor wheel especially on smaller-displacement engines or low-rpm performanc to spin faster, in order to generate more low-end power-great when less exhaust gas is collected through turbine.

Since the majority of compressor housing are linked to the trim of compressor wheel, the performance of compressor is insensitive to A/R in general.
 

Have you wonder why your turbo works so noisy? It might caused by the balancing problems of turbo. Under the high speed rotation, a small error in compressor end of turbo such as the errors on the shafts, compressor wheel, thrust collar or lock nut may lead to balancing problem which make turbo noisy and may cause the premature bearing failure.
There are two ways to correct the imbalance Cartridge by measuring the vibration response at the high speed, that is change the assembly position or remove the material from the nose (lock nut).
In this process, the turbo need to runs under the normal service operating speed which is around 100,000 to 200,000 rev/min according to the size of the wheel.