It's important to note here that, contrary to popular belief, compressor size affects turbo lag little. Lag is mostly associated with the speed at which the shaft spins, in other words, the turbine wheel. But that doesn't mean there aren't consequences from an overzealous compressor selection. As compressor size increases, efficiency drops and heat rises, never a good thing for performance. Sure, 10psi is 10psi no matter the size of the turbo, but while air quantity may be equal, air quality differs between varying turbos, as will power. As a turbo loses efficiency, it produces less dense air which, in turn, yields less volume of incoming air for the engine to inhale.
Surge
There are two places you don't want to end up on a compressor map: the choke point and the surge point. Points to the right of the choke line represent the lowest efficiency points, excessive shaft speeds and mean a larger compressor is in order; pretty straightforward. Points to the left of the surge line are bad news and require further explanation.
You'll recall we mentioned that compressor maps may reveal numerous candidates fit for the job. Selecting the most efficient one will also be the one with the lowest surge airflow limit. In case you didn't know, surge is bad and its consequences range from a slight power loss to severe bucking and jerking. This occurs when the engine is unable to inhale what the compressor wants to feed it. As a result, air backs up in the intake tract and inside the compressor, in turn, wielding itself furiously against the compressor wheel - not a good thing for those turbo thrust washers. Surge may sometimes be identified by a chattering sound mistaken by many a newbie for a blow-off valve. As far as compressor maps are concerned, be sure and select a compressor that stays to the right of the map's surge line.
Trim
Trim is the relationship shared between the minor and major diameter of a compressor or turbine wheel. On the compressor side, these are referred to as the inducer and exducer, respectively and vice versa for the turbine. Its calculation is simple and its results influence a turbo's flow characteristics. For the most part, larger trims equal more flow, assuming all else remains constant. As expected, there are tradeoffs as far as trim selection is concerned. When dealing with compressor wheels, larger wheels are generally less efficient. The solution is to increase trim size without increasing the overall diameter of the wheel. As for turbine wheels, bigger trims will reduce ackpressure at the expense of increased spool time.
A/R
A/R ratios are ways of further sub-classifying compressors as well as turbine housings by offering differing flow characteristics within equally sized housings. The A/R is calculated by dividing the inlet (compressor) or discharge (turbine) diameter's cross-sectional area by the distance between the center of the wheel's shaft and the center of the measured inlet or discharge area. When calculated correctly, the A/R ratio will remain constant throughout the housing. Varying the A/R affects compressor performance little but the same cannot be said of turbine housings.
Turbine
sizing While the compressor brings the air in, it's the turbine that actually powers the compressor wheel through a shared shaft, but you knew that so let's get back to sizing. Because of this relationship, smaller turbine wheels spin the compressor wheel faster and vice versa. While keeping that compressor wheel spinning fast can be a good thing, small turbine wheels/housings can prohibit exhaust gas flow, causing buildup between the combustion chamber and the turbine, also known as backpressure. Despite all this, there's some leeway here on turbine sizing, unlike the finicky nature of compressors.
There are two things to consider when selecting a turbine: size and A/R. In most cases, turbine sizing is dependent upon its exducer bore size; a larger bore will potentially yield more power. The key is keeping the turbine wheel diameter within 15 percent of the compressor wheel's. This 15-percent relationship is clearly illustrated in many of the popular T3/T4 combinations that have proven time and again to work well on small-displacement 4-cylinders.