On turbines, the A/R is just as important as turbine size. You should care about the area part of the equation because it dictates how well exhaust gases are evacuated by maintaining their velocity. Erring on the small side might get you a faster spool-up but can lead to reversion back into the chambers. Upsizing will yield some more ponies on the top-end but at the risk of a late spool-up or worse, no spool-up. The radius portion of the equation is important in that it influences turbine speed. Increasing the radius supplies the turbine shaft with additional torque for turning the compressor wheel. While this may seem like a sneaky way to stuff a larger compressor wheel in place, most turbine radiuses are fixed and experimentation lies mainly with turbine area. Perhaps the best way to select the proper A/R ratio is from experience. There really is no magic calculation here. Too many factors are involved including exhaust gas pressure, turbine inlet pressure and boost pressure.
The bottom line: There's no longer any reason to consider turbo sizing black magic. With a few compressor maps, a pencil and a calculator, there's just no excuse for hooking up that oversized, boost-lagging, beast of a turbo and feeling good about it. Despite what they say, size does matter.
How to plot points on a compressor map:
Calculate airflow for the X axis: measured in lb/min, airflow = (HP target) x (air/fuel ratio) x (BSFC/60) where BSFC is lbs fuel / (hp x hr) If you don't have access to BSFC data, you can plug in an estimated value between .50-.60.
Calculate volumetric efficiency = (actual CFM / theoretical CFM) x 100 where theoretical CFM = (RPM x CID) / 3456 and actual CFM = 90%, a good average number if you don't have access to measuring this. Multiply by .0610237 to convert cubic centimeter displacement into cubic inches.
Calculate manifold absolute pressure: map = [(airflow) x (639.6) x (460 + intake temperature)] / (VE) x (RPM/2) x (CID) Upwards of 100 degrees F is a good estimate for intercooled intake temps if you don't have access to measuring this. Plan on adding about 1psi to take into account pressure drops related to the intake and/or intercooler.
Calculate pressure ratio for the Y axis = (14.7 + map) / 14.7 If you're at sea level, 14.7psi will work. As elevation varies, this figure will need to be adjusted.
Note: Don't worry about the constants like 3456, 60 or 639.6, they're simply there to tidy things up and convert units.
VE = volumetric efficiency
BSFC = brake specific fuel consumption
HP = projected flywheel horsepower
CFM = cubic feet per minute
CID = cubic inch displacement
Other useful calculations:
Trim = (minor/major)squared x 100
Trim describes the relationship the minor diameter and major diameter of a compressor or turbine wheel shares with each another. The inducer, or minor end when referring to compressor wheels, is where the air enters. Air exits through the exducer portion.