Try saying that title 5 times fast. Sweet...
I recently got some PMs about turbo specs, like AR, trim, etc and how they affect performance. So I'm going to write up one of my technical novels here so every can benefit from the info and ask questions, or add to my explanation.
Once you have decided what basic type of turbo you are gong to run, you need to firgure out the specifics. What I mean by that is, are you going to run a Mitsu turbo, a Garret turbo, or a combination of the two? The center section of the turbo you choose will determine what oil and water lines you need to make it work. A Mitsu turbo center section will bolt right on to a 1g, or a 2g using the feed line from all of the installation kits to add mitsu turbos to 2Gs. A garret center section will require oil/water line kit that goes with your turbo and car.
Once you know what center section the turbo will have, there is still compressor wheel, compressor housing, turbine wheel, and turbine houisng. What you choose here will DRASTICALLY affect the power potential of the turbo, as well as the lag, and what supporting mods you need to get for this turbo, which is VERY important.
First we'll look at compressor wheels. The trim people often quote for the wheel is NOT the size of the wheel. It is simply the ratio of the quare of the size inducer (smaller diameter, that you see in the inlet) to the size of the exducer (the larger diameter, you can only see this when the wheel is not in a housing), times 100. So its inducer squared, divided by exducer squared, times 100. So a wheel that measure 50mm on the inducer and 65 mm on the exducer is a "59 trim." The trim of the wheel determines the flow and boost characteristics of the wheel. One tirm may be happy running high boost, but doesnt want to move a ton of air, while another trim may like to move a ton of air at low boost. Keep in mind that you can take one compressor wheel, double the size of the inducer and exducer, and the trim doesnt change, but one wheel is obviously much larger than the other.
Typical Mitsu wheels are the 9b, 13g, 14b, 15g, 16g, big16g, EVOIII 16g, 17c, 18g, 19c, 20g. The simplest way to explain the system is the number is the basic size, and the letter is the "trim." For example, the 15b, 15c, and 15g are the same basic size wheel, but the trim is different between them. For the TO4E wheels used in the T3/T4 turbos, you have the 40, 46, 50, 54, 47, 60, and 60-1 trims, in asending order.
The compressor housing will have a rating in "A/R." This is the ratio of the area of the nozzle to the radius of the volute, or some part of the housing. I'm not sure. Someone can look this up. The only thing you need to remember about AR is a lower number will have less lag but less flow capability, and a larger number means more lag, but more flow (and therefore more power) capacity. Typical housings are the Mitsu tdo5h found on the 14b through 19c size turbos, the tdo6 found on the 20g, and the "cyclone" cover found on the red, green, M50/60, etc. I'm not sure anyone knows the actual ARs of those housings, but they are in order from smallest to largest. For garret, you have the housng found on the t3/t4 steups. The compressor side is the T4 side. So you have the TO4B, TO4E, and TO4S (also called TS04), again from smallest to largest. I dont know the ARs of those housings, but the TSO4 is .70, which is considered very large.
The turbine housings are also rated in AR. Mitsu uses the 5cm (13g turbo found on 1g auto tranny cars), 6cm (found on 14b turbos from 5spd 1g), 7cm (found on 16g and up, used on reds that run into the nines), and 8cm housings found on large turbos. There are other sizes, but htey are rarely used. The 6cm equates to a .41 AR, the 7cm is a .49 AR, and the 8cm is a .57. Note how small these are compared to the garret housings. Typical T3 housings (the exhaust side of the turbo is the T3 in the T3/T4 combo) are the .48 and .63 housings. .63 is known to spool very well on our cars with large turbos, while the .82 is going to lag a bit more. There are other sizes as well. Its been said that the only way the mitsu turbos get away with such small housings is because thier turbine wheels are so good. Who knows.
Typical Mitsu turbine wheels are the TDO4 (13g), TDO5h (14b to 19x, custom 20Gs), TDO6 (20g), and TDO6H (20g). There is also the TDO7 25g stuff but we never use it. For the garret T3 wheels found in the T3/T4 turbos, there is the Stage 1 (also called "standard"), Stage 2, Stage 3, and Stage 5. Stage 3 is the most popular wheel used with DSMs, with stage 5 being too laggy or most to use. You will very rarely see a T4 turbine side on a DSM, but there is a whole slew of other wheels and housings in the T4 turbine family.
So the key points to take away from this:
1) Big AR flows more but lags mores, small AR is vice versa
2) Trim is not the size of the wheel, but how it will behave
3) T3 is the exhaust side of the turbo, to reduce lag. T4 is the compressor side for big flow
4) These parts can all be mixed and matched as you want to. This is why some turbos work, and other dont. Its all about a matched system.
5) A small turbine side will spool well, but will choke a large compressor side. A large turbine side will flow more, but will lag more, espeically on large compressors. It has to be matched.
6) The type of turbo combination you choose must not only be well matched in itself, but to your needs as well.
7) You MUST support the turbo you choose. What injectors, pump, IC, etc you use are ALL determined by what turbo you are running.
8) Look for a compressor wheels flow rate in Lbs/min. Multiply this number by 10 to get the horsepower capacity of that wheel. A 50 trim wheel moves up to 50 lbs/min, and well tuned will make about 500 hp. Etc.
9) Getting a turbo too large for your goals will lag more, but allow room to grow. A turbo too small will perform well but you may be wanting more before long. Its a tough balance.
10) as with anything else in life and physics, there is no free lunch, its all a comrpomise. Choose your weapon wisely.
This is a very basic overview. Feel free to ask more questions to get into some more of the specifics.