I've noticed a lot of questions about 'What turbo should I get?' followed by 'Why should I get this turbo?'. In most of these posts I keep seeing people focus on the A/R ratio and skipping over the compressor and turbine size and it's importance. So I did some more research and browsed through the Turbo Education thread but still didn't find any good explanations of the wheels and the importance of the size of Inducer and Exducer's. So here is the information I've used to help compare turbo's and determine what turbo is going to deliver what I want.
First some basic terms. (I know some of this has been covered before but I hate having to bounce between threads to look up things.)
A/R Ratio:
A = (Area)Housing Inlet Diameter
R = (Radius)Distance from the center of the Turbine to Center of Housing Inlet
A divided by R = A/R Ratio
A smaller A/R will spool faster but at the cost of high RPM power.
A larger A/R will spool later but provide a better high RPM power band.
A/R can only be cross compared between same housings, T4 to T4, T31 to T31, etc.
Trim:
Trim is a subjective measurement of the Exducer squared divided by the Inducer squared.
Knowing what trim a turbo is doesn't mean as much as knowing the size of the Inducer and Exducer. For ease of explanation I am going to only focus on the compressor side of the turbo.
The Inducer swallows the air into the turbo. The larger the Inducer the more air that can be swallowed, however, the more potential for higher lag. The more air it swallows the harder the turbo has to work.
The Exducer sends the air into the volute(snail) compressing it simultaneously. The larger the exducer the faster the air gets compressed into the volute.
In essence Increasing the size of the Exducer while keeping the same size Inducer can decrease spool up time. This is because the tipspeed of the the larger Exducer is faster for the same shaft speed as the smaller Exducer. The faster tipspeed also increases the force with which the air is being pushed into the volute. This leads us to the diminishing return phenomenon. In short you can't put a massive Exducer with a small as hell Inducer and expect it to perform with any effeciency.
We'll use the SCM3231R, SCM3431R and the SCM4431R from PTE with a T04E compressor housing to help illustrate based on an average hp goal of 533hp. These 3 turbos have the same Turbine wheel(as illustrated by the 31 in their title) and slightly different Compresser wheels(as illustrated by the 32, 34 and 44). The "R" means the turbos are Ball Bearing.
(The turbine trim, turbine housing A/R, portwork, Journal or Ball Bearing centers and internal or external wastegate, etc. will also affect spool times. That is why I chose 3 turbos with the same Turbine specs and centers.)
The dimensions of the Compressor wheels are as follows:
SCM32
Inducer: 2.170"
Exducer: 2.950"
Trim:54
490hp
SCM34
Inducer:2.230"
Exducer:2.950"
Trim:57
545hp
SCM44
Inducer: 2.290"
Exducer: 2.950"
Trim: 60
565hp
(Horsepower given is an average output based on safe operating hp. All turbo's have minimum and maximum effective hp ranges.)
You can relatively figure that the SCM32 will spool a bit faster than the SCM44. Spool rates vary based on level of supporting mods, the A/R of the housing used whether it be .48 or .63 for these turbo's, and Turbine Trim and A/R.
For my application I am aiming for a hp around 500awhp. I plan on going to the track frequently but also want to dominate those pesky S/C V8's and NO2 Honda's.
This is where the Trim comes into play. A larger ratio between the Inducer and Exducer leads to the turbo performing better at higher boost levels with lower flow. They also make peak boost quicker(not sooner) and more abruptly. A lower ratio leads to the turbo spooling sooner and at a more linear curve.
For my application, based on just these 3 turbos, I would choose the SCM3431.
I know I did not cover the Turbine wheel and housing. This is because I have not learned enough about them to explain their effect on the overall performance of the turbo. That is why I used 3 turbos with the same Turbine to make my illustration.
