Turbo: Choose the right turbocharger

How to choose the right turbo?

In order to know which turbo to use, there are three very simple things to take into account:

Engine volume
Max RPM
Max horsepower

Engine volume and maximum RPM indicate which turbine part (turbine wheel and turbine housing) to be used. Max horsepower tells you which compressor (compressor wheel and compressor housing) should be used.

Choosing a compressor is easy as turbo manufacturers state this clearly. Simply choose the compressor that can handle the power you want. Read more about this further down under "Convert Horsepower to Airflow (HP to CFM)".

Choosing a turbine is more difficult, here you need to have a lot of data or simply see what has worked for others.

Size of turbine housing is usually measured in A/R or cm2. This is simply explained the space for exhaust gases that is in the turbine housing.
Larger space = Power higher up the power band and higher peak power.
Smaller space = Power further down the power band and lower peak power.

In addition to the size of the exhaust housing, the turbine wheel has a large part in what flow the exhaust side receives. A turbine wheel with 11 blades flows worse than one with 9 blades. Fewer blades give a worse spool but more space for the exhaust gases to pass through and thus cope with higher peak power. More blades give better spool but not as high peak power.

The Turbine's design is also of interest here. What is the angle of the blades, are all the blades the same size, what trim does the turbine wheel have.

The different parts of the turbo

If you dismantle the turbo in the simplest way, you get a compressor part, a bearing part and a turbine part. This is how we usually see turbo units in stores.

Compressor part = Compressor cover
Bearing part = The center part with the contents such as shaft, bearing, turbine wheel and compressor wheel
Turbine part = Turbine housing

When we have to choose a turbo for a certain application or power, we want to divide the turbo into 4 different parts as follows:

Compressor cover
Compressor wheel
Turbine housing
Turbine wheel
More information on turbo connections can be found here.

Convert horsepower to airflow (HP to CFM)

CFM = (fpm * area). FPM is Feet Per Minute and CFM is Cubic Feet per Minute. No, we will not calculate this, it easily gets wrong. We'll keep it simple. The turbo manufacturer has measured this and states this in theoretical horsepower that the turbo can handle and the airflow is included in the compressor map.

The manufacturers that do not distribute a compressor map are usually meant for an OEM application. Then this is already taken into account and no compressor map needs to be distributed to the public. The aftermarket, on the other hand, has compressor maps available and the data is clearly presented on the theoretical power and air flow that a turbo compressor can handle.

In other words, we see what the manufacturer states for theoretical max HP or CFM and if this is not presented, we have to rely on what works for others.

What turbocharger do others use?

When we know more about which data fits our engine, we want to confirm this by looking what turbochargers other people use.

Here we will present what turbo other people use on their engines and as we said before, there are many parameters to take into account in order to find the right compressor map and see which turbine to choose. By looking at what turbo has worked well for others, you can get a lot of help in choosing the right one. This can even be a decisive factor if weighing between two different turbochargers.