Turbocharging Basics 101: Session 2:
An article from Import Racer! By Dino G. Tadokawa


All diagrams courtesy of Garrett Engine Boosting Systems

In today's session of Basic Turbocharging 101, we will cover the concepts of turbocharger matching for your daily driven street vehicle. In this lesson we will introduce the basic process of selecting the proper size of turbocharger. The size of the engine, the intended application and your technical ability will influence the choices you make.

Size is everything! In order to select the proper turbocharger for your needs you must first determine your personal performance objectives!

To begin the process you must first determine your specific vehicle performance requirements. A good question to ask yourself is whether or not your vehicle will be used for daily driving or for all-out, full-race competition. There are a lot of tradeoffs associated with sizing for either application. Let's examine what affects turbocharger geometry (turbocharger component sizing) has on selecting the proper turbocharger for your intended application:

Housing A/R = is the aspect ratio Where A is the volute (scroll) cross-sectional area (not at the inlet flange), and R is the mean radius of that cross-sectional area. The R trends with wheel size and is a constant for a family of housings, different A/R have different A.

Here's a simplistic look at comparing turbocharger geometry with different applications. By comparing different turbo component specifications and build up, it is often possible to determine the intended use of the system. Following is a table listing three different turbochargers and their basic geometry.

Hypothetically, let's imagine that we have three identical engines. The only difference being that each has a different turbo (see Turbocharger Comparison Chart below). What can we infer about the intended use and the turbocharger matching?

Let's establish Engine#1 as the baseline and make some qualitative statements about how Engine #2 and #3 will behave differently.

Engine#2: This engine is using a turbo that is biased more towards low-end torque and optimal vehicle response. For most of us, we would experience this as being more “fun” to drive on the street than Engine #1. Normal daily driving habits with optimal transient response within the standard operating range (rpm band) would be the focus here. It would, however, sacrifice some high-end power.

Some of the design features tell us while it has the same compressor wheel OD as Engine #1, it is a smaller trim and this means it will work better at lower airflow rates (and thus lower rpm). At higher rpm this turbo may not be able to provide the amount of air needed to support the same level of power as Engine #1.

Also this engine has the exact same turbine wheel as Engine #1, it uses a smaller A/R turbine housing. This will also help in achieving higher boost levels at lower rpm than Engine #1. The smaller A/R housing provides higher turbine exhaust velocity into the turbine wheel, therefore it helps the response of the turbo and thus minimizes turbo lag. At high rpm this smaller A/R housing will result in more back pressure than in Engine #1, which can also equate to the loss of top end power.

Now in comparison to Engine #3: This engine is using a turbo that is biased more towards high end power while sacrificing response, low speed torque and drivability. We can make the following observations: While it has the same compressor trim as Engine #1, the wheel OD is larger. This means that it will perform better at the high air flow rates needed for peak power at high engine speeds. At low engine speeds the compressor will not operate as well and you would have wait until higher rpms are achieved before seeing the onset of usable air flow. The turbine wheel OD is also larger (but still the same trim) and it has a larger A/R housing. Both of these will contribute to minimizing back pressure at high rpm, to the benefit of engine peak power. On the other hand, this will also make the rpm at which the turbo can provide boost higher, thus reducing time to boost response.

Turbocharger Comparison Chart
Engine Turbo Type Comp Hsg A/R CW Trim CW Dia (in/ex) Turb. Hsg A/R TW Trim TW Dia (ex/in)
#1 GT30 0.70 56 2.25/3.00 0.82 84 2.16/2.36
#2 GT30 0.70 52 2.17/3.00 0.63 84 2.16/2.36
#3 GT35 0.70 56 2.41/3.22 1.06 84 2.45/2.68

General Rule of Thumb
For any car that will be driven on the street, you should choose the smallest turbocharger that will meet your performance and power objectives. This maximizes the drivability and response. Instant power and response is ultimately the factor that puts a smile on your face every time you slam the pedal to the metal. Also, choose your power goals sensibly: low speed torque is fun, high-speed power is fast, how often do you really need to go fast?

Match Making
Now we will look at actually choosing a turbocharger match. The are many methods to calculating a turbocharger match. To help simplify this process we will present to you some basic calculations, simplified extrapolated graphs, and a basic turbocharging matching scenario for a proposed street application. Also, by running through this exercise it will help prepare you for the more detailed analysis that will be covered in the next session on racing turbocharging matching.

How to select your turbo
Theory: The overriding principle in matching the turbocharger to the engine is that to increase the air mass flow rate through the engine (and thus increase power) you must increase the air density in the intake manifold. The engine is a volumetric machine that will pass a certain volume of air for a given engine displacement, rpm and volumetric efficiency. Manifold air density is what determines how much air mass will pass through the engine for a given volumetric flow. The air mass flow is what determines how much fuel can be burned and thus how much power can be made. This is what turbocharging is all about: increasing manifold density.

Once you have done what is possible to reduce manifold temperature (by using an inter-cooler and assuming that the compressor is of reasonable efficiency), the only way to increase air mass flow rate is by increasing boost. Simply put, it takes boost to increase flow rate, you cannot increase air flow rate (and thus power) by simply using a turbocharger with a higher flow rating, you must also increase the boost pressure.

Now we'll present a way to quickly have enough information to begin selecting a turbocharger for most applications. By using the equations noted below and some other common ones we can create the following Pressure Ratio vs. Engine Flow requirements chart. This chart is similar to many others found in turbocharger distributor catalogs, it has just been done in a slightly different manner.

Reading the Graph
In Figure No.1, along the horizontal axis we have expressed air mass flow in a way that allows it to be easily scaled to any size engine and rpm. Multiplying the values along the horizontal axis by the displacement of the engine and the rpm of interest will give you the total airflow rate. The vertical axis represents compressor pressure ratio, which will be slightly higher than “boost” depending on how well your intake system is designed. The four different lines represent different levels of assumed performance and optimization.

The chart can be used in two ways. If you have a target horsepower goal (and thus airflow) you can find that along the horizontal axis and then move up until you find the line that matches your application. The vertical axis will then tell you the compressor pressure ratio required. If you have a boost level (close to pressure ratio) target, you can find that on the vertical axis and then move over until you find the line that matches your application. The horizontal axis will then give you the airflow. Using the airflow/pressure ratio values you can then start looking through compressor maps until you find one that performs well at those values that you required.

Plotting the Map
So, let's start using it. We are interested in choosing a turbo for use on a 300 hp street application two-liter engine with some modifications. In our case, we know that we are not going to be running any levels of boost greater than 15 psi, so we will use the second method discussed above. We will be using an inter-cooler, so we will be using the “baseline-inter-cooled” line on the chart to account for the modifications to the engine. Because of pressure loss in the system we will look at Pressure Ratio = 2.1 on the vertical axis. For the worst case inter-cooled line this gives an airflow rate (horizontal axis) of 1.87 lb/min per liter per 1,000 rpm. For the rpm points we are interested in, this gives:

RPM Flow Rate Pressure Ratio
3000 13.00 2.1
4500 20.00 2.1
6000 26.50 2.1
7000 31.00 2.1

Choosing The Turbo
By looking through various compressor maps we decide that the T28 turbocharger looks like it will work well. (see Figure No.2 T28 Compressor Map). By plotting these four points on the compressor map below we can see that they all fall within the operating region of the compressor.

After deciding that this T28 turbocharger compressor map looks promising based on the quick analysis we just performed, we plotted the four operating lines on this same map. We can see that for the 4,500 and 6,000 rpm points, the quick method worked well, and this validates the assumption made in the quick analysis.

The 2,000 and 3,000 rpm points are interesting because we see that we will not be able to achieve the boost we would like to make. This is because the turbine is not able to drive the compressor to those levels of boost. If we used a smaller turbine housing A/R we would be able to improve this while sacrificing top end power. However, as it is we will be able to see some boost as low as 2-3,000 rpm that will result in minimal lag and good street drivability.

For reference, at 6,000 rpm we have increased the air flow rate by ~80% over naturally aspirated, and at 3,000 rpm by ~35%, which should result in a corresponding power increases. This is a good match. As noted above, turbine stage tuning can further tailor the boost “onset” and this further optimizes your performance objectives.

Matching a Racing Turbocharger
At our next session of Basic Turbocharging-101 we will look at the turbocharger matching of a high horsepower full-race bred engine. We will perform this turbocharger match calculation based on actual engine data taken from one of Gary Kubo's record setting Honda B18A engines. Till then, keep it a ‘lil sideways, but always under control.