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Turbochargers 101

This article is intended to help understand the selection of a turbocharger for your car. Whether you plan to change a normally aspirated, NA, car to turbocharged or to change, upgrade the factory turbo to a bigger unit. The article assumes you have a basic automotive knowledge.

To properly size a turbo for your vehicle you need to determine the airflow requirements of the engine. The best way to determine the engine's airflow is to measure it. Unfortunately most people do not have the ability to measure it. Therefore we do the next best thing and calculate it based on estimates. This article is not a lesson in math and therefore we'll refer to using the Auto Math calculators on this site to do the work for us. SMOKEmUP has written a neat turbo calculator which will allow you to enter information about your engine. Based on these inputs the calculator will output approximate airflow requirements of your engine. Using these airflow requirements we can then take this and plot it on turbo maps to help determine the correct compressor map for your application. So lets get started.

Understanding the turbocharger compressor map. The left side of the map has a line called the surge limit line. If the compressor operates in this region the compressor becomes unstable and turbo failure may result. The two axis of the compressor map are typically pressure ratio and air flow in lbs/min. These correlate to the engines pressure ratio (how much boost you're running) and the airflow requirements of the engine, we'll let the calculator do the math for us and figure this out as explained below. The center of the map has ovals or islands as they are commonly referred too. The islands have numbers associated to them which is the efficiency of the compressor in this area. The center island is the most efficient and each outer ring has a slightly less efficiency. The goal is to place the engine's most desired RPM range in the most efficient part of the map. Typically you try to place the air flow demands so the peak RPM is at least 65% and the peak torque falls on the most efficient part of the map. This is easier said than done.

First things first we're using SMOKEmUP's Turbo Calculator. The left side of the page contains the inputs for your engine.

  • Engine Displacement - Engine displacement is entered in cubic inches. This is the size of your engine. If you do know the displacement you can calculate it here. For our example we'll use 122.
  • Engine Type - The options are two stroke, four stroke, or rotary. Most street engines are four stroke. For our example we'll use 4 stroke.
  • Volumetric Efficiency (%) - This number is very critical for accurate results. The bad part is most people do not know the VE of the engine without measuring it. If your engine has been on the engine dyno you'll know exactly what the VE is. Otherwise we'll use estimates. Most stock engines have VE's between 80 - 85 %. Race engines can exceed 100%. For our example we'll assume the VE is 90%.
  • Boost Low Octane (PSI) - Enter the boost level you plan to run on low octane gas. For our example we'll use 14psi.
  • Boost High Octane (PSI) - Enter the boost level you plan to run on race gas. For our example we'll use 24 psi.
  • Compressor Efficiency (%) - Enter the compressor efficiency from the turbo map you select. For our example we'll use 74%.
  • Number of Turbo's - Enter the number of turbos you plan to run. For our example we'll use single turbo.
  • Intercooler Efficiency (%) - Enter the intercooler efficiency. Again the best method to obtain this number is to measure and calculate it. For our example we'll use 70%.
  • Air Temp (°F) - Enter the intake air temperature. For our example we'll use 77 °F.
  • Air Fuel Ratio - Enter the desired Air Fuel ratio. This is used to aid in sizing your fuel system. For our example we'll use 11.5.

The inputs for our example are similiar to the motors used in the Mitsubishi Ecplise, and Eagle Talon. Once we enter our information in we press the calculate button and the computer does the work in calculating the engines airflow requirements, much easier than by hand. The output from the calculator looks like below.

Low Boost Results:
Pressure Ratio1.95
Compressor Heat Added Ideal (° F)111.88
Compressor Heat Added Actual (° F)151.18
Compressor Inlet Air Temp (° F)77
Compressor Outlet Air Temp (° F)228.18
Intercooler Inlet Air Temp (° F)228.18
Intercooler Outlet Air Temp (° F)122.36
Density Ratio1.8
Low Boost
RPM Total CFM Total lb/min 11.5:1 A/F
11.5:1 A/F
1000 57.2 4.24 3.69 13.97
2000 114.39 8.48 7.38 27.93
3000 171.59 12.72 11.06 41.86
4000 228.78 16.96 14.75 55.83
5000 285.98 21.2 18.44 69.8
6000 343.17 25.45 22.13 83.76
7000 400.37 29.69 25.81 97.69
8000 457.56 33.93 29.5 111.66
High Boost Results:
Pressure Ratio2.63
Compressor Heat Added Ideal (° F)169.13
Compressor Heat Added Actual (° F)228.56
Compressor Inlet Air Temp (° F)77
Compressor Outlet Air Temp (° F)305.56
Intercooler Inlet Air Temp (° F)305.56
Intercooler Outlet Air Temp (° F)145.57
Density Ratio2.33
High Boost
RPM Total CFM Total lb/min 11.5:1 A/F
11.5:1 A/F
1000 74.17 5.5 4.78 18.09
2000 148.33 11 9.56 36.18
3000 222.5 16.5 14.35 54.31
4000 296.66 22 19.13 72.41
5000 370.83 27.5 23.91 90.5
6000 445 33 28.69 108.59
7000 519.16 38.5 33.47 126.68
8000 593.33 43.99 38.26 144.81

What the calculator did for us is based on the input parameters it calculated the engines airflow requirements. Now we can take this information and select different turbo chargers to plot this information on. Again we don't want to print out a bunch of turbo maps and try to figure out where these points are on the turbo map so we let the calculator do the work for us. SMOKEmUP has a list of over 40 different turbo's you can choose. Simply select which two turbos you want to compare and press plot. The computer will plot the calculated information on the maps for you.

Since this example is based on the Mitsubishi engine we selected two turbo's. The stock 14B turbo and a Garrett T04B V1/V2. Below is the ouput from the calculator on these two compressor maps.

Ok...Now what? What you have is a plot of the engine's peak airflow requirements plotted on the turbo maps you selected. Let's look more closely at the turbo's we selected. The compressor map on the left is for the 14B turbo which comes stock on the car. The line in red is the engines airflow requirements for the engine running at the low boost level. The 1K, 2K .... through 8K points are the engine airflow at each 1000 rpm increment. You can see that at approximately 6500 rpm the airflow requirements are off the map. Now looking at the high boost plot, in blue, you'll notice the engine is off the map at approximately 6000 rpm. Also notice that the plot shows the turbo is performing outside of the peak efficiency of the turbo. In general Mitsubishi did a good job in sizing this turbo for the application.

So your friend has this big Garrett turbo that he'll sell to you for cheap money and it supports making much more power than your wimpy 14B turbo. So you buy the turbo get all the parts to swap for the Garrett conversion. You take the car for a ride and it's a dog below 5500 rpm. Let's see why. Now looking at the Garrett turbo map you'll see that the engine at 4K rpm is below the surge line on the map (not good). Since most of the usable power band is below the surge line this turbo is not a good choice for your vehicle.

The goal of selecting a turbo is to select a turbo where the most amount of usable RPM band for the engine falls on the most efficient part of the turbo map.

Happy Turbo Hunting!

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