Technical Tuesday - Aftermarket Turbo Sizing Basics


It's finally race night and you find yourself lined up next to that guy who's been smack talking your ride all day. Your engine is half the size, but you're not even a little worried. The sound of him yelling, "There's no replacement for displacement!" starts to fade as you pull into the staging lane and engage your launch control. You slam the accelerator to the floor and the engine RPMs jump to the limiter. The turbo spools in a matter of seconds as the engine screams to life. Each bang and associated flame being thrown out of the tail pipe seems to be slowly wiping the smirk right off his face. The light turns green, you dump the clutch and sink several inches into the back of your seat as the car flies off the line. You hear yourself saying the words "Sorry, car!" every time you quickly slam the transmission into the next gear. You fly across the finish line and look over at his lane, there's not a v8 in sight! As you leisurely cruise back to the start line you pat your dash a few times and reaffirm that it's definitely the turbo life for you. You know he'll be back next year with a 300 shot of NOS, but you'll be ready. You've got the Firing Order Aftermarket Turbo Sizing Basics Guide and a need for speed.


Whether you're adding a turbocharger to a naturally aspirated engine, or upgrading an existing turbocharger, there's a lot of variables that you'll need to take into account when choosing the turbo that's right for you. In some cases you will also need to modify your Fuel System, Exhaust Piping, Ignition System, and much more to work with your new turbocharger. Stay tuned for future articles that cover these subjects. For now we're just looking at the variables required to ensure your new turbocharger will not only bolt up to your car, but also spool efficiently and move the right amount of air for your specific requirements.

To start, we'll look at a few common turbo flange types and how to identify them. Then we'll look at how turbo's are sized based on Exhaust and Compressor sizing using A/R. Additionally, we'll briefly touch on the different bearing types available before jumping into calculating your engine's CFM throughput. Once you're able to calculate your CFM throughput we'll get into how you can use that information to see exactly how a turbo will react on your motor based on the turbo's compressor map.

Without further adieu, let's get nerdy.

Flange Types and Identification

The first set of specifications you're going to want to find out are your requirements for the physical mounting of your new turbocharger. You will need to find out what the current sizing for the following connections are. 

  • Exhaust Inlet flange type
  • Exhaust Outlet flange type
  • Compressor Inlet flange type
  • Compressor outlet flange type

If you plan on changing your downpipe, exhaust header, or turbo piping, you will want to plan these modifications out before proceeding with your turbo purchase. 

The exhaust inlet flange is what connects your turbocharger to the exhaust manifold. There are many standard flange types available, such as T3, T4, T25, T28, or T6 for example. It's good to be aware that there are also many variations of these and the many other standard flanges, so when you're selecting a turbo, make sure to check that the subset of the flange types are compatible between your engine and exhaust manifold. For more detailed information on the available exhaust inlet flanges and how to identify them, check out this article from

The remaining few flanges will be pretty straight forward. A few quick google searches based on your year/make/model or turbo model should net you the information you need for your exhaust outlet flange, and compressor inlet and outlet flanges. 

Once you have collected this information you will have almost all the information you need to ensure the turbo will bolt up. You will also want to check for clearances based on the compressor and exhaust sizing. 

Turbo Housing A/R and Sizing

The first thing you will need to know is how turbo housings are sized. The sizing of a housing is based on Area/Radius, or A/R for short. A/R is used to describe the geometric characteristics of all compressor and turbine housings. The measurements are based on the housing inlet, or discharge on compressor housings, cross sectional area divided by the radius from the turbo centerline to the centroid of that area.

While A/R can be used to describe both Compressor (Cold Side) and Turbine (Hot Side) Housings, most turbo manufacturers only offer A/R variations on the turbine housing. A/R changes on the compressor housings tend to have a much smaller impact on the turbo's performance than A/R changes on the turbine side. A/R sizing on the compressor side can be used to tune for a quick spool and low end power with larger housings, or a slower spool with more high end air throughput and density with a larger housing. 

Turbine Housings are much more sensitive to A/R changes. It is primarily used to adjust the flow capacity of the turbo. A smaller A/R on the turbine housing will spool quickly and provide low end power sooner. However, it can run out of throughput on the top end. Turbine housings with a large A/R tend to spool more slowly, however provide better air flow and density in the top end of an engine's RPM range. 

For a more in-depth look at A/R sizing, check out the Turbo By Garret article on this subject here.


There are two types of bearings used in modern turbochargers. Journal and Ball Bearing. Some manufacturers also offer hybrid options that feature a Journal exhaust side and ball bearing Compressor side.

Journal bearings are similar to an engine's crank bearings in that they utilize oil to keep a metal shaft lubricated as it rotates. Ball bearings utilize ball bearings to significantly reduce drag and oil requirements. Ball bearing turbos also have a significant advantage when it comes to throttle response.

For more information on bearing types, check out the Turbo by Garret article on the subject here.

Finding your Engine’s CFM Throughput

To find your engine's CFM Throughput, you will need to know the following three things about your engine.

  • Cubic Inch Displacement
  • Volumetric Efficiency
  • Engine RPM

CFM is equal to Cubic Inch Displacement times Volumetric Efficiency times Engine RPM divided by 3456.

Engine RPM and Cubic Inch displacement should be incredibly obvious to you if you've made it this far into the article. However, calculating your engines Volumetric Efficiency is a little more involved. To calculate your Volumetric Efficiency, check out the formulas defined here. That being said you may simply be better off searching google for "My Engine Volumetric Efficiency".

Once you have gathered all this information, head over to to this handy CFM throughput calculator and input your engine's specs.

Reading Turbo Efficiency Maps


Once you have all the above information, you can reference it against a turbo's compressor map. 

The horizontal axis on a compressor map represents the corrected mass air flow per unit of time. Pounds per minute, for instance.  

The vertical axis represents the pressure ratio of the compressor itself. It is calculated by taking the absolute outlet pressure and dividing by the absolute inlet pressure. Pressure ratio is used instead of boost because atmospheric pressure changes with altitude and weather conditions.

The rings in the middle of the graph are known as Efficiency Islands, as you get closer to the center ring the turbo becomes more efficient.  

On the far left edge of the efficiency islands lives a line known as the Surge Line, this is the maximum amount of pressure the turbocharger can produce while flowing the least amount of air. On the oposite side of the map on the far right edge is the Choke Flow Region, which is the maximum amount of air that the compressor side can flow at a given pressure ratio.

The horizontal lines that run across the efficiency islands are known as Speed Lines. These lines represent a specific speed of the compressor wheel.

You can then plot a horizontal and vertical line based on this the data you collected above to see how well your engine fits into the efficiency of the turbo. A general rule of thumb is that one lb/min roughly equates to support 10 horsepower

For more detailed information on reading turbo maps, check out this article.

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