How to Go Fast Faster: The Math Behind Turbocharging. Part 6: Turbine Selection

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Part 1 is here.

Part 2 is here.

Part 3 is here.

Part 3b is here.

Part 4 is here.

Part 4b is here.

Part 5 is here.

Part 5b is here.

     After we’ve selected our compressor, we can now focus on the turbine.  The turbine is what ultimately decides when the power will be most accessible/most efficient.  With the choosing of the compressor, we have a select range of how much power we can make, but the turbine is what decides how we’ll make use of the compressor.  In short, the exhaust gases that are being expelled from the cylinders are turning the turbine to power the compressor.  How big our turbine is will be will decide whether the power we’ll make will be low-rpm torque, high power torque, or somewhere in between (or maybe even both). When choosing our turbine, we need to deal with two quantities; the Exducer Bore (in) and the Area/Radius Ratio.

     A larger turbine will obviously produce a higher amount of power, while a smaller one will produce less.  The simplest way to measure a turbine’s size is to measure its Exducer Bore.  The Exducer Bore is the inner diameter of the turbine outlet (the connection between the turbo and the exhaust).  Take a look:

page31top

 ”Fig. 3-9.” Chart. Maximum Boost, Designing, Testing and Installing Turbocharger
     Systems. By Corky Bell. Cambridge, MA: Bentley Publishers, 1997. 31.
     Print.

     The size of the exducer bore basically determines how much Airflow Rate (cfm) the compressor can deal with using that sized exducer bore.  Here is an approximation:

page31bottom

 ”Fig. 3-10.” Chart. Maximum Boost, Designing, Testing and Installing Turbocharger
     Systems. By Corky Bell. Cambridge, MA: Bentley Publishers, 1997. 31.
     Print.

     You should obviously match your exducer bore to the Turbo Airflow Rate (cfm) that you’ve calculated before.  You should lean towards a larger diameter in case you’d like to increase your power a bit, but this graph isn’t definitive.  You should contact the turbo manufacturer, or anyone with enough experience to find the best fitting turbine for your goals.

     The second quantity is the Area/Radius Ratio.  This is a bit more important than the Exducer Bore, as this is the general sizing of the turbine.  The Area/Radius Ratio will determine the specifics of your turbo.  This is the ratio between the Area (in2) of the turbine outlet (or inlet) and the Radius (in) from the center of that cross-sectioned area to the center of the turbine.  Here is a picture to simplify it a bit

page32

 ”Fig. 3-11.” Chart. Maximum Boost, Designing, Testing and Installing Turbocharger
     Systems. By Corky Bell. Cambridge, MA: Bentley Publishers, 1997. 32.
     Print.

     This is the basic picture of the turbine, and it shows that throughout the whole turbine, the Area/Radius Ratio is always constant; from the turbine inlet all the way to the turbine outlet.  The Area determines the velocity at which the exhaust gases exit the turbine scroll and power the blades.  The exhaust gases are what are rotating the turbine scroll, which is connected to the compressor side and in turn rotates the compressor wheel (which compress our intake air).  The Radius determines the amount of power that the exhaust gases will have.  The larger the radius, the more torque the compressor wheel will have to compress the air.

     Basic knowledge states that a larger Area/Radius Ratio will give you more power, while a smaller ratio will give you better(meaning quicker) response.  Here is another graph:

page34

 ”Fig. 3-14.” Chart. Maximum Boost, Designing, Testing and Installing Turbocharger
     Systems. By Corky Bell. Cambridge, MA: Bentley Publishers, 1997. 34.
     Print.

     The problem is that there is no “perfect” or “right” Area/Radius ratio.  If you want more power, by all means go for a higher(bigger) ratio, but the engine/turbo response might be too laggy for  you.  Choose a smaller Area/Radius Ratio, and your car might feel too jumpy, since the turbo will spool up too quickly.  A small Area/Radius Ratio can also lead to not enough torque spinning your turbine scroll, which means your compressor also won’t work to its maximum efficiency.  This is where it becomes trial and error, as different people have different ideas of what “enough” power and what “enough” response is.  Research if vital here, and putting together a turbine and compressor which match doesn’t always mean you’ll like the result.  Play with the numbers, do some research, and ask some questions, as this will be a costly purchase.

     Next I’ll go into intercooling, and then wrap up the turbo section.  I’ll work in some examples, and then devote a final post to putting together the examples into one coherent (hopefully) step by step process.

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