How to Go Fast Faster: The Math Behind Turbocharging. Part 7: Intercooler Selection


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.

Part 6 is here.

     After we choose our turbo and match it to our engine, we can start working on choosing our intercooler.  Intercoolers these days are becoming  more and more popular as people want more and more power to go with that turbo kit.  An intercooler is basically a radiator for the compressed air from the turbo.  It goes in between the turbo and the engine (not literally, but in terms of airflow) so that after the air has been compressed, we can make it even denser by cooling it down (The Pressure-Temperature Law) with the intercooler.  The problem is that a poorly designed intercooler system will decrease performance rather than increase it, so we have to make sure our intercooler purchase will be as efficient as possible.  The main enemy we have to tackle when we’re designing our own intercooler system is Pressure Drop. Pressure Drop is when the pressurized air going through the intercooler loses some of its pressure due to aerodynamic losses (due to twisting piping).  If we lose too much pressure, then the act of cooling the air will be meaningless since we could’ve just skipped the whole intercooler and routed the air into the engine directly.  So what we have to do is increase our cooling as much as possible without losing too much pressure.  This is where all the  math comes in.  Here’s a diagram of an intercooler’s internals, and the terminology of the parts:


 ”Fig. 5-11.” Chart. Maximum Boost, Designing, Testing and Installing Turbocharger
     Systems. By Corky Bell. Cambridge, MA: Bentley Publishers, 1997. 54.

     The first step would be to figure out how “big”  our intercooler has to be, or how much Internal Flow Area (in3) we have.  That’s basically how much area the air has in the intercooler so it can be cooled.  There are two ways to finding a suitable Internal Flow Area, one of which is using this graph.  You’lll be needing your Turbo Airflow Rate (cfm) or you Desired Bhp (Hp):


 ”Fig. 5-13.” Chart. Maximum Boost, Designing, Testing and Installing Turbocharger
     Systems. By Corky Bell. Cambridge, MA: Bentley Publishers, 1997. 55.

     The second way would be to use the suggested value of 6 square inches (in²) per 100 bhp.  Using this way would require just your Desired Bhp to plug into this easy equation:

Internal Flow Area (in3) = Internal Flow Area

Just plug in your Desired Bhp and multiply by 6, then divide by 100.

     The second step is to figure out the exact sizing of our intercooler.  Most cores require 45% of their Internal Flow Area to be the Charge Air Face (the area of the “top” of the intercooler, not the front).  So calculate that to find the Area of the Charge Air Face:

Area of the Charge Air Face (in2) = Area of the Charge Face

Simple.  Just divide the Internal Flow Area by 0.45.  That will be the area of the “top” of your intercooler, not the front/face.

     and with that, we can find the length of the intercooler.  Divide the Charge Air Face Area by the depth of our intercooler (most are either 2 or 3 inches) to find the width of our intercooler.  Now that we have the Internal Flow Area, the depth, and the width of our intercooler.  Remember that a 2 inch thick core intercooler will require a wider intercooler to achieve the same Charge Air Face Area. Also, a 2 inch thick core will offer better cooling (if our Internal Flow Area stays the same) due to the increased Frontal Face Area’s (in2) access to pure, undisturbed cool air.

     The last step would be to determine our Pipe Diameter.  This is just as important as the sizing  of our intercooler, as an incorrect pipe size will decrease performance as well.  Science states that air flows most efficiently between Mach 0.4 and 0.5.  Anything faster than that, and drag starts to negatively affect our flow.  To find out a suitable pipe sizing, we need the Turbo Airflow Rate (cfm).  Then we can solve for an efficient Pipe Size using this formula. To do this, we’re going to solve for the Pipe Diameter using an air velocity (ft/sec) of Mach 0.4.  Keep in mind any velocity between Mach 0.4 and 0.5 works and that intercooler piping increases by factors of 0.25 inches.  With this equation, I’ve factored in the change in units so you don’t have to worry about that.:

Pipe Diameter (in) = Pipe Diameter

The original equation was to solve for the Velocity by dividing the airflow by the area of a cross-section of the piping.  It’s been changed around to solve for the diameter of the piping.  You can use whichever velocity you like, but once again, I suggest staying in between the Mach 0.4 and Mach 0.5 speeds.  The speed of sound is approximately 1,100 ft/sec, so Mach 0.4 is 440 ft/sec.  The constants are a result of converting the area of the cross-section so you can solve for the diameter, as well as converting the units from inches to feet, and minutes to seconds.

     This will give you the best pipe diameter for a certain Airflow Rate for a velocity of Mach 0.4.  I would round up to the next increment of 0.25 inches, so as to now choke the flow.  Also keep in mind that smooth piping is also important, as air is like liquid, and doesn’t like to change direction too quickly.  But also try to minimize the amount of piping as well.  Try to balance the two when adding an intercooler, and make sure you have enough room in your engine bay (or elsewhere) to fit it.  Also important is ducting, as that can increase your intercooler efficiency by up to 20%.   For ducts, as  long as the inlet area is larger than 25% of the Frontal Core Area, you’re fine.  Just don’t go any smaller.  Also important is the way you duct them.  Try to lead the air towards the intercooler without any space for the air to escape.  Also, try to have the mouth of the duct to be smaller than the Frontal Face Area so the duct becomes larger as it gets closer to the intercooler (like a cone).

     The next part will be a bit backwards, as we’ll focus on calculating a certain intercooler’s efficiency when we already know its specifications.  Stay tuned.


7 Responses to “How to Go Fast Faster: The Math Behind Turbocharging. Part 7: Intercooler Selection”

  1. 1 Wayne October 1, 2009 at 3:43 pm

    The series is very educational and informative. I’m in the process of info gathering to piecing together a rotary powerplant for an experimental aircraft and this has been very helpful.

    Best regards, Wayne

  2. 3 Dan March 20, 2010 at 5:25 pm

    I believe you made an error in your section discussing the area of the charge air face. Unless I am reading this incorrectly, if you really wanted to find 45% of the internal area, you would multiply, rather than divide, by 0.45.

    Furthermore, and more importantly, you are converting inches cubed to inches squared simply by dividing by a unitless number. This does not make any sense at all with dimensional analysis.

    That being said, the rest of the stuff is great. Thanks a lot for putting this info together.

  1. 1 How to Go Fast Faster: The Math Behind Turbocharging. Part 6b: Intercooler Efficiency « Take me beyond the Horizon Trackback on September 20, 2009 at 11:31 pm
  2. 2 How to Go Fast Faster: The Math Behind Turbocharging. Part 7b: Intercooler Efficiency « Take me beyond the Horizon Trackback on October 26, 2009 at 4:32 pm
  3. 3 How to Go Fast Faster: The Math Behind Turbocharging. Part 8: An Example « Take me beyond the Horizon Trackback on November 23, 2009 at 4:19 pm
  4. 4 How to Go Fast Faster: The Math Behind Turbocharging. Part 8b: The Result « Take me beyond the Horizon Trackback on November 23, 2009 at 4:20 pm

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