Determining The Air Flow of an Engine

Recently I came to the conclusion that I need a bigger turbo for my truck, but since I’m cheap, I decided to commandeer a stock turbocharger from another factory built engine.   Used stock turbos are almost always the cheapest to buy because people that drive newer factory turbocharged cars upgrade them for bragging rights.  Some factory cars come with really nice turbochargers that shouldn’t be discarded as often as they are.  Newer Volkswagens, GM & Ford diesels, and Mitsubishi’s all have great turbos on them that are just asking to be mounted onto engines that they don’t belong on.  The problem is that you need to figure out which turbo is the best size for your application.  As you may guess, this is when math enters the building. The good news is that I have done most of it for you.

I created a turbo sizing spreadsheet that calculates the engine air flow with or without a turbocharger in CFM and Lbs/Min based on inputs of your choosing.  Just change the numbers in the green boxes to your own specification to display your air flow.  Once your numbers are all in there, you can compare them to any turbo compressor map.  This will help you determine the which size turbo is best for your application.

Step 1: Cubic Inches.  Type in the cubic inches of your engine into the upper green box.  In my case, I am starting with a 2.0L engine which is 122 cubic inches. If you don’t know your cubic inches, I made a conversion box for that too.  Cell H19, check it out.

Step 2: Volumetric Efficiency.  Now you need to take an educated guess at your volumetric efficiency.  In the most basic of words, this is a measurement of how efficiently your engine can move air/fuel into and out of your engine.  Unless you have a really polished high performance engine, your volumetric efficiency is probably somewhere between 75% and 90%.  I have read that most modern 4 valve engines are in the neighborhood of 85%, so that’s what I’m going with for mine.

Step 3: Boost!  This is how much pressure you expect your turbo to make when you are beating the heck out of it.  I’m going with 20 psi here as a starting point.

Step 4: Ambient Temperature is the temperature outside.  85 degrees sounds like a perfect sunny day!

Step 5: Compressor Efficiency Range.  This is found on a compressor map like this one from You can see that towards the bottom of each ring, there is a number in the 60’s and 70’s.  That is your compressor efficiency within that lobe of the compressor map.  You want to land above 70% when all the math is done. Throw “70” in that box as a starting point.  The more efficient your turbo is, the less  heat your turbo makes.  As the efficiency goes down, the outlet temperature of the turbo goes up.  Simple enough right?

Step 6: Intercooler Efficiency / Intercooler Pressure Drop / Air Filter Pressure Drop.  I have read the most “decent” intercoolers are around 60-65% efficient, with .5-1 PSI of pressure drop.  The average air filter has .5-1 PSI of pressure drop before the turbo.  Some high performance air filters actually list this stat, which is pretty awesome.   Naturally you want the least amount of pressure drop possible across the intercooler and air filter, with the highest efficiency intercooler.  Little changes in these areas pay huge dividends in horsepower.  Play with the numbers a bit and look how much the air flow changes.

Step 7: Engine Compression Ratio.  This is something that you will need to look up, or figure out mathematically for your engine.  It represents the ratio of the combustion chamber volume; its largest capacity to its smallest capacity. In my case 8.5 to 1.

With these inputs, you can see how the air flow changes in columns B, C, and D based on RPM (Column A).  If you have a compressor map for the potential turbocharger for your vehicle, you can determine if it will be within the turbochargers efficient range based on your engine specs.

Let’s see an example, shall we?

I am too scared to rev my engine past 7000 rpm, so why don’t we assume that 7K is my red line.  At that rpm, my max air flow is 31.53 lbs/min (426.24 CFM).  My Density Ratio (Cell K4 in the spreadsheet), is 2.03.  (Density Ratio is more accurate to use on compressor maps because it accounts for temperature.)  Cross those two points on the compressor map (seen in red), and you notice that I would actually be ~73% efficient at 7000 rpm.  This sounds like a great place to be, except that I rarely hang out at 7K rpm.  The majority of my time is spent at 2500-4500 rpm, which would put me below 70% on the compressor map.  This is hard on the turbo, and worse for performance.  So for me, this turbo is just too big!  I would need something smaller for my engine to be happy at lower rpm’s.


Does any of that make sense? Should I try again?

Jeremy Nutt

Hi, I'm Jeremy.

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