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Home Articles Compressor Flow Maps and Calculations

Compressor Flow Maps and Calculations

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By Joseph Obernberger

There seems to be a lot of confusion about which turbocharger to choose for which application.  These are some notes and calculations for the MR2 Turbo and which maps I think are best for this 2.0 liter engine.  If anything is wrong here, please let me know, and I will make the corrections.

The Compressor

The turbocharger compressors that I will be comparing are the TD06 20G compressor from Mitsubishi that comes with a couple of the Greddy kits, the T04E-46, T04E-50, and the T04E-60 trim compressor wheels from Garrett.  This method and formulas was taken from A. Graham Bell's excellent book, 'Forced Induction Performance Tuning'.
First we need to calculate the engine air flow rate (CFM).  The formula for this is:

CFM = L x RPM x VE x Pr
5660

Where L = engine capacity in liters
RPM = maximum engine speed (we'll adjust this later)
VE = engine volumetric efficiency.  From A. Graham Bell's book Forced Induction Performance Tuning some good values for VE are:
Stock 2-valve = 85%
Stock 4-valve = 90%
Street modified = 93%
Competition = 105%
Pr = pressure ratio

To calculate the pressure ratio you need to know what boost pressure you want to run and then plug that into the following formula:

Pr = 14.7 + Boost
14.7

So - let's plug in some numbers and then apply them to the compressor maps.  Say we want to run 18psi of boost.  The pressure ratio comes out to be (14.7 + 18) / 14.7 = 2.22
Now lets calculate airflow.  I think it's best to calculate airflow at at least two different RPM points.  For our example, let's say we want to have full boost by half of max RPM.  Redline on the MR2 is 7200RPM.  So we'll calculate airflow for 3600RPM and 7200RPM, and then see which map works out best for these values.  We'll choose 90% for volumetric efficiency (VE).
For 7200RPM:
CFM = (2.0 x 7200 x 90 x 2.22) / 5660 = 508.32 or 35.6lb/min.  To convert this value to lb/min take CFM and divide by 14.27.
For 3600RPM:
CFM = (2.0 x 3600 x 90 x 2.22) / 5660 = 254.1 or 17.8 lb/min.  As a side note, since half the RPM will result in half the airflow, 254.1 is indeed half of 508.32

Check out the new CFM calculator. It will generate a table with varying RPM given boost, volumetric efficicency, max engine RPM, and engine size.

Now we can look at some compressor maps and see where these points fall.  Let's take a look at the TD06 - 20g compressor map first.

We need to plot our two points on this map:

From these two points (where the above red lines intersect), this compressor looks like a great fit.  According to the compressor map it can make 18psi by 3600RPM, and at 7200RPM it is in the 76% efficiency 'island'.  The higher the efficiency island the lower the outlet temperature of the compressed air, and hence the more power you can make.  This map also includes compressor RPM as denoted by the numbers on the graph 55000rpm, 75000rpm and so on.  At 7200 engine RPM, and making 18psi of boost, the compressor RPM is between 105,000 and 120,000RPM.  This means that the compressor is capable of supplying this type of spool up, but only when matched with a correct sized turbine for your application.  See 'The Turbine Side' below.

There is yet another item we can determine from the compressor map.  We want to make sure that the turbocharger selected will not operate to the left of the 'surge line'.  The above graph does not specify a surge line, but it is to the left of any point within the largest island on the graph.  Check out the T04E-46 and 50 trim maps below, they include a surge line on the graph.  A good approximate method for checking to be sure we are away from the surge line, is to plot one more point on the compressor map.  This is 20% airflow at a pressure ratio of 1.0.  Then connect that point with the 3600RPM point.

20% airflow, in our example is 101.6CFM or 7.12lb/min.  Plotting this point and connecting the dots:

Great!  The line plotted from 20% max airflow to our 3600RPM point falls to the right of the surge line.  If this line fell to the left at any point, this compressor would not be a good choice.  If the turbocharger operates to the left of the surge line, the compressor will be unstable, and will eventually damage the compressor.  From this, the TD06-20G compressor is a wonderful fit for the MR2 turbo.  Now that we have these numbers, we can plot them on other compressor maps.

For the T04E-46 trim compressor map, we need the values in lb/min.
At 7200 RPM we have 35.6lb/min.
At 3600RPM we have 17.8lb/min.
20% max airflow is 7.12lb/min.

Plotting our points results in:

Here at 18psi of boost and 7200RPM we are at about 73% efficiency.  This turbo will also make 18psi by 3600RPM, and we are very safe from surge.  Looking at this map, it is clear that this compressor is smaller than the TD06-20G and will spool faster.  Just for fun, we can see where on this plot we will be at 25psi of boost.
Pr = (14.7 + 25) / 14.7 = 2.70
CFM = (2 x 7200RPM x 90 x 2.70) / 5660 =  618 or 43.3lb/min.
Valid intersections are denoted by arrows.  Plotting this point for the T04E-46

Whoops! - Off the graph.  So we know that this compressor and the MR2's 2.0 liter engine cannot run 25psi at 7200RPM.  Doing the same for the TD06-20g compressor:

This compressor can produce 25psi with the 2.0 liter engine at an efficiency of  73%.  Looks like 25psi, at 7200RPM is about all this compressor will do.  Keep in mind, that more boost can be made at lower RPMs, but it will start to drop off as the RPMs rise.  The TD06-20g should be able to hold 25psi to redline, but can certainly make more boost at lower RPMs.

Having fun?  I sure am.  How about the T04E-50 trim compressor (my favorite).

Looks like we cannot make 18psi by 3600RPM with this compressor, so it is going to have slightly more lag than the above two compressors.  At 7200RPM, we are just out of the 78% efficiency island, and with room to spare for more boost.  So how do we make sure that we are not to the left of the surge line?  If you draw a line from 20% airflow (7.12lb/min) to the 3600RPM point (17.8lb/min), the line is to the right of the surge line right up until the intersection.  The method for determining if you are to the right of the surge line is an approximate one, and suffice it to say, will not work if the compressor doesn't make full boost by half of redline as the method prescribes.  That said, given this compressors record, it does not operate in surge with the 2.0 liter MR2 engine.  An explanation of this is beyond the scope here.  Surge usually can be noticed by a pop or backfire out of the compressor inlet.
Plotting 25psi:

This compressor has room to spare, even at 25psi of boost and 7200RPM it is on the 76% island!  So how far can we go?  We'll try 29psi of boost.
Pr = (14.7 + 29) / 14.7 = 2.97
CFM at 7200RPM = (2.0 x 7200 x 90 x 3.08) / 5600 = 680.6CFM or 47.6lb/min

Just makes it!  Again, keep in mind that more boost can be made at lower RPMs, but it will just start to drop off to about 29psi as the RPMs rise.

How about that 60 trim Garrett compressor.

Clearly there is more lag with this compressor, but it is over 78% efficient at 7200RPM and 18psi of boost!  Again, surge is a consideration, and from word of mouth from those using this compressor, it doesn't seem to exhibit surge problems.  That said, I have not tried it myself.
25psi:

Makes 25psi at 7200RPM - still at 75% efficiency.  
Here is an interesting one, this compressor, despite being larger, cannot make the 29psi that the 50 trim can.  The graph doesn't even extend to a pressure ratio of 3.0.  At boost levels of around 18-25psi, however, it is extremely efficient and hence will make more power at those boost levels.

Great resource for compressor maps.

The Turbine Side

The compressor is only half the story for turbochargers - the other half is the turbine side.  Important numbers to consider are the A/R ratio on the turbine side, and the exducer bore diameter.  Exducer bore can be seen in the following figure.

Some general numbers for exducer bore are:
200HP - exducer bore between 41mm and 51mm
300HP - exducer bore between 52mm and 60mm.
400HP - exducer bore between 61mm and 70mm
500HP - exducer bore between 67mm and 78mm

A/R is the real important dimension for judging a turbines potential.  It is determined by dividing the area of the turbine nozzle A by the radius R from the center of the turbine axle to the center of the housing throat.

There doesn't seem to be a very good way to find out what A/R is best for your particular application.  Usually one must resort to other's experience. A small A/R turbine housing will make more boost at lower engine RPM at the expense of reduced maximum power at high RPMs because of exhaust restriction.  Consequently a larger A/R will take longer to spool, but make more power at high RPMs.  For example (these are for example only!), a 0.63A/R may make 12psi of boost by 2500RPM, a 0.82A/R may make the same 12psi by 3500RPM, and a 1.06A/R may take as high as 5000RPM.
The above two diagrams were pulled from 'Forced Induction Performance Tuning'.

Some Comments/Opinions

Since this is the internet and I can write whatever I want....

I've noticed that many turbo kits have been released to the aftermarket recently.  This is really great stuff.  However sometimes claims are made about turbochargers and kits put together using a compressors that no one knows about, and for which no maps are available.  If you come across a kit that has wonderful claims about power production, but is using a compressor that you have not heard about and do not have a map for, ask them for the compressor map.  Do the calculations yourself and plot them on the map before plunking down your cash.  It seems that some compressors are used just because somebody told someone else that it was a really great fit.  It's not that hard to find out, if you have the map.  If the map is not available, or the map sent to you looks suspect, I would forget about it.  Keep in mind that if the company/vendor/whomever building the kit doesn't have the map, how could they determine that it was a good fit for your application!  

Click here for a CFM Table of the 2.0 liter MR2 engine.
Check out the new CFM calculator. It will generate a table with varying RPM given boost, volumetric efficicency, max engine RPM, and engine size.

 

 

 

Last Updated on Saturday, 01 January 2011 21:01