How to Read Turbo Compressor Flow Maps
This post is to help you get past of the "bigger is better" when it comes to turbos, and get you thinking, "properly sized is better".
For the purposes of this document, it is assumed that anyone reading is familiar with some basic turbo terminology and function along with some simple math.
So, lets take a look at a compressor map. This is the map for the TD05H small 16G.
* refer file no.1
Understanding Information within the Map:
It's really pretty simple.
Pressure ratio is just that a ratio of pressure.
Pr = (Bp+Ap)/Ap
Bp = boost pressure
Ap = air pressure (14.7psi you should know this, if not go back to school)
so if we want Bp = 15 psi, PR = (15 + 14.7)/14.7 = 2.02
(Here bar more popular, so 15psi is act = 15/14.7 = 1.02 bar)
Now the column on the bottom is Air flow CFM, but sometimes they will be in lb/min specially for the Garrett turbo families.
Here is how to calculate CFM
CFM = (L x RPM x VE x Pr)/5660
and to convert CFM to lb/min simply multiply CFM x 0.07
L = engine size in liters
RPM = what rpm your plotting the point for (normally at full load or redline)
VE = volumetric efficiency
2 valve engines 85%
4 valve engines 90%
ported and polished 95%
race heads like whoa 103%
These are just estimated numbers, but should get you pretty close to what you need.
Pr is taken from the calculation we did earlier.
5660 = constant
So if we have a d16 engine, what kind of CFM do we need?
CFM = (1.6 x 7000 x 90 x 2.02)/5660 = 359.82
and that in lb/min is 25.2
so lets plot that point.
* Refer file no.2
Not bad, at redline that turbo at 15psi is just at the end of the center island of 76% efficiency of the turbo. (that’s a good thing) but there has to be more right? Yep, we are going to have to plot 2 more points. With these next 2 points we are going to make a few assumptions. (but that’s ok, because they are almost always right.)
The 2nd point we need to plot is 50% max rpm.
Assumption 1. The turbo will make full boost by this rpm. Usually it will, or it will be really close. This is easy oddly enough the engine flows 1/2 the CFM at 1/2 the rpm (yea yea, that’s an assumption too, but again its fine)
So to plot this point we keep the Pr the same, but divide the CFM in half. That gives us a new CFM of approx = 180
So lets plot that second point.
* Refer file no.3
This is good; the point falls on the right side of the surge line. Had it been on the other side, all hell would break loose, cats and dogs living together, real wrath of god kinda stuff and twisted to the dark side u are….
A quick note about compressor surge… If that point (or any point) falls on the other side of the line, it is similar to letting off the throttle to shift and not having a BOV. (only a little different) The other side of that line is where the turbo isn’t pushing air out of the compressor housing. Instead the air is just spinning with it, and with the exhaust side still spinning it, it can/will create pressure build up at the turbo outlet. This is where damage to the turbo can occur, as the air can/will try reverse flow and go back though the impeller.
The last point we need to plot is the 20% air flow to make sure we don’t cross that surge line between then and 50%. We will plot this point at 1 Pr (atmospheric pressure, no boost) and take 360 CFM and divide that by 5 (20%) roughly 72 CFM, and then run a line from there to the 50% point.
* Refer file no. 4
By looking at that map with these points, you can see that this turbo on a D16 is a pretty damn good match.
So figure those 3 points out, and go to town plotting compressor maps and find the right turbo for your application.
Notes
The 3 points are 3 different rpm/boost ranges
the red is the highest rpm you will see and full boost (take a look at the formula)
the blue is 50% of that, and full boost
and the green is 20% of the red, and 0 boost (but full vacuum).
If that line (the green one) falls on the other side of the surge limit line the turbo will surge... basically a pressure wave will enter the exit... this will cause all sorts of issues with the turbo, and possibly damage it.
Also note that if you want to size your turbo based off of a specific horsepower goal you can do this. Simply take the lbs/min flow from your compressor chart and multiply by 10. This is estimation of how much horsepower your turbo is capable of at this point.
This post is to help you get past of the "bigger is better" when it comes to turbos, and get you thinking, "properly sized is better".
For the purposes of this document, it is assumed that anyone reading is familiar with some basic turbo terminology and function along with some simple math.
So, lets take a look at a compressor map. This is the map for the TD05H small 16G.
* refer file no.1
Understanding Information within the Map:
- The oblong ovals on the chart or “islands” as they are called represent the efficiency of the turbo in that range. As you can see on this map, the most efficient operation (77%) is in the very center of the chart. This is general characteristic of most turbochargers. Without getting into the thermodynamics of adiabatic heat-pumps, we’ll just say that efficiency is a measure of how much excess heat the turbo puts into the compressed air coming out of the outlet. So intuitively, more efficient is better.
- Wheel rotational speed is simply the rpm at which the compressor wheel is spinning.
- The choke point, which is usually not indicated on flow maps, is the maximum flow rating the turbo is capable of regardless of pressure or efficiency.
- Beyond the surge limit on the left of the plot, compressor surge occurs. In laymen’s terms, this phenomenon is caused by a backpressure wave entering the exit of the compressor housing and disrupting flow through the compressor wheel. Surge will kill turbos and is to be avoided at all costs.
It's really pretty simple.
Pressure ratio is just that a ratio of pressure.
Pr = (Bp+Ap)/Ap
Bp = boost pressure
Ap = air pressure (14.7psi you should know this, if not go back to school)
so if we want Bp = 15 psi, PR = (15 + 14.7)/14.7 = 2.02
(Here bar more popular, so 15psi is act = 15/14.7 = 1.02 bar)
Now the column on the bottom is Air flow CFM, but sometimes they will be in lb/min specially for the Garrett turbo families.
Here is how to calculate CFM
CFM = (L x RPM x VE x Pr)/5660
and to convert CFM to lb/min simply multiply CFM x 0.07
L = engine size in liters
RPM = what rpm your plotting the point for (normally at full load or redline)
VE = volumetric efficiency
2 valve engines 85%
4 valve engines 90%
ported and polished 95%
race heads like whoa 103%
These are just estimated numbers, but should get you pretty close to what you need.
Pr is taken from the calculation we did earlier.
5660 = constant
So if we have a d16 engine, what kind of CFM do we need?
CFM = (1.6 x 7000 x 90 x 2.02)/5660 = 359.82
and that in lb/min is 25.2
so lets plot that point.
* Refer file no.2
Not bad, at redline that turbo at 15psi is just at the end of the center island of 76% efficiency of the turbo. (that’s a good thing) but there has to be more right? Yep, we are going to have to plot 2 more points. With these next 2 points we are going to make a few assumptions. (but that’s ok, because they are almost always right.)
The 2nd point we need to plot is 50% max rpm.
Assumption 1. The turbo will make full boost by this rpm. Usually it will, or it will be really close. This is easy oddly enough the engine flows 1/2 the CFM at 1/2 the rpm (yea yea, that’s an assumption too, but again its fine)
So to plot this point we keep the Pr the same, but divide the CFM in half. That gives us a new CFM of approx = 180
So lets plot that second point.
* Refer file no.3
This is good; the point falls on the right side of the surge line. Had it been on the other side, all hell would break loose, cats and dogs living together, real wrath of god kinda stuff and twisted to the dark side u are….
A quick note about compressor surge… If that point (or any point) falls on the other side of the line, it is similar to letting off the throttle to shift and not having a BOV. (only a little different) The other side of that line is where the turbo isn’t pushing air out of the compressor housing. Instead the air is just spinning with it, and with the exhaust side still spinning it, it can/will create pressure build up at the turbo outlet. This is where damage to the turbo can occur, as the air can/will try reverse flow and go back though the impeller.
The last point we need to plot is the 20% air flow to make sure we don’t cross that surge line between then and 50%. We will plot this point at 1 Pr (atmospheric pressure, no boost) and take 360 CFM and divide that by 5 (20%) roughly 72 CFM, and then run a line from there to the 50% point.
* Refer file no. 4
By looking at that map with these points, you can see that this turbo on a D16 is a pretty damn good match.
So figure those 3 points out, and go to town plotting compressor maps and find the right turbo for your application.
Notes
The 3 points are 3 different rpm/boost ranges
the red is the highest rpm you will see and full boost (take a look at the formula)
the blue is 50% of that, and full boost
and the green is 20% of the red, and 0 boost (but full vacuum).
If that line (the green one) falls on the other side of the surge limit line the turbo will surge... basically a pressure wave will enter the exit... this will cause all sorts of issues with the turbo, and possibly damage it.
Also note that if you want to size your turbo based off of a specific horsepower goal you can do this. Simply take the lbs/min flow from your compressor chart and multiply by 10. This is estimation of how much horsepower your turbo is capable of at this point.
