kicker22705
Member
- :
- 03.5 Ti MSP
I found this link on how to read a compressor map. it has great info, which can then aid you to find the right turbo for your car and for your particular future plans.
i thought this would be helpful since a lot of people ask, what turbo should i get if i plan to run ##psi.
i went to a lot of different sites to learn how to read a compressor map, and i found this one to be the easiest to understand.
http://www.nissanperformancemag.com/march05/nerds/
Too small of a compressor match Too big of a compressor match
Properly sized compressor match
i thought this would be helpful since a lot of people ask, what turbo should i get if i plan to run ##psi.
i went to a lot of different sites to learn how to read a compressor map, and i found this one to be the easiest to understand.
http://www.nissanperformancemag.com/march05/nerds/
Probably the most critical step in selecting which turbo you should use is figuring out which compressor will work the best. Compressor matching to the engine is also where the most beginner mistakes are made. By using the formula we will soon discuss and looking at various compressor maps, you will be able to closely estimate which compressor will work the best for your intended use.
The math involved in choosing the correct turbocharger is somewhat involved and requires that some technical information from the manufacturer be available to make the correct judgment. The most important bit of information is the compressor map. The compressor map is a graph of the compressors efficiency when the boost expressed, as pressure ratio is the Y-axis of the map and the flow as expressed as pounds of air per minute is the X-axis. The compressor map is two-dimensional and the area efficiencies of the compressor are called islands. The area of compressor surge (area of operation where the air column inside the compressor hits mach speed and the flow starts to oscillate and becomes unstable, basically stopping) is a line bordering the islands on the far left side of the map. Compressor maps are available from the Garrett Turbo website, Turbonetics, in their catalog, The Innovative Turbo Systems catalog and sometimes from calling the manufacturer of the turbos.
To determine if the compressor is a good match for the motor, you must plot the engines flow requirements over its operational rpm range against the compressor map. Ideally the plot will fall over the maps best efficiency island and stay out of the surge range. The calculations are somewhat involved but if you understand high school algebra, its not too bad. Those that are computer literate may find this a useful tool and can put these equations into an Excel spreadsheet to make it user friendly.
Here is how it goes, bust out the calculator and sharpen your pencil.
Figure out the maximum level of boost that you plan to run. Most stock import motors with proper fueling, etc on 92-octane pump gas can handle at least 7-10 psi. Some very strong stock motors like the Nissan SR20DE can take up to 20 psi if enough fuel is provided and detonation is controlled. If you plan on running race gas, figure at least a few more safe psi. If you are building your motor to lower the compression to a turbo friendly 8-8.5:1 compression, figure that you can run 14-18 psi on pump gas and over 20 psi on race fuel. This is all very general and much of it depends on how good your tuning is, how big your budget is and how much intestinal fortitude you have.
Lets do a compressor match on a SR20DE.
Assume:
Boost- 20psi is realistically what a stock motor on C16 can safely take.
Pressure drop across intercooler- assume 1.5 psi in most cases. If you have a Spearco intercooler, you can look up the pressure drop in their excellent catalog.
From this you can calculate absolute pressure out of the compressor or Pco:
Pco=Boost pressure + Atmospheric Pressure + Pressure drop across the intercooler
(assume Atmospheric pressure is 1 bar or 14.7 psi)
Pco=20+ 14.7+ 1.5
Pco=36.2 psi
From this you can calculate the pressure ratio or Pr:
Pr= Pco/Atmospheric pressure
Pr=36.2/14.7
Pr=2.463
Next guess what the post intercooler temperature might be. We use 130 degrees F as a good starting point as we have found that this is fairly representative of what we have measured with many turbocharged cars with a fairly good aftermarket intercooler.
We will now calculate the approximate density of the air post compressor and intercooler or Di:
Di= Boost pressure+ Atmospheric pressure/ R x 12 x (460+ post intercooler temp)
R= 53.3 (this is a constant from the ideal gas law), 12 is to preserve the inch units in the equation and 460 is to convert degrees F to degrees Rankin(absolute temperature). You don't need to understand this, just always plug these into the equation.
Di= 20+14.7/53.3 x 12 x (460+130)
Di= 9.19 x 10 to the -5th power, lb/cubic inch
From this we can calculate Mf or the mass flow rate of the engine at the rpm where we want to do the match at:
Mf= Di x Engine displacement in cubic inches x RPM of match point/2 x Volumetric Efficiency
For Volumetric Efficiency we can assume 90%, which is typical for most modern 4 valve DOHC sport compact motors. For an old school2 valve motor, you might want to plug in 80%
Mf= 9.19 x 10 -5th power x 122 (for a 2 liter SR20DE motor) x 7500 rpm/2 x .90
Mf= 37.84 lbs per minute
Now we need to find the corrected mass flow or CMf to get us in the ballpark:
CMf= (Mf x the square root of compressor inlet temp/ 545 temperature in degrees Rankin)/ (Atmospheric Pressure/Corrected Compressor Inlet Pressure)
CMf= (37.84 x the square root of 545/545)/ (14.7/13.95)
CMf= 35.9 lb/minute
Boost- 20psi is realistically what a stock motor on C16 can safely take.
Pressure drop across intercooler- assume 1.5 psi in most cases. If you have a Spearco intercooler, you can look up the pressure drop in their excellent catalog.
From this you can calculate absolute pressure out of the compressor or Pco:
Pco=Boost pressure + Atmospheric Pressure + Pressure drop across the intercooler
(assume Atmospheric pressure is 1 bar or 14.7 psi)
Pco=20+ 14.7+ 1.5
Pco=36.2 psi
From this you can calculate the pressure ratio or Pr:
Pr= Pco/Atmospheric pressure
Pr=36.2/14.7
Pr=2.463
Next guess what the post intercooler temperature might be. We use 130 degrees F as a good starting point as we have found that this is fairly representative of what we have measured with many turbocharged cars with a fairly good aftermarket intercooler.
We will now calculate the approximate density of the air post compressor and intercooler or Di:
Di= Boost pressure+ Atmospheric pressure/ R x 12 x (460+ post intercooler temp)
R= 53.3 (this is a constant from the ideal gas law), 12 is to preserve the inch units in the equation and 460 is to convert degrees F to degrees Rankin(absolute temperature). You don't need to understand this, just always plug these into the equation.
Di= 20+14.7/53.3 x 12 x (460+130)
Di= 9.19 x 10 to the -5th power, lb/cubic inch
From this we can calculate Mf or the mass flow rate of the engine at the rpm where we want to do the match at:
Mf= Di x Engine displacement in cubic inches x RPM of match point/2 x Volumetric Efficiency
For Volumetric Efficiency we can assume 90%, which is typical for most modern 4 valve DOHC sport compact motors. For an old school2 valve motor, you might want to plug in 80%
Mf= 9.19 x 10 -5th power x 122 (for a 2 liter SR20DE motor) x 7500 rpm/2 x .90
Mf= 37.84 lbs per minute
Now we need to find the corrected mass flow or CMf to get us in the ballpark:
CMf= (Mf x the square root of compressor inlet temp/ 545 temperature in degrees Rankin)/ (Atmospheric Pressure/Corrected Compressor Inlet Pressure)
CMf= (37.84 x the square root of 545/545)/ (14.7/13.95)
CMf= 35.9 lb/minute
Note: 545 Rankin is 85 degrees Fahrenheit (Rankin is absolute temperature, used in many engineering calculations, 0 degrees R is absolute zero), which also happens to be the standard temp Garrett uses in their compressor maps. So for matching to Garrett compressors, we use this figure. We will also use it for our ambient temp just to make things easier for us. You can subtract whatever the actual temperature you want to use from 545, but we figured that 85 degrees is a good rough estimate of average underhood intake temp and it makes out math easier. Convert your ambient temp degrees from Fahrenheit to Rankin by adding 460 to it. As for the ambient air pressure, it is 14.7 psi. Garrett uses 13.95 psi as the corrected compressor inlet pressure considering the pressure drop across your typical air filter.
So you have your pressure ratio of 2.46 and your mass air flow of 35.9 lb/minute. Next plot these points on the compressor map that you are considering to see how well your engine matches the compressor map.
If you want to plot more points you can do so by plugging in different RPM values remembering that no turbo is going to produce 20 psi at idle. I would figure for something between 4500 to 7500 RPM for the SR20DE. Like we said before, ideally you want to stay away from the surge line and fall across the areas of maximum efficiency. See these three matches below for what a too small, too large and a good match will look like.
Too small of a compressor match Too big of a compressor match
Properly sized compressor match
The first match is with a small GT30 compressor; note that the SR20 is completely off the map. Its sucking hot thin air here!
The second match is a huge GT50 compressor, near redline the SR20 is hard into the compressors surge zone. This is gonna break something!
The third time is the charms with the GT35. Near redline the GT35 is still in the 74% efficient island. This means that the engine is in the compressors fat part of the efficiency nearly all through the rev range. This is a pretty close to an ideal map.
Last edited:
