How to choose the proper size Turbocharger
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Joined: Sep 2003
Posts: 10,301
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From: New Jersey
How to choose the proper size Turbocharger
I was sitting in my engineering thermodynamics class today and I got to thinking, how do you truly decide how to pick the proper size compressor for a specific engine application (most turbines are similar so its really down to the compressor).
Well I sat down and started playing around with the basic equations. After a little bit I saw what I needed to calculate and how to relate everything. I did some research on the internet and I've come up with some generalized equations to help us, FI guys, with the decision of choosing the proper compressor using the manufacturer's compressor maps.
With that being said, here is how you can get an idea of what size compressor you want.
WARNING, some math is involved. I've simplified the equations to basic numbers but I will also explain what I am doing along the way. If your not interested in the concepts behind the equations, just plug your values in. All text in italics will be the numbers YOU plug in for your setup.
Here are two commonly used compressor maps (GT3071, GT3076, and GT35). Remember that every compressor will have a slightly different map so refer to your manufacturer to get the proper information about your compressor.
As you can see by these charts we need to calculate two different numbers, Pressure ratio and something called "corrected flow."
Pressure ratio is simply the absolute pressure you plan on running in your engine divided by your compressors inlet pressure + absolute pressure. In equation form we have:
Pressure Ratio = (Your intended boost + 14.7)/(compressor inlet pressure + 14.7)
In my case, I will be running 21psi (gauge), however you will plug in your intended boost pressure that you will be running. A simple assumption of -.5psi can be used for compressor inlet pressure. 14.7 is atmospheric pressure at sea level.
Plugging in my numbers: (21+14.7)/(-.5+14.7)=2.51
Now that we have Pressure Ratio, we will need to calculate the "corrected flow." Corrected flow is simply the lbs/min of air coming into the engine at standard temperature and pressure. Here is how we calculate this number.
First we must calculate the volume of air our engine ingests.
V=(RPM * Engine size (in^3))/(1728*2)
F20C=122in^3
F22C=132in^3
In this equation, you can select where in the RPM range want for maximum power. For example, you want a autoX car? Select 5000rpms for nice midrange power. You want to drag race? Select 9000rpms for maximum peak power! In my case I will be selecting 9000rpms as I am into drag racing. So here we go:
V=(9000*122)/(1728*2)=317cfm
If your interested where the 1728*2 comes from, they are constants from PV=nRT at standard temperature and pressure. Since the compressor maps assume standard pressure and temperature you can use these numbers as a simplification.
Next we must calculate the mass of air flowing.
n=[(Your intended boost + 14.7) * (Volume) * 29]/[(Your Intake Temperatures + 460) * 10.73]
Like before, plug in your intended boost levels in psi (gauge pressure). Plug in your volume number from the previous step. You will also need your intake temperatures, for this step I assumed ~130F. Here is the equation with my numbers:
n=[(21+14.7)*316*29]/[(130+460)*10.73]=51.67 lbs of air/min
As before a lot of the numbers are constants from other equations or are for converting to proper units.
Now engines are not 100% efficient as far as ingesting all the volume that we calculated it needs. With that being said, Most 4-cycle DOHC engines are between 85-90% volumetrically efficient so for our case I will assume an efficiency of 90%.
n*90%=46.5 lbs of air/min
Now its time to plug everything in to get our corrected flow.
Corrected flow=CF=[(n*90%) * ((ambient air+460)/545)^.5]/[(compressor inlet pressure+14.7)/13.949]
Because we are calculating this under standard conditions use ambient air as 70F and earlier we assumed compressor inlet pressure as -.5psi. Using my numbers this is what we get:
CF=[(46.5)*(.9861)]/[(70+14.7)/13.949]=45.04~45
And there we go, we have both the CF number and the pressure ratio number that we need to find where we are on the compressor efficiency charts!
I've shown the point on the GT3071, GT3076, and GT35 compressor charts with MY numbers to give you an idea of how it will be:
As shown by the graphs, at 21psi and 9000RPMs I am clearly out of the efficiency range of a GT3071. If I was to choose a GT3071 I would be making more heat then I would power at 21psi thus my power/psi ratio would be dropping or I would actually be loosing power the more psi I want to run. On the GT35R chart, I am basically at maximum efficiency for this particular compressor (78% vs maximum of 79%) thus a GT35R is a GOOD choice for my specific application!
The 76 is definitely better then the 71 but at 9000rpms and 21psi, the 35 is still slightly more efficient (also shown as it makes more peak power and torque in real life application).
On a final note, you must understand this is a VERY generalized way of determining what compressor you want for your setup. I did not take into account turbine manifold, intercooler piping, intercoolers, throttle bodies, uneven pressure distribution, back pressure, etc etc. However, despite my assumptions, this should give you a very good idea of what size compressor you want; this can be applied to ALL turbo kits using any compressor assuming you have a compressor map from the manufacturer.
And thats it. Please fee free to ask me any questions or to tell me I screwed up a calculation somewhere
. Either way feedback is welcomed 
Jon
Well I sat down and started playing around with the basic equations. After a little bit I saw what I needed to calculate and how to relate everything. I did some research on the internet and I've come up with some generalized equations to help us, FI guys, with the decision of choosing the proper compressor using the manufacturer's compressor maps.
With that being said, here is how you can get an idea of what size compressor you want.
WARNING, some math is involved. I've simplified the equations to basic numbers but I will also explain what I am doing along the way. If your not interested in the concepts behind the equations, just plug your values in. All text in italics will be the numbers YOU plug in for your setup.
Here are two commonly used compressor maps (GT3071, GT3076, and GT35). Remember that every compressor will have a slightly different map so refer to your manufacturer to get the proper information about your compressor.
As you can see by these charts we need to calculate two different numbers, Pressure ratio and something called "corrected flow."
Pressure ratio is simply the absolute pressure you plan on running in your engine divided by your compressors inlet pressure + absolute pressure. In equation form we have:
Pressure Ratio = (Your intended boost + 14.7)/(compressor inlet pressure + 14.7)
In my case, I will be running 21psi (gauge), however you will plug in your intended boost pressure that you will be running. A simple assumption of -.5psi can be used for compressor inlet pressure. 14.7 is atmospheric pressure at sea level.
Plugging in my numbers: (21+14.7)/(-.5+14.7)=2.51
Now that we have Pressure Ratio, we will need to calculate the "corrected flow." Corrected flow is simply the lbs/min of air coming into the engine at standard temperature and pressure. Here is how we calculate this number.
First we must calculate the volume of air our engine ingests.
V=(RPM * Engine size (in^3))/(1728*2)
F20C=122in^3
F22C=132in^3
In this equation, you can select where in the RPM range want for maximum power. For example, you want a autoX car? Select 5000rpms for nice midrange power. You want to drag race? Select 9000rpms for maximum peak power! In my case I will be selecting 9000rpms as I am into drag racing. So here we go:
V=(9000*122)/(1728*2)=317cfm
If your interested where the 1728*2 comes from, they are constants from PV=nRT at standard temperature and pressure. Since the compressor maps assume standard pressure and temperature you can use these numbers as a simplification.
Next we must calculate the mass of air flowing.
n=[(Your intended boost + 14.7) * (Volume) * 29]/[(Your Intake Temperatures + 460) * 10.73]
Like before, plug in your intended boost levels in psi (gauge pressure). Plug in your volume number from the previous step. You will also need your intake temperatures, for this step I assumed ~130F. Here is the equation with my numbers:
n=[(21+14.7)*316*29]/[(130+460)*10.73]=51.67 lbs of air/min
As before a lot of the numbers are constants from other equations or are for converting to proper units.
Now engines are not 100% efficient as far as ingesting all the volume that we calculated it needs. With that being said, Most 4-cycle DOHC engines are between 85-90% volumetrically efficient so for our case I will assume an efficiency of 90%.
n*90%=46.5 lbs of air/min
Now its time to plug everything in to get our corrected flow.
Corrected flow=CF=[(n*90%) * ((ambient air+460)/545)^.5]/[(compressor inlet pressure+14.7)/13.949]
Because we are calculating this under standard conditions use ambient air as 70F and earlier we assumed compressor inlet pressure as -.5psi. Using my numbers this is what we get:
CF=[(46.5)*(.9861)]/[(70+14.7)/13.949]=45.04~45
And there we go, we have both the CF number and the pressure ratio number that we need to find where we are on the compressor efficiency charts!
I've shown the point on the GT3071, GT3076, and GT35 compressor charts with MY numbers to give you an idea of how it will be:
As shown by the graphs, at 21psi and 9000RPMs I am clearly out of the efficiency range of a GT3071. If I was to choose a GT3071 I would be making more heat then I would power at 21psi thus my power/psi ratio would be dropping or I would actually be loosing power the more psi I want to run. On the GT35R chart, I am basically at maximum efficiency for this particular compressor (78% vs maximum of 79%) thus a GT35R is a GOOD choice for my specific application!
The 76 is definitely better then the 71 but at 9000rpms and 21psi, the 35 is still slightly more efficient (also shown as it makes more peak power and torque in real life application).
On a final note, you must understand this is a VERY generalized way of determining what compressor you want for your setup. I did not take into account turbine manifold, intercooler piping, intercoolers, throttle bodies, uneven pressure distribution, back pressure, etc etc. However, despite my assumptions, this should give you a very good idea of what size compressor you want; this can be applied to ALL turbo kits using any compressor assuming you have a compressor map from the manufacturer.
And thats it. Please fee free to ask me any questions or to tell me I screwed up a calculation somewhere
. Either way feedback is welcomed Jon
Joined: Mar 2004
Posts: 2,160
Likes: 35
From: Decatur, GA
This looks great. I remember thinking about how to take everything I learned in a turbomachinery course and apply it to turbocharging my car.
Two comments quick comments.
1. For anybody not at sea level, you will need to use something other than 14.7 for atmospheric pressure. This does change things quite a bit. Be careful.
2. You did the calculations for 21 psi and 9000 rpm. When I chose my turbo, I wrote a spreadsheet to calculate multiple RPM and boost levels. I would calculate from 150 KPA to 300 KPA (I think I did every 20-30 KPA). I also did every 500 rpm from 2500 rpm to 9000 rpm. When you look at the compressor map over the entire range of the engine, the calculations come out a bit different. I actually then went back and plotted compressor efficiency for every point and color coded my spreadsheet. Finally, I calculated the average efficiency over a few ranges.
If anybody wants to see the spreadsheets, please let me know (PM your e-mail address). The color coding isn't automatic and they aren't perfect, but they should be able to calculate mass flow versus PR for a variety of engines.
Tim
Two comments quick comments.
1. For anybody not at sea level, you will need to use something other than 14.7 for atmospheric pressure. This does change things quite a bit. Be careful.
2. You did the calculations for 21 psi and 9000 rpm. When I chose my turbo, I wrote a spreadsheet to calculate multiple RPM and boost levels. I would calculate from 150 KPA to 300 KPA (I think I did every 20-30 KPA). I also did every 500 rpm from 2500 rpm to 9000 rpm. When you look at the compressor map over the entire range of the engine, the calculations come out a bit different. I actually then went back and plotted compressor efficiency for every point and color coded my spreadsheet. Finally, I calculated the average efficiency over a few ranges.
If anybody wants to see the spreadsheets, please let me know (PM your e-mail address). The color coding isn't automatic and they aren't perfect, but they should be able to calculate mass flow versus PR for a variety of engines.
Tim
Joined: Mar 2004
Posts: 2,160
Likes: 35
From: Decatur, GA
My interpretation of the graphs. The GT30R will be more efficient at low RPM and lower PR's. The GT35R only beats it when you get near the redline. For a daily driver, the GT30R is probably a better match. If you're planning on extending your redline, running big cams, and/or going racing, then the GT35R may be a better match. Even though the GT30R is out of the center of it's efficiency, it's still over 70% efficient- more than most superchargers!
Plotting a series of points at a variety of RPM should back this up. I can post some scans of a compressor map showing what I'm talking about when I get home (if I can get my scanner to work).
Tim
Plotting a series of points at a variety of RPM should back this up. I can post some scans of a compressor map showing what I'm talking about when I get home (if I can get my scanner to work).
Tim
Thread Starter
Joined: Sep 2003
Posts: 10,301
Likes: 18
From: New Jersey
Originally Posted by timg,Apr 24 2008, 06:51 PM
My interpretation of the graphs. The GT30R will be more efficient at low RPM and lower PR's. The GT35R only beats it when you get near the redline. For a daily driver, the GT30R is probably a better match. If you're planning on extending your redline, running big cams, and/or going racing, then the GT35R may be a better match. Even though the GT30R is out of the center of it's efficiency, it's still over 70% efficient- more than most superchargers!
Plotting a series of points at a variety of RPM should back this up. I can post some scans of a compressor map showing what I'm talking about when I get home (if I can get my scanner to work).
Tim
Plotting a series of points at a variety of RPM should back this up. I can post some scans of a compressor map showing what I'm talking about when I get home (if I can get my scanner to work).
Tim
If you could post them up, it would be great to see how efficiency changes based on rpm as I only did it at one point.
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thats not a gt30r, its a gt3071r...thats a step smaller than a gt3076r which most people use. I would agree that in your case the gt35r is the better choice, but to say the "gt30 is clearly out of efficency" is incorrect. Yes its LOSING efficency but its still at 73%....thats pretty damn good honestly, and thats the 3071r which has a 45lb compressor -vs- the 3076r which has a 52 lb compressor.
Also the vtec engine is better than 90% VE. When you add a turbo to an engine the VE also goes up so keep that in mind as well. Otherwise good stuff.
Also the vtec engine is better than 90% VE. When you add a turbo to an engine the VE also goes up so keep that in mind as well. Otherwise good stuff.
Thread Starter
Joined: Sep 2003
Posts: 10,301
Likes: 18
From: New Jersey
Originally Posted by AndyFloyd,Apr 24 2008, 10:28 PM
thats not a gt30r, its a gt3071r...thats a step smaller than a gt3076r which most people use. I would agree that in your case the gt35r is the better choice, but to say the "gt30 is clearly out of efficency" is incorrect. Yes its LOSING efficency but its still at 73%....thats pretty damn good honestly, and thats the 3071r which has a 45lb compressor -vs- the 3076r which has a 52 lb compressor.
Also the vtec engine is better than 90% VE. When you add a turbo to an engine the VE also goes up so keep that in mind as well. Otherwise good stuff.
Also the vtec engine is better than 90% VE. When you add a turbo to an engine the VE also goes up so keep that in mind as well. Otherwise good stuff.
I never said a GT30 wasn't good, I was just saying for my DRAG RACING setup a GT35R is a smarter choice. The point of this post was so everyone else can plug in their own numbers so they can see what is right for what they want
. Also note that the efficiency "islands" (as I will call them) are very close together at the edge as in a slight shift to the right yields a "large" drop in efficiency. It still might be fairly efficient (which it is) but your nearly off the chart suggesting your loosing a good amount of power/psi.As far as efficiency, you can always change the number in my equation; I just took a guestimation on what it was.
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Thread Starter
Joined: Sep 2003
Posts: 10,301
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From: New Jersey
Since you did bring up a good point, AndyFloyd, I added the compressor charts for the GT3076R in my original post!
The 76 is definitely better then the 71 but at 9000rpms and 21psi, the 35 is still slightly more efficient (also shown as it makes more peak power and torque in real life application).
The 76 is definitely better then the 71 but at 9000rpms and 21psi, the 35 is still slightly more efficient (also shown as it makes more peak power and torque in real life application).
