daryl337
08-13-2007, 10:30 PM
I have noticed that alot of people come onto this forum asking "what kind of turbo should I get?" I just wanted to share with you the knowledge that I have pieced together about reading compressor maps so you can get an idea of what kind of turbos you want to use for your set ups.
For this quick lesson, I am going to reference the Garrett Website alot (I hope they dont mind) mostly because they have alot of information readily available, and to be quite frank they are way more experienced than I am. I am not claiming to be a turbo "expert" but I believe I have a pretty firm grasp on the ideas. If I give any misleading information, please let me know so I can better my own education, but I think I have pretty much got it. Ok, here we go.
Lets start with the semi-basics (since I am assuming pretty much everyone in here knows the gist of how a turbocharger works... ball bearing vs journal bearings, compressor vs turbine, so on and so forth)
First, lets familiarize our selves with the terminology
Absolute pressure - atmospheric pressure(14.7 psi at sea level) + boost/vacuum pressure
Pressure Ratio - The ratio of the absolute pressure measured at the compressor outlet vs the absolute pressure at the compressor inlet. In for form of : outlet/inlet.
Let me elaborate on this one: if your compressor is sucking in air, you are lowering the pressure inside of the intake tube portion of your intake system by a given amount. On the reverse side, the outlet of the compressor is of course creating your "boost" by compressing more air into the given volume which increases your outlet pressure. So in a sense, the pressure ratio is a ratio of boost over vacuum created by each side of the compressor itself.
Mass flow ( lbs/min) or Volumetric flow (cfm) - to put it in lamens terms, this is amount of air being moved through the motor in a given amount of time. You can calculate this in a few ways. I personally use volumetric flow since it is easier for me to grasp the concept of cfm than it is lbs/min. With a stock bore/stroke sr20det for example I know that I have a given cc for my cylinders, multiply by your desired peak power RPM operating range and you will end up with ccm. use simple math to convert cubic centimeters to cubic feet and you will have pretty close to your desired CFM for peak power. Garrett suggests that as a rule of thumb, it takes 1 lb/min to generate about 10 hp. So this means that if you want a top notch 400hp engine you want a turbo with a flow rating that can reach about 40. To convert CFM into lb/min, you simply divide your CFM by the air density in your area, this should give you a good idea of lbs/min.
Compressor Efficiency - this is a percentage term which relates to (uh) the efficiency of your turbo. Of course, the higher the number, the better. The effeciency of a compressor essentially measures the benefits to each additional measurement of work being produced by the turbo vs the costs in both volumetric efficiency as well as compressor damage. For example, Ever wonder why you cant just let your little t-25 boost to 50 lbs to make your car fast? The reason is that after 12-14 psi or so the compressor loses efficiency drastically, and so each additional lb of boost put more of a strain on the turbo, as well as your intake charge. By finding a turbo that is slightly larger you are moving your compressor efficiency islands (I will show you with pics soon) more in the area that is suitible for your desired boost.
Surge line - this is pretty much self explanitory. Compressor surge is when your compressors flow backs up causing your fins to attempt to stop. Most people are familiar with this problem when they have an inoperating BOV, but it is also a common problem with a turbo that is too large for the vehicle. Lets say that you have a t88 on a 1.5 liter motor that doesnt rev very high. Now, imagine you are trying to get 11 lbs of boost out of the turbo, in this low of an operating range. Your engine would not be able to consume the air fast enough for the compressor to flow properly, so the air would constantly be backing up into the intake system.
compressor rpms - self explanitory. The rate at which your compressor wheel is spinning
Now, lets take a look at some compressor maps shall we? http://www.turbobygarrett.com/turbobygarrett/images/catalog/Turbochargers/gt25_images/GT2554R_471171_3_comp_e.gif
This is a Gt2554 R, which has characteristics similar to the turbos we have stock on our SRs. Notice how on the right hand side you have your choke point, which is where the compressor no longer produces any air at an acceptable efficiency rate. notice also, that inbetween 12 and 21 lb/min that little island in the middle has the highest effeciency rate at about 71%. this means that using the pressure ratios described, and flowing inbetween 12 and 21 lbs/min, you get the best effeciency out of this turbo. What does this mean for you? Running at peak efficiency, if you use garretts rule of thumb, this turbo would be good for about 210hp... but dont let that fool you into thinking that is the MAX it can produce. If you pushed the turbo to its limit, notice at about 60% efficiency you can produce almost 280HP. Do you want to though? Depends on how reliable you want your set up to be.
Lets take a look at another one...
http://www.turbobygarrett.com/turbobygarrett/images/catalog/Turbochargers/gt30_images/GT3071R_comp_t.jpg
Notice the peak efficiency island on this turbo. Starting at about 23 and ending at about 37 lbs/min, this turbo proves to be pretty versatile. a turbo that has a 77-78% efficiency from 230-370HP? This turbo is one of the more versatile turbos on the market. For lower boost applications (look at how low the pressure ratio starts for the island) the turbo maintains a high efficiency. As a matter of fact, this turbo really doesnt drop much under 70% until about 430 hp.
Lastly, lets take a look at a larger turbo so we can get a solid view of compressor surge.
http://www.turbobygarrett.com/turbobygarrett/images/catalog/Turbochargers/gt35_images/GT3582R_714568_1_comp.gif
This is the garrett GT3582R. notice it holds a peak efficiency from 26 to about 45 lbs/min. Now you might say "well why get the gt3071R if this turbo has a better top end and its efficiency starts at only 3 more lbs/min?"
The answer is this: notice the upper left corner of the graph? How it has that large "dent" in it? That is your surge line. Unless I run the motor in an RPM range that keeps this turbo up in the 26-45 lbs/min range, this compressor will surge more than your pants if Jessica Alba walked by nude. sure, this compressor has alot better top end flow characteristics, but its low end ability to safely produce power is not there. Also, notice its requirements necessary to keep the efficiency at a high level. You have to increase the pressure ratio dramatically as your increase your flow rate, or the compressor chokes out, like wise, if you choose to flow less air (operate at a lower RPM) you need to drop your pressure ratio in order to keep it happy. So this turbo has a fairly limited use compared to a gt3071r. However, for certain people (high boost or high rpm situations) this may be a better turbo for them.
Please feel free to ask any questions since I seem to have tapped out my "originality" function of my brain and I can't think of what else I need to explain. If you don't like my way of explaining things, you can visit the garrett website at www.turbobygarrett.com. They go into more detail from an engineering standpoint on selecting turbos. I am simply here as sort of a translater to all of that info for you. :)
For this quick lesson, I am going to reference the Garrett Website alot (I hope they dont mind) mostly because they have alot of information readily available, and to be quite frank they are way more experienced than I am. I am not claiming to be a turbo "expert" but I believe I have a pretty firm grasp on the ideas. If I give any misleading information, please let me know so I can better my own education, but I think I have pretty much got it. Ok, here we go.
Lets start with the semi-basics (since I am assuming pretty much everyone in here knows the gist of how a turbocharger works... ball bearing vs journal bearings, compressor vs turbine, so on and so forth)
First, lets familiarize our selves with the terminology
Absolute pressure - atmospheric pressure(14.7 psi at sea level) + boost/vacuum pressure
Pressure Ratio - The ratio of the absolute pressure measured at the compressor outlet vs the absolute pressure at the compressor inlet. In for form of : outlet/inlet.
Let me elaborate on this one: if your compressor is sucking in air, you are lowering the pressure inside of the intake tube portion of your intake system by a given amount. On the reverse side, the outlet of the compressor is of course creating your "boost" by compressing more air into the given volume which increases your outlet pressure. So in a sense, the pressure ratio is a ratio of boost over vacuum created by each side of the compressor itself.
Mass flow ( lbs/min) or Volumetric flow (cfm) - to put it in lamens terms, this is amount of air being moved through the motor in a given amount of time. You can calculate this in a few ways. I personally use volumetric flow since it is easier for me to grasp the concept of cfm than it is lbs/min. With a stock bore/stroke sr20det for example I know that I have a given cc for my cylinders, multiply by your desired peak power RPM operating range and you will end up with ccm. use simple math to convert cubic centimeters to cubic feet and you will have pretty close to your desired CFM for peak power. Garrett suggests that as a rule of thumb, it takes 1 lb/min to generate about 10 hp. So this means that if you want a top notch 400hp engine you want a turbo with a flow rating that can reach about 40. To convert CFM into lb/min, you simply divide your CFM by the air density in your area, this should give you a good idea of lbs/min.
Compressor Efficiency - this is a percentage term which relates to (uh) the efficiency of your turbo. Of course, the higher the number, the better. The effeciency of a compressor essentially measures the benefits to each additional measurement of work being produced by the turbo vs the costs in both volumetric efficiency as well as compressor damage. For example, Ever wonder why you cant just let your little t-25 boost to 50 lbs to make your car fast? The reason is that after 12-14 psi or so the compressor loses efficiency drastically, and so each additional lb of boost put more of a strain on the turbo, as well as your intake charge. By finding a turbo that is slightly larger you are moving your compressor efficiency islands (I will show you with pics soon) more in the area that is suitible for your desired boost.
Surge line - this is pretty much self explanitory. Compressor surge is when your compressors flow backs up causing your fins to attempt to stop. Most people are familiar with this problem when they have an inoperating BOV, but it is also a common problem with a turbo that is too large for the vehicle. Lets say that you have a t88 on a 1.5 liter motor that doesnt rev very high. Now, imagine you are trying to get 11 lbs of boost out of the turbo, in this low of an operating range. Your engine would not be able to consume the air fast enough for the compressor to flow properly, so the air would constantly be backing up into the intake system.
compressor rpms - self explanitory. The rate at which your compressor wheel is spinning
Now, lets take a look at some compressor maps shall we? http://www.turbobygarrett.com/turbobygarrett/images/catalog/Turbochargers/gt25_images/GT2554R_471171_3_comp_e.gif
This is a Gt2554 R, which has characteristics similar to the turbos we have stock on our SRs. Notice how on the right hand side you have your choke point, which is where the compressor no longer produces any air at an acceptable efficiency rate. notice also, that inbetween 12 and 21 lb/min that little island in the middle has the highest effeciency rate at about 71%. this means that using the pressure ratios described, and flowing inbetween 12 and 21 lbs/min, you get the best effeciency out of this turbo. What does this mean for you? Running at peak efficiency, if you use garretts rule of thumb, this turbo would be good for about 210hp... but dont let that fool you into thinking that is the MAX it can produce. If you pushed the turbo to its limit, notice at about 60% efficiency you can produce almost 280HP. Do you want to though? Depends on how reliable you want your set up to be.
Lets take a look at another one...
http://www.turbobygarrett.com/turbobygarrett/images/catalog/Turbochargers/gt30_images/GT3071R_comp_t.jpg
Notice the peak efficiency island on this turbo. Starting at about 23 and ending at about 37 lbs/min, this turbo proves to be pretty versatile. a turbo that has a 77-78% efficiency from 230-370HP? This turbo is one of the more versatile turbos on the market. For lower boost applications (look at how low the pressure ratio starts for the island) the turbo maintains a high efficiency. As a matter of fact, this turbo really doesnt drop much under 70% until about 430 hp.
Lastly, lets take a look at a larger turbo so we can get a solid view of compressor surge.
http://www.turbobygarrett.com/turbobygarrett/images/catalog/Turbochargers/gt35_images/GT3582R_714568_1_comp.gif
This is the garrett GT3582R. notice it holds a peak efficiency from 26 to about 45 lbs/min. Now you might say "well why get the gt3071R if this turbo has a better top end and its efficiency starts at only 3 more lbs/min?"
The answer is this: notice the upper left corner of the graph? How it has that large "dent" in it? That is your surge line. Unless I run the motor in an RPM range that keeps this turbo up in the 26-45 lbs/min range, this compressor will surge more than your pants if Jessica Alba walked by nude. sure, this compressor has alot better top end flow characteristics, but its low end ability to safely produce power is not there. Also, notice its requirements necessary to keep the efficiency at a high level. You have to increase the pressure ratio dramatically as your increase your flow rate, or the compressor chokes out, like wise, if you choose to flow less air (operate at a lower RPM) you need to drop your pressure ratio in order to keep it happy. So this turbo has a fairly limited use compared to a gt3071r. However, for certain people (high boost or high rpm situations) this may be a better turbo for them.
Please feel free to ask any questions since I seem to have tapped out my "originality" function of my brain and I can't think of what else I need to explain. If you don't like my way of explaining things, you can visit the garrett website at www.turbobygarrett.com. They go into more detail from an engineering standpoint on selecting turbos. I am simply here as sort of a translater to all of that info for you. :)