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Thread: How To: Read Compressor Maps

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    How To: Read Compressor Maps

    This guide isn't meant to be an in-depth look at turbo sizing, but rather to give the reader a working knowledge of how to read compressor maps. In other words, to decipher what all those swirling lines mean when choosing a turbo compressor.

    Terminology

    Compressor - This is the "cold" side of the turbo that sucks in intake air and compresses it for the engine to later combust with fuel.

    Turbine - This is the "hot" side of the turbo. Hot exhaust gasses pass through it, expanding and cooling. This expansion spins a turbine wheel that drives the compressor wheel via a shaft. Unfortunately turbo manufacturers don't make turbine maps available to the general public.

    Absolute Pressure - This is pressure referenced from a pure vacuum. Most calculations done involving compressors use absolute pressure. Note - 1 atmosphere = ~14.7 psia (Absolute pressure in pounds per square inch) = 0 psig (gage pressure in pounds per square inch). Your boost gauge reads in psig, referenced to local atmospheric pressure.

    ONTO THE MAPS

    Surge - This is lowest amount of airflow a compressor can supply at a given pressure ratio(getting to that). Any pressure above this at this airflow, the compressor will "gulp" air. This is not good for your turbo, or your power output. Fortunately you have to saddle a pretty huge compressor with a small turbine to really worry about this effect.

    Here is a compressor map with the surge line highlighted in red.



    On the X-axis(horizontal) you'll notice the mass airflow of the compressor in lbs/min. On the Y-axis there is the Pressure Ratio. Pressure ratio is defined as follows:

    Atmospheric Pressure + Boost Pressure = Pressure Ratio
    Atmospheric Pressure

    So the astute reader will notice a pressure ratio of 1.0 is the exact same as atmospheric. A pressure ratio of 2.0 is equivalent to 1 atmosphere or ~14.7 psig in your intake manifold. Without concrete data proving otherwise, it is always the best course of action to assume the pressure is ambient at the compressor inlet and make note of the pressure drops of the system will in the end cause less horsepower to be produced than the mass flowrate of the turbo would suggest.

    The oval shaped rings on the compressor map are efficiency islands. These are regions where the compressor has approximately the same efficiency at compressing the air. Of course, the higher the efficiency the better since the compressor will be introducing less unneeded heat into the charge air. Note that as you go away from the maximum efficiency island, you always go down in efficiency. By the time you're off the map you're usually in the <60% range, which is not a good thing.

    The lines that slope from the surge line to the right and down across the efficiency islands are constant speed lines. This would be really useful if you could match up the speed of the compressor to the speed of the turbine and find out its efficiency and mass flowrate for that shaft speed, but since we don't have turbine maps we're kind of at a disadvantage there for picking the ultimate turbo match. The maps used here out of Garrett's publicly available catalog aren't too detailed. Some maps will have much more data like putting RPM values on the speed lines, more efficiency islands etc.

    I won't go into the hard equation to calculate the mass airflow of the engine, as it really doesn't gain anybody any further insight into the turbo selection process. The only important things to understand that the big factors in how much mass airflow an engine is consuming are:

    • Engine Displacement
    • Volumetric Efficiency(how well the engine breathes)
    • Pressure at the inlet valves(BOOST!)
    • RPM


    By altering these things(more displacement, cams to increase VE, more boost, more RPMs) you can make the engine combust more air and make more power. I'll be attaching a spreadsheet that makes calculating the airflow of an engine an easy matter. It does over simplify things since it doesn't vary VE by RPM and whatnot, but it is a reasonably close approximation. I use a VE of 90% in most my calculations. It is pretty close to what a modern 4 valve engine gets in high RPMs, and tends to be conservative on less modified engines.

    So go ahead and download the spreadsheet and we can look at a compressor map.

    Here I'll look at a GT30R turbo on an S52B32 engine with a VE of 90%, displacement of 3.2L and maximum RPM of 7000. For the first go, we'll see what happens at a modest boost level of ~8.7 psi(pressure ratio of 1.6).



    How I evaluate compressor maps is to note the airflow at 2000 RPM. Find that on the X-axis and draw a straight line from that point at a PR of 1 to the airflow at 3000 RPM at your desired PR(1.6 in this case). This gives you an idea of how a typical turbo will look when spooling up, and let you know if it's at a risk of surging. From there, the line should stay at that PR all the way to the airflow at redline ~39 lb/min here.

    As you can see, surge is not a problem here, but this turbo sure does look a bit too small for this sized engine! It goes off the map just before redline, so that means it is very inefficient at higher revs.

    Let's see what happens when we up the boost to ~17.4 psi(PR of 2.2).



    No risk of surge due to this being a large engine for the turbo, but boy does it ever get REALLY inefficient at higher revs. Past 6000 RPM it is again off the map.

    Let's go to a slightly larger turbo, a GT35R, to see the difference. Same boost of 17.4 psi(PR of 2.2).



    Now that is more like it! Notice how the engine spends a good amount of time in the really efficient islands, and the turbo is still at 72% efficiency all the way to redline. I'd think this turbo would be putting out in the 450-500rwhp range at this boost on this engine, and that's probably being a bit conservative. If the VE of the engine was even higher(which it can be), this turbo could still put out even more power. The compressor map also suggests it has a bit more headroom on this particular engine.



    I hope that was helpful to everybody, and gives people a start in the right direction on reading compressor maps themselves. If you want to modify the Excel airflow chart I attached, just extend the RPM row and copy the formula in the airflow cells for CFM and lb/min over below the RPM and it should fill in correctly. You can change the displacement, VE and pressure ratio in the parameters section to get an idea of how these change things.


    Enjoy boost junkies!

    -edited for pics and engrish-
    Attached Files Attached Files
    Last edited by Def; 09-22-2006 at 07:07 PM.

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    Pleeeeaassseee fix the pictures, this is definitely a great thread!
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    sticky!
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    It's stuck.......you need to fix the pics though to a more reliable (and long term) host.
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    Sorry, that's about all I could find for free hosting. If anybody wants to offer up a place to host the pics that is reliable I'll edit the post.

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    Def, zip up all the pics, email them to me (nick@techniquetuning.com) and I'll host them (and fix the links in your original post).
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    Quote Originally Posted by Def
    Sorry, that's about all I could find for free hosting. If anybody wants to offer up a place to host the pics that is reliable I'll edit the post.
    Use http://imageshack.us/index3.php for now.
    Anyone wants to buy my e36 m3 please contact me at soldbycam@gmail.com

    Quote Originally Posted by jszy25 View Post
    Get drunk with Mike Radowski they said, it'll be fun they said...A broken toe, the worst hangover known to man, and bite marks in my arm said otherwise

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    Quote Originally Posted by NickG
    Def, zip up all the pics, email them to me (nick@techniquetuning.com) and I'll host them (and fix the links in your original post).
    Sent you an email Nick. Just need to know the path to where the files will be residing.

    Sorry about the pics and diction folks - you can really tell I was writing a lab report for 6 hours straight right before I wrote this up - the writing errors are horrid! I'll give it a quick edit later tonight.

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    Quote Originally Posted by Def
    Sent you an email Nick. Just need to know the path to where the files will be residing.

    Sorry about the pics and diction folks - you can really tell I was writing a lab report for 6 hours straight right before I wrote this up - the writing errors are horrid! I'll give it a quick edit later tonight.
    You can rehost them really quick with the link I gave above. I use it for my sig & averything...never a red X at all!

    You can open up a few pages & send multiple pics at a time to speed up the process. I find that 800 width works just about perfect for this forum. Doesn't stretch it out in order for scrolling to occur.
    Anyone wants to buy my e36 m3 please contact me at soldbycam@gmail.com

    Quote Originally Posted by jszy25 View Post
    Get drunk with Mike Radowski they said, it'll be fun they said...A broken toe, the worst hangover known to man, and bite marks in my arm said otherwise

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    Hope that fixes everything guys. My brain has been fried lately with all this worthless school stuff.

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    Question when you are considering the compressor maps don't you have to add or factor in addition loss like intercooler, air filter, and piping what is that loss normally 3 to 4 psi ?

    So then you would have to factoring in what the compressor put out 3 to 4 psi more ?
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    Quote Originally Posted by M3TurboCa
    Question when you are considering the compressor maps don't you have to add or factor in addition loss like intercooler, air filter, and piping what is that loss normally 3 to 4 psi ?

    So then you would have to factoring in what the compressor put out 3 to 4 psi more ?
    Def's info was really helpful to me, much appreciated. Before his post I thought compressor maps were like modern art! interesting but meaningless to a Philistine like myself.

    If you want to go the next step, you can calculate a closer approximation for PR and for corrected airflow. The Garrett site has a PDF with this info:

    PR = (Boost + Intercooler Drop + Atmosphere)/(Atmosphere - Air Filter Drop)

    Example: Intake manifoold pressure (boost) = 12 psi
    Pressure drop over intercooler = 2 psi
    Pressure drop through air filter = 0.5 psi
    Atmosphere = 14.7 psi at sea level

    PR = (12+2+14.7)/(14.7-0.5) = 2.02 in that example

    The corrected airflow allows for ambient air temperature and pressure.

    Corrected flow = (Actual Flow*(Square Root(Air Temp+460)/545))/(Baro/13.95)

    Example: Engine air consumption (a la Def spreadsheet) = 50 lb/min
    Air temp = 60oF
    Baro = 14.7 psi

    Corrected airflow = (50*(Square Root(60+460)/545)/(14.7/13.95)) = 46.3 lb/min
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    I don't really think approximating values like that is a smart thing to do. What are you basing them off of, and how do you know if they're even close to your setup? Just arbitrarily choosing values to get the answer "closer" to what you *think* it is is rarely a good idea - in fact, that's something you NEVER do in a scientific experiement because you just can't even begin to quantify decisions like that. The best course of action is to do the calculations with known data, THEN realize that you'll have some amount of error. How much is a question of your setup and the ambient conditions.

    Corrected mass flowrate is also a useful thing, but who's to say how hot the air is going into the inlet of the compressor, its humidity etc. Again, you can just calculate it straight then realize there is some error(unless you feel like gathering the data at the compressor inlet and averaging this to get your mass flowrate slightly more accurate).

    Glad you guys have gotten something out of this post. I learned to read these things when I got into turbos, so I figured I'd pass it along.

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    how do i find the flow rate for my engine? at a given rpm?
    e30 turbo

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    Download the spreadsheet I have attached at the top of this thread.

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    Unzip it and open it with Excel. Change top values and read off the airflow at the bottom. Look up a compressor map and compare.

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    i dont have excel but i appreciate everyones help
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    I haven't used it, but I think this stuff is free and will read .xls files.

    http://download.openoffice.org/1.1.3/index.html


    PS - you should really get MS Office or some equivalent, it's pretty much a standard format for documents/files these days.

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    After reading some info on turbonetics is states: "Select a compressor such that the engine torque peak operating point is near to the compressor peak efficiency island as possible while maintaining the engine rated speed point in the compressor 60% or higher efficiency region."

    When you size a compressor then where would you want the eff in torque peak or where you flow max on the top end ?

    Thanks
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    Help

    Jacob,

    Thanks for your info. I wonder if I'm missing something here?
    Last edited by MrBlonde; 11-11-2004 at 03:19 AM.
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    Quote Originally Posted by MrBlonde
    What am I getting wrong here?

    I think you're looking at turbos that a tad small for your power goals. 35 psig on an 8.5k RPM 3.2L engine is going to flow GOBS of air(as you see from your mass air calculation).

    Looks like you might need to step up to an even bigger turbo if you want to run really efficient there.

    Although from your plotting, it looks like the GT42 would be an "alright" match. Power would probably roll off up top due to it dropping in efficiency though.



    How much power are you trying to make with this engine? 900+rwhp? You might want to look into what some of the BIG hp Supra guys are running and start looking at those compressor maps instead of the GT series(which has large gaps between compressors at high power levels as you see).

  22. #22
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    Quote Originally Posted by M3TurboCa
    After reading some info on turbonetics is states: "Select a compressor such that the engine torque peak operating point is near to the compressor peak efficiency island as possible while maintaining the engine rated speed point in the compressor 60% or higher efficiency region."

    When you size a compressor then where would you want the eff in torque peak or where you flow max on the top end ?

    Thanks
    If you want a really strong top end powerband, then I'd size the compressor so you're just getting into the peak efficiency island as you near torque peak. That way the compressor maintains peak efficiency after the volumetric efficiency starts dropping due to passing the torque peak. That'll help do away with the torque "pyramid" that some poorly sized turbo setups have *cough*all AA's kits*cough*

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    Jacob thanks for the information very helpfull.

    A question if Im running say 12 psi do I figure pr as .8 or pr of 2.0 for system loss as the compressor is actually flowing say 14.5 psi but getting 12 in the manifold.
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    FYI.
    I noticed that your calculation of boost pressure is incorrect in the spreadsheet.

    I changed my to input boost and calculate pressure ratio - (boost + 14.7)/14.7

    p.s. thanks for the spreadsheet

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    is there anyway someone can do this for me cuz im very confused and i cant open the program

    i have a m20 2.5l... with a t3/t4 60-1 with a stage 3wheel and a .63ar. running 10-15 psi. at that boost is my turbo efficient in there??
    e30 turbo

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