compressor maps!
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This is a discussion on compressor maps! within the Tuning: Electronic Engine Management forums, part of the Tech & Modifying & General Repairs category; Hey everyone im somewhat new here, and im a little confused on how to read a compressor map. if anyone ...

  1. #1
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    compressor maps!

    Hey everyone im somewhat new here, and im a little confused on how to read a compressor map. if anyone could offer any help?

    I saw a lot of threads explaining it im just still a little confused.

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    Registered User poly_poly-man's Avatar
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    So, the graph is actually 4dimensional, with 2 independent variables (inputs) and 2 dependent variables (outputs).

    Along the x-axis, is air flow (or similar) - this scaled for units is called load in the ecu. Basically, it's relative to RPM, VE, throttle angle, and much more. The y-axis is usually PR, which is the ratio of the total pressure out of the compressor to the total pressure into the compressor. The higher your boost, or the lower the atmospheric pressure, the higher this number is (2.0 PR means about 14.7 psi boost - in other words, the pressure of the atmosphere on a gauge).

    If you knew the engine load (airflow) and the boost you wanted to run at that airflow, you'd put a point on the graph. Which "oval" this point is in determines how efficient the turbo is (how much work is converted to boost, how much is converted to heat), and an interpolation of the horizontal lines will tell you at what compressor rpm you are. In reality, people general start with a known airflow range (based on logging or theoretical calcs), and look to see how much boost they can run at each rpm. A funny part about the TD04 map, for example, is that, as much as it looks like you can run 18-20 psi, you really can't hold it all the way to redline - the efficiency is still good, but you're over-revving the compressor.

    Anyway, here's one of my logs superimposed on a compressor map for my turbo:


    What you should be able to garner from that: I have a ton of room for expansion in the middle rpm range, especially if I got a 3-port BCS so as to keep it steady when I get there. The little drop around the middle of the range is just because my clutch slipped for that run, so it skewed that data weirdly. Basically, I'll eventually aim for about 2.35 at about 3-4k (corresponding to a boost of about 20 psi), tapering down with the compressor rpm as my limiting factor.
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    what do those percentages mean?

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    Registered User poly_poly-man's Avatar
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    the percentages are labelling the ovals, they are compressor efficiencies. Basically, 100% would mean that the compressor is not heating up the air whatsoever while it is compressing it (this really isn't possible), and 0% would mean that it can no longer compress the air; it is just creating heat. It's good to stay in the centermost ovals as much as possible, or in other words, try to stay in the 70's most of your rpm range.
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    how can you convert the pressure ratio over to PSI

    and is the left side of the map not used at all? i guess thats where im somewhat confused

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    Registered User poly_poly-man's Avatar
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    Quote Originally Posted by Kurtisg123 View Post
    how can you convert the pressure ratio over to PSI

    and is the left side of the map not used at all? i guess thats where im somewhat confused
    PR is basically (boost + atmospheric pressure)/(atmospheric pressure) - although I believe IC pressure drop might add to it too.

    Everything to the left of the last oval is compressor surge - basically your turbo makes a nasty noise and isn't really making any boost. With the TD04, you really don't have to worry about it, since you can't spool up that early, even with all the spoolfast mods.

    Here's a good link for more info about that: TurboByGarrett.com - Turbo Tech103
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    Quote Originally Posted by poly_poly-man View Post
    especially if I got a 3-port BCS so as to keep it steady when I get there.
    Also sorry for this stupid question but whats a BCS ?

    And thank you im just trying to figure out how to read maps and how to figure out what MAX psi is on a map, and just trying to familiarize myself with everything.

  9. #8
    Registered User poly_poly-man's Avatar
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    the boost control solenoid is basically the bridge between the electronics in the ECU and the boost system. Its fundamental operation is keeping the wastegate closed when you're building boost, and properly cycling so as to allow just the right boost. An aftermarket EBCS (E=electronic) is a little more sensitive, and more stable, so it can more accurately give you the WGDC (wastegate duty cycle) that you need, especially where the stock starts falling off in the higher boost. You may want to read up on how subaru's boost control works for a little insight into that department.

    As for reading max PSI from a map, first of all, that doesn't mean much, secondly, it really depends what you're willing to do.

    Let's take a couple examples.


    This is one of garrett's smaller turbos, more adequately suited for a 4cyl engine than most. You'll only be able to run, regardless of what engine you put this on, maybe up to a PR of 2.5 (although most would probably try to keep it under, but close to that, tapering off following that 112krpm line) - that's about 21psi. However, 21psi on that is no where near the same amount of power as 21psi on a td04, because of the difference in airflow. Anyway, the .15 line on the mistubishi map corresponds with about 24 lbs/min airflow. In other words, for a 2.0l engine at least, this turbo is a tad big, but not excessively big. You'll get noticable lag, but probably somewhere in the high 200's or low 300's for whp. It's a little better, but not too much better on a 2.5l. Now take this one:

    Again, no matter what the engine, I'd keep the boost mostly on the south side of 2.5 PR (about 21psi) (actually, there is a bit of room near 2.75 in the middle of this thing's range, but take the same PR for the sake of argument) - however, that 21psi is going to make A LOT more power on a car that can take it, because of all the airflow. Unfortunately, a tiny little engine like ours is going to have a hell of a time feeding that thing all the air it needs to spin - that .15 mark on the mitsu map, corrseponding with somewhere in the 5k-6k range, when the engine is almost done spinning, is right about where that turbo is going to start spinning. This is a great turbo for a v6 or v8, but honsetly, even on the 2.5's, this is a tad ridiculous.

    Hope that helped...
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    okay its making more sense thank you..

    DO you want to stay in the very center circle?

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    Quote Originally Posted by poly_poly-man View Post
    You may want to read up on how subaru's boost control works for a little insight into that department.
    also you have any links off hand where i can find this?

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    Moderator Donkey's Avatar
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    The problem with compressor maps is just that,they only involve the compressor. The turbine wheel and housing A/R can make a large effect on how the compressor operates and the speed of the compressor.

    Stealth 316 - Turbocharger Compressor Flow Maps
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  13. #12
    Registered User poly_poly-man's Avatar
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    Quote Originally Posted by Kurtisg123 View Post
    also you have any links off hand where i can find this?
    romraider forums, but particularly the "subie newbie tuning guide" should help.
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