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    How to Read Compressor Flow Maps (TURBO for BOT and Upgrade)


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    10092008 How to Read Compressor Flow Maps (TURBO for BOT and Upgrade)

    Post by blugu

    This article is meant to help those who are
    considering either turbo charging their N/A car or upgrading their
    factory turbo to a bigger unit. For the purposes of this document, it
    is assumed that anyone reading is familiar with some basic turbo
    terminology and function along with some simple math.

    Turbo terms to know:

    1. Compressor and Turbine wheels
    2. Wheel “trim”
    3. Compressor and Turbine Housing A/R

    These items are the key elements that define a turbocharger and
    determine its flow characteristics. If you don’t already know how to
    interpret information about A/R or wheel trims and sizes, try searching
    the boards here at AF or looking online. I know Garrett has a lot of
    info on their site. (

    For those ready to dive into the vast world of turbo selection, we’ll start by looking at a sample flow map.

    *** Will be updated ****

    From the top of the chart we can see that this is a Garret TO4B turbo with and S-3 compressor wheel.

    Understanding the Axis:

    1. First we will start by looking at air flow through the turbo
    measured on the x-axis. Garrett uses lb/min on their maps while other
    companies like Mitsubishi use cubic feet per minute (cfm). Since I
    think it’s easier to work with cfm, we’ll convert. Every 10 lb/min is
    equal to 144.72 cfm, remember this.

    2. The pressure ratio measured on the y-axis is merely the ratio of
    air pressure leaving to the turbo to air pressure entering the turbo.
    Since atmospheric pressure at sea level is 14.7 psi, if you were to run
    29.4 psi of boost, the pressure ratio would be 2.

    Understanding Information within the Map:

    1. The oblong ovals on the chart or “islands” as they are called
    represent the efficiency of the turbo in that range. As you can see on
    this map, the most efficient operation (73%) is in the very center of
    the chart. This is general characteristic of most turbochargers.
    Without getting into the thermodynamics of adiabatic heat-pumps, we’ll
    just say that efficiency is a measure of how much excess heat the turbo
    puts into the compressed air coming out of the outlet. So intuitively,
    more efficient is better.

    2. Wheel rotational speed is simply the rpm at which the compressor wheel is spinning.

    3. The choke point, which is usually not indicated on flow maps, is
    the maximum flow rating the turbo is capable of regardless of pressure
    or efficiency.

    4. Beyond the surge limit on the left of the plot, compressor surge
    occurs. In laymen’s terms, this phenomenon is caused by a back pressure
    wave entering the exit of the compressor housing and disrupting flow
    through the compressor wheel. Surge will kill turbos and is to be
    avoided at all costs.

    Selecting a Turbo

    Calculating your Engine’s Flow Requirements

    Now that you can read and understand a compressor flow map, its
    time to figure out how to match a turbo to your engine, this involves
    selecting the proper compressor and turbine wheels along with the right
    combination of housing A/R. A mismatched turbo could not only result is
    extreme lag, but also wasted potential as a turbo can easily outflow an
    engine. I.e. bigger is not always better.

    The only real calculation that needs to be done is to determine how
    much air you engine is actually flowing. This depends on a number of
    things including the RPM, absolute temperature (Rankin, equal to 460 +
    Fahrenheit temp), absolute manifold pressure (psi, equal to boost
    pressure plus atmospheric pressure), and lastly the engine volumetric
    flow or EVF in cfm.

    First to calculate EVF use the following equation:

    *****Will be updated*****

    Next we’ll use EVF to calculate the amount of air in lb/min the
    engine is flowing under boost and at temperature using this equation:

    ****will be updated*****

    Were N is the airflow in lb/min, P is the absolute pressure in psi, and T is the absolute ambient temperature in Rankin.

    Finally, multiply N by the volumetric efficiency of your engine
    (VE). This compensates for the fact that upon every cycle of the
    engine, not all of the old air/fuel mix in the cylinders is forced out
    the exhaust. Thus there is a difference between the actual airflow
    through and engine and the predicted airflow. This discrepancy is
    equated to a VE. There is literally thousands of hours worth of online
    reading about volumetric efficiencies for just about every production
    engine. To get the most accurate results from this step I would suggest
    researching your engine and coming up with the most realistic VE
    possible as this does have a significant affect on engine flow. If you
    are just messing around with compressor flow maps and need a value for
    VE just to experiment with, 85% efficiency is a nice conservative
    number for most modified turbocharged cars at high rpm (6500-7500).
    Keep in mind though that on a forced induction setup VE can easily
    exceed 100% so again it will be very beneficial to research your

    Determining the Best Wheel Trim-Housing A/R Combination

    With the flow rate you have just calculated, you can look at
    compressor maps of different turbo chargers to see which ones give you
    the air flow you need at the pressures and efficiencies that you want
    to run.

    When selecting a turbo, it is important to do the above calculations
    for a number of different RPM’s and boost pressures because you will
    not always be at redline under full boost while driving you car.
    Checking the turbo performance at various engine speeds and pressures
    will give the overall picture of how well the turbo is sized to your

    Matching a flow map to your engine flow requirements will allow you to
    pick the compressor wheel trim for your application. However before you
    can go out and purchase that new turbo, you still have to settle on an
    exhaust wheel and turbine A/R. The real determining factor in this
    selection is maintaining compressor wheel speed. Remember the wheel RPM
    lines on the flow map? Well a properly sized exhaust wheel/housing
    combination will keep the compressor wheel operating within the maximum
    and minimum wheel speeds on the map as often as possible. Since
    different “hot side” combinations can affect your turbo’s performance,
    (i.e. a little more lag in return for more top end, or quicker spool up
    at the cost of overall power) the best thing to do is to contact a
    turbo manufacturer or distributor (,,
    and they will be able to tell you the exact effects you can expect from
    all of the various hot side combos available for your turbo model.


    This article has covered the basics of understanding compressor flow
    maps, how to read them, how to select compressor wheels which best suit
    your engine and the best way to determine wheel trims and housing
    A/R’s. Hopefully, this sheds some light on the mysteries of turbo
    selection and should give you a little more confidence when purchasing
    that next thousand ringgit upgrade.

    Turbocharger Basics

    A turbocharger is an air pump that is driven by exhaust gas. It
    pressurizes the intake to allow more air and fuel to go into the
    cylinder. More air and fuel means more horsepower.

    The two sides of the turbo are the turbine side, which is spun by
    exhaust gases as they leave the engine, and the compressor side, which
    is driven by the turbine side and which compresses the intake air.

    The size of the turbine and compressor wheels, and the size and
    shape of their housings, affects the efficient range of the turbo.

    The turbine size has an impact on how much power the engine can
    produce. A large turbine will pose little resistance to outgoing
    exhaust gas, so the engine will be able to make more horsepower, but a
    large turbine will spin up to speed ("spool up") much more slowly. A
    small turbine will spool up quickly, but will choke down the exhaust
    and limit horsepower.

    The compressor side has less of an effect on spool-up, but
    generally a larger compressor will produce cooler compressed air, and
    will be able to generate more turbo boost, but will take a little
    longer to spin up to speed.

    The moral of the story is there is no free lunch. A larger, more
    efficient turbo can produce more boost, but will spin up more slowly
    and will not work as well at low rpms. A smaller turbo will have fast
    spool-up but weak high-rpm horsepower.

    What does this mean? Factory turbos are usually sized for
    around-town driving, not top-speed autobahn use, so they're smaller
    turbochargers with low lag and modest maximum boost levels. The 95-99
    turbo cars have very small turbochargers, but also exhibit almost no
    "turbo lag." The 90-94 cars had a bigger turbo, with a bit more lag,
    but still very fast spool-up.

    At the other end of the spectrum are monster race turbos. These
    turbos will require very high volumes of exhaust gas just to get them
    spinning. This means no usable boost until 5000 rpms or more. This kind
    of turbo can produce enormous boost levels and huge horsepower numbers,
    but the car will be practically undrivable on the street, with weak
    low-rpm power, abrupt on-boost transition, and huge top-end power.

    Mitsubishi 16G

    This is factory issue on several Mitsubishi products in Japan (like the
    Lancer Evolution), and bolts directly to the Eclipse exhaust manifold.
    (2G cars will require adaptor hardware for oil lines and the outlet
    pipe.) It uses an internal wastegate.

    There are two variants of the 16G. The "small" unit maxes out
    around 350-375 horsepower. The "big" unit has a larger compressor
    section that is good for close to 400 horsepower, but produces less
    boost at low rpms.

    Garrett T-25

    This is the smallest turbo listed here. It's factory equipment on
    the 2G (95-99) Eclipse and Talon turbos. It spins up almost from idle,
    and makes for good low-end torque on the 2G cars. It has a hard time
    making more than about 10psi of turbo boost above 5500 rpms.

    If you use a mechanical boost controller to try to get more boost
    at high rpms, you may eventually ruin the bearings from running it
    faster than its design parameters allow.

    Maximum horsepower for a motor equipped with this turbo is about 250.

    XS Engineering "16G Killer"

    This turbo is an IHI model that is popular with Japanese tuners --
    it is used in A'PEXi turbo upgrade kits, for example. It has higher
    flow than the small 16G turbo, but a ball-bearing center section means
    it can spin up faster, and saps less horsepower from the car when it's
    spinning. It mounts to the Mitsubishi exhaust manifold using a custom
    adapter plate.

    Garrett T3/T4 and T4/T4

    These Garrett turbos can be custom-made to almost any
    specification. The biggest problem with them is the Mitsubishi exhaust
    manifold does not bolt to the Garrett turbine housing, so you have to
    fabricate an adaptor plate, or a whole custom exhaust manifold to bolt
    them up. You see them mostly on high-end race cars as a result.

    Mitsubishi 14B

    The 14B is the stock turbo on the 1G (89-94) turbo
    Eclipse/Talon/Laser. It's a little bigger than the 2G turbo, so it
    spins up slower, but is good for more top-end boost. It can maintain
    12psi to redline, and can produce 20psi at lower rpms.

    Maximum horsepower: About 300

    Mitsubishi 20G

    This turbo is identical to the 16G on the turbine side, but has a
    much larger compressor section. The small turbine means good spool-up,
    but the large compressor section can produce very high airflow volumes
    to make 25psi of boost or more, and a maximum of around 450 horsepower.

    The high volumes of exhaust produced by this much boost mean the
    internal wastegate is no longer adequate, and a 20G is normally
    installed with the internal wastegate welded shut and replaced by an
    external wastegate.

    "Frankenstein" turbos

    The Frankenstein series turbos are assembled by Texas Rebuild
    Turbos. The name comes from the hybrid nature of these turbochargers.
    Most of them use Garrett center sections and compressor wheels, with
    Mitsubishi housings and turbine wheels.

    The ball-bearing center section makes for good spool-up and durability.

    Here are a couple of Frankenstein models and their specs. The other
    numbered turbos have performance that ranges between these.
    Frankenstain 5 and 6 are race-only turbos that can produce 600
    horsepower and beyond.

    Other Tweaks

    As we mentioned before, the second-generation MAS is MUCH larger
    and better flowing than the first-generation. Plus, it can flow more
    air before it starts to lose accuracy. That naturally leads to...why
    not put a 2G MAS on your 1G car?

    Well, you can. There are a couple of things you have to do to make it work.

    First, the wiring is different. You'll need to splice a few wires to adapt the sensor to your car.

    Secondly, the 2G sensor outputs different values, so your car won't
    run right with it. HOWEVER, in a convenient coincidence, it happens to
    run around 20% too lean. If you put in 550cc fuel injectors, your car
    will run about 20% richer at all times. And these two seem to just
    about cancel each other out. No guarantees here, but we've heard of
    this being done more than a few times.

    Fuel Cut

    "Fuel Cut" is the common term for what happens when the ECU detects
    what it thinks is too much air entering the engine. Under normal
    conditions, the car would almost never see this much air volume, so if
    you modify the car (say, by turning up the turbo boost) and more air is
    flowing in, you may reach the point where the ECU says "no more!"

    When the ECU sees this level of air flow, it immediately reacts by
    SHUTTING OFF fuel to the engine. If you're in a low gear, it feels like
    you hit a wall. Even in third of fourth, it can be a pretty scary
    feeling, as the engine just shuts down for a second or two, then comes
    back online.

    There are a couple of solutions to this. One is to fool the
    computer into thinking there is LESS air coming in than there really
    is. There's room for this, since the factory fuel program is so
    conservatively rich. You can modify the airflow sensor, use the Super
    AFC, or both, to reduce the airflow measurement that the ECU is seeing.

    The other (more precise) way of getting around this problem is to
    have your ECU reprogrammed. Usually we see cars exhibiting fuel cut at
    around 18psi of boost at normal temperatures. At colder temps, it can
    happen earlier, and it seems to vary from car to car as well. Note that
    if you're seeing fuel cut at lower, or normal, boost levels, you may
    have an intercooler pipe leak that is letting pressurized air escape
    under boost.

    "I already pulled all the honeycombs out of my MAS..."

    "...and now my car runs like crap. What can I do?"

    We get this question all too often. You've probably heard of the
    old carpenter adage, "Measure twice, cut once." Here it's READ UP on
    this stuff before you just rip parts out of your car.

    Without the honeycombs in your MAS, it CANNOT figure out how much
    air is entering into the motor. The car will not run well under part or
    full throttle. Gas mileage will probably be terrible. It's very
    difficult to get the honeycombs back in there -- they usually get
    destroyed when you remove them. So the remedy is usually to get a new
    or used replacement airflow sensor.

    Info's from the other forum, posted by an OTAI using the nick of Super91
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      Current date/time is 22/10/2018, 12:56 pm