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. (www.egarrett.com)

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
engine.

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
vehicle.

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 (www.extremeturbo.com, www.forcedpeformance.com, www.turbochargers.com)
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.

Summary

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