Just a quick guide I wrote up on turbocharging today. There seem to be a few forced induction threads out there and hopefully this will help demystify turbocharging for those considering it. Enjoy!
Turbocharging Information
It seems in the mustang world that the supercharger is king and rightfully so. Paxton, Eaton, and KB have all left their mark in the mustang world with proven combinations that make great power. The success of the 03/04 Cobras is a testament to the maturity of mustang supercharging. While not as rich of a history, turbocharging also has had it's place in mustangs. The SVO of the 80's one such car. To this day there are ardents of the 2.3 turbo platform abound. With the turbocharger designs and pricing we have today we are prime for a new renaissance of turbocharged cars. Hopefully this "Cliff's Notes" guide to turbocharging will help you make an informed decision should you decide to forcefeed your car.
How does it work?
If you are already familiar with superchargers, thats easy. It uses exhaust gases intead of a belt to spin your compressor wheel. You may now go on to the next section. :)
For those of you not familiar, this picture may help:
Image source: http://www.turbomustangs.com/turbotech/main.htm
when the exhaust gases pass over the turbine wheel it causes it to turn. The turbine wheel is connected to the compressor wheel by a shaft. This way when the turbine wheel turns, the compressor wheel turns. The compressor wheel spinning is what pressurizes the air between the compressor outlet and your engine.
Turbine Details
The turbine wheel of a turbocharger is enclosed in a housing that will direct exhaust gas over the turbine wheel to make it spin efficiently. The consequence of this is some restriction in exhaust gas flow. The restriction of flow creating backpressure becomes more extreme as you try to push more and more exhaust gas through the housing (i.e. when your engine's RPM increases). A properly sized turbine housing will allow efficient spinning of the turbine wheel relative to exhaust gas flow. This is where the turbine housing's A/R comes into play.
What is A/R Anyway?
The A/R of a turbo's housing is a ratio that shows the relationship between the cross sectional area of the throat and the distance from that cross sectional area to the center of the wheel. See the graphic:
Image source: http://www.junkyardturbos.com/Turboc...Area-Ratio.php
As the above illustration shows, as the area increases, so does the radius proportionally so that the ratio stays constant.
What this means is that for two turbos that have the same overall housing size, a lower A/R number means that the cross sectional area of the throat is smaller. This in turn means that for two turbos with the same housing size, the turbo with a lower A/R number will spool faster compared to a higher A/R number but will hit its ability to flow gas sooner (read: become a performance impediment sooner).
What Is "Trim"?
In addition to the diameter of a compressor or turbine wheel, the "trim" of the wheel gives more details about the behaviour of the gas flow. This term applies to both compressor and turbine wheels but is more common in compressor wheel discussion.
If you look at a turbine or compressor wheel from the side view, you will see that the fins of the wheel are "wider" at the bottom than at the top. For a compressor wheel, the diameter at the top of the wheel is called the Inducer Diameter. The diameter at the bottom of the wheel is call the Exducer Diameter. The positions of the inducer and exducer diameters are reversed on a turbine wheel due to the direction of gas flow across the wheel.
Image source: http://www.junkyardturbos.com/Turboc...Area-Ratio.php
Image source: http://www.junkyardturbos.com/Turboc...Area-Ratio.php
The trim of a compressor wheel is a numerical value derived from the following formula:
Compressor Trim = (I/E)^2*100
Where I = Inducer Diameter
E = Exducer Diameter
For example, if we have a 78mm wheel with a 55mm inducer diameter, then:
Compressor Trim = (55/78)^2*100 = 49.72 ~ 50
So this compressor wheel would be a 78mm, 50-trim compressor.
A compressor wheel's trim is an indication of the tradeoff between flow and efficiency. The higher the trim, the better the flow but the lower the efficiency. The "50-trim" turbos are a great compromise between flow and efficiency which is why these turbos are so popular.
So, What Size Turbo Do I Use?
We're going to ignore the turbine side for the moment so that we can discuss how to select a compressor using a compressor map. Everything I am going to discuss here can be found at Garrett's website:
http://www.turbobygarrett.com/turbob...o_tech103.html
In a nutshell, its flow, not PSI, that determines your potential power output. The relationship between flow and power is expressed in the following equation:
Wa = HP * Afr * BSFC/60
Where:
Wa = airflow required
HP = horsepower target
Afr = air-fuel ratio
BSFC = Brake Specific Fuel Consumption (0.55 is a good middle-of-the-road number to use)
So for 500hp at an air-fuel ratio of 11.5:1 with a BSFC of 0.55, we need an airflow of:
Wa = 500 * 11.5 * 0.55 / 60 = 52.71 lb/min of flow.
Notice that the airflow does not rely on engine size or RPM. However, how much boost you need to achieve the desired flow does depend on engine size and RPM as shown in the following equation:
MAP = (Wa * R * (460 + Tm))/(Ve * N/2 * Vd)
Where:
MAP = Manifold Absolute Pressure (psia) required to meet the horsepower target
Wa = Airflow in lb/min
R = Gas Constant = 639.6
Tm = Intake Manifold Temperature (degrees F)
Ve = Volumetric Efficiency
N = Engine speed (RPM)
Vd = engine displacement (Cubic Inches, convert from liters to CI by multiplying by 61.02, ex. 2.0 liters * 61.02 = 122 CI)
In our example, if we are looking at getting 500hp out of our Mustang 5.0, lets make the additional assumptions based on a well-intercooled turbo system:
Wa = 52.71 lb/min as calculated above
Tm = 130 degrees F
Ve = 90% at peak power (not unrealistic assuming you have a good set of heads on the engine)
N = 6000 RPM
Vd = 302 CI
Then:
MAP = (52.71 * 639.6 * (460 + 130))/(0.90 * 6000/2 * 302) = 24.39 ~ 24.4 psia (or 9.7 psig boost).
(Note that both calculations above are using a single turbo system. If you are going to use a twin turbo setup, then cut your airflow in half (since each turbo will handle half the airflow) and use half of your engine's displacement for the MAP calculation.)
Now that we know both the airflow and the amount of boost we need (neglecting pressure losses going through an intercooled system), we can look at the compressor maps of various turbos in order to figure out a suitable turbo.
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Originally Posted by biminiLX
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