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Turbo Systems Engineering

4. System Components

Air Filter

It is important to appropriately size the air filter for the maximum flow rate of the application.
For our specific example, we are looking for target face velocity of <=130 ft/min at redline to
minimize restriction so as to provide the turbo with all the air necessary for it to function optimally.
If the turbo does not have access to the proper amount of air, excessive restriction can occur and cause:

  • Oil leakage from the compressor side piston ring, which results in oil loss, a fouled CAC
    and potentially smoke out of the tailpipe.
  • Increased pressure ratio, which can lead to turbo overspeed.
  • Overspeed will reduce turbo durability and could result in an early turbo failure.

Determining the correct air filter size

Face Velocity = 130 ft/min
Mass Flow = 40 lbs/min
Air density = 0.076 lbs/ft3

Mass Flow (lbs/min) = Volumetric Flow Rate (CFM) x Air Density (lbs/ft3)

 Volumetric Flow Rate (CFM) = Mass Flow (lbs/min) / Air Density (lbs/ft3)

Volumetric Flow Rate = 526 CFM

**For twin turbo setups, simply divide the flow rate by two.

Face Velocity (ft/min) = Volumetric Flow rate (CFM) / Area (ft2)

Area (ft2) = Volumetric Flow rate (CFM) / Face Velocity (ft/min) 

Area (ft2) = 526 / 130 = 4.05

Area (in2) = 4.05 x 144

Area = 582 in2

Now that we know the required surface area that our air filter must have, we need to determine
the correct air filter size using information provided by the filter manufacturer. We will
need to know the following information about the filters we are considering:

  • Pleat height
  • Pleat depth
  • Number of pleats


Pleat Height = 9.00

Pleat Depth = 0.55 in.

# of Pleats = 60


Area (in2) = pleat height x pleat depth x # of pleats x 2

Area (in2) = 9.00 x 0.55 x 60 x 2

Area = 594 in2

Actual Filter Area (594 in2) > Calculated Area (582 in2)

Since the actual filter area (594 in2) is greater than the required area, this air filter will work for our application.

Oil Supply & Drainage

Journal Bearing Turbo

Journal Bearing TurboJournal-bearings function similarly to rod or crank bearings in an engine - oil pressure is required to keep components separated. An oil restrictor is generally not needed except for oil-pressure-induced leakage. The recommended oil feed for journal bearing turbochargers is -4AN or hose/tubing with an ID of approximately 0.25.
Be sure to use an oil filter that meets or exceeds the OEM specifications.

Ball Bearing Turbo

Ball Bearing TurboAn oil restrictor is recommended for optimal performance with ball bearing turbochargers. Oil pressure of 40 - 45 psi at maximum engine speed is recommended to prevent damage to the turbocharger’s internals. In order to achieve this pressure, a restrictor with a 0.040' orifice will normally suffice, but you should always verify the oil pressure entering the turbo after the restrictor in insure that the components are functioning properly.
Recommended oil feed is -3AN or -4AN line or hose/tubing with a similar ID. As always, use an oil filter that meets or exceeds the OEM specifications.


Oil Drain
In general, the larger the oil drain, the better. However, a -10AN is typically sufficient
for proper oil drainage, but try not to have an inner diameter smaller than the drain hole
in the housing as this will likely cause the oil to back up in the center housing. Speaking
of oil backing up in the center housing, a gravity feed needs to be just that! The oil
outlet should follow the direction of gravity +/-35° when installed in the vehicle on level
ground. If a gravity feed is not possible, a scavenge pump should be used to insure that
oil flows freely away from the center housing.

  • Undulations in the line or extended lengths parallel to the ground
  • Draining into oil pan below oil level
  • Dead heading into a component behind the oil pan
  • Area behind the oil pan (windage tray window) where oil sling occurs from crankshaft

When installing your turbocharger, insure that the turbocharger axis of rotation is parallel
to the level ground within +/- 15°. This means that the oil inlet/outlet should be within
15° of being perpendicular to level ground.

Water Lines
Garrett_water_cooling_instructionsWater cooling is a key design feature for improved durability and we recommend that if your turbo has an allowance for watercooling, hook up the water lines. Water cooling eliminates the destructive occurrence of oil coking by utilizing the Thermal Siphon Effect to reduce the Peak Heat Soak Back Temperature on the turbine side piston after shut-down. In order to get the greatest benefit from your watercooling system, avoid undulations in the water lines to maximize the Thermal Siphon Effect.



Negative degrees: water outlet of center housing is lower than water inlet
Positive degrees: water outlet of center housing is higher than water inlet

For best results, set the orientation of the center housing to 20°.

Significant damage to the turbo can occur from improper water line setups.


Charge Tubing
The duct diameter should be sized with the capability to flow approximately 200 - 300
ft/sec. Selecting a flow diameter less than the calculated value results in the flow
pressure dropping due to the restricted flow area. If the diameter is instead increased
above the calculated value, the cooling flow expands to fill the larger diameter, which
slows the transient response.
For bends in the tubing, a good design standard is to size the bend radius such that it is
1.5 times greater than the tubing diameter.
The flow area must be free of restrictive elements such as sharp transitions in size or

For our example: 

  • Tubing Diameter: velocity of 200 - 300 ft/sec is desirable. Too small a diameter will increase pressure drop, too large can slow transient response.
  • Velocity (ft/min) = Volumetric Flow rate (CFM) / Area (ft2)



Again, for twin turbo setups, divide the flow rate by (2).

Charge tubing design affects the overall performance, so there are a few points to keep in
mind to get the best performance from your system.

  • Duct bend radius:
       - Radius/diameter > 1.5
  • Flow area:
       - Avoid area changes, sharp transitions, shape changes
  • Available packaging space in the vehicle usually dictates certain designs


Selecting a Charge Air Cooler (aka intercooler) has been made simple with's intercooler core page. Each core is rated for horsepower,
making it as easy as matching your desired power target to the core.
In general, use the largest core that will fit within the packaging constraints of the application.


Another important factor in selecting the correct intercooler is the end tank design. Proper
manifold shape is critical in both minimizing charge air pressure drop and providing
uniform flow distribution. Good manifold shapes minimize losses and provide fairly even
flow distribution. The over-the-top design can starve the top tubes however. The side
entry is ideal for both pressure drop and flow distribution, but it is usually not possible
due to vehicle space limitations.


Proper mounting of the intercooler increases the durability of the system. Air to air
charge air coolers are typically "soft-mounted", meaning they use rubber isolation
grommets. This type of mounting is also used for the entire cooling module. The design
guards against vibration failure by providing dampening of vibration loads. It also
reduces thermal loads by providing for thermal expansion.


Benefits of Isolation:
  • Guards against vibration by damping loads
  • Reduces thermal loading by providing for thermal expansion


Blow Off Valves (BOV)
Using the proper blow off valve (BOV) affects the system performance. There are
two main types to consider.

  • MAP (Manifold Absolute Pressure) sensor uses either a vent-to
    atmosphere valve or a recirculation valve.
    • Connect signal line to manifold source
    • Surge can occur if spring rate is too stiff

  • MAF (Mass Air Flow) sensor uses a recirculation (bypass) valve for best drivability. Garrett_Bypass_Valve
    • Connect signal line to manifold source
    • Position valve close to the turbo outlet for best performance (if valve can handle high temp).
    • Surge can occur if valve and/or outlet plumbing are restrictive.


Wastegates Garrett_Internal_Wastegates

Internal wastegates are part of the turbo and integrated into the turbine housing. Two connection possibilities exist for signal line. The first is to connect line from compressor outlet (not manifold - vacuum) to the actuator. The second is to connect a line from compressor outlet to boost controller (PWM valve) and then to the actuator. Manifold pressure is limited by the spring rate of the actuator. Most OEM style actuators are not designed for vacuum, and thus, the diaphragm can be damaged resulting in excessive manifold pressure and engine damage.

External wastegates are separate from the turbo and integrated into the exhaust manifold rather than the turbine housing. Connection to the manifold greatly affects flow capability, and correct orientation of the wastegate to the manifold is essential. For example, placing the wastegate at 90° to the manifold will reduce flow capacity by up to 50%! This greatly reduces the control that you have over the system and puts your entire drivetrain at risk. Instead, the ideal connection is at 45° with a smooth transition.

There are two connection possibilities for signal line:

  • Connect a line from the compressor outlet (not manifold - vacuum) to the actuator
  • Connect a line from the compressor outlet to a boost controller (PWM valve)
    and then to the actuator
Again, manifold pressure is limited by spring rate of actuator.
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