Turbo System Optimization (2024)

The GarrettMotion.com website contains a lot of helpful information. It contains product information about turbos, intercoolers, and turbo kits. It explains Garrett’s model nomenclature, it goes over Garrett’s technology and product development as well as our involvement with OEM’s and Motorsports. It contains tech tutorials written by the engineers from Basic, to Advanced, up to the Expert level where detailed formulas are used to plot operating points on compressor maps to help select the correct turbo.

It also contains News and what events we’ll be at throughout the year as well as a distributor locator. The website has extensive information, both general and technical. No matter what your experience level, you’ll find information that will help you with your specific application or simply increase your knowledge of turbos and turbos systems.

Application Information

The most important things to understand before designing a system is the application use and your horsepower target. Is it going to be used for road racing, drag racing or drifting or maybe a street driven car? The intended use greatly affects the turbo selection as well as the system components.

A turbo system that works well for a 9 second drag car is most likely not going to work well for a drift car or road race car (find out more about how a turbo works). You also need to have a target flywheel horsepower in mind. The horsepower value will be used to help design the entire system. A turbo that is too large will spool very slow and a turbo that is too small will not make the power you desire.

Turbo Matching

Go to www.GarrettMotion.com or Boost Adviser.com

Every turbo has a range for horsepower and displacement. These values are on all of the performance turbo pages. The key to identifying your potential turbo matches lies within these ranges. What is your target horsepower? What is your engine displacement? Next, find the turbochargers that align with your requirements.

  • Click on Turbo Tech • Read Turbo Techs 101, 102 and 103.
    • Using formulas in Turbo Tech 103, calculate mass flow and pressure ratio (PR) at redline for your specific application.
    • Plot mass flow and PR on several compressor maps to determine the best fit.
    • For the example in this presentation, the “application” will be a 400 flywheel hp street car using pump gas, therefore the estimated mass flow ~ 40 lbs/min

Air Filter Selection

It is important to appropriately size the air filter for the maximum flow rate of the application. A target face velocity of ≤130 ft/min at redline is used to minimize restriction and to provide the turbo with the air necessary for it to function optimally. If the turbo does not have access to the proper amount of air, excessive restriction will occur and cause:

• Oil leakage from the compressor side piston ring, which results in oil loss, a fouled intercooler 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 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/mn) / Air Density (lbs/ft³)

Volumetric Flow Rate = 526 CFM
For twin applications divide the flow rate by 2

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

Area (ft²) = CFM / Face Velocity (ft/min)

Area (ft²) = 526 / 130 = 4.05

Area (in²) = 4.05 x 144

Area= 582 in²

How to determine filter size once you know the calculated area

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

Area (in²) = 9.00 x .55 x 60 x 2

Area = 594 in²

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

Oil Supply & Drainage

Ball Bearing Turbo

An 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 to ensure the components function 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.


Journal Bearing Turbo

Journal-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.

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 +/- 15° when installed in the vehicle on level ground. If a gravity feed is not possible, a scavenge pump should be used to ensure that oil flows freely away from the center housing.

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.


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

Water Line Optimization

Water 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 co*king 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 water-cooling system, avoid undulations in the water lines to maximize the Thermal Siphon Effect.

For best results, set the orientation of the center housing to 20°. Significant damage to the turbo can occur from improper water line setups.

Click here to read the full white paper on water cooling

Calculating the Correct 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 1.5 times greater than the tubing diameter. The flow area must be free of restrictive elements such as sharp transitions in size or configuration.

For our example: Velocity (ft/min) = Volumetric Flow Rate (CFM) / Area (ft²)
• 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

Selecting a charge air cooler (aka intercooler) has been made simple with the Intercooler core page. Each core has a horsepower rating, making it 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 provides 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 damping 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 turbocharger 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.
– 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.


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 to an external wastegate:
• 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.

Oil Leakage

A properly installed turbo should NOT leak oil. There are, however, instances where oil leaks occur. Here are the most common causes, depending on the location of the leak.

Leakage from compressor and turbine seals

– Excessively high oil pressure
– Inadequate drain – drain is too small, does not go continuously downhill, or the location of the drain inside the oil pan is located in a section that has oil slung from the crank causing oil to back up in drain tube. Always place oil drain into oil pan in a location that oil from crank is blocked by windage tray.
– Improper venting of crankcase pressure.
– Excessive crankcase pressure.
– Oil drain rotated past the recommended 35°.

Leakage from compressor seal

Excessive pressure across the compressor housing inlet caused by:
– Air filter is too small.
– Charge air tubing too small or has too many bends between the air filter and compressor housing.
– Clogged air filter.

Leakage from Turbine seal

– Collapsed turbine piston ring from excessive EGT’s.
– Turbo tilted back on its axis past recommended 15°

Testing Your Turbo System

Many problems with turbo systems can be identified before the catastrophic happens through simple system testing.

Pressurize system to test for leaks

  • Clamps – Check tightness
  • Couplers – Check for holes or tears
  • CAC core / end tanks – Check for voids in welds


The turbo system in your car should be monitored to insure that every aspect is functioning properly to give you trouble-free performance.

Instrumentation used to monitor / optimize system: The most accurate way to calibrate and optimize a system is through data logging!

  1. Oil Pressure (Required to monitor engine operation)
    2. Oil Temperature (Required to monitor engine operation)
    3. Water Temperature (Required to monitor engine operation)
    4. A/F Ratio (such as a wideband sensor; required to monitor engine operation)
    5. Manifold Pressure
    6. Turbine Inlet Pressure
    7. Exhaust Gas Temperature
    8. Turbo Speed Sensor

Manifold Pressure

– Calibrate actuator setting to achieve manifold pressure required to meet hp target
– Detect over-boost condition
– Detect damaged actuator diaphragm

Back Pressure

– Monitor pressure changes in turbine housing inlet
– Affect of different turbine housing A/R’s
– Increased back pressure decreases Volumetric Efficiency thus decreasing ultimate power


– Monitor exhaust gas temperature (EGT) in manifold / turbine housing
– Adjust calibration based on temperature rating of turbine housing material or other exhaust components Turbo Speed
– Determine operating points on compressor map
– Determine if the current turbo is correct for the application and target hp
– Avoid turbo over-speed condition, which could damage turbo

11 Point Checklist

  1. Application Information – target horsepower, intended use of vehicle, etc.
    2. Air filter sizing – determine size for application needs
    3. Oil Supply – restrictor for ball-bearing turbo
    4. Oil Drain – proper size and routing
    5. Water Lines – set up for greatest thermal siphon effect
    6. Charge Tubing – determine diameter for application needs
    7. Charge-Air-Cooler – determine core size for application needs, design manifolds for optimal flow, mount for durability
    8. BOV – VTA for MAP engines and by-pass for MAF engines
    9. Wastegate – connect signal line to compressor outlet, smooth transition to external wastegate
    10. System Testing – pressurize system to check for leakage, periodically check clamp tightness and the condition of
    11. System Monitoring – proper gauges/sensors to monitor engine for optimal performance and component durability
Turbo System Optimization (2024)
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