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When Does Your Turbo Need an Oil Restrictor?
One of the most common questions we receive is whether a turbocharger requires an oil restrictor. The answer depends on several factors, including the turbocharger’s bearing system, the engine’s oil pressure, the oil feed setup and the manufacturer’s specifications.
Many ARASHI turbochargers are supplied with a suitable oil restrictor or oil inlet fitting, whether they use a journal-bearing or ball-bearing system. However, you should never select a restrictor based on bearing type alone. Proper oil pressure must be maintained to ensure reliable lubrication without overwhelming the turbocharger’s internal oil-management system.
What Does a Turbo Oil Restrictor Do?
A turbo oil restrictor is a fitting with a calibrated opening installed in the turbocharger’s oil feed line. Its purpose is to control the amount of oil entering the bearing housing.
Contrary to a common misconception, a restrictor does not increase oil pressure. It reduces the available oil flow and can help lower excessive pressure at the turbocharger inlet.
This is particularly important when the engine produces more oil pressure than the turbocharger requires. Excessive oil supply can overwhelm the bearing housing’s oil-management system, potentially allowing oil to enter the compressor or turbine housing.
However, using a restrictor that is too small can be equally harmful. Insufficient oil flow may reduce lubrication and cooling, leading to excessive heat, accelerated bearing wear or premature turbocharger failure.
Do Journal-Bearing Turbos Need an Oil Restrictor?
You may have heard that journal-bearing turbochargers do not require oil restrictors. In many applications, this is correct.
A journal-bearing system uses pressurized oil to create a hydrodynamic film between its moving components. This film prevents direct metal-to-metal contact while lubricating and cooling the shaft and bearing surfaces.
Because journal bearings generally require a greater volume of oil than ball-bearing systems, an oil restrictor is usually unnecessary unless the oil pressure entering the turbocharger is excessive. A restrictor is generally not required for ARASHI journal-bearing turbochargers except when oil-pressure-induced leakage occurs.
Installing an unnecessarily small restrictor on a journal-bearing turbo may limit the oil needed to maintain the hydrodynamic film. Therefore, the restrictor should not be used simply as a precaution or as an automatic solution for oil smoke.
Do Ball-Bearing Turbos Need an Oil Restrictor?
Ball-bearing turbochargers generally require less oil volume than journal-bearing turbochargers. For this reason, many ball-bearing models require a restrictor.
The restrictor helps keep the oil pressure within the operating range specified for the bearing cartridge. If the pressure is too low, the bearings may not receive sufficient lubrication and cooling. If it is too high, excessive oil may collect inside the bearing housing and contribute to oil leakage from the compressor or turbine side.
For many ARASHI ball-bearing performance turbochargers, the recommended oil pressure is approximately 40–45 psi at maximum engine speed. An orifice of approximately 0.040 inch, which is close to 1 mm, will often achieve this range. However, a 1 mm restrictor should not be treated as a universal specification for every ball-bearing turbocharger. Always follow the requirements for the exact turbo model being installed.
When Is a Turbo Oil Restrictor Required?
An oil restrictor may be required under the following conditions:
The Turbo Manufacturer Specifies One
The turbocharger manufacturer’s specifications should always take priority. Use the supplied fitting or the recommended restrictor size whenever one is included with the turbo.
The Engine Produces Excessive Oil Pressure
High-performance engines, upgraded oil pumps and certain custom oil-feed locations may supply more pressure than the turbocharger is designed to receive.
In this situation, a restrictor may be used to reduce the oil supply. For engines with particularly high or widely varying oil pressure, a dedicated turbo oil-pressure regulator may provide more consistent control than a fixed restrictor. A restrictor reduces pressure by limiting flow, while a regulator is designed to maintain a target outlet pressure.
A Ball-Bearing Turbo Requires Controlled Oil Flow
Many ball-bearing cartridges require a lower oil volume than journal-bearing systems. A properly selected restrictor prevents the engine from supplying more oil than the bearing housing can drain effectively.
The Oil Pressure Has Been Measured Above Specification
The decision should ideally be based on an actual pressure reading taken at the turbocharger’s oil inlet with the engine oil at normal operating temperature.
Engine oil pressure can change with RPM, temperature, viscosity and oil-pump configuration. A restrictor size that works on one vehicle may not produce the same pressure on another.
How to Check Whether Your Turbo Needs a Restrictor
Before choosing or changing an oil restrictor, follow these steps:
1. Identify the exact turbocharger model and bearing system.
2. Check whether the turbo or supplied oil fitting already contains a restrictor.
3. Review the manufacturer’s recommended oil-pressure range.
4. Inspect the oil feed and drain lines for restrictions or installation problems.
5. Confirm that the crankcase ventilation system is functioning correctly.
6. Install an oil-pressure gauge at the turbocharger inlet.
7. Bring the engine oil to normal operating temperature.
8. Measure the pressure at idle and at higher engine speeds.
9. Select the restriction according to the measured pressure and the turbo manufacturer’s specifications.
10. Measure the pressure again after making any changes.
Do not assume that a particular orifice size will deliver the same pressure on every engine.
A turbo oil restrictor is not a universal component that should be added to every installation. Its purpose is to control excessive oil supply and help maintain the pressure required by a specific bearing system.
Journal-bearing turbochargers generally require a greater oil volume and often operate without a restrictor unless excessive pressure is present. Ball-bearing turbochargers commonly require more carefully controlled oil pressure, but the correct restrictor depends on the exact cartridge and engine setup.
Always use the fitting supplied with your ARASHI turbocharger when specified, inspect the complete oil-feed and drain system, and measure the actual pressure at the turbo inlet before making changes.
