“Holsets don’t require a restrictor!” “Yes, they do!” I have seen so many threads in forums about whether or not to run a restricted fitting in a Holset turbo but the argument is flawed from the get-go because it’s the wrong question altogether. The question should be what are the oil requirements for the turbo? For that, we can get information from the manufacturer from the Holset HX30/32/35/40 Service Repair Manual. Here are the requirements:

6. Oil should be filtered to 10μm with efficiency of 60% TWA (Time Weighted Average) /20 μm with efficiency of 85% TWA. Efficiency assessed using ISO Standard 4572/SAE J 1858.
7. The oil quality must be as specified by the engine manufacturer and will be a minimum API SE – CD (MIL - L - 2104C) specification. Improved life can be obtained by using super high performance diesel (SHPD) oils, particularly in industrial applications which use extended oil drain periods.
8. Normal oil temperature is 95+/-5°C (203+/-9° F). It should not exceed 120°C (248°F) under any operating condition.
9. Any pre-lube oil must be clean and meet the minimum CD classification.
10. The orientation of turbine housing, bearing housing and compressor cover is fixed according to application. During installation, do not attempt to rotate these components. Inclined turbocharger installation is not recommended. If an installed angle is necessary, oil inlet centreline must be +/- 10 degrees from vertical and rotor centreline +/- 5 degrees from horizontal.
11. Holset permits oil return pipes to decline at an overall angle of not less than 30 degrees below horizontal. All turbocharger applications require a pipe of internal diameter greater than 19 mm which has integrated connectors. To ensure oil returns into the engine under all operating conditions, the return connection into the engine sump must not be submerged and the outlet flange of the turbocharger must be 50 mm above the maximum oil level of the engine sump pan. Crankcase pressure should be limited ideally to 0.8 kPa (0.12 lbf/in2) but 1.4 kPa (0.20 lbf/in2) can be accepted by reference to Holset.
12. Oil pressure of 150 kPa (20 lbf/in2) must show at the oil inlet within 3 - 4 seconds of engine firing to prevent damage to turbocharger bearing system. A flexible supply pipe is recommended.
13. The minimum oil pressure when the engine is on load must be 210 kPa (30 lbf/in2). Maximum permissible operating pressure is 500 kPa (72 lbf/in2) although 600 kPa (88 lbf/in2) is permitted during cold start up. Under idling conditions pressure should not fall below 70 kPa (10 lbf/in2).
14. Recommended oil flows for the turbochargers are 2 litre/min at idle and 3 litre/min above maximum torque speed.

Let’s talk about the bolded content above.
  • Orientation. I have seen pictures of some pretty questionable setups with respect to how a turbo sits on the manifold. The turbo should be as close to plumb and level as possible. Five degrees off level and ten degrees rotation on the vertical axis is not a lot, so get those turbos positioned the way they should be.
  • Drain. The drain tube should be 19mm or greater. That is a 12AN fitting, not a 10AN. Additionally, it needs to drain ABOVE the oil line, not the side of the oil pan. If you use a smaller drain, then you will most likely need to restrict the oil coming into the turbo or it will cause an issue. I am guilty of running a 10AN line!
  • Oil Pressure. The oil pressure requirements are self-explanatory but to the opening questions, it doesn’t give a regulator specification because there isn’t one. It specifies the pressure the turbo needs in order to have a long life.


Currently, when someone buys a turbo from us, we provide a 4AN restricted fitting. If the engine falls within the oil requirements above, the fitting does NOT need to be restricted and can be drilled out easily. If the pressure is too high or the drain is too small, then restricting the oil intake is probably warranted.

In some cases, we have also provided 10AN flanges. I run a 10AN on my HX40 Super with no restrictor. I should not be doing this but myself and a lot of other people do the same. Again, if running a smaller drain, you may want to slow the oil intake down a little or better yet, switch to a 12AN/19mm setup.

To wrap things up, Holset turbos are extremely tolerant of oil pressure but there are requirements and all the answers are not always found in the forums. Consult the manufacturer’s service manual and evaluate your own engine’s performance to understand what you need to do to give you turbo a long productive life.  If you are still worried about it, check out the Turbosmart Turbo Oil Pressure Regulator


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