om617 Restrictor Size for the HE200 Series Holset

A while back, I posted a couple of blogs and videos on the need for limiting oil pressure on the Holset turbos and om617 oil pressure specifically.  My recommendation thus far has been to use an oil pressure regulator but not everyone can flip another $100 for one and the cost for additional oil lines.

When I originally made these posts, I failed to realize that customers were still unsure of how to proceed with their installs until I had a conversation with my customer, Sean, this week.  He let me know that there appears to be a gap in the market on the M10 fittings and not sure where to turn.  He tried buying an oil line kit, but the reseller would not complete the order because he didn’t buy the turbo from them and he didn’t want to come out of pocket for the oil pressure regulator from Turbosmart.

I didn’t realize that the community was struggling for this information.  I know looking for info in forums can be questionable, but I thought solutions were readily available.  Therefore, to put this to rest, I commissioned A. Elshenawy who holds a Bachelor of Mechanical Engineering from Alexandria University, Germany to give me an answer to our question.  This is specific to the Holset HE200 series and the om617!

I will be sourcing fittings and lines to make customer installations easier and appreciate the dialog with Sean to bring this to my attention.

Below is Mr. Elshenawy’s response:


    The HE200 series Holset turbos need a continuous operating pressure lower than 50 PSI, but the engine lube oil pump is capable of generating as much as 105 PSI, this will damage the turbo sealing if we didn’t reduce it, and this is what a restrictor will do.

    What is the restrictor?

      This is a mechanical part used to reduce the oil pressure before entering the turbo, in our case we need to reduce the pressure from 105 PSI to 50 PSI.  The restrictor applies the law of hydraulic losses (Bernoulli equation) to make pressure drop when the oil passes through it.

      What do we mean by hydraulic losses?

      Hydraulic losses are due to two main factors:

      1. Friction losses due to flow through a pipe with surface roughness
      2. Local losses due to sudden change of pipe area or flow direction.

      The restrictor is affected by the local losses as it has a hole with a small diameter, which means a sudden change in the area, and then pressure drops across it.

      Note: we can neglect the friction losses as the hole length is not big enough to make a pressure difference and quality machining should limit its impact

      Formula for the Calculation:

      1. Frictional losses (major losses):

        This is the loss due to the oil friction inside the pipe and it depends on the flow speed and the pipe material, but we will not work on it as we need to see the effect of the restrictor only.  As explained above, we will ignore this.

      2. Local losses (minor losses):

        This is the pressure drop across the fitting (restrictor) and it depends on the shape and area ratio between the pipe and the connection.

        hL ……………………………. Local head losses (m).
        KL ...…………………………. Local losses dimensionless coefficient.
        V ……………………………. Flow speed through the restrictor (m/s).
        𝛒  ……………………………. Oil density at 90oC (15w40) (835.2 kg/m3).
        g ……………………………. Gravitational acceleration (9.81 m/s2).
        𝛥P …………………………. pressure drop across the restrictor (bar).

      Restrictor Calculation

      1. Inputs:
        1. The maximum pressure for the turbo.
        2. The engine pressure (to get 𝛥P).
        3. The oil density.
        4. The geometric shape of the restrictor (to get KL).
        5. The engine oil flow rate (6 L/min)
      2. Outputs:
        • The oil velocity through the restrictor.
        • The cross-sectional area of the hole / The hole diameter.
        1. Calculated values:
        2.  Turbo max pressure (PSI) 


          Restrictor hole diameter (mm)



          Calculation steps to check the conditions of 3 bar engines:

          If we reverse all our calculations, and we get the pressure drop using the hole diameter above we get the values in the table below (we assume that the flow rate become 3 L/min): 

          Turbo max pressure (PSI)


          Actual pressure (psi) as engine pressure become (3bar) 30.63


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