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Engine Modification

  1. May 24, 2017 #1

    I'm an automotive technician with a question on engine oil flow following a performance modification.

    Can you folks help me with that?

    Thank you,

  2. jcsd
  3. May 24, 2017 #2


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    Rather than tell us you have a question, why not ask the question?
  4. May 25, 2017 #3
    Yeah we can help you with that. What is your question? For future reference, just post the actual question in the OP.
  5. May 25, 2017 #4
    OK. Thanks for your help.

    If there are any classic Japanese muscle-bike enthusiast reading, this is a 1982 KZ1000 "J" engine.

    The attached picture shows the cylinder block and head. This engine delivers oil to the camshaft bearings through the space between the 4 outer mounting studs, and the drilled holes in the block and head. The stud on the right is original equipment with an OD of 8mm. The stud on the left is heavy duty with an OD of 9.4mm. This is to strengthen the hold on the block and head in performance applications.

    To me, the larger OD studs in the same ID hole obviously lessens the space for oil to travel, and therefore restricts the volume of oil delivered to the camshaft bearings.

    Question #1: This is more geometry. The hole in the cylinder head is 10.8mm. Replacing the 8mm with 9.4mm studs, what ID should I enlarge the stud hole to accurately compensate for the reduced area for oil travel?

    Question #2: The stud holes in the block are 11.8mm, while the holes in the head are 10.8mm. Will the larger space for oil travel in the block, stepped down to the smaller space in the head before reaching the cam bearings, have any effect on oil flow looking at the adjustment I'm trying to make?

  6. May 26, 2017 #5

    Ranger Mike

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    Do not do a thing to the oil system. If you do you risk blowing the engine big time. The upper half of the engine gets plenty of oil. When you increase volume by hogging out the stud hole you risk taking away oil meant for the bottom end. You may swamp the top half with too much oil that will bleed thru the valve seals. It may stay top side and not be able to drain properly. Now you got a quart of taffy fluid under the valve cover and the valve train is submerged! My thinking is the engineers know oil control is key to proper horse power and already accounted for the volume differences. In out round track and formula cars we restrict top end oil to keep the problem outline above from happening.

    Don’t forget, oil lubricates and it COOLS. My take is - don’t monkey with it!
  7. May 26, 2017 #6
    That larger stud decreases the available cross-sectional area in that hole by almost 50%.

    If you increase the cylinder head's hole size to 11.8mm inside diameter, then the cross-sectional area that is open for oil flow will return to where it was before you installed the larger stud. However, then it will be the same size as the holes in the block. That will potentially change the flow characteristics.
  8. May 26, 2017 #7
    Hey Mike. I know exactly what you're saying, and I'm trying to maintain OEM oil flow. These aftermarket/oversized studs reduce flow to the top-end, I'm just looking into returning flow to what the Kawasaki engineers designed.
  9. May 26, 2017 #8
    I was never good in geometry, but I was thinking taking a mm off the head ID was about right, like you confirmed. And thanks also for responding to question #2.

    I was thinking there must have been a reason for the Kawi engineers to make the holes 1mm larger in the block than the head. So is that an advantage in oil flow? And will having 11.8mm across the board have a negative or positive effect on flow?

    I cannot enlarge the holes in the block equally to maintain the original step-down flow due to gasket hole IDs.
  10. May 28, 2017 #9
    Here's a terrible sketch showing what I'm looking at: original oil passage, after installing oversized studs, and enlarging the head hole ID to 11.8mm equaling the block hole ID.

    Can anyone tell me the difference in flow from original to oversized studs, with enlarged head hole to equal original oil passage area through the head, but losing the effect of larger to smaller ID from the block to head? And if there is a difference, is there a way to compensate without enlarging the block hole ID? Possibly a little less enlargement of the head hole ID?


  11. May 28, 2017 #10
    My amateur view shows me lost turbulence from eliminating the step-down, therefore the modification may overcompensate. And/or a loss of velocity in the head which may decrease flow, and the modification may undercompensate. Anyone with the correct formula?
  12. May 28, 2017 #11


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    Head Cross Sectional Area for Oil Flow:

    [(hole radius)^2-(stud radius)^2] x Pi

    (5.4)^2-(4)^2 = 29.16-16 = 13.2xPi = 41.3sq.mm. (100% of design)

    OEM w/HD stud:
    (5.4)^2-(4.7)^2 = 29.16-22.09 = 7.07xPi = 22.2sq.mm. (54% of design)

    Modified w/ HD stud:
    (5.9)^2-(4.7)^2 = 34.81-22.09 = 12.72xPi = 39.96sq.mm. (98% of design)

    Assuming there is enough structural material in the head, I would go with the 11.8mm enlarged head hole.
  13. May 28, 2017 #12
    Hey Tom,

    So you don't see any significant effect on flow no longer having a a step-down from block to head ID?

    Thank you...
  14. May 28, 2017 #13


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    No, in fact it seems to be a requirement from a flow standpoint. Just watch out for the structural integrety of the head and that a seal is maintained around the oversized holes.
  15. May 28, 2017 #14
    What do you mean "a requirement from a flow standpoint"? You mean equal block and head ID is a requirement?

    And the head ID at 11.8mm should not be a problem. The top of the head is sealed with a crush washer, and the head-to-block is sealed with an O-ring fitted for the 11.8mm ID of the block.
  16. May 28, 2017 #15


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    With the larger studs, yes. Without enlarging the holes, the larger studs block 1/2 of the oil flow capacity to the top end. Not conducive to "Live long and prosper."
  17. May 28, 2017 #16
    Let's say the original design oil flow was slightly reduced due to the larger passage in the block going into the smaller passage in the head which caused turbulence or something. Now I enlarge the head hole to equal the original oil flow area, but also eliminate that turbulence by having equal ID in the head and block (strait through flow). Is it possible that could cause an overcompensation of flow. If the head and block were both 10.8mm, and I drilled both to 11.8mm, I wouldn't have this question. See what I mean?

    Too much volume up top means not enough in the bottom end for the crankshaft.
  18. May 29, 2017 #17


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    Well, that's beyond my knowledge. My personal opinion is any turbulence would have had a minor effect.
    Perhaps @boneh3ad or @Nidum or @Chestermiller can shed more light on this.
  19. May 29, 2017 #18


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    (1) Useful information and diagrams post #459441

    (2) The oil system on this engine follows the common pattern of having a positive displacement oil pump and a system of oil ducts , nozzles and orifices to control the flow of oil in different amounts to the several places in the engine where it is needed .

    Ideally in such a design the ducts should be generously sized and the flow controlled by the nozzles and orifices alone . In this particular engine though the oil to the crankshaft and connecting rod journals is not directly controlled . It is sort of controlled by default depending on how much oil flow is going to other places in the engine .

    (3) Question asked is whether changing the flow area through that hole with a stud in it could cause problems . Best guess answers :

    (a) If the flow area is maintained at original value by opening out the hole as well as enlarging the stud then probably not .
    (b) If flow area is significantly reduced then near certainty that it will .

    Really depends on whether the flow area around the stud is more or less than the summed area of all the nozzles and orifices it is feeding . Generously more is ideal , about the same is problematic and definitely less is going to cause uncertain and erratic oil feed .

    (4) There are secondary considerations regarding altering dimensions of the stud and hole .

    (a) Any significant step increase in flow area in an oil duct can cause frothing .

    (b) Any significant step decrease in flow area can leave dirt traps
  20. May 29, 2017 #19
    What you would like to have is the same amount of oil flow through the annulus in the new design. Are any of the surfaces moving, or are they all stationary?
  21. May 29, 2017 #20
    Thank you all for your helpful advise!

    These oversized aftermarket studs have been out for many years with no public hardcore failures. That tells me the reduction in flow is not severe, but may have a high mileage or high RPM effect on the camshaft bearings. So I'm going to split the difference and enlarge the the cylinder head stud holes .5mm. This way I know it will help with any adverse effects of limited top-end flow, at the same time reduce any chance of over compensation.

    Thanks again.
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