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Oil pressure Questions/Concerns

792 Views 18 Replies 3 Participants Last post by  yaggs
Hey everyone, need to pick your brains:

Earlier this year I rebuilt my stock k20a2 (new oil pump). I used honda bearings on the crank so clearances were pretty spot on. The rod bearings I needed were not available from honda so I went with king xp standard rod bearings. The clearances were on the tight side of specs. I could have gotten a second smaller set to pair with the standard, but this would put me on the loose side of the spec. I opted to do one set of bearings and stay on the tighter side.

My current oil pressures with Mobil One 5w30 are (at 185 deg F oil temp):
32 psi idle (975 rpm for MM vibrations)
82 psi at 3k rpm, 87 psi at 4k rpm
At cold start-up and regular driving it gets to mid 90s psi

I’ve noticed the oil pressure relief valve opens around 80-85 psi (which coincides with info I’ve found). Basically, anytime I’m over 3k rpm it is open. I’m not sure on how the pressures are supposed to work in a healthy k20a2. I would assume that the valve should be closed when the engine is up to temp (throughout the rev range).
  • Am I over thinking this? Is it totally fine that my relief valve is open during most of my drive cycle (I’m on highway to/from work everyday @ around 3.5k to 4k rpm). My gut says that 100% flow at 70 psi is better than a “relieved”, “bled off”, or “bypassed” flow at 85 psi.
  • I’m considering trying out 0W20 to see where my pressures sit. This brings up a few concerns like film strength and the effect on the looser clearance items like camshafts and crankshaft.
  • I’d rather not change out the rod bearings if not absolutely necessary.
What would you all do if it was your DD?

Thanks everyone!
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Am I over thinking this? Is it totally fine that my relief valve is open during most of my drive cycle (I’m on highway to/from work everyday @ around 3.5k to 4k rpm). My gut says that 100% flow at 70 psi is better than a “relieved”, “bled off”, or “bypassed” flow at 85 psi.
Yeap, regarding efficiency. Once the valve opens, more flow is pumped around as is used for the tribological feed. The reverse flow but is needed for higher engine speeds to increase the capability for a wider bandwidth of engine speeds. Without it would aerate the oil much earlier on the sucking side. My DAMPFHAMMER runs at 3.4-3.5 bar from 3300 till 9300 rpm (~51 psi) on a 5W30 Amsoil Signature Series, bearings looking fine, no signs of wear.

If your engine goes well above 90 psi I would be concerned about the oiling system. The oil pump is a positive displacement pump, which pumps a volume, where efficiency and pressure are results of the resistances on the oil path. The pressure you see is on the high side, inefficiently high, but still ok.

I’m considering trying out 0W20 to see where my pressures sit. This brings up a few concerns like film strength and the effect on the looser clearance items like camshafts and crankshaft.
Are you still within the Honda specification of clearances? Your consideration is correct, but not only there, also in the main and journal bearings. With 0W20 I would go only with very good additives like the 0W20 of Amsoil Signature Series. It may fit, but I would be interested in the clearances for both.

I’d rather not change out the rod bearings if not absolutely necessary.
If within the spec, there is no reason to do so.

What would you all do if it was your DD?
Just doing what I've asked above.
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So are you saying that the bearings don't see a significant hit on flow with the valve open?
The pump pushs a defined volume into the pressure chamber, this volume is quite always the same and just influenced by suction conditions. If the resistance of the oil path sees an higher pressure, the flow balance between oil path and reverse path searches a new balance. Comparing two engine:
  • 50 µm clearance engine: oil pressure is at 3000 rpm 4.0 bar with 5W30 and 80 °C (as a victual example)
  • 35 µm clearance engine: oil pressure is at 3000 rpm 5.2 bar with 5W30 and 80 °C
The reverse flows of both are different, because of the higher restriction in the oil path of the low clearance system, therefore valve will open more to balance the pressure new. The valve controls the pressure in the chamber and is of course more opened with an higher pressure, but is easy on max, so that the pressure in the chamber can increase further. Finally the bearing oil flow is reduced by an certain amount in the 25 µm clearance engine. That's the cause I warn from using higher oil grade oils, it decreases the oil flow through the bearings and mostly comes with a greater use of viscosity enhancers, which can thermal crake. The only thing keeping the tribological film on life is oil flow, not pressure, hence the higher pressure comes with concerns.

Actually I have no simulation model of the oil system, not sure if I take that challenge, I would have to reduce my time, pretty busy on building the engine dyno exhaust system.

So besides being inefficient, is there any real cause for concern given the info I've just provided?
Yeap, but a bit on distance. If the clearances are small enough the tribologic distance of the upper bearing side get small, once the pressure increases further there is a likeliness to spin a bearing shell increases. We had a case here of a spun journal bearing, where pressure was massively increased. I just don't remember the member name, the thread title began with "Another..." and a word which was new to me, I believe for attempt and it was about a K20 engine built. That guy used Mahle 12 ccm pistons and his avatar included a road directory sign in green, so a high way directory sign. You find that guy in the Joe McCarthy 9.0 and header discussion. Search for his last post and you find the thread I mean.
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Reduced model of the oil flow system:
What you see below is a oil tank, an oil pump, a pressure chamber with two outlets: one to the engine and one to reverse flow path. The pump, which is connected to crank speed only, conveys the oil from the tank into the engine oil system as a main function. The the pressure of the oil system can be controlled, a too high pressure can disturb the function of the tribologic system, a relief valve is placed in that chamber. It opens at a certain oil pressure, balanced by the spring. As the spring delivers a force, F = spring rate * compression height difference, it is clear the pressure must increase to further open that valve. The distance from closed to fully open is around 3 mm, while around further 3 in the beginning are just from closed to slightest open. It is no linear opening at all.

Handwriting Rectangle Slope Font Line

For a model simplification we can say there is a opening pressure and a full open pressure which are different, but their distance is quite close together.

Scenario tight bearing:
The main pressure drop happens at the bearings obviously. The smaller the clearances the higher the restriction. The oil flow sees a restriction and immediately pressure rises as the positive displacement pump has no mercy, all must go off the pump into the pressure chamber. Here the oil follows the engine oil path till the point the relief spring give some opening to that valve (drawing double triangle = valve system). Thence a part of the oil flow, so much that the oil pressure get lowered to the release level by bypassing the engine oil path into the reverse feed (= suction line, see thick point connection). The reverse path is directly to the pump connected, maybe the drawing does not apply to locations, just on process rows for abstraction reasons.

If pump speed increases further more reverse flow is created while no higher oil flux goes into the engine oil path. Now you understand why clearance design and oil weight consequently define how much oil goes through the engine. If the pressure further increases, less oil sucking flow will go into the pump as the efficiency goes down and more is just pumped around the bypass and only a slight little more goes into the increase of engine oil path. The system is sort of saturated, efficiency controlled. This safes the engine integrity.

Scenario loose or right bearing:
At the releasing oil pressure more oil flow is supplied to the bearing, which gives it a better tribological integrity and more tolerance on higher engine speeds and every extra force on the bearings. The engine speed at which the relief valve opens is at higher engine speeds. But also here: there is a point where the system stalls and further increase of oil pressure just leads to tiny increases of oil mass flow through the engine and any increase goes into bypass flow increase until the system is saturated.

What is the outcome:

  • A remark here to the consequences: the valvetrain get more oil. The oil will be cooler and less peak temperature will be seen on hot spots like the small end rod pin or journal bearings. The viscosity design is a bit easier, as one can play with pump speed gear ratio, engine oil temperature and so on.
  • Same is valid for viscosity. A lower viscosity corresponds to loose or right bearing clearance and a higher to more tight bearing clearance. Thence for the sake of a higher oil film height less oil film is pumped through the engine. Is this more safe? It depends, but for sure a 5W50 is less safe then a 10W40. It might be worth testing to see if a 5W30 is better fitting the system as an 10W40 oil. For sure a 0W20 is not improving over a 5W30, as the oil film height is quite engine speed sensitive with those, especially at hot start up and low engine speeds.
The K20C1 comes with a highly Molybdenum containing 0W20 oil but has the same clearance recommendations as the K20A2 engine. That means fabrication tolerances has to be decreased to make that system working. I have contact to a oil passionated K20C1 Green Hell driver and he reports quite good wear results with 0W20 on the Gree Hell use. But the K20C1 has a higher ECT level as the K20A-series, no one knows why ;). Did I already elaborate why I don't like boosted engines 🤣?! Anyway, a 0W20 oil approach can be run on the K-series, but lower maintenance intervals and only good additive packages and base oils, no aftermarket oil from Penzoil, Castrol, ...I recommend Amsoil SS, Ravenol VSW or RSP, Quarker State FS, as they have shown good results even down to 0W20 levels.
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I suppose it depends on what rpm we are looking at. in either scenario, it seems like high rpm could prove harmful. But again, which is worse?
On a Newtonian Fluid we would see on any oil weight an increase in oil film height with engine speed increase, just because of the relative velocity increase over the oil film height. The thinner the oil the more it has a sensitivity regarding minimal oil film height with respect to bearing clearance. It just flows more easily out of the bearing during the rotational movement. In a scenario of 0.0011"-0.0016" this is no issue, but beyond 0.0030 this gets very critical. So, for our scenario the reduced oil supply is more critical than the oil film height loss. I recommend for that always OEM bearings, as the bearing fabrication tolerances and quality get more significant with thinner oils.

What type of oil change interval would you suggest?
You would do an amazing job if you do oil analysis application depended on every 2000 miles at Oil-Test companies to see how things develop. This is the best method of controlling the engine oil, this is the way we do it in racing. Otherwise for a daily with full year use and K-series like driving style (Iiiiiiiiiiiiiiiii want to hear the engine souuuuuuuund ;)) maybe annually one time or if shorter 10,000 miles. Dilution of the oil is a topic with those 0W20.
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I could not source the correct sized oem bearings.
What else did you use 👀?

I'm looking at quaker state 5w20 since it has a slightly higher viscosity at 100deg C versus the 0w20. You are 100% on the better additive package.
The Quaker State Ultimate Durability Full Synthetic is in 0W20 or 5W20 a top level oil regarding engine protection, which is in the level of the Mobil1 FS 0W40. Regarding Oil filtering, I would recommend BOSCH Premium oil filter if you have the clearance. It has one of the best filter results, which is preferable over slight pressure drop disadvantage (compared to high flow filters like K&N and stuff) on a SAE 20 oil weight. Low fabrication tolerances and a high filter quality support the low oil weight approach!
King xp standard rod bearings. I did get the oem crank bearings tho.
King is claiming for having smallest production tolerance in the market. I never measured them, so no validation of that from my side.

A german recommending bosch...hmm. jk. I have actually been looking in to those they seem good quality for the price.
I just recommend the Premium model of them because it is the best filter in the market regarding filter quantity. They fullfill the ISO 4548-12 testing with 99 % or 14 g, no other filter does this. The 20 grader oils need support on particle side, as it is more critical as on a higher weight oil. If you know a filter with a higher filter quality, take it. It is all about filter capability.
It shows a bunch of links to articles from King Bearings R&D director
Yes, I know Dmitri Kopeliovich and his two website's, SubsTech and SmoothSliding, who claims to be a "World leading expert (30 years of experience) in design, technology and materials for Engine bearings in applications such as automotive, renewable energy, aviation, racing and others". I didn't know him from the scientific publications side, just from his website. His scientific record is quite short and the number of quotes, I haven't found a single one, I found 2 primary author papers about material design of him. Anyway, as I am interested in oil stuff and made some experience with it at an OEM I read a 3 digit number on papers regarding oil and bearing stuff, not a single one among it from Mr. Kopeliovich. I can't copy his claim.

Anyway, the data he shares on his website regarding oil viscosity, oil film height and bearing clearance height is something I can copy, beside it is obvious it is important to know where the thresholds are once you go down in oil viscosity. But don't take the graphs for general, each bearing diameter-width combination has its own threshold were the minimal oil film height goes down. On the K-series the 0W20 is safe within the main bearing clearances, but I wouldn't go the maximum rod bearing clearance, to stay below 0.0020" would be better, especially if the application and engine setup has an inefficient oil cooling and the oil temperature goes well above 100 °C.

A slight indicator for that can be oil pressure difference of a oil weight step from e.g. 5W30 to 0W20, which should be in the area of 0.2-0.3 bar. If you see much more it can indicate that one or more of the clearances may on the bigger side, but not telling you about it to be inefficient in minimal oil film height. It is just an indicator. The step between the 0W40 of Mobile1 FS down to the OE 0W20 viscosity on the K20C1 led to 0.6 bar, according the K20C1 racing oil enthusiast I chat with from time to time. Thence a quite linear correlation as it seems to be, but I doubt it is always like that as 0W20 oils have much higher friction reducers concentrations compared to typical OE oils, just because the fact these oils are an instrument for keeping their CO2-emission low.
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Update: changed out the mobil one fs 5w30 with the quaker state fs 5w20. There's almost no change in pressure. Idle went from 33 to 30 and psi went down by 1 at 4k rpm
Thanks for your feedback, that are great news. The QSFS doesn't may look as a major step ahead regarding oil pressure, but the wear protection by that stuff is quite awesome. We didn't find any signs of wear on a race engine after half the season. Oil flow is quite good on it, you may see it in the MPG on your commute.
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