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Old 05-20-2010, 05:29 AM   #1
williaty
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Default Increasing Pump Size to Increase Flow While Reducing Oil Weight to Reduce Pressure?

OK, stay with me on this one, it's a bit different of a tactic than a lot of people take here. First of all, keep in mind that this is a NON TURBO MOTOR. In fact, it's a new crate Subaru EJ253 shortblock.

With a factory EJ253 shortblock and the stock 7mm rotor oil pump, the idle oil pressure has always been kinda scary low (single digits) while the running oil pressure has been higher than it needs to be. It gained about 20psi per thousand RPM, which is twice the guideline. This meant that I was very low at idle but cruised on the freeway at 60psi and put the pump in bypass at 3800RPM.

However, there's a 9mm rotor oil pump with the same relief valve spec available from the factory as well. I've been seriously considering this pump as it would help my idle by flowing about 30% more oil. However, the downside to that is that it'll flow 30% more oil This means my oil pressure with raise even faster and I'd have the pump in bypass just driving down the freeway. It would give me more oil flow at low RPM, but not change the flow at high RPM where I might need it (it'd just wast more power into bypassing the pump). Not really that great of a solution.

Then I got to thinking about changing oil viscosity at the same time as I changed the pump. If I dropped down to something like the Redline 0w20 (I use 5w30 now), idle and low-RPM flow would increase due to the larger rotor and higher RPM flow would increase due to the reduction in viscosity (I skipped like 3 steps of fluid dynamics there, work through it if you have to) once the pump went into bypass. Additionally, the pump would go into bypass at a higher RPM due to the change in viscosity. In fact, since the change in viscosity would be about the same as the change in pump flow, I should get the same system pressure with 30% more flow volume at all engine speeds.

Thoughts?
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Old 05-20-2010, 05:47 PM   #2
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Are you running over 130psi at redline (6.5K)?
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Old 05-20-2010, 07:38 PM   #3
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No, from 3,800RPM up to the 7,000RPM redline, the bypass valve in the pump is open and the oil pressure is capped to 72psi.
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Old 05-20-2010, 11:49 PM   #4
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I like the thinking behind it and no particular reason why it would not work unless you are using NOS or some other power booster that creates big cylinder pressure and heat. Too bad there is no 10wt oils available!


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Old 05-22-2010, 02:07 PM   #5
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The system pressure or flow aren't the only things to check...
The pressure layer on the bearings can actually surpass the oiling system pressure several times (due on how work the hydrodynamic bearings)
Using thinner oil actually you are allowing the oil to escape more easily under such extreme pressures, weakening the hydrodynamic bearing layer.
High flow is ok for remove the temperature from the bearings, but not in expense of the strenght of the layer...
I not see this theory work well in a racing enviroment, but maybe will work well in a road car, allowing less losses (due to less viscosity/cohesion of the oil), and saving a bit of fuel.
In a racing engine you want a strong layer between the bearing surfaces AND good flow.
So, while maintaining identical system pressure (shimming or bigger oil pump) you enlarge the bearing play for allow more flow (actually enlarging the play decrease a bit the pressure of the hydrodynamic layer) AND a more dense oil (that allow more cohesion, so a more strong layer that recover the loss of the more play).
This work because at the same oiling system pressure we have a bit more strong hydrodynamic layer (due to more dense oil), with more flow (that recover the more heat generated by the more dense oil).
Take in mind that using only a too viscous oil with low flow actually adds more heat (so thins the oil), lowering the total pressure of the layer at the bearings.
This is why most of racing oils are something like -60 grade.... But must be used with engine tollerances diferent from the oem specs

Last edited by masterx81; 05-22-2010 at 02:16 PM.
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Old 05-23-2010, 10:36 AM   #6
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The oil pressure only exists to promote flow, increasing flow by using a thinner oil but maintaining the same pressure will certainly remove more heat from the bearings. But as detailed above, it's the oil's surface film strength that protects the bearings, and thinner oil isn't is good at acheiving this. On engines with low loads this is fine, but with high compressive rod loads from high torque low speed engines, or very high speed engines this becomes insufficient and the bearing will scuff on the crank.


Subaru spent a lot of time and money on developing the oil system, but everyone thinks they can reinvent it for the better. The absolute best thing you do is to make sure you have good quality oil and the right temperature that isn't aerated to hell or suffering from massive surge.
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Old 05-23-2010, 11:23 AM   #7
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Quote:
Originally Posted by Pavlo View Post
The oil pressure only exists to promote flow, increasing flow by using a thinner oil but maintaining the same pressure will certainly remove more heat from the bearings. But as detailed above, it's the oil's surface film strength that protects the bearings, and thinner oil isn't is good at acheiving this. On engines with low loads this is fine, but with high compressive rod loads from high torque low speed engines, or very high speed engines this becomes insufficient and the bearing will scuff on the crank.


Subaru spent a lot of time and money on developing the oil system, but everyone thinks they can reinvent it for the better. The absolute best thing you do is to make sure you have good quality oil and the right temperature that isn't aerated to hell or suffering from massive surge.
Subaru has for sure spent a lot of time enginnering the oil system, but it isn't perfect... Is has a tremendous high rate of bearing failures due to the oiling scheme used. For sure reinventing (or modifying) it isn't an easy task, but there must be a way to made it better...
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Old 05-23-2010, 01:54 PM   #8
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Hell, that's something I've been wondering as well. Does my little NA engine, which its max engine load of 1.3g/rev and a 6.5kRPM redline even count as a "heavy" load on the bearings considering they're the same bearings as are used on 4g/rev 7.5kRPM STi builds?
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Old 05-23-2010, 02:19 PM   #9
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Quote:
Originally Posted by masterx81 View Post
Subaru has for sure spent a lot of time enginnering the oil system, but it isn't perfect... Is has a tremendous high rate of bearing failures due to the oiling scheme used. For sure reinventing (or modifying) it isn't an easy task, but there must be a way to made it better...
There isn't alot wrong with the oil system, most of the failures I've seen (out of hundreds) are atributable to something in paticular, and mostly they are not due to the architecture of the oil system. If you want to suggest that the crank bearing journals are too narrow for long term reliability then I wont argue. But for the purpose of getting oil to the bearings it's not bad at all. But if you overheat the oil, or get oil surge it's going to fail, as the narrow bearings just don't have a margin for error!
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Old 05-23-2010, 03:22 PM   #10
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Quote:
Originally Posted by Pavlo View Post
There isn't alot wrong with the oil system, most of the failures I've seen (out of hundreds) are atributable to something in paticular, and mostly they are not due to the architecture of the oil system. If you want to suggest that the crank bearing journals are too narrow for long term reliability then I wont argue. But for the purpose of getting oil to the bearings it's not bad at all. But if you overheat the oil, or get oil surge it's going to fail, as the narrow bearings just don't have a margin for error!
Yes, a lot of problems are caused by oil sarvation/pickup problems, too hot oil, and sometimes by det. But here the bearings fail also on oem untouched engines on normal usage. The oiling scheme it's quite complicated. Those restriction in the main galleries are simply a bad compromise, like the n2 and n4 smaller bearings. The rod bearings 2 and 3 are getting pressure from the same n3 main bearing. This is a quite strange configuration. And the KISS rule doesn't miss a beat. The OEM system works *quite well* with the requirement/variables that subaru had in mind for a road car. But when you change something that alter the established equilibrium (rpm, flow, pressure, etc), you encounter a lot of throubles. A lot of people heave destroyed rod bearings only due to a bit of more rpms... And here comes in play the strange drilling scheme of the spec c crank, maybe for get little more psi at the bearings for permit a bit higher rev limiter.
Please, take in mind that under normal combustion the bearings are higly stressed by the inertia of the alternating mass (the peak is in the overlap), so an n/a engine have quite similar bearing stress of a turbocharged engine (at the same rev range - excluding small variables like turbo backpressure - I'm speaking about bearing stress, not rods or other mechanical components)
There are out there some engines that rev like the hell, with similar bearing loads and stock oiling system and in some case with worst leverages, and the bearings are the last thing to fail. This means that the subaru oiling system MUST have something wrong... Hey, if they need to time the main bearings oil passage for gain few rpm, means that are quite in shorages of ideas

In any case, the subaru crank it's amazing on how it's short. Minimal torsional deformation, lightweight and strong...

Last edited by masterx81; 05-23-2010 at 04:31 PM.
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Old 05-23-2010, 11:43 PM   #11
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The bearings may have the inertial stress but the oil is not undergoing the thermal stress of a turbo motor and that's a big difference.

Finally someone is talking about the massive forces the main bearings are under in a boxer motor. It's the price of the compromise for perfect primary balance. In Porsche's experience the mains endure over 100g...

The solution to the oiling issues are not simple, that's the point. Subaru made an engineering compromise and the downside is limited output capability. It's not the only significant compromise as the heads are fodder for a whole other thread.
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Old 05-23-2010, 11:48 PM   #12
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Soobaviator, what's your take on my initial question?
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Old 05-24-2010, 12:23 AM   #13
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Like I said the thinking is sound. You may however end up with cooler oil which may increase the pressure a bit over your expectation. This is not a bad thing necessarily.

Single digit pressure at idle is too low. Your higher RPM pressure is definitely too high so bigger volume with lower vis may be a nice happy medium.
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Old 05-25-2010, 02:04 PM   #14
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Or you could just idle the motor up a little and experiment with mixed amounts of the same oil brand between viscosities. And adjust the formula for the seasons. Once the pump is too big only a external bypass will help even with 0-20w. Changing ambient temps will always be a challenge. Are you monitoring the oil temps now? 200 fdegree oil across the whole spectrum would be great with 10psi at 900 to 1k rpms as you know. 100 degree days with the ac on will change idle oil pressure with light vis oil though.

The pressure you have had was a different sb yes? The new one will have different clearances yes? You might want to stay with what the factory sb was to begin with. In my old man opinion most wrx motors have young ex honda guys that are maintaining them and they don't check the oil often enough. The oil gets thinner and maybe even fuel diluted on the hotrodded ones at about 4k miles and then who knows what till the next oil change. Everyone knows turbo subys use oil yes? Not sure about a na motor, that might more closely resemble the honda.

The toyota service writer for my wifes car says oil needs to get broken in, he says leave it for 7500 miles thats best, maybe so, but I still will change it every 3-4k because I'm old and hard headed. Course I also think the ignition switch turned off and gear selector in neutral will stop a runaway totota.

Last edited by charliew; 05-25-2010 at 02:10 PM.
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