BMW S54 Rod Bearing Oil Pressures

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Greetings all! This is my first post.. I've been following this BITOG forum for many years as a guest...a tremendous wealth of knowledge on this forum! I've been searching the forums for over an hour and haven't come across anyone that's answered this issue adequately, and I'm thinking that this could be a solution for all S54 owners:

As the engine-builder's rule of thumb goes, one should have around 10 psi oil pressure per 1000RPM for standard bearing widths.

As we are all too familiar with, our rod bearings are narrower than the typical rod bearing found in cars.

One can then safely assume then that there's no way our bearings should be experiencing anything under 10psi/1000RPM, and, if anything, should be set to at least 12-14psi/1000RPM.

After taking a look at the TIS documents on checking oil pressure on the S54, it is clearly stipulated that there should be minimum 0.7bar (10.1 psi) at idle speed at operating temperature. It then goes to say that the pressure is then regulated to a constant 4bar (58 psi). This pressure is regulated by a sprung piston situated in the oil pump which limits the output pressure to a maximum of 4bar (58psi), (experts please step in and correct me if I'm wrong here).

At this point it becomes apparent that the bearings are under-lubricated at RPMs over 4800RPM (using a conservative guideline that 12psi are required per 1000RPM)

This makes me think that a far simpler solution to eliminate the rod bearing issues would be to remove the oil pump pressure regulator, or at least modify the existing setup to maintain higher oil pressures at the higher rpm.

1. Does anyone have experience with this or considered this?

2. Why would BMW fit a pressure regulator to the oil pump in the first place?

I just find it really bizarre how this could have been overlooked by the engineers in Germany, as this is the 101 of engine building, and I must be missing something...
 
To increase the pressure, just fit a heavier spring into the pressure regulator. This may or may not be easy depending on its location. You may also be able to 'space' the stock spring tighter with something behind it so it puts more force on the valve, but you have to make sure the valve can still reach its full open position before you bind the coils of the spring.
You need to regulate the pressure to prevent the pump from being damaged by high pressure. Its a positive displacement type pump, which means every rotation it outputs X ccs of fluid. If the oil is cold, there will be quite a lot of resistance to flow through the oil galleries. The pump will keep pumping the same amount fluid in on every rotation increasing the pressure until something gives.
 
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I would hazard a guess that if BMW think 4 bar is fine then it probably is.
Oil flow is just as important as just pressure. The oil at the bearings need to be constantly replaced to keep the bearing cooled.
If you plug the relief valve my money is on the filter bursting from a cold start, maybe with the oil pump drive breaking as a covering bet.

Claud.
 
Originally Posted By: Claud
I would hazard a guess that if BMW think 4 bar is fine then it probably is.
Oil flow is just as important as just pressure. The oil at the bearings need to be constantly replaced to keep the bearing cooled.


The temperature in the big end is DUE to the viscous friction in the bearing and the oil.

Heat flow is OUT of the bearing, and into both the con-rod and crankshaft at high RPM.

There has to be enough oil available to replenish the side leakage ...
 
Originally Posted By: Shannow
Originally Posted By: Claud
I would hazard a guess that if BMW think 4 bar is fine then it probably is.
Oil flow is just as important as just pressure. The oil at the bearings need to be constantly replaced to keep the bearing cooled.


The temperature in the big end is DUE to the viscous friction in the bearing and the oil.

Heat flow is OUT of the bearing, and into both the con-rod and crankshaft at high RPM.

There has to be enough oil available to replenish the side leakage ...


Absolutely, the hot oil needs to be replaced before it breaks down.

Claud.
 
welcome2.gif
to BITOG Simon
 
IIRC, the 10psi "rule" applies to the Small Block Chevrolet and is not some universal guideline.

The oil pressure relief spec pressure simply means that the bypass cracks at that point, it doesn't mean that's the maximum pressure you are going to see. It is very possible; probable even, to observe pressures significantly higher than the relief as the pump over-drives the relief circuit and continues to build pressure in the rest of the system.
 
+1
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And, yes and no. Most USA V8's have somewhat similar bearing sizes, so it has become a rule of thumb for USA engines. But all bets are off on bearings with other sizes and different clearances...

I'd just run a good high HTHS oil in the appropriate grade and keep it clean. If you want to do track days, think about loosening the clearances by a few thou and be done with it
laugh.gif
 
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The displacement 3.2L is the same as the the M30 in my '83 533i that puts out only 160 hp. The trouble is that oil needs to changed more often. These cars are victims of poor care and hard usage. Most have a tender cooling system that contributes to overheats.Replacing plastic parts before 100K is advisable. Having a tank split open at full hoon will often result in a major overheat. Something about the new stuff being more bio degradable in the waste stream.
 
Worry more about HTHS rating of your Oil.
Mains pressure just puts oil into the bearings and provides little bearing cooling as there should be minimal heat transfer due to windage in this situation.
 
Originally Posted By: BrocLuno
+1
smile.gif


And, yes and no. Most USA V8's have somewhat similar bearing sizes, so it has become a rule of thumb for USA engines. But all bets are off on bearings with other sizes and different clearances...

I'd just run a good high HTHS oil in the appropriate grade and keep it clean. If you want to do track days, think about loosening the clearances by a few thou and be done with it
laugh.gif



Reinforcing this...tTry to find an oil that has a high HTHS for it's grade, and drop back on Kinematic viscosity a little.

For example I'd use
https://www.redlineoil.com/Content/files/tech/15W50_MO_PROD_INFO.pdf

over either
https://www.mobil.com/english-gb/passenger-vehicle-lube/pds/glxxmobil-1-extended-life-10w60
https://msdspds.castrol.com/bpglis/FusionPDS.nsf/Files/B97CA7A6D4E6B82580257F8E006DA735/$File/BPXE-A9VFNF.pdf

As you get the same protection (HTHS) with about 15% lower KV, which means less pressure loss along the oil galleries and the feed port through the main to big end transfer port...a little bit, but in the right direction.

Also, being highly additised, when the oil film IS in distress, there's additives to help out.
 
I was told by someone with lots of knowledge on the S54 (has a 900 horsepower one in his race car - big turbo, nitrous) that the rod bearing issues are due to low-RPM, daily driving. S54 bearings seem to live long lives at the track in high-RPM high-oil pressure use. FWIW, he has stock bearings in his car with no issues.
 
The rod bearing "issues" in the S54 and S85 are, in my opinion, due to people driving with their foot to the floor while the engine is cold. If the people that daily drove and short tripped them did so with 0W-40 in the sump, they'd probably be fine.
 
Thanks for all the responses!

rooflessVW, I can assure you that the damage is not caused by driving hard while cold... I've nursed my car since day 1, always drove with low load and below 3000rpm until oil temp reached 180 degrees and had to do the rod bearings at 120k miles as they were on the copper.

It's exactly as 1JZ says... they don't like low RPM and its the preignition which kills the bearings...its alot less common on track cars. And indeed, mine is a daily city driver..

Shannow, your suggestion on reducing kinematic viscosity makes alot of sense... I was going to take your advice and go with the Redline 15W50 until I just found out that here in South Africa it costs $35 a quart, so would you suggest the Mobil 1 10W60 over the Castrol Edge Sport 10W60?
 
Originally Posted By: SlipperySimon
Thanks for all the responses!

rooflessVW, I can assure you that the damage is not caused by driving hard while cold... I've nursed my car since day 1, always drove with low load and below 3000rpm until oil temp reached 180 degrees and had to do the rod bearings at 120k miles as they were on the copper.

It's exactly as 1JZ says... they don't like low RPM and its the preignition which kills the bearings...its alot less common on track cars. And indeed, mine is a daily city driver..

Shannow, your suggestion on reducing kinematic viscosity makes alot of sense... I was going to take your advice and go with the Redline 15W50 until I just found out that here in South Africa it costs $35 a quart, so would you suggest the Mobil 1 10W60 over the Castrol Edge Sport 10W60?


FWIW, I'm running Redline 10w60 in my S85. It is speced for Castrol TWS 10w60, but I liked how the Redline oil performed, better. I never load the engine heavily while cold, or lug it. The rod bearing specs are on the tight side. If I ever have a problem, I'll open up the clearance a bit more.
 
Originally Posted By: SlipperySimon
Thanks for all the responses!

rooflessVW, I can assure you that the damage is not caused by driving hard while cold... I've nursed my car since day 1, always drove with low load and below 3000rpm until oil temp reached 180 degrees and had to do the rod bearings at 120k miles as they were on the copper.

It's exactly as 1JZ says... they don't like low RPM and its the preignition which kills the bearings...its alot less common on track cars. And indeed, mine is a daily city driver..

Shannow, your suggestion on reducing kinematic viscosity makes alot of sense... I was going to take your advice and go with the Redline 15W50 until I just found out that here in South Africa it costs $35 a quart, so would you suggest the Mobil 1 10W60 over the Castrol Edge Sport 10W60?


What does Motul 300V cost down there?
 
Originally Posted By: SlipperySimon
The Motul actually seems even more expensive!

Ok I've managed to find a Redline supplier who is reasonable with their pricing, but it now seems as though I have an option and I'm not sure which one to consider (they're both exactly the same price):

https://www.redlineoil.com/Content/files/tech/15W50_MO_PROD_INFO.pdf

or

https://www.redlineoil.com/50wt-race-oil-15w50


Let me rather give my question a bit more direction...

1. I can't find the HTHS values of the 50WT and am wondering if they're higher than the standard 15W50?

2. Furthermore would there be any differences in the base stocks between the two products?

3. I'm sure that the racing oil will have to be replaced more frequently, but why is this?
 
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