Oil flow -vs- pressure: a thought about 928 engine. (Yes, again)
04-16-2010, 06:26 PM
#1
Racer
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Oil flow -vs- pressure: a thought about 928 engine. (Yes, again)
First of all: sorry for yet another thread on this "highly flammable" subject. 
I'm currently looking for proper oil for my 928gt and I was considering trying the Delvac 5w40 oil that Doug Hillary recommended: actually this post was first supposed to be a reply to Delvac discussion thread (https://rennlist.com/forums/928-foru...truck-oil.html), but unfortunately it's locked.
Anyways, I have read quite a bunch of oil discussion threads and it looks like that most "oil professionals" tend to recommend lower grade oils, as long as they provide sufficient "hot" viscosity or "film thickness": one that is above Porsche specs. I've encountered a post with someone recommending 5w20 or something similar for a Ferrari Maranello IIRC - but for 928 the "common recommendation" usually is 0w40 or 5w40.
Main reasons named are:
I do agree with above statements, so I was considering Mobil 1 0w40 or Delvac 5w40 or something similar.
On the other hand, though, what has never let me stop thinking about the whole "oil deal" and just happily use Delvac or M1:
We have a lot of people successfully use 15w50 Mobil1, we have M. Kibort with his Amsoil, we have a load of other peeps running 20w50 or similar oils, with great success.
So, when I thought about it some more and factored in that the 928 engine is well known for its 2/6 failure, I came to the following idea: As you all know, the main source for the bearing problem is yet to be found, but I guess you all would agree that these two factors are pretty important:
AND, If I understand everything correctly, increasing oil flow through 928 engine with its oiling issues actually makes those problems worse: if the flow increases, oil "reserves" drain faster from pan - therefore engine ingests air earlier; oil gets sprayed more from bearings, - and I would think that this contributes to more oil foaming.
Thicker oil in similar conditions would flow somewhat slower and therefore allow for longer cornering without oil starvation, while also reducing the "oil spray" from bearings.
So, I'll just repeat the main idea: while lower viscosity oils seem to generally be better for engines, the 928 engine due to above mentioned issues seems to be unsuitable for them and "likes" higher grade ones more. This "conclusion" has been mentioned numerous times and is not really new to anyone, but usually the reasoning sounds like "I have used it and it works for me" or "the 20 years old user manual recommends it" and therefore was not convincing enough. So I'm trying to find some "scientific"
explanation - please let me know what you think about my theory: I would be happy to read any input on the subject.
Also, perhaps someone has already expressed similar thoughts - I apologize for repeating then, but I have not yet encountered such thread.
P.S. Please apologize me for my english.
I'm currently looking for proper oil for my 928gt and I was considering trying the Delvac 5w40 oil that Doug Hillary recommended: actually this post was first supposed to be a reply to Delvac discussion thread (https://rennlist.com/forums/928-foru...truck-oil.html), but unfortunately it's locked.
Anyways, I have read quite a bunch of oil discussion threads and it looks like that most "oil professionals" tend to recommend lower grade oils, as long as they provide sufficient "hot" viscosity or "film thickness": one that is above Porsche specs. I've encountered a post with someone recommending 5w20 or something similar for a Ferrari Maranello IIRC - but for 928 the "common recommendation" usually is 0w40 or 5w40.
Main reasons named are:
- Lower grade oils flow much better at lower temperatures, therefore are way easier on engine on cold starts (when the most wear occurs anyway).
- Also, lower grade oils flow better at operating temps, and therefore provide better engine cooling.
I do agree with above statements, so I was considering Mobil 1 0w40 or Delvac 5w40 or something similar.
On the other hand, though, what has never let me stop thinking about the whole "oil deal" and just happily use Delvac or M1:
We have a lot of people successfully use 15w50 Mobil1, we have M. Kibort with his Amsoil, we have a load of other peeps running 20w50 or similar oils, with great success.
So, when I thought about it some more and factored in that the 928 engine is well known for its 2/6 failure, I came to the following idea: As you all know, the main source for the bearing problem is yet to be found, but I guess you all would agree that these two factors are pretty important:
- Oil starvation while cornering at high RPM and on sticky rubber
- Excessive oil aeration and foaming at higher RPM
AND, If I understand everything correctly, increasing oil flow through 928 engine with its oiling issues actually makes those problems worse: if the flow increases, oil "reserves" drain faster from pan - therefore engine ingests air earlier; oil gets sprayed more from bearings, - and I would think that this contributes to more oil foaming.
Thicker oil in similar conditions would flow somewhat slower and therefore allow for longer cornering without oil starvation, while also reducing the "oil spray" from bearings.
So, I'll just repeat the main idea: while lower viscosity oils seem to generally be better for engines, the 928 engine due to above mentioned issues seems to be unsuitable for them and "likes" higher grade ones more. This "conclusion" has been mentioned numerous times and is not really new to anyone, but usually the reasoning sounds like "I have used it and it works for me" or "the 20 years old user manual recommends it" and therefore was not convincing enough. So I'm trying to find some "scientific"
explanation - please let me know what you think about my theory: I would be happy to read any input on the subject.Also, perhaps someone has already expressed similar thoughts - I apologize for repeating then, but I have not yet encountered such thread.
P.S. Please apologize me for my english.
04-16-2010, 06:42 PM
#2
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My 2 cents : (New to Rennlist but not to Porsche's and Cars/Motorcycles in general)...
Lower weight oils offer faster circulation and return to the pan quicker. On motorcycles (such as Hondas and Yamaha's ) I notice a cooler operation (especially Air cooled bikes/cars) from using a lower viscosity with a narrower ranger (Ie-5w30 or 10w30). Colder environments also benefit from the lower viscosity because they tend to flow better and protect better at startup :
Higher viscosity oils tend to function better for Hot climates, or extreme RPMs, and higher mileage vehicles (cars with looser internal clearances).
That said, if you dont have any leaks, and the engine is broken in already, why not follow the recommendations? 5w20 - 10w30 etc work well unless you are in extremely hot climates or hot weather (Like stop and go in the summer in Florida or Texas or California.. U get the idea). If you have the $$ Redline is good, so is Golden Spectro and Amsoil. I play to switch over to Golden Spectro once I finish my internal work . Semi synthetics and synthetic oils work very nicely (if you do not have any oil control problems!) and tend to outlast conventional oils.
As always YMMV.
Lower weight oils offer faster circulation and return to the pan quicker. On motorcycles (such as Hondas and Yamaha's ) I notice a cooler operation (especially Air cooled bikes/cars) from using a lower viscosity with a narrower ranger (Ie-5w30 or 10w30). Colder environments also benefit from the lower viscosity because they tend to flow better and protect better at startup :
Higher viscosity oils tend to function better for Hot climates, or extreme RPMs, and higher mileage vehicles (cars with looser internal clearances).
That said, if you dont have any leaks, and the engine is broken in already, why not follow the recommendations? 5w20 - 10w30 etc work well unless you are in extremely hot climates or hot weather (Like stop and go in the summer in Florida or Texas or California.. U get the idea). If you have the $$ Redline is good, so is Golden Spectro and Amsoil. I play to switch over to Golden Spectro once I finish my internal work . Semi synthetics and synthetic oils work very nicely (if you do not have any oil control problems!) and tend to outlast conventional oils.
As always YMMV.
04-16-2010, 09:10 PM
#3
Race Director
It all depends on your driving style....if you drive purely on the street and do not drive "hard" then just about anything will be fine.......
If you drive on track or under "spirited" conditions then your needs are different....
I will only run synthetics in 15-50 or 20-50 (I do live in CA where its warm) in any 928 I own...including my race 928's....
The bottom line is this...if you run high enough RPM with high G turns...no 928 engine will live long, regardless of oil...
If you drive on track or under "spirited" conditions then your needs are different....
I will only run synthetics in 15-50 or 20-50 (I do live in CA where its warm) in any 928 I own...including my race 928's....
The bottom line is this...if you run high enough RPM with high G turns...no 928 engine will live long, regardless of oil...
04-17-2010, 04:45 AM
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On the more serious note: I do agree that it mostly depends on driving style and oil choice actually does not matter much: as long as oil is not -too- thin.
Personally I was running 5w50 M1, and I think I'm gonna try new product: 10w60 oil instead of going lower (5w40 or 0w40).
04-17-2010, 09:26 AM
#5
Nordschleife Master
Crankshaft oiling considerations
Alex --
I agree with you that the most important part of the 928 oiling problems is that the oil pickup sucks air. This is either because the oil is in the pan but g-forces uncover the pickup or because to oil is not in the pan (windage cloud, heads, etc.)
Here are my thoughts on one small piece of the puzzle, relating to the transfer of oil from the main journal to the rod journal. For the oil to make it to the rod journal, the oil obviously has to first make it thru the crankshaft main journal. Below is what I've figured is the worst case scenario in terms of the required oil supply pressure.
In order for the oil to make it thru the main journal, the supply pressure has to overcome two effects. First, the crankshaft main journal surface travels at a very high speed. The oil needs to somehow be accelerated to the same speed as the journal surface. In the worst case scenario, all this work needs to be done by oil pump. The pressure loss due to this effect is called acceleration losses.
Second, once the oil has been accelerated to the speed of the journal surface, it has to fight against the centrifugal force (or perhaps more accurately the oil pressure must provide the centripetal force) to make it inside the crankshaft main journal that is spinning rapidly. The pressure loss due to this effect is called centrifugal losses.
The centrifugal losses are the largest if the oil passage goes thru the crank centerline. The 928 crankshaft is drilled in this worst-case way. For cranks that are drilled with straight shot oiling paths, the passage doesn't go very close to the crank centerline and the centrifugal losses are therefore smaller.
There is a third effect that depends on whether there's air bubbles in the oil. If there are no air bubbles in the oil, the crank journal being on an outer radius circle creates a siphon effect. This will create in the best case a zero absolute pressure for the oil pump to push against. In the absolute worst case, a big air bubble forms inside the passage. For the absolute worst case, I am assuming that the air bubble has atmospheric pressure that the oil pump has to push against.
I've put together a small spreadsheet that computes the minimum required oil supply pressure at the main journal in the worst case scenario, attached:
WorstCaseReqOilPressure.zip
According to the spreadsheet, the 928 crank in the worst case needs 100 psi oil pressure at 7000 rpm.
Now, this is the worst-case scenario that assumes inviscid fluid and smooth crank journal surfaces. In reality, things may be better for the following reasons:
- The oil is not inviscid. When oil comes in contact with the crank journal, the viscous oil in the main journal bearing oil groove is accelerated by the crankshaft. Not all of the work is done by the oil pump, so the required supply pressure is lower.
- The crank is not perfectly smooth. In particular, if there is a lead-in groove or a tear-drop in the main journal oil transfer hole, again the crankshaft is doing some work.
I think that these two mitigating effects are larger when the oil is has higher viscosity. Higher viscosity oil could then help in some circumstances.
Now, let me just clarify one more time that crankshaft drilling considerations are pretty much irrelevant if the oil pan pickup gets uncovered. The point I am trying to make is that 928 needs a high oil pressure to support high rpm even if one manages to keep the oil pan pickup submerged in oil. Higher viscosity oil might plausibly reduce this required oil pressure because higher viscosity oil might lead to lower acceleration losses.
Best, Tuomo
I agree with you that the most important part of the 928 oiling problems is that the oil pickup sucks air. This is either because the oil is in the pan but g-forces uncover the pickup or because to oil is not in the pan (windage cloud, heads, etc.)
Here are my thoughts on one small piece of the puzzle, relating to the transfer of oil from the main journal to the rod journal. For the oil to make it to the rod journal, the oil obviously has to first make it thru the crankshaft main journal. Below is what I've figured is the worst case scenario in terms of the required oil supply pressure.
In order for the oil to make it thru the main journal, the supply pressure has to overcome two effects. First, the crankshaft main journal surface travels at a very high speed. The oil needs to somehow be accelerated to the same speed as the journal surface. In the worst case scenario, all this work needs to be done by oil pump. The pressure loss due to this effect is called acceleration losses.
Second, once the oil has been accelerated to the speed of the journal surface, it has to fight against the centrifugal force (or perhaps more accurately the oil pressure must provide the centripetal force) to make it inside the crankshaft main journal that is spinning rapidly. The pressure loss due to this effect is called centrifugal losses.
The centrifugal losses are the largest if the oil passage goes thru the crank centerline. The 928 crankshaft is drilled in this worst-case way. For cranks that are drilled with straight shot oiling paths, the passage doesn't go very close to the crank centerline and the centrifugal losses are therefore smaller.
There is a third effect that depends on whether there's air bubbles in the oil. If there are no air bubbles in the oil, the crank journal being on an outer radius circle creates a siphon effect. This will create in the best case a zero absolute pressure for the oil pump to push against. In the absolute worst case, a big air bubble forms inside the passage. For the absolute worst case, I am assuming that the air bubble has atmospheric pressure that the oil pump has to push against.
I've put together a small spreadsheet that computes the minimum required oil supply pressure at the main journal in the worst case scenario, attached:
WorstCaseReqOilPressure.zip
According to the spreadsheet, the 928 crank in the worst case needs 100 psi oil pressure at 7000 rpm.
Now, this is the worst-case scenario that assumes inviscid fluid and smooth crank journal surfaces. In reality, things may be better for the following reasons:
- The oil is not inviscid. When oil comes in contact with the crank journal, the viscous oil in the main journal bearing oil groove is accelerated by the crankshaft. Not all of the work is done by the oil pump, so the required supply pressure is lower.
- The crank is not perfectly smooth. In particular, if there is a lead-in groove or a tear-drop in the main journal oil transfer hole, again the crankshaft is doing some work.
I think that these two mitigating effects are larger when the oil is has higher viscosity. Higher viscosity oil could then help in some circumstances.
Now, let me just clarify one more time that crankshaft drilling considerations are pretty much irrelevant if the oil pan pickup gets uncovered. The point I am trying to make is that 928 needs a high oil pressure to support high rpm even if one manages to keep the oil pan pickup submerged in oil. Higher viscosity oil might plausibly reduce this required oil pressure because higher viscosity oil might lead to lower acceleration losses.
Best, Tuomo
