Oil starvation problem at high rpm
02-07-2004, 12:15 PM
#91
So we want the V drilling because it has more oil flow at higher RPMS?
02-07-2004, 01:22 PM
#92
Nordschleife Master
Yes.
The paper is pretty clear in that it flows more at all oil pressures and RPMs.
There's some discussion about cavitation that was hard to follow. (Maybe lost in translation) Sound like "V" is better, and worse, for that in the doc. If it has consistently higher pressure, I don't understand how that could be worse.
The paper is pretty clear in that it flows more at all oil pressures and RPMs.
There's some discussion about cavitation that was hard to follow. (Maybe lost in translation) Sound like "V" is better, and worse, for that in the doc. If it has consistently higher pressure, I don't understand how that could be worse.
02-07-2004, 02:25 PM
#93
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I'm not sure this applies exactly to the 928 cranks. I checked a book I have that shows the "I" style which is just a single passage drilled from the main journal hole to the con rod journal hole. Actually in the book by A. Graham Bell he emphatically states that cross drilling the main journal - exactly as is shown in the paper - is useless. This paper obviously shows that he's dead wrong.
Anyways, there's not a lot of "discussion" in the paper, but it seems to assume that the benefit is having the two holes in the main journal, which the pictures in this thread show the 928 crank having. There's no discussion of the "V" or the angle of the V. So it's hard to draw a conclusion because the 928 cranks do have the two oil pickup holes.
I was also confused about whether the "V" style was good or bad for aeration. I had to re-read it a couple of times. If you look at Fig. 6 it shows that at the same RPM (1800) aeration happens in the I style at a higher pressure, which is bad. The axis of the graph is a little confusing. So my take is the V style is better in every way.
Not to rehash this whole thread but - I don't think the crank passages are the *cause* of the oiling problems - they might make it a little worse. But I think the real root cause is improper design of the passages in the block or girdle that cause the main to get less oil flow/pressure. Of course all of the sloshing, foaming problems are real too - but most engines have these to some degree when driven hard.
--Brian Morris
89 951
Anyways, there's not a lot of "discussion" in the paper, but it seems to assume that the benefit is having the two holes in the main journal, which the pictures in this thread show the 928 crank having. There's no discussion of the "V" or the angle of the V. So it's hard to draw a conclusion because the 928 cranks do have the two oil pickup holes.
I was also confused about whether the "V" style was good or bad for aeration. I had to re-read it a couple of times. If you look at Fig. 6 it shows that at the same RPM (1800) aeration happens in the I style at a higher pressure, which is bad. The axis of the graph is a little confusing. So my take is the V style is better in every way.
Not to rehash this whole thread but - I don't think the crank passages are the *cause* of the oiling problems - they might make it a little worse. But I think the real root cause is improper design of the passages in the block or girdle that cause the main to get less oil flow/pressure. Of course all of the sloshing, foaming problems are real too - but most engines have these to some degree when driven hard.
--Brian Morris
89 951
02-07-2004, 04:10 PM
#94
Nordschleife Master
Checking the pics on 928 crank drilling vs. the "V" style I start to see why this service is so expensive. DEVEK offers it for around $700! If the oiling system was converted from the existing way to the "V" way that'd be a lot of drilling and plugging.
I know some 944 cranks are "perp" drilled where an extra passage is added straight out the top from the center of the rod journal. Any ideas on the usefulness of this for the 928? Looks like a lot less money and much lower risk.
I know some 944 cranks are "perp" drilled where an extra passage is added straight out the top from the center of the rod journal. Any ideas on the usefulness of this for the 928? Looks like a lot less money and much lower risk.
02-17-2004, 11:08 PM
#95
Intermediate
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left a porsche club meeting tonight with a question that came up concerning bearing issues on 968's vs 944/928's and that was that no one knew of 968's having the classic bearing problem. first does that sound accurate and second if so what was different.
thoughts
glenn
thoughts
glenn
02-18-2004, 08:21 PM
#96
Hi
I've been studying this problem for some time, and have the following to add:
1. High rpm alone is not the issue. Normally it seems to be this in combination with high speed left hand bends is worse, because this is when the oil level is most likley to fall below the pickup
2. A GTS baffle does help, but not much.
3. porsche have changed the design of later S4/GT cranks - I've compared about 5 with a brand new one bought recently from the factory - the oil ways are at a different angle and are a different size.
4. The only real solution is a dry sump set-up - accusump only works for short bursts.
Also Porsche did use 928's in racing, though not to the same extent as 911's. I will try to find out if/what modifications were made re Oil starvation.
Adrian
2 X black 928SE's
I've been studying this problem for some time, and have the following to add:
1. High rpm alone is not the issue. Normally it seems to be this in combination with high speed left hand bends is worse, because this is when the oil level is most likley to fall below the pickup
2. A GTS baffle does help, but not much.
3. porsche have changed the design of later S4/GT cranks - I've compared about 5 with a brand new one bought recently from the factory - the oil ways are at a different angle and are a different size.
4. The only real solution is a dry sump set-up - accusump only works for short bursts.
Also Porsche did use 928's in racing, though not to the same extent as 911's. I will try to find out if/what modifications were made re Oil starvation.
Adrian
2 X black 928SE's
06-09-2004, 12:07 PM
#97
Drifting
just revisiting this subject and looking at martins very helpfull pics, ive just clicked that they are actualy pics from main no.1 and rod bearing 1/5. it seems odd that the oiling problem shows up on rod bearing 2/6 and not the others, especialy if they are all drilled the same way?? doesnt this point to a problem with supply at the main rather than the crank drillings per-se?
why should the supply to main no.2 be any different or susceptable to poor supply conditions?
why should the supply to main no.2 be any different or susceptable to poor supply conditions?
06-09-2004, 03:16 PM
#98
Rennlist Member
didnt al unser drive a viper around willow springs in 100 degree heat with no oil using slick 50 !????? hm, maybe we should just toss in some "teflo-oil"??
hey, im still a fan of 6200rpm max, be smart about the revs in long turns, and no hitty, the rev limiter! (and of course, use of Amzoil 20-50 racing oil)
Now on my 3rd season of some pretty tough racing. No accusump, stock oil cooler, no baffle, no anything other than a stock engine.
still knocking on wood!!
MK
hey, im still a fan of 6200rpm max, be smart about the revs in long turns, and no hitty, the rev limiter! (and of course, use of Amzoil 20-50 racing oil)
Now on my 3rd season of some pretty tough racing. No accusump, stock oil cooler, no baffle, no anything other than a stock engine.
still knocking on wood!!
MK
06-09-2004, 04:31 PM
#99
Burning Brakes
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Sorry if this is a re-run but wanted to say thanks Nicole for sharing this very interesting thread from the PFF (DE). I've tried to join that group before but was never contacted by the sysop/admin there. Is there any magic to it? 
Two comments:
1) Will someone be sure to bring up this topic at the OCIC meet in Kansas?
2) Could the type of "spirited driving" (read spiral curves) that folks did either with the group or alone on the Blue Ridge Parkway at SITM 2004 cause said oil starvation?
3) What happens when there are no more GTS cranks available due to the classic 2 & 6 bearing failures?
Again...thanks for the enlightenment...this is one reason I love this board!
Cheers,
Tim
Two comments:1) Will someone be sure to bring up this topic at the OCIC meet in Kansas?
2) Could the type of "spirited driving" (read spiral curves) that folks did either with the group or alone on the Blue Ridge Parkway at SITM 2004 cause said oil starvation?
3) What happens when there are no more GTS cranks available due to the classic 2 & 6 bearing failures?
Again...thanks for the enlightenment...this is one reason I love this board!
Cheers,
Tim
06-09-2004, 06:48 PM
#100
Drifting
I emailed Jon Milledge to get his thoughts...here's what his reply which is helpful.
quote:
--------------------------------------------------------------------------------
Rick,
Ok, here is the real reason rod bearings (mostly #2) fail in 944s.
In 1985, when we won Firehawk Grand Sport, we had to run on Firestone S660 tires. The bottom end of the engine was never disassembled and we never had a bearing failure. At the end of the year, the engine was disassembled and the rod bearings were great. Why? The tires had no stick. Cornering never exceed .96G, we didn't hit curbs, drive off the track, and braking didn't exceed .98G. We shifted at 6400 rpm because the rev limiter cut in at 6500.
Here's why the rod bearing fails. There isn't sufficient oil at the rod bearing to lubricate and cool it. Bearing cooling is one of the most important functions performed by lubricating oil.
Why isn't the oil supply sufficient? There are a number of reasons. The first is the oil pump design and the oil pump supply. The gerotor pump design should never be driven faster than 70% of crankshaft speed. On the 944, it's locked to the crankshaft. At high revs, air can be drawn through the oil in the oil pan when the oil covering the pickup is marginal and enter the oil system. When this air reaches the oil pump, there is an instantaneous loss of oil pressure until the air is compressed. When the air reaches the oil gallery, it is mixed with oil and traveling at quite a velocity. The oil inlet port to the main oil gallery is opposite the oil supply to #1 main bearing. #1 main bearing supplies oil to #1 rod bearing. Oil has considerably greater mass per unit volume than air so oil goes to #1 main bearing and air displaced and forced down the gallery to #2 main bearing supply. #2 main bearing supplies oil to #2 rod bearing. Main bearings can live with marginal lubrication much longer than can rod bearings. Rod bearings fail rapidly when subjected to air and oil mixed. This is the total reason.
What can be done about this? 0. Run tires with no grip 1. The oil pan can be baffled better than the factory baffling to keep the oil level higher at all times. 2. The oil pickup can be modified to reduce the ability for air to enter. 3. Air can be removed from the oil before the air gets to the engine. 4. Bearings can be coated with an oil retaining film that makes the bearings more durable when supplied insufficient oil. 5. Modifications can be done to the oil pan to remove more oil from the crankshaft (may not be legal in PCA) and return this oil to the deep part of the sump. 6. Use an accusump with an anti-backfeed one way valve to keep the instantaneous pressure loss from occurring (I feel this is the least important but OK with the rules makers). 7. Never run the oil level over full as this allows the crankshaft to whip more air into the oil. The question here is just what is full? Try checking the oil level this way and run it half way between the full and add line. Engine fully up to temperature, remove the dipstick and plug tube with finger. Rev engine to 5000rpm and hold for 15 sec. Switch off engine. Immediately after crank stops turning, insert dipstick and remove, reading level.
Hope this helps
Jon
quote:
--------------------------------------------------------------------------------
Rick,
Ok, here is the real reason rod bearings (mostly #2) fail in 944s.
In 1985, when we won Firehawk Grand Sport, we had to run on Firestone S660 tires. The bottom end of the engine was never disassembled and we never had a bearing failure. At the end of the year, the engine was disassembled and the rod bearings were great. Why? The tires had no stick. Cornering never exceed .96G, we didn't hit curbs, drive off the track, and braking didn't exceed .98G. We shifted at 6400 rpm because the rev limiter cut in at 6500.
Here's why the rod bearing fails. There isn't sufficient oil at the rod bearing to lubricate and cool it. Bearing cooling is one of the most important functions performed by lubricating oil.
Why isn't the oil supply sufficient? There are a number of reasons. The first is the oil pump design and the oil pump supply. The gerotor pump design should never be driven faster than 70% of crankshaft speed. On the 944, it's locked to the crankshaft. At high revs, air can be drawn through the oil in the oil pan when the oil covering the pickup is marginal and enter the oil system. When this air reaches the oil pump, there is an instantaneous loss of oil pressure until the air is compressed. When the air reaches the oil gallery, it is mixed with oil and traveling at quite a velocity. The oil inlet port to the main oil gallery is opposite the oil supply to #1 main bearing. #1 main bearing supplies oil to #1 rod bearing. Oil has considerably greater mass per unit volume than air so oil goes to #1 main bearing and air displaced and forced down the gallery to #2 main bearing supply. #2 main bearing supplies oil to #2 rod bearing. Main bearings can live with marginal lubrication much longer than can rod bearings. Rod bearings fail rapidly when subjected to air and oil mixed. This is the total reason.
What can be done about this? 0. Run tires with no grip 1. The oil pan can be baffled better than the factory baffling to keep the oil level higher at all times. 2. The oil pickup can be modified to reduce the ability for air to enter. 3. Air can be removed from the oil before the air gets to the engine. 4. Bearings can be coated with an oil retaining film that makes the bearings more durable when supplied insufficient oil. 5. Modifications can be done to the oil pan to remove more oil from the crankshaft (may not be legal in PCA) and return this oil to the deep part of the sump. 6. Use an accusump with an anti-backfeed one way valve to keep the instantaneous pressure loss from occurring (I feel this is the least important but OK with the rules makers). 7. Never run the oil level over full as this allows the crankshaft to whip more air into the oil. The question here is just what is full? Try checking the oil level this way and run it half way between the full and add line. Engine fully up to temperature, remove the dipstick and plug tube with finger. Rev engine to 5000rpm and hold for 15 sec. Switch off engine. Immediately after crank stops turning, insert dipstick and remove, reading level.
Hope this helps
Jon
06-13-2004, 09:51 AM
#101
Developer
I love this thread - lots of helpful, useful info.
I have one or two things that we have tried and found succesful to help
anybody racing a 16v 928:
Try this exercise: remove the oil filler cap and separator screen from your 16v 928 and have an associate start the motor and take it up to about 2K with several 4k blips. Hold your hand over the inlet port of the circular oil filler chamber....
We found the amount of solid oil droplets (not so much a mist) thrown up into that chamber to be amazing. And this was AFTER the installation of a DEVEK louvered oil baffle in the crankcase vent. When we were installing that louvered oil baffle, I remember looking straight down and seeing a connecting rod and crankshaft counterweight directly below. That meant to me, as well as all the oil "misting" problems you guys have all correctly written about - oil was just plain being SLUNG up there by that counterweight and rod! I love Porsche too - but I gotta say - that location to take the vent off of the crankcase is a really poor one. Anyway - that's what we have to deal with.
Two things that I have not seen mentioned in this thread that have helped our race car:
1) we found that we would "pump" more oil up into the oil separator/vent under hard braking rather than hard cornering. We first thought it was oil coming up the drain hose from the front of the motor so we added a one-way check valve to that thing. Did not help. So we went the other way, and we increased the size of the hose that drains out of the vent can down and into the top of the oil pan. This helped quite a bit. We think that, on hard braking, the forward crank counterweight can get submerged in oil and a tremendous amount slung up into the separator chamber - apparantly more than can drain back into the crankcase at one time. I do not think the bigger hose is passing more oil (as the inlets and outlets are the same) but that the bigger hose is functioning as an "accumulator" to hold the oil until I am off the brakes, on the gas, and it can drain back into the pan.
2) the second thing that has helped in a big way is to vacuum the crankcase to lower the pressure. The early hot rodders discovered a long time ago that, when they could draw a vacuum - or a partial vacuum - in the crankcase - the rings would seal tighter and they would gain HP. In fact, positive crankcase pressure starts to lift the rings off the cylinder walls and even more blow-by will result! This is now so common among drag racers that they mount an external belt-driven pump onto the motor just to pull a vacuum in the crankcase.
But the early soultion - the low-budget solution - that still works VERY well
was to weld a hose nipple into your headers at an angle so that the passing gasses would draw a vacuum on the hose. The other end goes up to the crankcase and you are all set. Simple, elegant, very effective. What I like best about it is that the higher the rev's, the more vacuum it draws - perfect!
In the attached picture below you can see both ends of this little system. In the picture on the left it starts at the top of my oil catch tank, and leads into a #10 AN braided steel hose. That hose runs under the car and attaches to the nipple welded into the collector. Done. That little round thing that looks like a vacuum cannister right at the exhaust nipple is a one-way valve so a backfire cannot go backwards up into my catch tank.
I found the oil nipple and backfire valve at Summit Racing, and the #10 stainless braided hose I got from Pegasus. Under $50 for the whole thing.
By accident, we discovered a second benefit to this system: in a long race once, I neglected to turn on the pump that returns the oil from the bottom of my catch tank back into the motor. The catch tank overfilled, but instead of spraying all over the top of my motor (and probably starting a fire) it was all plumbed into the tailpipe and safely away from the motor. The sudden plume of blue smoke reminded me of what I had done (or not done, as it were) and I hit the pump switch. Problem solved! Nice to not have to clean up a nasty oil spill all over the motor.
When we were diagnosing blow-by and crankcase pressure in 2001 (our first year with the supercharger on the 928) we installed a vacuum pressure/boost gauge on the crankcase and mounted it in the cockpit. Under WOT and full boost (9 PSI) at 6,000 RPM, we would spike just briefly as much as 2.5 psi in the crankcase before it would go down again. We were worried about blowing gaskets.
After the installation of this exhaust vacuum system, I cannot make the vacuum/boost needle on that gauge move at all. And we run more boost now than before - but no matter what I do the crankcase stays at Zero.
That has helped the rings stay seated and seal better, and we have noticed that we have to drain the catch tank far less often.
http://forums.rennlist.com/upload/cr...cuumsystem.jpg
I have one or two things that we have tried and found succesful to help
anybody racing a 16v 928:
Try this exercise: remove the oil filler cap and separator screen from your 16v 928 and have an associate start the motor and take it up to about 2K with several 4k blips. Hold your hand over the inlet port of the circular oil filler chamber....
We found the amount of solid oil droplets (not so much a mist) thrown up into that chamber to be amazing. And this was AFTER the installation of a DEVEK louvered oil baffle in the crankcase vent. When we were installing that louvered oil baffle, I remember looking straight down and seeing a connecting rod and crankshaft counterweight directly below. That meant to me, as well as all the oil "misting" problems you guys have all correctly written about - oil was just plain being SLUNG up there by that counterweight and rod! I love Porsche too - but I gotta say - that location to take the vent off of the crankcase is a really poor one. Anyway - that's what we have to deal with.
Two things that I have not seen mentioned in this thread that have helped our race car:
1) we found that we would "pump" more oil up into the oil separator/vent under hard braking rather than hard cornering. We first thought it was oil coming up the drain hose from the front of the motor so we added a one-way check valve to that thing. Did not help. So we went the other way, and we increased the size of the hose that drains out of the vent can down and into the top of the oil pan. This helped quite a bit. We think that, on hard braking, the forward crank counterweight can get submerged in oil and a tremendous amount slung up into the separator chamber - apparantly more than can drain back into the crankcase at one time. I do not think the bigger hose is passing more oil (as the inlets and outlets are the same) but that the bigger hose is functioning as an "accumulator" to hold the oil until I am off the brakes, on the gas, and it can drain back into the pan.
2) the second thing that has helped in a big way is to vacuum the crankcase to lower the pressure. The early hot rodders discovered a long time ago that, when they could draw a vacuum - or a partial vacuum - in the crankcase - the rings would seal tighter and they would gain HP. In fact, positive crankcase pressure starts to lift the rings off the cylinder walls and even more blow-by will result! This is now so common among drag racers that they mount an external belt-driven pump onto the motor just to pull a vacuum in the crankcase.
But the early soultion - the low-budget solution - that still works VERY well
was to weld a hose nipple into your headers at an angle so that the passing gasses would draw a vacuum on the hose. The other end goes up to the crankcase and you are all set. Simple, elegant, very effective. What I like best about it is that the higher the rev's, the more vacuum it draws - perfect!
In the attached picture below you can see both ends of this little system. In the picture on the left it starts at the top of my oil catch tank, and leads into a #10 AN braided steel hose. That hose runs under the car and attaches to the nipple welded into the collector. Done. That little round thing that looks like a vacuum cannister right at the exhaust nipple is a one-way valve so a backfire cannot go backwards up into my catch tank.
I found the oil nipple and backfire valve at Summit Racing, and the #10 stainless braided hose I got from Pegasus. Under $50 for the whole thing.
By accident, we discovered a second benefit to this system: in a long race once, I neglected to turn on the pump that returns the oil from the bottom of my catch tank back into the motor. The catch tank overfilled, but instead of spraying all over the top of my motor (and probably starting a fire) it was all plumbed into the tailpipe and safely away from the motor. The sudden plume of blue smoke reminded me of what I had done (or not done, as it were) and I hit the pump switch. Problem solved! Nice to not have to clean up a nasty oil spill all over the motor.
When we were diagnosing blow-by and crankcase pressure in 2001 (our first year with the supercharger on the 928) we installed a vacuum pressure/boost gauge on the crankcase and mounted it in the cockpit. Under WOT and full boost (9 PSI) at 6,000 RPM, we would spike just briefly as much as 2.5 psi in the crankcase before it would go down again. We were worried about blowing gaskets.
After the installation of this exhaust vacuum system, I cannot make the vacuum/boost needle on that gauge move at all. And we run more boost now than before - but no matter what I do the crankcase stays at Zero.
That has helped the rings stay seated and seal better, and we have noticed that we have to drain the catch tank far less often.
http://forums.rennlist.com/upload/cr...cuumsystem.jpg
04-25-2010, 12:45 AM
#104
Rennlist Member
What if any, changes to opinions / solutions have surfaced in the 7 years since this thread was started? Seems like there are a number of causes. Does MK still swear by Amosil and dislike M1?
Does Doug Hillary still want us to use Truck oil in our flat tappet motors....
Does Doug Hillary still want us to use Truck oil in our flat tappet motors....
04-25-2010, 10:04 AM
#105
Nordschleife Master
What if any, changes to opinions / solutions have surfaced in the 7 years since this thread was started? Seems like there are a number of causes. Does MK still swear by Amosil and dislike M1?
Does Doug Hillary still want us to use Truck oil in our flat tappet motors....
Does Doug Hillary still want us to use Truck oil in our flat tappet motors....
There are a few other long, involved and vigorous discussion on bearing failure, crank drilling, dry sumps and windage systems. Look for those.