Rennlist Discussion Forums   classifieds | membership | rennlist | photo album    
sponsors | upload photo | chat    
 
Go Back   Rennlist Discussion Forums > Water Cooled Technical Discussion Areas > 924/931/944/951/968 Forum
Sign in using an external account
Register Forgot Password?
Search


Reply
 
Thread Tools Search this Thread
Old 08-26-2005, 05:03 PM   #1
eohrnberger
Addict
Rennlist Member
 
eohrnberger's Avatar
 
Join Date: Jun 2005
Location: Troy, MI
Posts: 1,565
Question #2 Rod bearing oil starvation problem?

Then I'm really confused on this issue, and ask everyone's help in resolving my confusion. Below are my notes about the subject.

====================
http://www.theautobahn.com/forum/sho...794#post157794
Serious Discussion about crank drilling: http://web2.pcasdr.org/phpBB2/viewto...1054c85e8e1a2d

The problem is Porsche only drilled two holes in the crank (where the connecting rod attaches) for oil AND drilled them at a spot where centrifugal force (at above ~5000+ RPMs sustained) [possiblity on a left bank] stops oil from flowing to the bearings. The solution is to drill two more holes (x 4 places) 90 degrees off set of the stock oil holes.
============================
There has been a great deal of discussion regarding this problem over the years. None of it seems to have helped one bit.

I saw 944's blow up in 1991 when I started racing and I see them blow up now. I hate to just say that the engine is unsuitable for racing, but it seems that that is really the case.

Here is what I know from first hand experience:

Accusumps do nothing.
Cross drilling the rod journals does nothing, and may make it worse.
The oil level is critical.
The hot oil pressure is not particularly important.
Oil/Air separators and crankcase vents do nothing.
There is no correlation whatsoever between fuel mixture distribution and rod bearing failures.

Here is what seems to help:

Keeping Rpm below 6000 rpm
Checking the oil level after EVERY session
Never running enduros
Keeping the oil cool
Keeping the oil pressure as high as possible. The racer's only means of doing this is by cooling the oil.
Modifying the oil pan so the effective oil level in corners is higher.

Here are some ways that a 911 differs from a 944. This is relevant because 911's just don't have oil related bearing failures.

911s have a simple but effective dry sump system.
911s have a very large pressure pump. They almost never run short of volume.
911s don't have an aerodynamic mess in the crankcase.
911s tend to have better control of oil temp
911s have large oil tanks, even when you are three quarts low, there is still plenty of oil.
911s have a very open crankcase breathing system.

_________________
Chris Cervelli
Technodyne Inc.

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.

Jon Milledge
================

So like what is the bottom line? If I rebuild my engine for street/track use, what do I do to reduce the chances of blowing it up like this to the very minimum, Rennlist collective conscious?
To remove this ad, register today or login if you already are registered!

__________________

eohrnberger is online now   Reply With Quote
Old 08-26-2005, 05:16 PM   #2
M758
Super User
 
M758's Avatar
 
Join Date: Aug 2002
Location: Phoenix, Az
Posts: 17,643
Default

Quote:
Originally Posted by eohrnberger

So like what is the bottom line?
Nobody... Not even the experts know how to fix it.


It would be rare to blow-up on street. Possible at the track even with all the precautions.

I have had two failures. One on a stock 45k motor after a number of years racing. Bottom end was destoryed.

Second was on newly rebuilt (main & rods), baffled pan, high quality oil motor. This time I caught it before I damaged the block, but crank was toast.

I have been given information about cross drilling only #2 & #3 rod journals. This person ran flowtesting on the crank and that combo seemed to balance the flow for all 4 journals. Even crossdrilling all show lesser oil flow on #2 & #3 rod journals.

His experience seems to think it works. I have yet to run this motor. Note that shop did not to the drilling on my crank so no money involved FWIW...
M758 is offline   Reply With Quote
Old 08-26-2005, 05:23 PM   #3
Mongo
Official Bay Area Patriot
Rennlist Member
 
Mongo's Avatar
 
Join Date: Nov 2001
Location: Holding the Line Against the Libs (SF Bay Area)
Posts: 27,850
mongo928 InsidiousMongo
Default

When did they integrate the baffle and would my car have it from the factory? my build date is 2/86. I'm a bit worried since I autocross.
__________________
Andy

1987 928 S4 #687
1999 Mercedes C280 Sport
2003 Mercedes ML350



“Beware the Fury of a Patient Man.”

-John Dryden
Mongo is offline   Reply With Quote
Old 08-26-2005, 05:25 PM   #4
M758
Super User
 
M758's Avatar
 
Join Date: Aug 2002
Location: Phoenix, Az
Posts: 17,643
Default

They never did.
the cars stock come with baffle, but it is a bolt in affair.

You can have welded in extra baffles. Autocross is pretty easy on the car anyway. I don't feel you will need it.
M758 is offline   Reply With Quote
Old 08-26-2005, 05:26 PM   #5
Yabo
Addict
Rennlist Member
 
Yabo's Avatar
 
Join Date: Nov 2003
Location: Boston (Watertown)
Posts: 11,362
Send a message via AIM to Yabo
Default

only S2 and turbo after 88 had the extra baffel I believe stig.
__________________
-- Devon

1986 951 (2), 1989 Range Rover. Past: 85.5 944
Never underestimate the 944's innate ability to break.
Yabo is offline   Reply With Quote
Old 08-26-2005, 05:27 PM   #6
Yabo
Addict
Rennlist Member
 
Yabo's Avatar
 
Join Date: Nov 2003
Location: Boston (Watertown)
Posts: 11,362
Send a message via AIM to Yabo
Default

Quote:
Originally Posted by M758
They never did.
the cars stock come with baffle, but it is a bolt in affair.

You can have welded in extra baffles. Autocross is pretty easy on the car anyway. I don't feel you will need it.
I read recently (can't recall where) that you can swap on an s2 or 88 (maybe only 89?) turbo oil pan and have the baffle.
__________________
-- Devon

1986 951 (2), 1989 Range Rover. Past: 85.5 944
Never underestimate the 944's innate ability to break.
Yabo is offline   Reply With Quote
Old 08-26-2005, 05:28 PM   #7
Mongo
Official Bay Area Patriot
Rennlist Member
 
Mongo's Avatar
 
Join Date: Nov 2001
Location: Holding the Line Against the Libs (SF Bay Area)
Posts: 27,850
mongo928 InsidiousMongo
Default

Aren't the bottom ends the same more or less? Or do the oil feeds (or areas of pickup) have different locations?
__________________
Andy

1987 928 S4 #687
1999 Mercedes C280 Sport
2003 Mercedes ML350



“Beware the Fury of a Patient Man.”

-John Dryden
Mongo is offline   Reply With Quote
Old 08-26-2005, 06:05 PM   #8
DanG
User
 
DanG's Avatar
 
Join Date: Nov 2001
Location: Costa Mesa, CA
Posts: 1,595
Default

Here's some "oil problem" questions and answers I gathered together in this thread...

http://forums.rennlist.com/rennforum...39#post2323539
Quote:
The engine failure has been diagnosed as a spun #2 rod bearing at the moment. The engine was supposed to reach the stand by COB today, they'll have the pan off and things apart by mid day tomorrow for a full report. I'm really crossing my fingers that the cylinder walls will be ok. If the walls are fine, it will be ready to go back together pretty friggin soon.

Some quick notes on the infamous 944 #2 rod bearing failure

Do some sustained high-g cornering, oil gathers on one side of the pan, pickup sucks air, good bye #2 bearing. I've read and heard a number of theories as to why is specifically #2, but regardless of that outcome, I'm going to work on preventing the cause.

First, a little discussion on the failure mode:
http://forums.rennlist.com/rennforum...d.php?t=121038

http://forums.rennlist.com/rennforum...d.php?t=191927

Next, the most comon methods of prevention (I'm doing at least the first 5 of these, probably add the 6th if I get too nervous):

1) Installing oil pan baffle(s) to keep the oil from sloshing too far away from the pickup


2) Weld a "collar" around the pickup to encourage it to draw the oil "up" from the bottom of the pans, rather than sucking air at the edges.


3) Install a crank scraper to strip oil from the crank, actually helping the oil situation, and making a few hp at the same time! Repeated tests show 3-5% "gain" (or more accurately, "reduced loss") in hp/tq. I'll take that all day long, especially for a whopping $69.95 shipped!


4) "Perp drill" the #2 and #3 rod bearing journals in the crank. "Perp" rather than cross, as the hole is drilled perpendicular to the original, facing radially outward. Centripital force (or lack thereof) helps draw the oil out into the bearing. Its done to #2 and #3 only, as its been proven #1 and #4 are almost never starved for oil, while #2 always goes first, and #3 is usually following about 50% behind it.


5) Calico CT-1 coated main and rod bearings. Dry film lubricant protects bearing on startup and low pressure conditions. Drastically improved bearing life.

6) Accusump - pressurized oil resevoir for "low pressure events". Pump pickup starts sucking air? Accusump pumps upto 3 quarts in at proper pressure, then instantly refils itself once the pickup is sucking oil again. Also, can be installed w/ an electronic valve that is linked to the ignition. Turn the key off, the sump stays charged w/ full oil pressure. Put the key in and turn to run, the sump pre-oils everything automagically! [red bull voice] BRILLIANT! [/red bull voice]
After consulting with "the experts" I've decided to forgo any drilling of the crank. I'll use Calico coated bearings, a crank scraper, a baffled pan, a collared pickup, and a carefully monitored oil level. I'll also likely install a KISS oil cooler. In the end, my car will really only see DE use, so its unlikely I'm going to be slamming curbs or reaching the extreme cornering grip levels of ultra-modified 944s. Hopefully this collection of "oil goodies" will keep my new engine together long enough for me to enjoy it. Although I have a feeling the first mile under boost will meet that requirement. So I guess my engine only needs to hold together for a mile!
DanG is offline   Reply With Quote
Old 08-26-2005, 07:17 PM   #9
Yabo
Addict
Rennlist Member
 
Yabo's Avatar
 
Join Date: Nov 2003
Location: Boston (Watertown)
Posts: 11,362
Send a message via AIM to Yabo
Default

Stig, i think the lower ends are the same, which is why the oil pan is a direct swap.
__________________
-- Devon

1986 951 (2), 1989 Range Rover. Past: 85.5 944
Never underestimate the 944's innate ability to break.
Yabo is offline   Reply With Quote
Old 08-26-2005, 07:28 PM   #10
Dave in Chicago
User
 
Dave in Chicago's Avatar
 
Join Date: May 2001
Location: Chicago Area
Posts: 1,979
Default

When I was tracking my '88 NA heavily, I would do two things:

A) Plan to replace the rod bearings between 40-60 hours of track time (street miles weren't counted even though I drove it daily)
B) Closely inspect the little magnet on the drain plug at every oil change (usually every 3 track days or so) for little tell-tale thin metal filings. These usually arranged themselves to form a nice little X-mas tree on the magnet. This little tree means "Change Rod Bearings NOW".

Both times, caught and replaced worn #2. Crank and rod okay, but did modify the pan and pickup after the second pull-the-whole-bloody-front-end-out R&R.

Watch the oil magnet and plan to change them as a PM if you are spending serious time at the track. Serious time at the track for me always meant flogging the car the entire time. I spent almost no time below 4500 rpm on track and ran each shift at full throttle right to the redline. If you drive the car easier and spend less time at 6200 rpm screaming down the straightaways, then your mileage may vary.
Dave in Chicago is offline   Reply With Quote
Old 08-26-2005, 07:32 PM   #11
Dave in Chicago
User
 
Dave in Chicago's Avatar
 
Join Date: May 2001
Location: Chicago Area
Posts: 1,979
Default

Almost forgot...

If you have to pull the pan for ANY reason, change the rod bearings (they are a wear item). Bearings are $50-$80, much less than the labor to get at them. It's worth it to get the full enjoyment out of these cars. Run that baby out to the redline regularly, it's what Dr. Porsche intended.

I have already PM'd the 968 rod bearings due to a leaking pan gasket (I hate cars that leak).
Dave in Chicago is offline   Reply With Quote
Old 08-28-2005, 10:53 AM   #12
eohrnberger
Addict
Rennlist Member
 
eohrnberger's Avatar
 
Join Date: Jun 2005
Location: Troy, MI
Posts: 1,565
Default Someone sent me this email

Someone has seen this thread, and sent me email on the topic. I submit this as from a known source (at least know to me), who wishes to remain anonymous.
Quote:
Soneone with personal experience with a former ’85 944 race car saw a significant reduction in rod bearing wear after the installation of an auxillary oil cooler. His current race car is a former S2 race car that regularly sees 6600+ RPM. It also has TWO auxillary oil coolers and no AC condenser to block airflow to the radiator. He removed the rod bearings after about 50 hrs of use just to check them and they were in beautiful condition. He’d easily feel comfortable going 100 hrs on the current set.

More points of fact:

S2’s and 968’s have a different oil pan baffle than the earlier cars. It’s a very subtle difference, but the baffle does somewhat limit the side to side sloshing in the oil pan compared to the earlier baffles. I installed on in my former race car too.

You never see a #2 bearing failure on the 968. My opinion is that it is because the factory changed the angle at which the #2 oil passage is drilled in the crank. This is also true of the late 928’s.

Oil lever IS critical and it is NUTS to suggest filling to the midway mark. Whether owners admit it or not (as it’s a little embarrassing), the vast majority of rod bearing failures on DE cars is due to the oil level getting too low. I know of at least 8 S2 racers, running with full race suspensions and Hoosier race tires that all subscribe to the OVERFILL practice and have NEVER had a rod bearing failure. My suggestion is to fill to the end of the machined part of the dipstick, just above the FULL mark.

He has an unfortunate, (and absentminded) friend who had BOTH a rod bearing failure AND a cam chain tensioner failure on his S2. His car had about 130k on it and had been very well maintained, but no tensioner replacement. He was lucky that his cams were able to be welded and re-ground to be re-used. Having a shop do all the work, his bill was over $3,000! His #2 rod bearing failure was the result of not checking the oil for probably 8-9 track sessions since it had been filled. The motor (other than the head) was irreparable and had to be replaced.

For my own purposes…. In his opinion, as long as you’re running a stock-ish suspension and street tires (or R-compound tires short of Hoosiers), run with plenty of oil and don’t worry about the #2 bearing.
seems pretty reasonable to me. What do you think?

Last edited by eohrnberger; 08-28-2005 at 02:33 PM.
eohrnberger is online now   Reply With Quote
Old 08-28-2005, 02:16 PM   #13
Dave in Chicago
User
 
Dave in Chicago's Avatar
 
Join Date: May 2001
Location: Chicago Area
Posts: 1,979
Default

Quote:
You never see a #2 bearing failure on the 968. My opinion is that it is because the factory changed the angle at which the #2 oil passage is drilled in the crank. This is also true of the late 928’s.
That's a good point. The other unique thing about the 968 is the addition of oil squirters shooting at the underside of the pistons (cooling effect was the desired outcome of this design change). I wonder if that also contributes to better rod bearing lubrication at the track? Who knows.

DO check oil between each track session. My '88 NA would not use ANY oil all winter (November - April) with generous usage on a daily basis. That same car could consume up to 1/2 quart a day at the track. Spending all of that time at WOT and north of 5000 rpm... Yup, better check that oil level!
Dave in Chicago is offline   Reply With Quote
Old 08-28-2005, 04:31 PM   #14
Scootin159
Super User
 
Scootin159's Avatar
 
Join Date: Oct 2002
Location: Syracuse, NY
Posts: 3,088
Default

Quote:
You never see a #2 bearing failure on the 968. My opinion is that it is because the factory changed the angle at which the #2 oil passage is drilled in the crank. This is also true of the late 928’s.
I have an '86 turbo crank and a 968 crank side by side in the garage right now. I just went and took a look at them, and there is no difference in the locations of the oiling holes. Actually the 968 rod holes are smaller with less camfer on the holes.

Then again I also noticed that my '968' crank has a '944' part #, is it possibly an S2 unit? Doubtful since I bought it off the owner of a very reputable and favorable sponsor of this site, but I guess it's possible.
Scootin159 is offline   Reply With Quote
Old 08-28-2005, 05:04 PM   #15
mark944turbo
User
 
mark944turbo's Avatar
 
Join Date: Jul 2002
Location: Massachusetts
Posts: 1,972
Send a message via AIM to mark944turbo
Default

Oil squirters would do nothing but take oil away from the rod bearings, no possible way for this to help. I did not know that the 968 crank had any different oiling, can anyone confirm?
mark944turbo is offline   Reply With Quote
Old 08-28-2005, 05:04 PM
Rennlist



Paid Advertisement
Reply

Tags
1, 2, 928, 944, bearing, bearings, cayman, crank, ct1, failure, oil, oiling, porsche, problems, rod, rodbearing, starvation, vw

Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Forum Jump


All times are GMT -3. The time now is 05:56 PM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
SEO by vBSEO 3.6.1
Copyright © 1998 - 2007 Rennlist.com

Advertise on Rennlist - Privacy Statement - Terms of Service - Jobs