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