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Jon Milledge Comment on Rod Bearings


944-Spec#94

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Interesting comments from Jon Milledge on #2 Rod bearing issues.

 

 

I emailed Jon Milledge to get his thoughts...here's what his reply which is helpful.

 

 

 

quote:

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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

 

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I'd welcome anyone's comments on these various options.

 

 

__________________

Rick

 

 

Interesing Comment about the oil pump speeds.

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Rules notwithstanding, I wonder if I should put my air-oil separator back on? It was installed just ahead of the oil cooler - wonder if that location really helps remove air bubbles soon enough? Wonder if Jon has any experience with these? Presumably he would have mentioned it if it helped. Presumably there's also no solution to the oil pump design. Seems like a knife-edged crank might also help reduce the chance of frothing.

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John sells a very effective separator for about $600 if I remember correctly. It uses a Canton-Mecca remote oil filter and bleeds the air/oil back to the cam housing.

 

I have his system installed on my ITS 944 and a teardown last year after 13 race weekends revealed perfect rod bearings. But you should know I also have done all the other things Jon recommends as well.

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John sells a very effective separator for about $600 if I remember correctly. It uses a Canton-Mecca remote oil filter and bleeds the air/oil back to the cam housing.

 

I have his system installed on my ITS 944 and a teardown last year after 13 race weekends revealed perfect rod bearings. But you should know I also have done all the other things Jon recommends as well.

 

Raceone,

I also had a Canton Mecca filter. Do you have a picture of your set-up?

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I'm sorry but I do not have any photos.

 

I can tell you I used a screw on oil filter plate with a braided stainless line to the Canton-Mecca housing mounted where my power steering resoivor(sp) had been mounted. The filter output goes into a 13 plate Mocal cooler that is mounted behind the fog light opening. The cooler output goes to a one-way valve and then a "T" fitting where the Accusump line feeds in. That line terminates back at the input connection on the filter plate.

 

Jon's bleed line is also braided stainless, but only about the size of a brake line. It attaches to the top of the Canton Mecca housing (which must be mounted vertically) at one end and with a special fitting which replaces one of the cam cover access screws draining foamed oil back to the engine.

 

I have the Accusump mounted on the passenger floor, but wish I'd put it in the spare tire well.

 

Does this help?

 

Denny

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