IMSB - Did I Dodge A Bullet

[Smallblock454]

Originally Posted by Jamesp

You've got the point. I made this video a year ago

https://www.youtube.com/watch?v=qzHwWUgU37k

There are 7 short ( 2 minutes or so) videos detailing precisely what you are saying, and showing how I approached this problem. I'm at just about 3000 miles since I made this simple modification to the IMS and all is well. I redline just about every day because it is fun. Click on my name under the video if your interested in seeing what I did. There's been an implication that Porsche blew the bearing design by overloading it. I have a hard time buying that unless the mistake was assuming grease in the bearing instead of oil when doing the load calcs. That is part of the reason I went back with a sealed high temp (high temp grease and Viton seals) greased deep groove steel ball bearing with a vented IMS. The other reason is that the fully sealed bearing feature coupled with venting keeps any trash in the oil out of my IMSB.

Hi jamesp,

some thoughts first:
Fluids do expand much more than air. So if you have oils in a sealed IMS tube the overpressure expands much more than you'll just have air in a sealed tube.

I would estimate that the inner volume of the IMS tube is around 0.5 litre (500 ml) and you said that there was a cup of oil in the tube - so around 100 ml? Would you agree with that? (because i want to calculate the overpressure at different temps).

If the car is on leveled ground the chains of the IMS tube run in engine oil, whereas the bearing is a little above the normal oil level. But if the car moves (accelerates, etc.), the oil level can be above the bearing. Also if the car stands or drives with the nose up a hill.

Watched your videos on Youtube:
01 http://youtu.be/qzHwWUgU37k
02 http://youtu.be/JXPMWDVn4Ec
03 http://youtu.be/PTk_xehieFc
04 http://youtu.be/Y5cgLrPsAdg
05 http://youtu.be/pKImRZ2reNY
06 http://youtu.be/SxhFdPCoYYs
07 http://youtu.be/1vrX6rmV_nI

Understand now what you mean with vented.

Where i struggle is if we really have a sealed IMS tube filled with around 50-100 ml of old engine oil, why isn't the oil pressed out of the tube / sealing, if we have overpressure (compared to the rest of the engine) in the IMS tube when the engine is hot?

If there is a way in, in general there also has to be a way out. The only way i could imagine is that materials expand when they get hot and so there is no way out, while materials shrink when they get cold and than there is a leak so that the oil can get in over a long time.

Also i'm not 100% shure that the system is 100% completely sealed system. The "weak" part might be the bolt that holds the inner part of the bearing. Also it could matter what engine oil you use. Too "thin" oil might get in faster than "thicker" oil.

You've drilled 2 holes in the IMS shaft - now i understand why you called it vented. I think it is good idea. Oil can get in and will get out when you start the engine, because of centrifugal forces, even if the oil level is a bit above the IMS shaft. Only thing to mention is that the bearing seal can be harmed over time by temperature changes and agressive oil. So it is still important to change engine oil and filters regularly.

Regards from germany
Markus

[Jamesp]

Originally Posted by Smallblock454

Hi jamesp,


If there is a way in, in general there also has to be a way out. The only way i could imagine is that materials expand when they get hot and so there is no way out, while materials shrink when they get cold and than there is a leak so that the oil can get in over a long time.

Markus

Markus, the way in and way out is at the IMS bearing seal to inner race interface. That area is sealed with a thin rubber lip much like the edge of a windshield wiper, but much smaller. When the air in the IMS tube heats up and pressurizes, that seal is no match for the delta pressure so gas escapes through it. The same thing happens in reverse when the tube cools down, only now there's a tiny bit of oil available to accompany the air through the bearing and back into the tube. That is why very nasty overcooked oil is found in the IMS when the bearing is removed. This process happens slowly over time every time the engine is heated then cooled, in other words every time the car us used.

[Smallblock454]

@ Jamesp.

Thank's for explanation. That makes sense.

So maybe the cars that were allways parked with the front downhill didn't suffer as much from the IMS bearing problem.



Whereas this car would have big problems - if it would be a 986.



Thanks again and regards from germany
Markus

[Gelbster]

I thought of getting a groove milled in my LN hardened+shot-peened oil pump drive. Then use a good quality 6204 bearing with the appropriate seal mods. But I if I did, I would never be able to look Jake in the eye again. :chicken:
But seriously, thanks for some great comments. I had abandoned any interest in IMSB threads because they usually are just repetitive 7 argumentative -but this is interesting.