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Spun bearing question

Old 08-23-18, 01:55 PM
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Spun bearing question

I understand that if you spin the rear rotor/stationary gear bearing you likely have an oil starvation problem. What might be a general cause for a front bearing spinning?
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Old 08-29-18, 01:49 PM
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Since you haven't detailed anything, I will answer in a generalized way. Generally speaking, oil starvation is high on the list of causes when you spin ANY bearing. I say that only to say that if you spun a front main or rotor bearing, then oil starvation could be the issue just as much as the rear. Additionally, I believe there is usually concern about the front main bearings anyway (as opposed to the rear) because I know some people (builders, racers) modify the front housing to establish a direct oil line to the bearing. I have never done it, myself, but I get the idea behind it. Some things that you could look at: adequate oil flow (no leaks, unclogged pickup and lines, adequate pressure, etc.), oil type, clearances, and to a somewhat lesser extent, detonation.
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Old 08-31-18, 10:59 AM
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Thank you. I figured that all things being equal that if oil starvation was the root issue the rear rotor/stationary gear would go first since it's first in line in the oil circuit. But that thinking probably is sort of backwards in that if oil flow is interrupted then last in line would lose oil first.

Details are that it's a ported S5 NA 24 Hours of LeMons car. It's running a T2 oil pump with modified oil pressure relief valve and RA OMP adapter to use a separate oil tank for the OMP. Front iron oil passages are ported/cleaned up for better oil flow. It runs 80-90psi at RPM. I generally run with maybe a quart or so extra to ward off the oil pickup sucking air. I suspect that we are just reaching the limits of what the stockish system will live on and probably will need to cut in the rev limiter at a lower RPM (don't recall exactly where it's set currently but I'd guess 8500). It's also possible that I'm not getting the oil cooler completely clean (yes, I'd taking the thermostat out and plugging the bypass hole first). I can't seem to find any details on a mod to add oil to the front bearings. I'm guessing they tap the front iron oil passage thats being the AC mount and feed oil in through there?
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Old 08-31-18, 08:02 PM
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Originally Posted by cheseroo View Post
But that thinking probably is sort of backwards in that if oil flow is interrupted then last in line would lose oil first.
Yeah. The front is the one that people fix, or try to prevent, a problem with.

I can't seem to find any details on a mod to add oil to the front bearings. I'm guessing they tap the front iron oil passage thats being the AC mount and feed oil in through there?
That's the modification. They drill and tap that area leading the oil passage. Just keep in mind, some people have had issues with cracks, while some have not.

Just another thought since you're actually racing the car (which means revving the **** out of it), I know that S6+ and MSP shafts journals are tapered at the mains, but I don't know if S5 shafts are. Maybe it's flexing???
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Old 09-01-18, 08:28 AM
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The loop line generally requires a straight thread with an O-ring washer. There is very little material on the front housing where the main oil drilling is to support tapered threads.

I am NOT a fan of the "large" oil pumps at all. The deeper the pump, the more likely it is to cavitate at higher RPM. I have no problem maintaining 90psi up to and past 10000rpm with a 12A oil pump, which is the same width as a S4 N/A and also smaller in diameter and have never noticed the "high RPM pressure drop" that other people claim happens.

The larger oil pumps are to boost pressure at low RPM where turbo engines can start making a lot of torque. Not an issue for a race engine.

Last edited by peejay; 09-01-18 at 08:31 AM.
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