Diff fluid friction modifier mix percentage?

[sc_tr0jan_m3]

Ive had just the Castrol fluid in my diff with no FM for a year and about 6k of track miles and haven’t had any issues. Granted I have solid bushings so the diff noise is prolly superseded by by solid bushings and zero interior noise.


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[Casa de Mesa]

When making tighter turns it will sound like a bit of a groan and you will physically feel it locking/unlocking as the car is moving. It's a slip stick effect. What likely happened was there was some residual friction modifier left on one of the previous fluid changes (some in the case and some worked in to the clutches) and now after the flush with the regular SAF-XJ fluid (no FM) you are experiencing the locking.

Any new noises you are hearing while driving perfectly straight are due to the solid aluminum differential mounts. You are likely hearing some pretty loud drivetrain noise (gear whine) due to these mounts. For anything short of a dedicated track car I recommend sticking to a new set of stock Genuine OE BMW mounts.

This helps a ton, thank you.

I think I have something else going on, or what I'm hearing is the bushings. I don't think it's the diff clutch discs.

This happens in a straight line. If I roll onto or off the throttle suddenly (not flooring it, just light switch "on off" at partial throttle - i.e., not smooth), I will get a noise like a Detroit locker engaging when you go around a tight turn. It's a very mechanical sound and very noticeable.

I'm hoping it's just the solid bushings transmitting noises through the subframe/chassis and it is now amplified thanks to zero deflection.


But what I'm worried about... I changed the front seal on the diff. I'm 100% sure that I got that big nut that preloads the pinion/bearings (the 32mm or 36mm one, can't remember exact size) in the exact same spot it was before disassembly. Counted threads, marked everything and got it all back - 100% sure of this. But is there some way that the preload is out of whack and what I'm hearing is the gear lashing from the front to the back side of the ring and pinion gears?

Car drives fantastic otherwise. Have several drives and probably 250 miles since the rebuild without any issues. I do have the gear whine - that I expected. What I did not expect is the 1960s muscle car Detroit locker gear noise.

[tedbm3]

4.6%

https://www.m3post.com/forums/showthread.php?t=155103

Good info, no idea why the bmw oil is so expensive

[bigjae46]

Ive had just the Castrol fluid in my diff with no FM for a year and about 6k of track miles and haven’t had any issues. Granted I have solid bushings so the diff noise is prolly superseded by by solid bushings and zero interior noise.


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It probably has a standard amount FM in it. Just doesn’t have the extra FM.


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