Differential whine
 

Pretty sure I already know the answer, but wanted to get some input. Yesterday I took on swapping my 4.1 torsen over to a 4.3 carrier and ring gear and the end product was my diff sounding like it has straight cut gears, whereas in the 4.1 carrier it was whisper quiet, so I don't really suspect bad bearings or the diff itself. The whine is faint with part throttle, increases with throttle/acceleration, and cuts out entirely off throttle. I'm guessing lash and/or preload are too tight, but before I rip everything back apart should I go ahead and plan for replacing any parts? Other acceptable answers "Take it to a professional, noob.", in which case I'd just pickup some kia gears and go full ham sammich.

I'm wondering if somewhere along the line someone messed with the 4.3 carrier as when I took measurements before disassembling either the 4.3 had no lash. It was easy to tell even before putting the dial indicator to it as it had zero play. The 4.1 had anything from high end of spec ~.004 to a bit out of spec at ~.006. I used the high end of preload on assembly as that's what the measurements were beforehand and set lash to ~.0035 to ~.005. I figure I need to get it back out, recheck measurements, and at the very least use the original lash. It seems getting it set perfectly on any one side of the ring leaves the opposite side out of spec on either the high/low end.

is it a torsen or open? i know the torsens are known to be a bit noisy as speed.

Torsen. A little noisy would be one thing, this thing is screaming at 30mph with anything more than maintenance throttle, then goes quiet on cruise. The 4.3 was an open diff and I brought my torsen over by installing the 4.3 ring gear and dropping it into the 4.3 carrier. It made no noise in the 4.1 carrier so I don't think it should in this one either. I suspect it's either too tight/too loose (backlash), guessing too tight. I ordered some marking compound so I can get a better idea of what's going on. From the research I've done there likely won't be a pattern on the accel side, but hopefully I can get some relevant information from cruise on whether the pinion is out of position. If I can determine pinion depth is good hopefully it'll just be a matter of finding proper backlash.

no they don't.

he needs to set his tolerances

^What he said. Just got to set it up right.

Word. I'll be taking it apart this weekend and using the marking compound to, hopefully, confirm measurements before throwing it back together. Something something do it once, do it right.

Marking compound and a used R&P will not get you terribly far. Once they start to wear getting a reliable reading goes out the window unless it is a really low mileage R&P, so don't try to match anything you find online that's supposed to be "normal" or you'll end up chasing your tail.

Find the specs for the pinion preload and R&P backlash, get the pattern as close as you think you can, and call it good. This is a relatively comprehensive guide: Differential Installation Instructions | West Coast Differentials

You'll need to load up the ring gear to create some resistance so that your pattern is indicated with any degree of visibility; put a stub shaft in the diff and hold it with one hand while you turn the pinion with the other, or use a prybar and a rag in one side and apply leverage. Mark both sides of 4-5 teeth on the ring gear in two locations 180* out. Make 2-3 full revolutions of the ring gear, and you will be able to read both the imprint in the teeth, and the printed pattern from transfer to clean teeth.

Good luck!

What I was afraid of since it's a used gear set. In all my searching I did find a couple of posts on grassroots from a guy who, apparently, does a lot of diff rebuilds. He had some pictures of a Miata diff with no pattern on accel side and almost textbook pattern on the cruise side. So, maybe I'll have something to work with. He also said he sets tolerance tight and lets it make noise until the gears wear in, but I'm not really comfortable with that and it doesn't sound like best practice. Right now it sounds like the gears are going to file themselves down to nubs, so would rather get it quiet :p I came across that link in my searching, looked like a good resource. Thanks for the info!

Worst comes to worst I'm throwing it back in the 4.1 R&P and will figure something out from there.

Got things apart right now, just on visual inspection the used 4.3 pinion looks far more worn than my 54k 4.1 pinion, I think. Now that I'm feeling for it I can feel roughness on the accel side just turning by hand. Clearly what becomes the howl. Not sure if these gears are toast. Gonna mess with it a bit, but likely going back to 4.1 for now.

Here's 4.1 (left) vs 4.3 (right) pinion.

Edit: definitely appears to be backlash. Cruise side was wiping clean, accel wasn't wiping anything off. I presume that means the pinion was jammed way up into the corner of the ring gear, causing the issue. Backing the ring gear away has both sides wiping clean, just need to check measurements and dial it in.

Turns out no amount of setting backlash would have fixed this problem, the gears are toast. Just finished up swapping the torsen back to 4.1 and it's quiet again so there's that.

I had this exact issue with my stock lsd. I cracked it open, took a sniff/peek, and junked it.

Bumping because I had the same problem this past weekend at the track. After two sessions, my junkyard type 2 Torsen developed a noticeable gear whine, just like I imagine a straight cut gearbox would sound like. Noise only on throttle, terribly noisy from 30-70 mph. I drove it home (100 mi), emptied the fluid and it was black, burned, and glittery. I don't want to crack it open yet because I don't have any driveway space (other project cars in the way) and its still (sorta) driveable.
I plan on swapping a new T2 4.1 differential in, but I think I may have damaged the housing and halfshafts due to driving so many miles on an overheating, dragging diff. Should I swap the housing, halfshafts, or both? I assume this was a wear item due to unknown miles, but perhaps i should invest in more frequent differential fluid changes.
Also, WTB 4.1 T2 Torsen.