2004 6MT VLSD into my 2004 5AT open-diff
 

Hi all, I've done a good search on this with not much returns. Plus a lot of the threads are 10+ years old with dead links and lost photos etc.

I've just been given the opportunity to make this swap so sort of need some answers quick!
A fella has a 2004 6MT with an LSD - He wants to straight swap for my open diff to weld.

Seems like the perfect opportunity to upgrade. Of course the first thing I'm going to do is check how much torque his diff can hold. If it is within the range for a good VLSD I'll move forward with the swap.

He said his has a ratio of 3.538 and that mine is 3.357 (sounds about right for 6MT vs 5AT)
Would I feel/notice much of a change with this difference in gear ratio?
Some quick calculations show I'll be doing 75mph when I used to do 80mph and when I was once doing 80mph at 3000rpm I'll now be closer to 3200rpm. If that's the case then it doesn't seem too much of an issue.

So my questions are:
1. What is the torque I should be expecting the diff to hold.
2. Anyone who's experienced a change in gear ratio, what should I be expecting? Any drawbacks or issues or daily driving worries?
3. Does anyone know for sure if the compete diffs will swap straight over. Not taking the gears out, just swapping the whole thing over. I think the driveshaft flange will need to be swapped from mine to his but the axle flanges are the same?
4. Also, is there any information out there I can use to verify my current gear ratio and verify his LSD gear ratio. Andddddd any where I could look to verify his diff is actually an LSD or OEM piece of equipment - stamps on the casing or something? He doesn't have brembo brakes but it is a 6MT and then I read that the LSD was optional so could have been on any car. I know spinning one wheel, the other should spin the same direction and it should have a break away torque of XX lb/ft but confirmation of some sort would be great.

Once the diff checks out and holds good torque my main concerns will be the effect of the new gearing and how much of a "straight" swap this should be as far as bolt on/off.

I know this upgrade is not the worlds greatest if I were aiming to track the car, but it's an opportunity to get a little more performance for the cost of some time. I just don't want to run into complications and end up wasting my time or going backwards somehow.

All advice is welcomed! Anything else you think I should be watch out for, check out properly or take note of please mention!
I'd really like to make this happen and update you all with the difference in driving experience.

6MT only came in the Sport package no? The Brembo 6MT was a standard methinks.

That's what I thought, the options are all over the place now.
Anyway I'm going to sit in the guys car and check his speedo is accurate with my gps to verify the diff is original to his car. Once that checks out it means he has the 3.538 gearing. I'll then break out the torque wrench and check it locks up to XX lb/ft and once all is good we will swap them over.

This is one of those mods there's no need to overthink. For a straight swap this is a no brainer. Diff will bolt up as is, the increase in low gear grunt is noticeable, you get a VLSD but if you drive a lot of highway miles know your gas mileage will suffer. I did the swap several years ago on my 05 sedan and haven't regretted it one bit.

Your calculations are spot on, at 80 MPH I'm indeed running at 3200 RPM so the VQ's working hard at not such a high speed. By contrast my GTO is a 6MT and at 80 it's humming along at 1900 RPM's and getting 23-24 MPG. The G is down to 21 on the freeway on trips but I still love her so much I don't really care. Also, the Goat is a much more comfortable, capable and fun car to take on road trips and I can't remember the last long trip I took in the G but I digress.

Do the swap, it's Blue Dream approved!!

Thanks for the helpful info, I really needed to hear that. I was definitely over thinking :bowrofl:... I guess all I will do is check the lock up torque to make sure it's worth breaking out tools to swap it.

I don't do much highway miles. To university and back (25 mile round trip, every day, no highway). The G really isn't a car to drive if fuel mileage is a major concern :) I was getting something like 17mpg but I'm sure it's gone up since changing the compression rod bushings. My front right tire was dragging badly and it ate out the inner edge very quickly.

I know what you mean about the v8 engines revving comfortable low on the highway. My dads C5 corvette is between 1500-2000rpm at highway speeds. Get's better gas mileage than me as well even with the bigger displacement engine. At least my current 285 30 20 tires will help with the gearing - by increasing my speed by about 2% from the 235 50 17.

So now, does anyone know the torque value and what fluid should I put in/how much etc as I may as well do this while it's sitting on the bench.
Also, I'm curious how much your fuel mileage dropped from? Not that it's going to stop me swapping it.

Just been briefed that this diff whines.
He says it's normal for the LSD's to have a slight whine to them......any one with an LSD, does it whine?

First time I've ever heard that in 8 years on the forum. Sounds like he's making excuses for a problem he doesn't want to disclose. I've been in numerous 350Z's and G's with the 3.5 diff and not a single one of them whined unless it had a problem.

To answer your other question gear oil is 75w90 and it's pretty common and easy to find. I've always used Amsoil Severe Gear but that's probably overkill, any quality brand will do. Torque specs not sure, maybe someone can chime in. Around town mileage stayed about the same and we already discussed highway use.

Mine doesn't whine. And it isn't normal for any rear end to whine.

If you can score a limited slip rear end and a little bit deeper gears, its always worth it other than the slight hit in mileage.

The car has 165k miles and was "owned by a friends grandma". Okay.
A while back we had a 05 350z with an LSD and never heard a noise out of that diff (besides the clicking sound from the axles).

Thanks for all the input guys, I too thought he's making excuses which is why I turned to the forum to verify if anyone had actually experienced any diff whines. I was looking forward to swapping out to an LSD but I don't want a whining diff. It could be his trans making that whine but it's not worth my time to investigate.