Looks like I need a new diff. 3.91 is the way to go, right?

[MattL]

In this, the latest installment of "Matt The Newbie's Carnival of Sounds" I learn the scraping sound coming from the passenger side of my 76 is my differential. Pinion bearing, to be precise. It would be a lot of work to rebuild, so I guess I'm in the market for an LSD a little earlier than expected. I still have the 4-speed transmission, but I hope to go to a 240 or 245 5-speed if I can find one in my area. 

 

I've searched the forum and read all the comparisons between the 3.64 and 3.91 ratios that I could find. Given the scarcity of these things, I don't know what I'll be able to find (and/or afford). Since I'm planning to go to a 5-speed at some point, though, it seems like the 3.91 is the one to search for, right?

Edited April 11, 2019 by MattL

[pierre²]

Not sure if a lsd diff. is necessary for street use. I had the impression the o2 was slower to accelerate at trafic lights after the 5sp swap.  So 3.91 is a good idea,  the lsd is up to your task with the o2.  3.91 open diff. should be easier to find,,  all the 320i with 5sp got them + 76 2oo2  

I have not put the open diff. on my car yet.  Personaly with Us 70' car that I had own that had lsd,  I had issue.  If you find a lsd 3.91 and can afford it GO for it .  The WOW !  factor will be priceless.

['76mintgrün'02]

11 minutes ago, pierre² said:

I had the impression the o2 was slower to accelerate at trafic lights after the 5sp swap.

 

Gears 1-4 are the same in the 4-5 speeds.  Fifth gear is where the 3.9 ratio is useful with the 5 speed.

 

I do appreciate the grunty feel of the (open) 3.90 in my '76, in spite of the higher rpms at highway speeds with the stock 4 speed transmission.

[pierre²]

Too bad,,  I was wishing the 3.90 would help on acceleration in first gear. 

 

['76mintgrün'02]

It does!  It is just that first gear is the same in the 4-5 speed transmissions.

[SaharaX2]

I have a MM 3.64 LSD in my ‘75 with a 5 speed XMSN and it does great.
The 3.64 keeps the rpm down at highway speeds.
It all depends on what you want to do with the car.

I take my car on long road trips where the 3.64 shines.

IF I was going to auto cross or track days a lot, I would install the 3.91 LSD and stick with the 4 speed. No need of the expense of going to the 5 speed XMSN for that usage. This gives you the acceleration of the ‘76 and keeps things simple.

The 3.91 with the 5 speed XMSN in 5th gear gives you approximately the same final drive ratio as the 4 speed with the 3.64 gears.





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[Mike Self]

3.64 vs 3.90 and 4 speed vs 5 speed...

Depends on how you use your car and whether you're gonna install a 5 speed or stick with the 4 speed...

 

4 or 5 speed:  If you plan to take long trips in your '02, stick with the 3.64.  With a 3.91 diff and a 4 speed you're gonna be turning 4600 or so rpms at 70 mph (ca 110 kph); with a 5 speed you'll be back to approximately the same 4k in 5th at 70 as you would in 4th with a 3.64.

 

With a 5 speed and a 3.64, you'll be right at 3200 rpm at 70 in 5th.  It makes a significant difference in noise level and will increase your gas mileage a bit--but not enough to make economic sense unless you drive 20k miles/yr.   I have a 3.64 with a 5 speed since 2004 and like the combination a lot, as I do take long trips in my '02.  I've climbed some pretty steep grades on the WV Turnpike without having to downshift.  And cruising is much more relaxed at the lower rpm--I can actually hear my radio!

 

mike

[2002Scoob]

I had the option recently to purchase a 323i euro LSD with the 3.45 that would be paired with my 4speed. I was curious if anyone had tried it, and was gunna ask about it, but the dude pulled outa' the trade. 

 

But I'm still curious... I wonder how much of a hit in acceleration I'd get, but then also for autobahn cruising at 150-160 kph+, it could be nice...

[TobyB]

The 3.45 makes the 2-3 gap even more apparent.   And 

it's only an incremental gain.  But the gears are pretty

common in manual e36's, so it would be easy to try.

 

t

[Oldtimerfahrer]

Hi,

I have the 3.64 lsd and 5-speed, I find the 1-2 gap more painful than 2-3 but maybe it's because I am taking a standing start on a hill climb as the bar.. You wind the piss out of it in first on a hill (say a little over 6000), you're still landing at the bottom of the sweet spot on hp/torque in second gear because you lose speed against the hill and maybe I shift like a grandpa because I hate crunching gears. I don't generally do a power shift or burnout in second because I'm not in the engine's sweet spot after the shift. It's not a great combo for hard acceleration, but great as a tourer. The 3.91 is an attractive proposition... because the 5-speed is overgeared in 5th and not evenly spaced.

Andrew

[tech71]

Is a diffs ratio marked somewhere on the case? 

[Oldtimerfahrer]

usually stamped on the side of the cast iron case in large but shallowly stamped numbers with a separator. there are a lot of pictures and identification threads on faq already on this topic of using them.

Andrew

[69Bimmer02]

3 hours ago, mike said:

3.64 vs 3.90 and 4 speed vs 5 speed...

Depends on how you use your car and whether you're gonna install a 5 speed or stick with the 4 speed...

 

4 or 5 speed:  If you plan to take long trips in your '02, stick with the 3.64.  With a 3.91 diff and a 4 speed you're gonna be turning 4600 or so rpms at 70 mph (ca 110 kph); with a 5 speed you'll be back to approximately the same 4k in 5th at 70 as you would in 4th with a 3.64.

 

With a 5 speed and a 3.64, you'll be right at 3200 rpm at 70 in 5th.  It makes a significant difference in noise level and will increase your gas mileage a bit--but not enough to make economic sense unless you drive 20k miles/yr.   I have a 3.64 with a 5 speed since 2004 and like the combination a lot, as I do take long trips in my '02.  I've climbed some pretty steep grades on the WV Turnpike without having to downshift.  And cruising is much more relaxed at the lower rpm--I can actually hear my radio!

 

mike

Mike, are you sure about your numbers?  Reason I ask, and please validate my thinking, a 5 Speed overdrive has a 5th of .81 final drive gear, which I believe puts the 3.91 down in the low 3's as it pertains to final ratio.  All the math I do puts a car with this combination at 70MPH in the 3000-3300 RPM range.  Others on the forum can validate my thinking but I know that's the case for my car with that combo.  So when I saw your 4K at 70 comment, I thought something just doesn't add up there.

 

thoughts?

 

Jeff

 

 

[69Bimmer02]

In addition, this is a handy website to add in tire size to calculate those numbers.

 

https://www.engineersedge.com/mechanics_machines/transmission-gear-ratio-calculator.htm

 

 

[jimk]

53 minutes ago, 69Bimmer02 said:

70MPH in the 3000-3300 RPM range

Calc - 76.7 mph @ 3500 rpm, 3.90 diff gears and 23.2 tire diameter.

Tach as inaccurate as it is reads about 75 mph @ 3500 rpm for me