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3-Series (E21, E30) Chat relating to the BMW 3-Series from 1975-1983 and 1984-1991 line. Specific models: BMW 315, BMW 316, BMW 318, BMW 318i, BMW 320/4, BMW 320i, BMW 320/6, BMW 323i, BMW 320i. E30 Family models include: BMW 325e, BMW 325i, BMW 325is, BMW 325ix.

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Old 12-14-2004, 09:14 PM   #1 (permalink)
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i have a chance to fix my differentail to either of these boxes...

1- 3.73.1
2- 4.10.1
used
what are the differences as im not sure
i do alot of freeway traveling and dont want a high rpm at say 70-80mph
and i need the lsd .
(my car is humming to me to fix this)
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Old 12-14-2004, 11:54 PM   #2 (permalink)
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the way gear ratios in the rear differential works is that a 3.73:1 means the driveshaft turns 3.73 times for every one time the axles turn...so the higher the gear ratio the better the acceleration but the shorter the gears...for instance....

on a factory differential you are at about 3000rpms in 5th gear at say 65mph with a higher gear ratio you will be at 3500 or 4000 rpms at 65...so you sacrifice top speed but gain accceleration....

i have driven 325i's with both....the 4.10's will RIPPPPP but you will reach redline in 5th gear around 125 or so but the 3.73 will offer a little more top speed and better gas mileage...

IMO if you are going to daily drive it go with a 3.73:1 but if a Auto X car is what you want go with a 4.10:1
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Old 12-15-2004, 12:33 AM   #3 (permalink)
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okay.
what tools do i need to replace the differntail?
is there a way to test the used one against a good-bad buy?
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Old 12-15-2004, 05:44 AM   #4 (permalink)
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i cant remember the tools of the top of my head but i would suggest consulting your bentley manual and if you dont have one buy one NOW..that is the BEST tool for a BMW...

as far as teting it im not sure...i bought mine from an extremely reputable BMW Dissmantler here in sacramento and they assured me it worked fine...

for safety sake i drained out the differential i bought and filled it back up with redline differential oil and red line LSD additive...grabbed like a champ...

make sure to get an LSD unit...LSD = Limited slip differential...means you will get better traction in rain, launches ETC...should not cost any more or less at a dissmantler at least from my experiance maybe $40 more...but well worth it...saved my arse a few times...


Good luck and happy again soon hopefully!


Edit: Forgot to mention...if the differential you are buying is still in the car...check the mileage of the car...find one with low mileage
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Old 12-15-2004, 02:23 PM   #5 (permalink)
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thanks.
is low mileage less than 200,000
ive found 2 and there both above 120,ooomiles.

is thre an easy way to replace the bearings?
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Old 12-15-2004, 03:37 PM   #6 (permalink)
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We were discussing better differentials in the 5-series form, and someone claimed that with a 3.64 open differential, and a chip, he could do 162 miles an hour.

And this is a car that comes stock with a 2.93 ... so, apparently, you don't really lose top-end.

But he did lose about 3 miles per gallon ... 19 was the norm before the switch, and now he gets between 16 and 17.
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Old 12-16-2004, 01:42 AM   #7 (permalink)
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are you serious?......you DO lose top speed....the rpms are higher at every speed meaning that redline in final gear is at a slower speed than with a lower differential gear...the only reason it may have gone faster is cause the speed limiter is gone, redline is higher and the gear wasnt to big for as much HP as it had...
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Old 12-17-2004, 07:59 AM   #8 (permalink)
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on a factory differential you are at about 3000rpms in 5th gear at say 65mph with a higher gear ratio you will be at 3500 or 4000 rpms at 65...so you sacrifice top speed but gain accceleration....

I hope that's just a generalization, cause my 86 325es pulls 80 mph at 3k on the tach. anyhoo, you answered what I needed to know about the gearing & an LSD. Sounds like I need to get one.
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Old 12-17-2004, 11:04 AM   #9 (permalink)
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i just cleaned my differentail to prepare for the transformation and i found the ration of my current diff

its 4:10-

it also has on that metal tab 4:10:1790 or 1970 or 7190 ..something at the end w/4 numbers...

do they make a open differential with 4:10 gearing?

why would mine not work like one..? i peeled out and one tire went"eeeeerrrrreeeeeeeeeeeerrrreeeeee" slippin and i did it in mud so i checkd the tracks...
wuold the lsd break or fail and the diff can still work?
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