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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 09-12-2005, 06:15 PM   #1 (permalink)
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Gear Ratio?

Is There Any Way To Make The Gear Ratio Bigger Because I Have An 88 325is And My Gears Are Way To Short And I Was Wondering If U Can Make Them Longer
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Old 09-14-2005, 08:27 PM   #2 (permalink)
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Originally Posted by driftin325s
Is There Any Way To Make The Gear Ratio Bigger Because I Have An 88 325is And My Gears Are Way To Short And I Was Wondering If U Can Make Them Longer

Let me get this right, you want a higher gear ratio (taller gears) so you can have (i'm guessing) better MPG or a lower crusing RPM? If that's what you want it's much easier to change the complete rear end assembly. I'm guessing you have the 4.10:1 LSD differential in that case you'll want 3.75:1 LSD differential. Check your local junk yards. You'll have no problem selling your 4.10 rear end (racers want them bad).

Good luck
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Old 09-15-2005, 04:29 PM   #3 (permalink)
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Star any chance you could get us some part numbers for the lovelys so we out there can tell the difference?

also you wouldn't off hand know of the ratio for the e21 rear end would you? (1980 320i with the five speed trans if that makes any difference)
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Old 09-15-2005, 04:47 PM   #4 (permalink)
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yea i want a lower crusing rpm thanks a lot for the help from what other bmw would i get the other rear end
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Old 09-16-2005, 11:15 PM   #5 (permalink)
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I'll check and see if the spare rear for my 86 325es will fit your car, I'll give you a good price, however, shipping will me killer (close to 80 lbs.)

When I'm looking in the yard they are mostly out and I just pull the cover and start counting teeth.

Where I got my set closed up last year, but check out this link, this place is located in the same state that I am moving to (I'll be looking them up), haven't had dealings with them yet, so I don't know how much they are.

Just scroll to the bottom.
http://www.kormanfastbmw.com/e30drive.htm

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Old 09-16-2005, 11:58 PM   #6 (permalink)
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you have a type 188 diff, also know as the medium diff. its called a 188, because that is the diameter in mm of the ring gear. unless you know the proper preload specs for the carrier and pinion bearings, and know how to set pinion backlash and depth, then you'll want a whole new diff. i can provide you with the specs for rebuilding the diff if you'd like.

as for what fits, there are lots of diffs that bolt on. any e30 diff besides the 318i,is will bolt up. various 5 and 7 series diffs will fit by changing the diff cover and flanges over to your stock pieces. also some z3 diffs will fit.

any type 188 diff ring and pinion will fit in, so that's like over 1/2 of the bmws made since the 80s.

as to telling what you have, or what a diff is that you find in a junkyard. there should be a tag bolted to the diff cover. if the first digit is an 'S', then its a limited slip unit. the next 3 numbers, or the first 3 numbers if there is no 'S' is the ratio. so it it says S293, its a limited slip with a 2.93 r&p ratio, if it says 373, then its an open diff with a 3.73 ratio. limited slip units will also have a big 'S' on the top of the diff.

if the tag is missing, unreadable, or you don't trust it there are two ways to figure out the ratio. 1. do as starmarvin suggested, count the teeth on the ring gear and the pinion shaft, divide one into the other and you'll have your ratio. 2. an easier, but less exact way is to raise the car up in the back and mark the pinion flange, where the driveshaft bolts to the diff. count how many times the flange rotates when you rotate the tires once. you can then match this up to the available ratios for the car, so if it looks like it rotated just a hair under 4 times, then if its and e30 it would be a 3.91 ratio. also note that if both tires rotate in the same direction, then its an lsd
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