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      01-12-2017, 03:42 PM   #1
mmmusty
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Which Diff Ratio to go with?

Hi guys, after upgrading suspension parts, RSFB, shocks and springs, I'm looking to change the diff ratio for my car to have shorter gearing so to speak.

I have a 128i with 18x8.5 wheels all around, manual transmission, DISA intake and AA tune. The car stock has 3.23 ratio, my friend who is phenomenal with car stuff, told me to look at about 20% change in diff ratio. I ran across this thread in my search http://www.1addicts.com/forums/showthread.php?t=1193231

seems that most people on the forum were leaning towards the 3.46 ratio rather than the 3.73.

What are your opinions for my usage as far as what diff to look into swapping with:

Daily drive the car in LOS ANGELES (so freeway speeds rarely get to anything passed even 70) no track time and dont plan on doing much tracking or autocross. Most of my spirited driving is canyon driving. The reason why i was looking at doing this upgrade is because i feel that although first gear actually is just right or slightly too short, the rest of the gears, particularly 2nd, are very tall and doesn't have the car in the right power band for the type of driving that is usual.

any recommendations and insight is greatly appreciated!

also, any recommendations on what diff to swap with??
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      01-12-2017, 04:07 PM   #2
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Quote:
Originally Posted by mmmusty View Post
Hi guys, after upgrading suspension parts, RSFB, shocks and springs, I'm looking to change the diff ratio for my car to have shorter gearing so to speak.

I have a 128i with 18x8.5 wheels all around, manual transmission, DISA intake and AA tune. The car stock has 3.23 ratio, my friend who is phenomenal with car stuff, told me to look at about 20% change in diff ratio. I ran across this thread in my search http://www.1addicts.com/forums/showthread.php?t=1193231

seems that most people on the forum were leaning towards the 3.46 ratio rather than the 3.73.

What are your opinions for my usage as far as what diff to look into swapping with:

Daily drive the car in LOS ANGELES (so freeway speeds rarely get to anything passed even 70) no track time and dont plan on doing much tracking or autocross. Most of my spirited driving is canyon driving. The reason why i was looking at doing this upgrade is because i feel that although first gear actually is just right or slightly too short, the rest of the gears, particularly 2nd, are very tall and doesn't have the car in the right power band for the type of driving that is usual.

any recommendations and insight is greatly appreciated!

also, any recommendations on what diff to swap with??
I'm VERY happy with my 3.64 diff. It's ~12.7% shorter, much better in 2nd through 6th, a comfortable 3331 RPMs @ 80 MPH (fairly typical SF Bay Area at the hours I commute) in 6th and still good to hit the 150 MPH speed limiter @ 6247 RPMs in 6th if I ever get the chance.

I would think a 3.73 would be quite similar.
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      01-12-2017, 05:08 PM   #3
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Quote:
Originally Posted by Suprgnat View Post
I'm VERY happy with my 3.64 diff. It's ~12.7% shorter, much better in 2nd through 6th, a comfortable 3331 RPMs @ 80 MPH (fairly typical SF Bay Area at the hours I commute) in 6th and still good to hit the 150 MPH speed limiter @ 6247 RPMs in 6th if I ever get the chance.

I would think a 3.73 would be quite similar.
Thanks John. thats not much of a change from stock at 80mph, looks like it'll be around 500rpm higher in the case of going with 3.73

Do you have a recommendation for which diff to swap with? Im looking to make this as straightforward and direct swap as possible
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      01-12-2017, 06:12 PM   #4
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I have a 3.73 and think it's awesome. Really woke the car up, and 1st is not at all too short. I can't even imagine how the 3.23 would feel anymore.

I imagine a 3.64 diff would be quite similar, almost imperceptibly different. That said, I would choose the 3.73 again because, why not? At the end of the day, I might suggest going with whatever is easier and cheaper, if there is a difference between the two ratios.

In any case, I would strongly recommend against the 3.46 rear end. I'm sure it's a difference, but believe me, these gearing changes are more subtle than a lot of people would have you believe. In fact, most of the nay-sayers I came across when I was doing my research were folks with no actual experience in gearing swaps - they were just looking at online calculators and such to come to various conclusions.

Granted - you will have to adjust your shifting to accommodate the new gearset, and I suppose it is possible that one might find 3.64 "perfect" and 3.73 "too much," but I think you have to be just as careful not to under do it. This is an expensive mod, if you do it right (i.e., LSD) and the worst thing in the world would be to be underwhelmed after spending all that money. I strongly believe I would have been greatly disappointed by a 3.46 rear end, which is what I recall everyone on the forums was recommending.
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      01-12-2017, 06:21 PM   #5
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Quote:
Originally Posted by Bimmer-Bob View Post
I have a 3.73 and think it's awesome. Really woke the car up, and 1st is not at all too short. I can't even imagine how the 3.23 would feel anymore.

I imagine a 3.64 diff would be quite similar, almost imperceptibly different. That said, I would choose the 3.73 again because, why not? At the end of the day, I might suggest going with whatever is easier and cheaper, if there is a difference between the two ratios.

In any case, I would strongly recommend against the 3.46 rear end. I'm sure it's a difference, but believe me, these gearing changes are more subtle than a lot of people would have you believe. In fact, most of the nay-sayers I came across when I was doing my research were folks with no actual experience in gearing swaps - they were just looking at online calculators and such to come to various conclusions.

Granted - you will have to adjust your shifting to accommodate the new gearset, and I suppose it is possible that one might find 3.64 "perfect" and 3.73 "too much," but I think you have to be just as careful not to under do it. This is an expensive mod, if you do it right (i.e., LSD) and the worst thing in the world would be to be underwhelmed after spending all that money. I strongly believe I would have been greatly disappointed by a 3.46 rear end, which is what I recall everyone on the forums was recommending.
Bimmer Bob, thats what my logic was also... seems to be more calculations and guesswork. I couldnt see how such a small change in ratio would have a discernible difference in the smile on my face.

Im not going the LSD route, as thats too expensive and the cars electronic diff isn't that bad for my use. I was looking for a swap with another BMW, a used diff if you will that i would be able to directly swap out

Any ideas?
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      01-12-2017, 06:47 PM   #6
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Quote:
Originally Posted by mmmusty View Post
Bimmer Bob, thats what my logic was also... seems to be more calculations and guesswork. I couldnt see how such a small change in ratio would have a discernible difference in the smile on my face.

Im not going the LSD route, as thats too expensive and the cars electronic diff isn't that bad for my use. I was looking for a swap with another BMW, a used diff if you will that i would be able to directly swap out

Any ideas?
Yeah, the 128i and 328i automatics have 3.73 open diffs, IIRC. Should be plenty around.
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      01-12-2017, 09:14 PM   #7
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Quote:
Originally Posted by Bimmer-Bob View Post
I suppose it is possible that one might find 3.64 "perfect" and 3.73 "too much,"
If I knew then what I know now I might well have gone to 3.73. Maybe.

Quote:
Originally Posted by Bimmer-Bob View Post
I think you have to be just as careful not to under do it. This is an expensive mod, if you do it right (i.e., LSD) and the worst thing in the world would be to be underwhelmed after spending all that money. I strongly believe I would have been greatly disappointed by a 3.46 rear end, which is what I recall everyone on the forums was recommending.
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      01-12-2017, 09:38 PM   #8
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Good call on that. I'm not looking to be underwhelmed. i think I'll buy a used 328i diff 3.73, sells for around 200, if its "too much", ill change it, inexpensive enough.

What diff fluid do you guys use? II'm looking into doing my tranny and diff change, getting the redline mtl 70/80 for manual tranny, but what about the diff? Anything else I'm not considering in this endeavor?
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      01-12-2017, 09:52 PM   #9
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Quote:
Originally Posted by mmmusty View Post
Good call on that. I'm not looking to be underwhelmed. i think I'll buy a used 328i diff 3.73, sells for around 200, if its "too much", ill change it, inexpensive enough.

What diff fluid do you guys use? II'm looking into doing my tranny and diff change, getting the redline mtl 70/80 for manual tranny, but what about the diff? Anything else I'm not considering in this endeavor?
Redline 75W140 in a diff with a clutch-type LSD. Redline "recommends" their 75W85 for "BMW 2012+ 1 Series, 3 Series and Z4 w/o LSD differential"
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      01-12-2017, 10:10 PM   #10
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Originally Posted by Suprgnat View Post
Redline 75W140 in a diff with a clutch-type LSD. Redline "recommends" their 75W85 for "BMW 2012+ 1 Series, 3 Series and Z4 w/o LSD differential"
That's interesting you say that, i called them regarding why they recommend the d6 oil and everyone on the forums recommends the mt90.... seems like its based off of what closest resembles factory oil. Ive gotten some great, invaluable information from you guys in the past few days.

Thanks supergnat
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      01-13-2017, 05:46 AM   #11
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Interesting thoughts,

I am close to doing the lsd. The auto from the wagon I have is a 3.91 . I think that might be too short. For me the 3.64 is the direction I'm going. At this point I have AA headers, 3 stage, (getting ready for tune) 135 brakes and square 18x8.5. the car is going to be a weekend fun and a hpde car.

anyone else think thats the way to go?
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      01-13-2017, 05:37 PM   #12
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FWIW, the 128i 6-sp that I bought came with a Wave Trac limited slip. The ration is 3.46, and to me, it is very nice. I can't compare with higher numerical ratios, but this seems effective for my driving.
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      02-04-2017, 08:20 PM   #13
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I have swapped my diff for 3.73 auto with out LSD - its derivable and improved my city fuel econ. Hwy Fuel is okay too if I keep it at 120km/h.
That said I do think 3.64 is the better ratio for balance of hwy and street.

My motivation for 3.73 was ease of finding a good deal, and easy fit because (I may be wrong, but) the 3.64 is only possible with custom LSD equipped pumpkins OR with a drive shaft mod to mate up to a junker 3.64
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      02-08-2017, 10:43 AM   #14
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What is the max speed in 2nd gear with the 3.73? I hae a 7500RPM redline, which goes to 70mph - that's too much, but good for autocross. I'm more interested in the torque multiplication.
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      02-08-2017, 02:36 PM   #15
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Originally Posted by TheAxiom View Post
What is the max speed in 2nd gear with the 3.73? I hae a 7500RPM redline, which goes to 70mph - that's too much, but good for autocross. I'm more interested in the torque multiplication.
60.72 MPH with a 225/45R17 @ 7500 RPM.
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      02-08-2017, 02:42 PM   #16
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Ohh, that's cutting it close.
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      02-08-2017, 04:04 PM   #17
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Originally Posted by TheAxiom View Post
Ohh, that's cutting it close.
Attached is a zipped gearing calculator I made. Play with this if you'd like.
Fields A2, B2 and D2 are Tire x-section, tire aspect ratio and wheel diameter.
Fields B7-12 are the transmission gear ratios. Entered are the stock 128i numbers.
Field B13 is the diff ratio.
B14 is engine red line, B15 is Torque peak and B18 is the stock diff ratio.

A5, B5 and D5 are the tire parameters calculated from Fields A2, B2 and D2.

Min and Max are calculated from the Torque Peak (Min) and Redline (Max). Redline shift is what the engine RPMs drop to when the transmission is shifted to the next gear from redline (for example, when shifting from 1st to 2nd at 7500 RPM with a 3.73 diff, the engine will drop to 4362 RPM).
Field H6 and below are stock values for comparison and field I6 and below is the difference in MPH from Field D6 and below to field H6 and below.
Attached Files
File Type: zip Gearing Calculator (1).zip (4.3 KB, 29 views)
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      02-08-2017, 10:43 PM   #18
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So many mods, so little moneysss... but this is a cheap and good one, gotta do this one soon!
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      02-09-2017, 02:43 PM   #19
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Quote:
Originally Posted by Suprgnat View Post
Attached is a zipped gearing calculator I made. Play with this if you'd like.
Fields A2, B2 and D2 are Tire x-section, tire aspect ratio and wheel diameter.
Fields B7-12 are the transmission gear ratios. Entered are the stock 128i numbers.
Field B13 is the diff ratio.
B14 is engine red line, B15 is Torque peak and B18 is the stock diff ratio.

A5, B5 and D5 are the tire parameters calculated from Fields A2, B2 and D2.

Min and Max are calculated from the Torque Peak (Min) and Redline (Max). Redline shift is what the engine RPMs drop to when the transmission is shifted to the next gear from redline (for example, when shifting from 1st to 2nd at 7500 RPM with a 3.73 diff, the engine will drop to 4362 RPM).
Field H6 and below are stock values for comparison and field I6 and below is the difference in MPH from Field D6 and below to field H6 and below.
Going to DL this when I get home. It's for the 6 speed manual right?
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      02-09-2017, 07:26 PM   #20
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Quote:
Originally Posted by TheAxiom View Post
Going to DL this when I get home. It's for the 6 speed manual right?
Axiom check this post for a software tool
http://www.1addicts.com/forums/showp...&postcount=964

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      02-09-2017, 08:45 PM   #21
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Originally Posted by TheAxiom View Post
Going to DL this when I get home. It's for the 6 speed manual right?
Yes, 6-speed manual. The one Andrey posted looks a lot fancier, though results should be similar.
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      02-10-2017, 10:43 AM   #22
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Im going to look for a 3.73 ratio diff to buy, what do you guys recommend I look for?

From what car?

Anything else to look out for or note?
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