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Diff / Ratio Options. Help/opinions Please

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Continuing work on my 2002 project.  I'm almost ready to replace the drivetrain. 

 

I have the complete 5-speed (G245) conversion ready to install. 

I have new Rota R20 rims which will have 195/50-15 tires mounted.

The engine is basically stock (for now). 

I intend to run A/C with a sanden compressor.

I plan on considerable highway cruising and spirited driving when-ever possible. 

Maybe even a few autox or club events.

 

I'm trying to decide which diff and ratio to install.  I've researched here and on the net for answers/opinions.  Everybody has one ;)

 

Based on my research I THINK I want something between 3.64 and 3.91.  Certainly LSD... I THINK 25%... Maybe 40%???

 

I have discovered that E36 diffs come in a 3.73, 25% LSD option.  This sounds ideal. The question is how to get this setup in my '75 2002.

 

2002Haus offers a custom subframe which allows the installation of a E36 diff OR is it possible to get E36 guts in my 2002 case? 

If it's possible who do you recommend and how much should I expect to pay for such a unit. (I'm in Las Vegas, NV)

 

I really like the look of the E36 diff with the Rogue cover buts that's a $$$ option for a car that's not pushing a ton of HP.

 

Thanks in advance for your HELP and constructive criticism.

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your research probably should have already found the answers to all your questions.......

 

main answer is how much do you want to spend?  low end just get a stock (25%) 3.91 lsd from an e21 and you are done.  maybe $500 with spacers.

 

high end..many thousand $$$ for custom subframe and trick diff.

 

from what you described as your use, the low end answer is what you need.

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Marshall's right, research will answer your questions

 

It's better to refer to the size of the differential itself SMALL (168mm ring gear), MEDIUM (188mm ring gear), and LARGE (210mm ring gear).

 

You cannot fit a 188mm differential pumpkin into a case meant for a 168mm pumpkin.  A medium case differential is not needed in your current build. 

 

Given your tendency to autocross you'd best start with the 3.91 and go up from there, but a 3.64 is nicer for highway speeds.

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If you're planning more than a little highway driving, I'd stick with a 3.64 diff (stock ratio except 76s).  That 5th gear will drop your revs about 800 on the highway (3200 vs 4000 at 74 mph) and it makes a real difference in noise level.  That's the route I took when I installed a 5 speed in my 73 back in 2004, and haven't regretted it since.  My '69 still has the original 4 speed, and a 500 mile trip in it when the 73 was not behaving convinced me that I had made the right choice.  I currently have a 3.64 limited slip (thanks, Greg) waiting to install when my original diff gets too noisy. 

 

But of course it's your call...

 

cheers

mike

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(edited)

I'm in the same boat. I have 3.64 with 5-speed overdrive mounted now. I would like to have a bit higher gearing but I'm afraid 3.91 is too much. Earlier I had 4.11 which is stock for 1502 which my car originally is.

I think that 3.73 small diff ratio was something like Japan only special so it would be pretty rare. Am I correct?

 

I'm gonna build lsd and I think I will try 3.91.

BTW if somebody has good source for 168 lsd friction discs please let me know.

 

  Tommy

Edited by Tommy

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So it's looking like a E36 3.73 LSD diff mounted in a 2002haus custom subframe is the way to achieve 3.73 in a 2002. At least I should be good if I ever decide to do a M20 swap ;)

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Your money is obviously yours to spend but you did ask our opinion, so...the difference in cost between (a.) the 3.64 or the 3.91 and (b.) the E36 3.73 option is a lot given the relatively small difference in actual performance. The overdrive 5th of your trans will make a much larger difference in revs than any rear-end ratio you choose from 3.64 to 3.91. As others have suggested already, if you think you might autocross a lot, get the 3.91; if you think your driving will include a lot of highway cruising, get the 3.64. Both of these ratios have satisfied thousands of 02 owners!

Good luck with your project!

Steve

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A small-case diff will handle an M20 if built correctly... look for the article in the "engine and drivetrain" article section.

 

If you are dead-set on the medium case diff (which it sounds like you are), then yes, the 2002haus piece is an option.  You can also make something work on your own (splicing in an E30 rear subframe) if you have the fabrication skills.

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For the price and the hassle you would go to for getting a 3.73 E36 diff, you could easily buy both a 3.64 and 3.91 and try them out for a while (really, its damn easy to just bolt one in), and then re-sell them for pretty much the same as you paid for them if you don't like them.

 

Really, try the 3.91, and if you don't like it, sell it.

 

I'm quite happy with my 3.91.  A few digits doesn't make as big of a difference as you think.  Hell, I've got a 5.14 in my truck with no overdrive, and its not that bad.

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Your money is obviously yours to spend but you did ask our opinion, so...the difference in cost between (a.) the 3.64 or the 3.91 and (b.) the E36 3.73 option is a lot given the relatively small difference in actual performance. The overdrive 5th of your trans will make a much larger difference in revs than any rear-end ratio you choose from 3.64 to 3.91. As others have suggested already, if you think you might autocross a lot, get the 3.91; if you think your driving will include a lot of highway cruising, get the 3.64. Both of these ratios have satisfied thousands of 02 owners!

Good luck with your project!

Steve

+1

 

folks...don't be afraid of a 3.91!  it is just not that big a difference in rpm than a 3.64 when connected to a 5spd, but it makes a noticable difference in acceleration.  a 3.73 is neither here nor there.  the difference between it and either a 3.91 or 3.64 is totally not worth the very large money layout to make it happen.  i would venture to say that you would not be able to tell which diff was in the car.

 

( I say that from purely a street/auto-x perspective.  depending upon the track in a full race car, small changes in diff ratio can make or break your lap times in a race.)

 

i have had 3.64, 3.91, 4.10 and 4.44 diffs in my car........they are all small case 168mm r&p's, and they seem to have no issues with the power output of a tweaked S14.

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I remember seeing 3.73 for e36 168 diff in realoem or somewhere. Can't find it again so I guess we agree there's nothing for small case between 3.64 and 3.91. It is pretty big gap but I believe we can live with it :)

 

   Tommy

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I have a 5-speed with a 3.91 LSD.  I think it is a great combo with my engine (dual side drafts and 292 cam), and the difference in acceleration is noticeable. 

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+1

 

folks...don't be afraid of a 3.91!  it is just not that big a difference in rpm than a 3.64 when connected to a 5spd, but it makes a noticable difference in acceleration.  a 3.73 is neither here nor there.  the difference between it and either a 3.91 or 3.64 is totally not worth the very large money layout to make it happen.  i would venture to say that you would not be able to tell which diff was in the car.

 

( I say that from purely a street/auto-x perspective.  depending upon the track in a full race car, small changes in diff ratio can make or break your lap times in a race.)

 

i have had 3.64, 3.91, 4.10 and 4.44 diffs in my car........they are all small case 168mm r&p's, and they seem to have no issues with the power output of a tweaked S14.

Marshall / palindrome / others,

So...please indulge me with an academic exercise, one to which we might not get an accurate answer. But I haven't a clue so I'd like your opinions.

My '76 49-state version came with the bona fide 3.90 diff that BMW stuck in those last U.S. models to compensate for the loss of power created by increased emissions controls. (The '76 3.90 diff is truly a 3.90 diff, separate from the later 3.91.)

Factory-published 0-60 time for the 1974 U.S. model (non-tii) was 12.3 secs.

Factory-published 0-60 time for the 1975 U.S. model (non-tii) was 12.8 secs.

Factory-published 0-60 time for the 1976 U.S. model (non-tii) was 12.8 secs. BMW never published, at least not that I could find, separate 0-60 times for the CA version (with thermal reactor, E12 head, and 3.64 diff, just like a '75) and the 49-state version (no thermal reactor, E21 head, and 3.90 diff). Thus, I always assumed the 12.8 secs. number was for the CA version. And, I have always assumed -- with no particular rationale -- that the 3.90 reduced the 0-60 time, for the 49-state version, back down to, say, 12.3 seconds, nothing more or less. So maybe a half second shaved off the 0-60 time.

Am I understating or overstating the acceleration benefit of my 3.90 over a 3.64?

Thanks and regards,

Steve

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Go to http://www.kormanautoworks.com/2002.htm and scroll down to the section on differentials.  Korman is clearly performance oriented, but since you're asking about 0-60 times . . . . . .  I've contemplated going to a 4.10, but I'd have to get a deal on one to go there.  4.22 just sounds like its intended for a track car. 

 

I maintain that a 3.90 is a good choice with a 5-speed.  After doing that and getting your suspension dialed in, you are probably looking at engine mods in the search for more performance (cam, bigger/more carbs, or fuel injection). 

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