Author Topic: 3.91 or 4.10 lsd? (Read 8359 times)

ahriley91 · « **on:** January 13, 2010, 09:42:26 PM »

So i just purchased a 1991 318is and am looking for a lsd. I live in western Washington and the mountain roads are extremely close to me and I am wondering which one would be better. I know that the 3.91 is a 60% lock up and the 4.10 is 25% lock up. Which one would be best for really good windy mountain roads? All the help is very much appreciated.

DesktopDave · « **Reply #1 on:** January 13, 2010, 09:55:43 PM »

I thought they were all 25% lock-up? I'm not sure. I put a 3.73 medium case in mine, from an '87 325is. I hear you can catch one in an e28, 318ti (might need output shafts swapped), and even a Z3. I was so tempted by the funky Z3M rear cover with the fins & sump but they were too expensive.

Perfect fit, just took me two hours and one squashed finger to get it to line up right. My cheap advice: DON'T bolt the diff ear up first. Catch a top bolt first, then the drive shaft, then the other three top bolts. After that the ear may require persuasion to fit correctly. Finally, the half-shafts & the exhaust hangar. If I had to do it again I'd drop the rear exhaust. I crushed my finger between the half-shaft & rear pipe.

BTW, the OEM 325is rear mount is far sturdier than the pathetic M42. It gives better feel to the rear of the car and isn't noisy or rough. I also noted (in the absence of grumbling diff input bearings) that the rear subframe mounts are thunking a little & need replacing too. I'll bet that would have been easy with the diff out, eh? Live & learn.

ahriley91 · « **Reply #2 on:** January 13, 2010, 10:08:10 PM »

So the lower gearing didn't compromise the acceleration too much?

tjts1 · « **Reply #3 on:** January 13, 2010, 11:50:43 PM »

The lower gearing actually helps real world acceleration. You'll hardly notice the loss in torque to the wheel but you will notice that first and second gear will be much more useful and 5th gear will be a lot quieter on the highway. For autoX my goal was to get to 30mph with first and 60mph with second. I think 3.73 is ideal of this car but I found a 3.64 for $50 and stuck with it. If a 3.73 LSD comes my way for the right price I'll jump on it. For now I'm happy with my setup.
http://www.m42club.com/forums/showpost.php?p=26477&postcount=28
cheers
Justin

KidneyBoy · « **Reply #4 on:** January 14, 2010, 12:16:12 AM »

3.73 works perfect in the 318is. I did mine years ago and have since autocross the car with plenty of success. It makes the car more drivable in real world and 'spirited driving'

DesktopDave · « **Reply #5 on:** January 14, 2010, 07:31:51 AM »

Yep, doesn't seem to make any sense but it's true. I usually start in 2nd too, unless I'm pointing uphill. I was worried about it but figured I'd do it temporarily. I've been very happy with it. Better fuel mileage, less noise at highway speeds. Third gear lasts forever & I can glide through stops in 2nd without the clutch.

Anything higher than a 3.91 would make 1st totally useless IMHO. Your 0-60 would suffer because you'd need 3rd to get there, but your 0-100 would improve. 100 would be a noisy place though.

ahriley91 · « **Reply #6 on:** January 14, 2010, 11:29:54 AM »

That's good to know. I will be adding more power later so the lower gearing will help as well. Thanks for the advice!

dvmotorsports · « **Reply #7 on:** January 14, 2010, 11:36:34 AM »

DesktopDave · « **Reply #8 on:** January 14, 2010, 12:45:52 PM »

Need the torque for climbing volcanos?

Just kidding, you must have ferocious acceleration. You're a braver man than I...speeding tickets would bankrupt me.

fabe · « **Reply #9 on:** January 15, 2010, 01:24:52 AM »

have used 4.10, 3.91 and 3.73... 3.73 may work well for autocrosses as u'll hardly need 3rd gear but not good for track driving...
4.10 is best for track... 3.91 is the best compromise for track, autocross and daily drive..

but now i'm using the Getrag 250 from the E36 with 5th gear ratio of 1.1. So now running 3.15 which is roughly similar to G240 with 3.91... but which more oomph at 5th gear with 1:1 ratio...

dvmotorsports · « **Reply #10 on:** January 15, 2010, 11:07:03 AM »

Quote from: DesktopDave;84681

Need the torque for climbing volcanos?

Just kidding, you must have ferocious acceleration. You're a braver man than I...speeding tickets would bankrupt me.

No , that was suggestion. I do want though. Just can't freakin' find one. I heard they are only in the automatic ti cars.

szed03 · « **Reply #11 on:** January 17, 2010, 02:03:24 PM »

Quote from: fabe;84746

have used 4.10, 3.91 and 3.73... 3.73 may work well for autocrosses as u'll hardly need 3rd gear but not good for track driving...
4.10 is best for track... 3.91 is the best compromise for track, autocross and daily drive..

but now i'm using the Getrag 250 from the E36 with 5th gear ratio of 1.1. So now running 3.15 which is roughly similar to G240 with 3.91... but which more oomph at 5th gear with 1:1 ratio...

I'm running a 3.91 in my e36...thing gets up and goes in 1st and 2nd and accelerating from 3500rpm in 3rd is quite bitchin too. Great for autox.

Planning to swap a 4.10 or 4.27 ring and pinion into a spare diff I have.

xwill112x · « **Reply #12 on:** January 17, 2010, 05:37:10 PM »

i love my 4:10 LSD from a 325is. but i think the 4:27 would be perfect.

JOMARO · « **Reply #13 on:** January 18, 2010, 08:11:26 AM »

I have the 4.45 in my e30 , and believe me its too short for a daily driven car ,1st gear is useless,although it pulls like a maniac,but u d never be able to get to it to go side ways in 1st for example its too short ,and the rev limiter will be your friend
Dunno about Autox But i shall know soon

gearheadE30 · « **Reply #14 on:** January 19, 2010, 09:23:49 AM »

I think that unless you are putting out significantly more than stock horsepower, the 4:10 is good. It is a little louder on the highway, but the 3.73 felt too long in the one I drove. For autocross, though, I can see how it would be better. I hate having to shift into 3rd on longer courses.