What's the best LSD diff for 325iC in the city?

[Wh33lhop]

Lots of conflicting opinions in this thread (what else is new?). It all depeds on your preferences and expectations. The stock 3.73 is pretty much the best all around gear for the car if you plan on highway driving to any degree. 4.10 if you want to sacrifice highway revs and shifting for a bit more oomph (which you might, since you're in a heavier vert).

Personally I run the equivalent of a 4.27 with my M50 which buzzes along at 4 grand on the highway. But I also don't mind shifting. It did get annoying for midnight runs to Safeway though.

[Cabriolet]

in my mtech2 vert i've had 4.1 and 3.73 the 3.73 is better but something like 3.91 ish would be best. but if you only have 2 options. the 3.73 is best.

[decay]

load of crap that i advise against something you yourself chose not to do. okay.

[mikeedler]

in an m20 vert? not if you want to get on the freeway, ever.

what a load of crap. 316-320's had that dif here in Europe, and there isn't even a speedlimit here. Your talking about a difference of about 500RPM at 70mph anyway. with a 4,27 you can hit a good 200kmh (120mph)before you hit the revlimiter.

I have a 3,91- only because the no speedlimit here.
and if you are getting worse mileage in town with such a small change in the diff- you are driving incorrectly- upshift then.

[Alpina]

Town driving,,,,,,,, 3.91

in and out,, 3.73

Longer distance,, 3.46-3.64

between lights 4.10

[Ian Bowers]

I have a 2.93 LSD in my vert, I'd do a 3.4, I think it'd be perfect. I cruise at 70 around 2200rpm, haha.

[LJ851]

WOW!

There are a lot of creative opinions in this thread. There are two things that affect rpm and torque output given a fixed (stock) engine /transmission combo: Tire diameter and rear end ratio. Lets ignore tire diameter for now.

The final drive ratio is absolutely personal preference, however those who are saying the 4.10 is not quicker than a 3.73 have their head up their ass.

The rear gear is a multiplier. The higher the number, the more torque is put to the ground for a given gear. There is no opinion about this, it is a mathematical fact. Your car will be faster in every single gear with a 4.10 vs 3.73.

One aspect a lot of people overlook is a product of this multiplication. With a numerically higher rear end the entire gear range gets compressed, all the gears get shorter and closer together. You end up shifting more in addition to having a higher rpm at cruising speed. It feels more like a close ratio sprinter.

With a lower numerically rear gear the gear range spreads out and there a larger gap between the gears. The car feels longer legged as it were.


OP, you are going to have figure it out yourself, it is too much a personal preference. If i rarely drove on the highway i would run a 4.10 since the M20 does not have a ton of torque.

If i was running unusually tall or short tires my answer might be different.

[MAXELHOFF]

Cool wheels though!

It's an M Tech II so I got stock euro basketweaves on it.

Overall tire height.

[Freude am Fahren]

oh and so far I'm back at square one. Equal number of people have suggested 3.73 vs. 4.10 I guess I just have to see which LSD ratio I can find available locally first :)

[Freude am Fahren]

What is the height of your tires from the ground, in inches?

It's an M Tech II so I got stock euro basketweaves on it.

[Vtec?lol]

I've tried them all.
I do a lot of highway driving in my 92 318ic vert. I swapped it to an m20 and put a 3.73 in and it sucked. it didn't really suck, it was the perfect ratio actually but then I bought an e39 for cruising the hwy. The e30 felt sluggish now. I put the 4.10 in and liked it, I then put the factory 4.27 back in and loved it. It's short but great for stop and go!
You can start from 2nd gear with plenty of pulling power and then jump to 4th or 5th if you don't like dicking around with the shifter in city traffic.

That being said, I think the 4.10 is perfect.

[MR 325]

I've tried them all. On your average M20 with a 5 speed behind it the 3.73 is the way to go.

[MAXELHOFF]

Do you go for some wine tasting often?

I live in San Francisco so not a lot of highway driving but I also don't want to burn a hole in my pocket if I decided to take the beast up to Sonoma for some wine tasting.

It's like 100 miles, round trip. +/- half a gallon of fuel difference.
What is the height of your tires from the ground, in inches?

[CabbE30]

Tell me why I'm wrong. It's a personal opinion, you can't really be wrong. First of all, a 4.10 will flat out accelerate faster than a 3.73, are you going to argue that? That's the only thing I said that I could possibly be "wrong" on because it's the only fact that I stated. Secondly, he's mainly driving in the city, san francisco (hello hills), which means lots of shifts whether he has a 4.10 or a 3.73. Faster acceleration off the line and at lower speeds is better for city driving.

By all means, a 3.73 is by far the best compromise for an m20b25 and has the best overall gearing for a combination of city and highway driving. But for mainly surface roads, I would personally go with the shorter diff. Sorry for going off on a bit of a rant, but just because you disagree with someone doesn't mean they're wrong.

I can guarantee even in the city the 4.10 will get worse gas mileage than the 3.73. I have what's called "personal experience". The 4.10 is fun but it gets pretty ridiculous in traffic. I used to just start in 2nd during rush hour. Even with the hills he's better off with the 3.73 & he wont have to worry about getting 20mpg on the freeway.
The acceleration between the 4.10 & the 3.73 is almost negligible. 4.10 feels torquier, but actually faster? Meh. Barely.

With that in mind, if I didn't daily my e30 I would have a 4.10 LSD in it.

[Vivek]

Tell me why I'm wrong. It's a personal opinion, you can't really be wrong. First of all, a 4.10 will flat out accelerate faster than a 3.73, are you going to argue that? That's the only thing I said that I could possibly be "wrong" on because it's the only fact that I stated. Secondly, he's mainly driving in the city, san francisco (hello hills), which means lots of shifts whether he has a 4.10 or a 3.73. Faster acceleration off the line and at lower speeds is better for city driving.

By all means, a 3.73 is by far the best compromise for an m20b25 and has the best overall gearing for a combination of city and highway driving. But for mainly surface roads, I would personally go with the shorter diff. Sorry for going off on a bit of a rant, but just because you disagree with someone doesn't mean they're wrong.