confusion about comparing 2 diff and engine

[markus_74]

hello, Friends

the reason of changing the engine to 325i is about the power it has so in my mind it will be more acceleration in 4 or 5 gear, torque to climb the hill and low rpm to run in the high way.

that I think this can be get with small motor like m40 318i with 4.10 diff ratio.

so if i use 4.10 diff in 325i it is no doubt that it will very fast acceleration, but if people change to 3.91, or 3.72 and 3.64 what is the limit before the car become slower comparing to m40 318i?

I don't want to built car with 325i engine but in the end it will be slower or no acceleration compare to my m40 318i.

I like going traveling in very long distance so the choice of diff still give good mpg.

should I choose 3.64? or 3.72

and sorry with my lack of knowledge about differential, is it important to install lsd to differential? because in my mind it is important to avoid the slip in the wet rain.

and can I up grade my differential with just buy lsd and then install to my diff?

thank you
markus

[Brianmoooore]

I ran a 325i touring for many years as my 'main' car, using it for all kinds of journeys, including occasional towing duties.
It was fitted with a 3.91 ratio from the factory, but I always felt that it could happily pull a lower ratio, and following the sudden demise of that diff., I fitted a 3.73 because that was all I had around, and this confirmed my hunch that this was the better ratio.
I've never tried a 3.64 in a standard 325i, but my feeling is that this would start to blunt the acceleration.
The advantages of LSDs on a road car are greatly over rated IMHO.

[jimbom30cab]

I have driven a lot in a 3.64 and 3.73 diff.

IMO the 3.64 is a great ratio. Crusing at 70mph gives 2800rpm. You can get 30mpg on a steady run. Acceleration is more than ample also. All depends what you want to use the car for. If it's a daily on the motorway a 3.64 is perfect. A weekend toy for enjoying the A roads and you might prefer 3.91.

[Brianmoooore]

All depends what you want to use the car for.

Exactly. I did say that I used the car for towing, and the nearest motorway to me is over 40 miles away, so sustained cruising is a relatively rare event for me.

[markus_74]

thank you for all the info brian and jimbom, but I mean that if both m40 318i and m20 325i compare, does use m20 325i with 3.64 or 3.73 has same acceleration compare with m40 318i with 4.10 diff?

i hope get result better or far better with m20 325i with 3.64/3.73 diff. is it correct?

[Blitz]

There's no comparison. A 325i will destroy a 318i regardless of which diff it has.

[HairyScreech]

I did go from an ex320 4.41 small case to a 3.64 medium when I killed the small case.

Personally did not think the acceleration was that effected, It just meant I was in 2nd where I used to be in 3rd etc.

Guarantee that in each gear it would be slower like for like, but really it just means you hold each gear a little longer.

After that swapped to a 1:1 5th box and a 3.15 (same as a 3:91 with E30 box) and again did not find much difference on the road, just changed the shift points around.

You have 5 gears at the front to chose from, these have a lot more effect.