316i Manual Conversion. Best Diff Ratio?

[joew23]

Hi Guys,

I am currently converting my 316i to a manual and doing the rear disc conversion among other things.

what Diff Should be in a 1990 316i Auto? (4.10??)

I also have a (small case) 4.27 diff from a 320i convertable lying around.

im guessing i should just use the one that is currently in the car, but would the 4.27 be better??

I Would rather double check with you guys, than go on my own assumption, as gear ratios confuse me a little!

Cheers!

[RB2]

My 1.6 auto 1990 has a 427 in it

I'd be 99% sure it's original

[jmc330i]

I think the 316i autos had a 4.45 diff, the manuals 4.27.

[RB2]

I am in the middle of doing some work to mine. It's defiantly stamped 427

I am running a 318is box now with an m52. Tops out about 115mph. Sits about 3.5krpm at 80mph.

Gearings a bit short but I like it that way. It's more of a B road blaster than a motorway commuter.

[joew23]

Ok, ill see what diff i actually currently have in it tonight. if it is a 4.45, the 4.27 will be more appropriate for a manual car?

[joew23]

Oh and, jmc330i & RB2 Thanks for the advice !!

[Shutthatdoor]

I am in the middle of doing some work to mine. It's defiantly stamped 427

I am running a 318is box now with an m52. Tops out about 115mph. Sits about 3.5krpm at 80mph.

Gearings a bit short but I like it that way. It's more of a B road blaster than a motorway commuter.

That's interesting as motorway cruising is doing my head in at those figures, what might return 80mph at 2.5krpm on a 318i? Any ideas? Cheers

[jimbom30cab]

3.64 will do 2750 rpm at 70mph

as a rule of thumb drop 100rpm for every 0.1 ratio drop

I have a 3.46 and it does just over 2500, probably 2550rpm at 70.

To get down to 2500rpm at 80mph you would need a 3.25 but this would be too tall for a 318i

A good motorway diff is the 3.64

[Shutthatdoor]

3.64 will do 2750 rpm at 70mph

as a rule of thumb drop 100rpm for every 0.1 ratio drop

I have a 3.46 and it does just over 2500, probably 2550rpm at 70.

To get down to 2500rpm at 80mph you would need a 3.25 but this would be too tall for a 318i

A good motorway diff is the 3.64

Cheers Jim I'll look into that

[jimbom30cab]

cool

all the numbers are assuming 205/55/15 and 0.83 overdrive standard 5th gear ratio

[Shutthatdoor]

cool

all the numbers are assuming 205/55/15 and 0.83 overdrive standard 5th gear ratio

Yep just read wiki

[jmc330i]

A good motorway diff is the 3.64

Behind which engine/gearbox Jim?

I'm driving my M44 16v touring with a 3.91 at the moment, but I feel that's too tall. Compared to the 4.27, it's sluggish, worse on fuel and I'm forever changing gears to maintain speed - 60mph and a slight hill means dropping to 4th to maintain speed.
I'm not sure I'd want a 3.91 diff with a standard M40, let alone anything taller.
I'll be changing back to the 4.27 when I get chance.

[joew23]

Right, as suggested my car has a 4.45, so would it be best to put the 4.27 in the car once it has a manual gearbox?

[Brianmoooore]

4.27, or 4.1 at the absolute limit. It may do a theoretical 70 MPH at 2750 RPM with a 3.64, but it'll take an age to get there, and maybe not at all into a headwind. You also won't be impressed with the fuel consumption, or the clutch wear when trying to get the thing off of the line.
BMW's engineers knew what they were doing when they chose the E30's final drive ratios, and you'd rarely want to change by more than the next available ratio without changing engine power.

[joew23]

Ok thanks that sounds good. It's only that it was suggested that a factory 316i manual came with a 4.27. And I'm converting an auto 316i that currently has a 4.45. Just want the car to be correctly geared to drive on a daily basis.

[joew23]

Just about to put the diff in to the Car, So consensus is that the 4.45 will be fine in for the 1.6 maual and not too short?

[Brianmoooore]

In fifth gear it will be exactly the same as it was with the auto when that was in top gear and the torque converter locked up, so perfectly usable. The 4.27 would give you slightly better fuel consumption.

[joew23]

Ok, Thanks very much for the advice!