316 diff into 315

Rbraz80 · Post by **Rbraz80** » Fri May 30, 2014 1:14 am

Hi,

has anyone ever fitted a 316 (4,11) diff into a 315 (3,91) for better cruising?

results?

thx!

R

Jeroen · Post by **Jeroen** » Fri May 30, 2014 8:33 am

I believe you mean the other way around? The 315 has a shorter 4.11 ratio diff, most 4 cylinder cars have a slightly longer 3.91. I don't believe it will dramatically affect acceleration times but it it also won't reduce motorway rpm's by a lot either. With a 5-6% ratio difference it is likely to be no more than 200-250 rpm @ 4000 rpm if I'm not mistaken.

Mihajlo · Post by **Mihajlo** » Fri May 30, 2014 8:34 am

315 is 4.10
316 is 3.91

I fitted it and it is noticable.
Some says difference can't be noticed.

If you are planing too highway cruise than it is realy so small difference that it makes no point swapping it. from 4.1 to 3.91
best would be find 3.64 diff or 3.45 (323i) but that would be overkill.
3.45 requires at least 150HP to drive in 5th gear otherwise you will torture your engine and acceleration would be like

Mihajlo · Post by **Mihajlo** » Fri May 30, 2014 8:35 am

always the second!
argh!!

Jeroen · Post by **Jeroen** » Fri May 30, 2014 8:37 am

you had sooo many hours to respond

thanks anyway for the additional input!

Rbraz80 · Post by **Rbraz80** » Fri May 30, 2014 9:39 am

yep, i meant the other way around sorry...!

but will the 315 have the guts to deal with a 4cil 320 diff?

i have a 4 speed gearbox and a 210000km engine...

thx!

R

ps. i know i´ll have a speedo problem after that but i reckon its worth it for the extra fuel economy... as long as i know the ratio!

Jeroen · Post by **Jeroen** » Fri May 30, 2014 9:59 am

It will make it slower. Perhaps fit a 5-speed?

Rbraz80 · Post by **Rbraz80** » Sat May 31, 2014 11:20 am

Yup, a 5 speeder would be ideal, but much more messy and expensive to fit...

petroscf · Post by **petroscf** » Wed Jun 04, 2014 11:09 am

My car is a 316 but I think the diff is the 4.11, not the 3.91
I think that 4.11 were fitted from factory on the cars equipped with M10B16 engines (aka 315, 316) and the 316 with 1.8 engine were fitted with the 3.91, like the 318

My question now: (Sorry to intrude this thread, but I guess the subject is very much in topic)

I have Nas's old lsd, and I want so much to drive my car in an lsd. According to Nas, this diff is a 3.64
So I 'm afraid to fit it, in a 1.6 car (even with a weber carb)...Nas lots of times suggested me to fit it, and that I won't even notice the difference. I 'm afraid of loosing the sporty feeling that it defo has now with the 4.11
Any suggestions? (I mean, it is a PITA job or a lot of money to the mechanic just to do a swap that I might finally not like, and I don't know how soon will I be going for a bigger engine!)

nas80 · Post by **nas80** » Wed Jun 04, 2014 12:54 pm

Just fit it and of you don't like it then switch em back. Takes an hour or less to swap the diff.

Scared to fit a diff lol. Grow a pair!

petroscf · Post by **petroscf** » Wed Jun 04, 2014 1:03 pm

nas80 wrote:Just fit it and of you don't like it then switch em back. Takes an hour or less to swap the diff.

Scared to fit a diff lol. Grow a pair!

lol

However, as I haven't got the space (and the technical skills!) to do it myself, I have to pay a mechanic to do it and then maybe pay him again to undo it!

(and, were you telling the other day me that I am going to become very poor??)

Was wondering however how easy might it be to put in your lsd my 4.11 rack+pinion, and then fit it...

Jeroen · Post by **Jeroen** » Wed Jun 04, 2014 1:17 pm

1. Diff specs: see specs pages

2. On a garage lift and with the right tools swapping the diff can be done in 30 mins so a mechanic should not be expensive, just make sure to use the right oil on the lsd

3. Can be done on the street/jack stands as well, but time consuming due to limited space

4. The 11% difference in ratio is not exactly small but it depends how fresh and strong the engine still is if it will cope. If you want a car that is better on driving long distances you will love the difference

petroscf · Post by **petroscf** » Wed Jun 04, 2014 1:26 pm

Jeroen wrote:1. Diff specs: see specs pages

2. On a garage lift and with the right tools swapping the diff can be done in 30 mins so a mechanic should not be expensive, just make sure to use the right oil on the lsd

3. Can be done on the street/jack stands as well, but time consuming due to limited space

4. The 11% difference in ratio is not exactly small but it depends how fresh and strong the engine still is if it will cope. If you want a car that is better on driving long distances you will love the difference

I may be persuaded to ask my mechanic to do it anyway...I want very much to try it!

nas80 · Post by **nas80** » Wed Jun 04, 2014 2:19 pm

Do it!

nas80 · Post by **nas80** » Wed Jun 04, 2014 2:20 pm

17mm nuts x4 holding the diff on
6 bolts each driveshaft
4 17mm nuts connecting the prop to diff.
19mm nut and bolt for the rear diff hanger mount

If you can undo and tighten a bunch of bolts up then I cannot see how you could possibly fail.

Obviously make sure you seal and tighten the diff rear cover and put the right oil in it.

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316 diff into 315

316 diff into 315

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Re: 316 diff into 315

Re: 316 diff into 315

Re: 316 diff into 315

Re: 316 diff into 315

Re: 316 diff into 315

Re: 316 diff into 315

Re: 316 diff into 315

Re: 316 diff into 315

Re: 316 diff into 315