E30 Diff Ratio Query

Post Thu Jan 20, 2005 12:34 pm

I picked my new (to me, at least) 320i, 1990 E30 2 door a couple of weeks ago, and on the 150 mile drive home from Bristol, I kept looking for 5th on the gears only I was already in it! It seemed to be screaming revs, and maked my Audi A6 seem like a relaxed cruiser in comparison.

Does anyone know any fixes for this? The simplest I reckon might be swapping the rear diff for a higher (or is it lower) ratio one.

All help appreciated!

Paddy, Sunny Kent

Post Thu Jan 20, 2005 12:42 pm

Hi there,

I think it's likely to be a 4.1 diff. You saying you're looking for another gear seems to confirm this, although I could be wrong.

I went from a 4.1 diff to a 3.38 diff from an e36 compact. It revs much lower on the motorway and gives better mpg.

I am going to be looking for a 3.91 LSD soon, so I could sell you my 3.38 diff! Not for a while though due to funds!

cheers,

Dave

Post Thu Jan 20, 2005 12:56 pm

Your car should currently have a 4.10 diff which was BMW's choice. You could try swapping for a diff from a 325i with a 3.64, 3.73 or 3.91 depending on what you can find.

Bear in mind the 325i will be a large casing and as well as the diff you'll need the output shafts which are shorter, the diff will fit fine tho.

Post Thu Jan 20, 2005 1:03 pm

Bob,

By that, do you mean the drive shafts? I though the only difference was the length, not the width? Which would mean the drive shafts would be the same, but you would need to adjust the length of the prop?

Am I missing something?

Also, my 3.38 diff was also a small case diff, which made the swap easier. But some may think the ratio is to high....

Post Thu Jan 20, 2005 1:14 pm

Dave - yep. The width of the cars is the same but because the diff is larger you need shorter driveshafts (output shafts) to fit in the small space left. (Makes sense in my head!

) The prop (input shaft) is unaffected.

Post Thu Jan 20, 2005 3:40 pm

I wouldn't go too far with the diff. BMW chose a 4.10 for a reason - the standard 320 hasn't got the grunt for a 3.38. Whilst you'd have relaxed and economic cruising the acceleration will be 0-60 in about half an hour!! I would stick to a 3.91 or 3.73 for better all round driveability. Everytime I think I know better than BMW I come unstuck!:thumb:

Post Thu Jan 20, 2005 5:37 pm

ive seen an 86 with a 315 diff on a m20b25

what would that be like on a b20

Post Thu Jan 20, 2005 5:50 pm

Bet it had a good top speed...eventually!!!

Post Thu Jan 20, 2005 6:03 pm

dunno, its up on a lift at the mo,car looks like the bigest barry boy heap of shite in the world with like 5 different colour body panels, and chips all over the place

Post Thu Jan 20, 2005 6:12 pm

To quote your sign off...Nuff Said!! :lol:

Post Thu Jan 20, 2005 6:15 pm

poor baby

Post Thu Jan 20, 2005 11:19 pm

The small and big diffs are the same width - it's just that the shafts on the big diff sit further in so that the overall width between the driveshafts is not altered. The only thing you have to do when fitting a big diff is to slacken the collar nut on the prop rear half and tap the prop inwards by about 10 -15mm. Bolt the new diff in and see how much you need to push the prop inwards. Tighten the 4 17 mm nuts and retighten the collar nut, job done. You'll need to remove the exhaust and the exhaust heat shield for this really.

The early (chrome bumper before mid '85) 320i used a 3.45 ratio diff (same as the 323i) which works fine although acceleration in 5th is a bit lame. After this date they went over to a 4.1 to try and improve the acceleration and it didn't really work. The ideal diff for a 320i is a 3.64 big casing 325i unit and yes, I have one for sale, Ãƒ”šÃ‚£40. this takes the edge off the revviness without bogging the thing down. Some 320i Tourings also used a 4.4 final drive which was just ridiculous.

Post Thu Jan 20, 2005 11:30 pm

MONSPORT42 wrote:Bet it had a good top speed...eventually!!!

Bet it didn't - too much aerodynamic drag

Post Fri Jan 21, 2005 8:47 am

There you go then ... MasterB with the official response.

Obviously i've misinterpretted the info about swapping to a big case diff.

Post Fri Jan 21, 2005 3:03 pm

No problems.

But even The Master

Makes the odd mistake

320i Touring has a 4.27 diff, not a 4.44 :lol:

Post Sat Jan 22, 2005 12:21 pm

M.Bates (!) wrote: "The ideal diff for a 320i is a 3.64 big casing 325i unit and yes, I have one for sale, Ãƒ”šÃ‚£40. this takes the edge off the revviness without bogging the thing down."

As I got a bit confused by all that... if I choose to buy this diff for my 320, do I need the halfshafts too, or do I try the collar trick?

Post Sun Jan 23, 2005 7:59 pm

you have to do the collar trick, and MrB is saying the halfshafts are the same length on all cars, but they sit further into the diff on a large case one?

I've just had my diff out, fitted a new gasket and oil and refitted it. It's a not a nightmare job to do just a bit trying at times!