what is the best diff for M20b25

[E30rocknrollstar]

Hi guys just after some advice.

my car was originally a m40 318i auto its now a m20b25 high compression engine manual box running motronic 1.3.

Im running the original diff at the moment but is not ideal first gear just spins the wheels and revs are very high at motorway speeds although acceleration is very good

iv got the chance to buy a 3.91 diff is this suitable or would a 3.64 diff be better.

whats the pro and cons of them both

cheers
guys

[Speedtouch]

The standard facelift 325i and 325iSE had the 3.64:1 open diff as standard, which is fine for normal use, being a well suited ratio.

The facelift 325i Sport had the 3.91:1 LSD as standard, which is to be honest, a bit on the low geared side, but it was fitted because the facelift Sport was fitted with a slightly lame low compression M20B25, so needed the lower ratio to compensate and give reasonably sporty acceleration, at the expense of top speed and relaxed motorway cruising.

Depending on the sort of driving you intend to do and whether you really need an LSD, I would probably opt for the higher ratio (3.64:1), which suits the high compression M20B25 pretty well.

[Brianmoooore]

325i touring came with a 3.91 diff., and one of the best mods. I ever made to mine was to change it to a 3.73. No noticeable reduction in performance, buut reduced noise levels and improved MPG.
3.64 isn't much different from 3.73, and with the highrt compression ratio, would be the one I'd choose out of the two you have available.

[reggid]

some sports had the close ratio box with 3.91 rear so 1st through 3rd are actually longer than the 3.64 with standard ratio box. 4th and 5th the boxes are the same ratio so its shorter overall due to the 3.91

ive got 3.91 with sports box and it cruises totally fine, 1st isnt too short.

A 3.91 with standard box would be too short in 1st IMO, even with 3.73 a standard box feels noticeably shorter than mine so 3.64 would be a good balance.

[E30rocknrollstar]

Cheers for the replys guys.

Are the early high compression gearboxes different ratio to face lift ones ?

mine is the early type with the 2 sensor wires coming out the box but now redundant as im running 1.3.

If i have 3.91 diff with my early high compression engine its going to make it feel very lively would it not ?

whats the difference in rpm at 70mph between 3.91 and 3.64 would it be very noticeable.

Are all 3.91 diffs lsd as the seller is saying its not?.

cheers

[Speedtouch]

Indeed, some 3.91:1 diffs are open type, not LSD:

http://www.e30zone.net/e30zonewiki/inde ... fferential

[Brianmoooore]

No change in gearbox ratios between early and late, although there was an optional closer ratio box available. Unfortunately, few were specified, outside of the very few 'special' editions, such as the UK 'Sport', where they were standard.
Same applies to all E30 diffs. LSDs were available for all ratios as a relatively low cost option, but again, very few were specified, except on the UK 'Sport', where the option was pre ticked.
Difference in RPM is the same as the difference in ratios. The speed you are travelling at 3000 RPM with a 3.91 diff. would require only 3000 x 3.64/3.91 = 2793 RPM with a 3.64 diff. Doesn't look that much, written down, but it's very noticeable in practice.