M52b28, which box is best?

Post Thu Jan 27, 2011 10:57 pm

I have m52b28 with gearbox and just bought m20 flywheel ,starter and clutch. It is going into my318i shell but what is best choice of box to use? Ihave the 2.8/m3 box but is m42 a better choice? I plan to do 5 stud conversion on rear so could possibly use 2.8 box with 2.8 lsd?

Post Thu Jan 27, 2011 11:31 pm

If you choose to use an m42 box you will need a 325 prop (minus the vibration damper) and a diff ratio somewhere between 3.91 and 3.64.

Post Fri Jan 28, 2011 9:32 am

The 2.8 gearbox is a tough old unit, but keeping it will give you hassles of getting a correct diff ratio,

E36 diff will not fit E30, you can get the guts of the E36 diff rebuilt into the E30 casing, but that will cost you a few quid. The only bolt in solution would be to fit an E30 M3 diff which usually is a 3.25 ratio or thereabouts.

Post Fri Jan 28, 2011 9:59 am

Z3 2.8 diff is a straight swap, with a ratio of 3.15, AFAIK, and is a slipper as a bonus. 525e E28 diff is 2.91, IIRC, and bolts straight in with a backplate and output flange change.
Other Z3 and E36 compact stuff probably fits as well, but I don't know the ratios.

Post Sat Feb 19, 2011 10:37 am

What would the best ratio be for the 2.8 engine and box? Am unsure wether to stick with the 2.8 box or go m42 box. I want the car to be rapid accelerating and not too bothered about top end 100+. Iam considering looking for z3 2.8 diff as I already have the 2.8 engine and box with obd1 inlet and bbtb and m20 flywheel and clutch,just need starter now.

Post Sat Feb 19, 2011 10:43 am

You can use your m40 box and a 2l m20 prop.

Post Sat Feb 19, 2011 3:02 pm

I would use the 328 box and 3.64 LD - that will give you 132mph at 7000 rpm and pretty nifty acceleration! Great for track, and not to shabby on the road either if you stick to speed limits...

Post Sun Feb 20, 2011 10:01 am

That doesn't sound too bad mate, do you know what the 2.93:1 would be like? It is going to be be a track weapon/sleeper with standard looks

Post Sun Feb 20, 2011 10:34 am

2.93 is more of a road ratio than for track. Would give the same top gear performance as the 3.64 with an E30 (overdrive top) box.
The 3.64 with the M52 box will have a similar top gear ratio to the highest ratio diff. found on E30's; around 4.5:1.

Post Sun Feb 20, 2011 2:21 pm

I have just been offered a diff from a h reg 325i with numbers 1210411 on it. Lad says is not sure if is LSD and is unsure how to check. Would this be any use with 2.8 box? Where could I find 3.64? Thanks for all the info and advice mate, when it comes to ratio's and gearing Im kak.

Post Sun Feb 20, 2011 5:37 pm

You can tell if a diff is an LSD if it has an 'S' on the end of the numbers - other way to check is turn the output flange and see if both flanges turn the same way - if they do then it's an LSD. Are you sure those numbers are right on the diff? Normally the first 3 numbers are the diff ratio, so 391 means 3.91 and so on. Replacement diffs were different - the first 4 numbers correspond to the crown wheel and pinion teeth - so 4011 would be 40 divided by 11 which equals 3.64...

Looking at those numbers above, I reckon it might be a 4.11??

Post Tue Feb 22, 2011 7:16 pm

Its a 3.73 on the tag mate, coming from a h plate 325. Would that make it LSD or not? Going to have look on saturday, might just get it to get car on road for now then track down another once its up and running.Would that be ok for acceleration?