Which diff??? & Which 'Box???

[miniblob]

Right then,

I need an LSD for my 325i touring. Only problem is I have no idea what the different ratios are or what they mean in the real world!! What should I be looking for?
I want a fast accelerating car, but also do a lot of motorway driving - normally cruise around 115 ish when no one is looking Would love a bit more top speed if it can be done without losing any acceleration!!

Also, it's time for a new clutch and 'box. Would ideally love a 6 speed, but judging by reactions on here that may not be easy!!
I'm sure there are different ratio 'box's but again i don't know what i'm looking at!

What would you lot advise/do?? What should i be looking out for??

Also, having some mild tuning done (maybe hit 200bhp?!?!) and determined this car will see a track or two (gets driven very hard anyway!) so is it worth upgrading the clutch - or just stick with bm parts?

I'm tired and getting confused so I hope the above makes some sense!!
Sorry for the long post and thanks in advance for your help!!
Rob.

[jmc330i]

If Ive got this the right way round , a 325i Sport box (closer 1st, 2nd and 3rd) and a 3.64 LSD would be the best compromise between acceleration and decent cruising speed, although I wouldnt say 115mph is a cruising speed

Also, having some mild tuning done (maybe hit 200bhp?!?!)

Would need a lot of tuning or a 2.7 conversion to hit 200bhp with an M20.

[miniblob]

Right then, I'll have to keep my eyes peeled for a sport 'box!!

Can anyone explain the diff ratios?? What does the 3.64 actually mean??

Is there any way to work out what difference the ratios would mean to accel and top speed?


By mild tuning I mean BTB exhaust and 6 branch, bbtb, itg kit, proper chip/remap and a few other bits to start, which should put me on the way to 200bhp and then i'll decide where to take the tuning from there depending on the results!!

[jmc330i]

3.64:1 is the diff or final drive ratio. The number has to do with the amount of teeth on the final drive gear and so how much the drive from the gearbox is reduced.

The higher the number, the quicker a car will accelerate, but its top speed will be reduced - using the 325i as an example, a 3.64:1 diff will give a higher top speed, but slightly slower acceleration than a 3.91:1 diff.
At 3k rpm the 3.64:1 diff will see about 70mph, where as the 3.91:1 will be at around 65mph.

Is there any way to work out what difference the ratios would mean to accel and top speed?

You have PM.

[Ziggy]

There's obviously a limit as to how high a gearing you can run though - for example if you were to run a 2:1 diff you wouldn't end up doing 250mph! (In the real world, it's the drop in economy you'll notice with a shorter diff, rather than the drop in top speed)

Just out of interest, does anyone know what the longest ratio is, that say a standard 325i can pull to the redline in 5th?

[Jhonno]

There's obviously a limit as to how high a gearing you can run though - for example if you were to run a 2:1 diff you wouldn't end up doing 250mph! (In the real world, it's the drop in economy you'll notice with a shorter diff, rather than the drop in top speed)

Just out of interest, does anyone know what the longest ratio is, that say a standard 325i can pull to the redline in 5th?

with a 3.91 i think yes

[Ziggy]

Yeah I'm fairly sure it could redline a 3.91 in 5th, but how much faster than that would it go? Would it actually have a higher top speed with a 3.64? (not that top speed makes any difference in the real world!)

[jmc330i]

Well using the gearing calculator on that unixnerd site and the program I have on my PC - at 6k rpm a 3.91 will top out at about 130-135mph, a 3.64 will see 140-145mph.

[Ziggy]

Well using the gearing calculator on that unixnerd site and the program I have on my PC - at 6k rpm a 3.91 will top out at about 130-135mph, a 3.64 will see 140-145mph.

But would it actually pull 145mph?

[jmc330i]

I dont know.

I have had an indicated 145 out of my Sport (3.91), but I have no idea what that would that be in terms of actual speed? Im guessing around the 130-135 mark???

I didnt hit the limiter (I ran out of private road and badly needed to brake), but I suspect it was close and TBH I didnt look at the revs, at that speed I was concentrating more on the road.

[gcorky]

I dont know.

I have had an indicated 145 out of my Sport (3.91), but I have no idea what that would that be in terms of actual speed? Im guessing around the 130-135 mark???

I didnt hit the limiter (I ran out of private road and badly needed to brake), but I suspect it was close and TBH I didnt look at the revs, at that speed I was concentrating more on the road.

and who can blame you...lol

[Morat]

So a 3.64 would be a decent choice for a turbo conversion then?
One for the notebook...

[gcorky]

So a 3.64 would be a decent choice for a turbo conversion then?
One for the notebook...

most try for a higher geared still than the 3.64-such as various m3 diffs in turbocharged e30s.


saying that im putting in a 3.91 lsd so ill get banned for speeds below 100

[jmc330i]

I dont know.

I have had an indicated 145 out of my Sport (3.91), but I have no idea what that would that be in terms of actual speed? Im guessing around the 130-135 mark???

I didnt hit the limiter (I ran out of private road and badly needed to brake), but I suspect it was close and TBH I didnt look at the revs, at that speed I was concentrating more on the road.

and who can blame you...lol

Ive got say that the E30 felt surprisingly good at those speeds. My old Cav SRi felt like it was going to fall apart anytime after 90mph and you really had to hang on, but the BM just felt solid. I guess thats German build quality for you

[miniblob]

Might start looking for a 3.91 lsd then!! Would like the extra accel and 130mph top would be enough i suppose!!

Any ideas what other ratios were actually available??

[miniblob]

Anyone got any more info on the other gearboxes available too???

I assume i would be best off looking for a 325 sport 'box??