Gearbox question

[aimlessrock]

Folks,

i need to replace my gearbox (320i M20b20-Cabby), i have a Getrag 240 in at present and i have just learned that the replacement i had sourced is from an E34 m20B20!”¦

whats better, a 240 or a 260 box? ..im not looking for off the line speeds, but cruising and lower revs on A roads.

i have seen the article on the wiki but am not sure i understand what the difference is in relations to the different ratios..

any help would be most welcome.

cheers

Gary

[Grrrmachine]

The final cruising speed won't make a difference - both boxes have the same ratio for 4th and 5th. The difference is the "acceleration" gears, and the 240 box will get you off the line quicker.

I run a 240 box and a 3.64 diff in my 325i Touring. It gives me decent acceleration with good economy in a heavy old chariot, so it's the perfect compromise.

[aimlessrock]

Thanks Grrr..looks like I had better post my wanted add now. Is any difference in the boxes (pre/facelift)..mine is a chromie but has a facelift engine...

cheers

[bmw9818]

The final cruising speed won't make a difference - both boxes have the same ratio for 4th and 5th. The difference is the "acceleration" gears, and the 240 box will get you off the line quicker.

I run a 240 box and a 3.64 diff in my 325i Touring. It gives me decent acceleration with good economy in a heavy old chariot, so it's the perfect compromise.

id have thought the standard 260 is quicker? ive got the same 3.64 lsd, and the 1st gear ratio on the 240 is .72 260 is .83

i was looking into a 240 as well, if it had higher ratios

[Grrrmachine]

1st gear tops out at 20mph anyway, so it's not really crucial. It's second and third gear that matter. Check out the ratios here:

http://www.e30zone.net/e30zonewiki/inde ... ain#Manual

Notice that the 320i box has almost identical ratios (beside 1st) to the 325i Sport box?

[Andyboy]

An E34 520i M20 has the same 240 Getrag as your 320i.

[bmw9818]

1st gear tops out at 20mph anyway, so it's not really crucial. It's second and third gear that matter. Check out the ratios here:

http://www.e30zone.net/e30zonewiki/inde ... ain#Manual

Notice that the 320i box has almost identical ratios (beside 1st) to the 325i Sport box?

yeah thats where i was looking at, sports usually came with a 3.91 lsd aint it, so id say with a 3.64 the standard 260 is the best one, for accelaration etc,

woudnt a 240 be weaker as well,

[Grrrmachine]

You might be understanding ratios wrong. A standard 260 box with a 3.64 diff would have the slowest possible acceleration out of any E30 box/diff combo. In contrast, a 240 box with a 4.45 diff would go like shit off a shovel (if there's power there to make that happen).

A 260 box with a 3.91 diff probably has the same acceleration as a 240 box with a 3.64 diff. But because 4th and 5th gear in both boxes are the same, the 3.64 diff will give you a better cruising economy, so it's the best of both.

And people do say the 240 box is "weaker", but having read the entirety of the Zone, I can't remember a single thread where one of them broke due to the unstoppable raw force of an M20 engine

[daimlerman]

And people do say the 240 box is "weaker", but having read the entirety of the Zone, I can't remember a single thread where one of them broke due to the unstoppable raw force of an M20 engine

Must agree with this.
I have been using a 240 box with my 2.7 M20 for years with no issues.
Also use an ex 318i type 240 with an M50 with no problems,so far,both cars have 3:64 diffs.

[Brianmoooore]

Must agree with this.
I have been using a 240 box with my 2.7 M20 for years with no issues.

The same doesn't apply to a small case/medium case differential, IIRC.

[bmw9818]

You might be understanding ratios wrong. A standard 260 box with a 3.64 diff would have the slowest possible acceleration out of any E30 box/diff combo. In contrast, a 240 box with a 4.45 diff would go like shit off a shovel (if there's power there to make that happen).

A 260 box with a 3.91 diff probably has the same acceleration as a 240 box with a 3.64 diff. But because 4th and 5th gear in both boxes are the same, the 3.64 diff will give you a better cruising economy, so it's the best of both.

And people do say the 240 box is "weaker", but having read the entirety of the Zone, I can't remember a single thread where one of them broke due to the unstoppable raw force of an M20 engine

oh right okay, im looking at the first gear ratio of the 260 box 3.83 and thinking the higher ratio the better? as opposed to 3.72 for the 240,

so yeah essentially im thinking cause i have a low ratio lsd 3.64 a higher ratio first gear will make for faster acceleration, with the lsds for example the higher ratio is more quicker,

which goes on to another question, ive got a 240 box sitting in my garden, in the rain etc... is it now a gonner, has rust inside the bellhousing on the splines etc, saves me some monies... anything in particular i should inspect to ensure its good?

[Grrrmachine]

Ok, let's get this sorted cos your ideas on gears are all over the place.

1) You're right that first gear in a 260 accelerates faster than in a 240. I'm sure that's great for the 0.5 of a second that it's used.

2) Trouble is, when you hit the red line in this gear, you'll be going slower with a 260 box (33.1mph) than with a 240 box (34.1mph).

3) Gears 2 and 3 are faster in a 240 than in a 260. These are the crucial "accelerating gears", as they cover anything from 15mph to 90mph. Again, redline the 240 box and you get 63/96mph, versus only 58/91mph for the 260 box. These speeds are calculated with a 3.64 diff.

4) Whether a diff is LSD or not has no impact on its acceleration. Also bear in mind that 25 year old LSDs will be so worn that they'll mostly operate just like an open diff.

5) The diff ratio affects ALL gears equally. Therefore a 3.64 diff will blunt a car's acceleration more than a 3.91 diff would, no matter what gearbox it has. The reason M20s usually have lower ratio diffs is that they have enough torque to get the car going, so they can benefit from the better cruising economy afterwards. Weaker four-pots don't have the luxury of torque, which is why they whine at motorway speeds.

6) A 240 isn't worth anything even if it's in good nick. Scrap it.

[aimlessrock]

An E34 520i M20 has the same 240 Getrag as your 320i.

”¦no is doesn't Andy the E34's used a ZF box and appears to be different..i know this as i purchased such a box (its in my shed now) and only when checking the part number did we release it was from an E34!