Diff and 'box query

Post by **decgforce** » 13 Jul 2008 11:24 pm

I have an 89 340 with a 4 speed 'box, found a 5 speed box with diff as a spare over the weekend, just wondering if I change gearboxs will using the 4 speed diff on the five speed box improve acceleration?

Any suggestions welcomed cos all this faffing about with diff ratio's and gearbox's makes my brain cell hurt

Post by **Vart** » 14 Jul 2008 05:50 pm

as i recall the 4 speed has the final drive ratio of a 5speed from the 1.7 something like 3.64. the 5speed from a 1.4 is 3.82 or similar. basically the 4 speed is taller to cope with the missing overdrive ratio i'd assume. i'm not entirely sure the diff will fit as the 4 speed box doesn't come apart in the same manner as the 5 speed (the 5th gear lives in a house between the two and the shafts may align differently) someone with more idea may be able to verify / discredit this.

best acceleration would appear to come from a 1.4 or 360glt final drive and worst from the 360 gle (i believe, but better economy (perhaps)). i'm sure someone on here had a 4.45 (or similar) ratio box but that might be a lie.

cheers

Post by **Bilbo** » 14 Jul 2008 05:53 pm

theres a little sticker on the diff casing with the ratio marked on it, if its fallen off over time you'll have to take the back plate off and count the cw and pinion teeth. the 5spd is about 100mm longer than the 4spd so check everything lines up, i know there are different diff casings and some driveshaft adaptors may be needed as the width differs on them aswell as length. someone may be able to tell you the exact way to go about it.....

Post by **Bilbo** » 14 Jul 2008 06:01 pm

ahh vart got in quicker than me

as far as i know the boxes have identical ratio's 4th being 1/1 and 5th being 0.83/1 [or something like that], basically they just bolted an extra gear on the back of the box hence the diff casings are different lengths to make way for the 5th gear extension.

Post by **kaos** » 14 Jul 2008 06:15 pm

my breaker ( ex volvo mechanic) has a list of the different fianl drive rations work out that i have the 3.62!

and thats an 89 340 1.4

next time i see him/email him i'll see if he can pass the information over to me!

Website · Post by **antiekeradio** » 14 Jul 2008 11:11 pm

you cannot use the diff from 4-speed on 5-speed box.

the 5-speed diff is much shorter, to accomodate for the extra case with gears for the 5th. the 4-speed diff fills the same space as the 5speed diff + the 5th gear casing.

on the other hand, changing out the pinion/great wheel, might just be possible!!! you really have to know what you're doing, though...

Post by **340GLT** » 15 Jul 2008 12:52 am

Kaos mate there is no final drive that is 3.62, yours will almost definately be a 3.82.

As already said fitting a 5 speed box wont make the car accelerate any faster as all gear ratios are the same, if you want faster acceleration you will need the crown wheel and pinion from a 1981 or earlier 4 speed box as that is 3.91.
Adam

Post by **foggyjames** » 15 Jul 2008 01:07 am

I would expect a 4 speed box with the (82+?) 3.64:1 diff to give slower accelleration than a 5 speed with its 3.82:1 diff. An early 4 speed (3.91:1 diff) is about the same as a 5 speed (with similar effective ratios), but it's really buzzing at speed in top gear...where it needs a 5th. The late ones are better, as the engine is spinning more slowly thanks to the taller diff.

I've always felt that the 3.64 diff in an injection 360 and 1.7 340 is neither fish nor fowl. It's not a 'short' diff, as such, but it's too short to make for relaxed high speed cruising, or optimum fuel economy.

Essentially...
Taller diff (lower number...tallest common diff is a 3.36:1 in a 360 with a carb) gives better economy, more relaxed high speed crusing, and you can go faster without hitting the limiter (assuming you can get there).
Shorter diff (higher number...shortest common diff is a 3.82:1 in a 340 1.4 5 speed) gives better accelleration, but poorer economy, and makes very high speed driving more noisy and unpleasent.

I have a 3.82 diff in my 343. That is attached to a high-revving, relatively non-powerful 2.0 8v engine. I want ready access to that high RPM powerband.

I will be putting a 3.36 diff in my 360 with a 2.3 turbo engine...which will have lots of torque (so the tall gear ratios aren't going to hold it back), and will mean I can cruise at 100+ on the autobahn without the engine screaming at me.

Hopefully that tells you a bit about where to find which diffs, and what I'd use them for. The M47 (5 speed) and 3.82 combo works really well with the 1.4. I'd fit it and be happy.

cheers

James