8 Speed gearboxes

[Welshbeef]

The new BMW 8 Speed box (or is it seven?))
This is currently only available on the 5 series and it appears to dramatically increase mpg.

SO the question is for those running older cars say an E90 BMW with say enough milage to fancy changing the gear box can this be done?

And if so what would the pay off be? - Guessing financially unviable from the project in isolation & the harm it would do for resale.

[Red&WhiteMonkey]

Would the cost of doing the conversion massively outweigh the fuel savings unless you run the car for mega high miles?

[Welshbeef]

From whati have read about the new gearbox it certainly gives a step change in increasing economy, the other element is that it is quicker for acceleration so gear changes I guess and different ratios plus a super mpg busing top gear8th. Paddles on the steering wheel too

[gizlaroc]

Changing the gearbox would cost far to much, you would need to be doing 150k miles a year to even justify it.
You could however change the rear diff and final drive.

Swap a 330d 3.91 final drive for one with a 3.4 ratio and this would drop the revs from something like 2800rpm at 90mph down to 1800rpm at 90mph and be saving you a decent amount of fuel, plus you can just go to a breakers and nick the diff off a car that had that ratio as standard.

[gizlaroc]

Forgot I had this, it is for the E90 but gives you an idea what engines get what drive ratios...

http://www.zen104526.zen.co.uk/BMW/E90%20specs.pdf

[Welshbeef]

gizlaroc said:

Changing the gearbox would cost far to much, you would need to be doing 150k miles a year to even justify it.
You could however change the rear diff and final drive.

Swap a 330d 3.91 final drive for one with a 3.4 ratio and this would drop the revs from something like 2800rpm at 90mph down to 1800rpm at 90mph and be saving you a decent amount of fuel, plus you can just go to a breakers and nick the diff off a car that had that ratio as standard.

I get 70mph = 1,750rpm and on a flat that as per instant readout with cruise on appears to be 50-60mpg.

90mph = 1,800rpm would equate to 1,400rpm at 70mph.... That currently is 56mph on my box in 6th which shows well into the 70's mpg. That would represent a significant fuel saving

[Mattt]

So we've gone from 4 to 5 to 6 to 7 to 8 speed boxes, surely CVT will be the only choice before long.

[y2blade]

Welshbeef said:

The new BMW 8 Speed box (or is it seven?))
This is currently only available on the 5 series and it appears to dramatically increase mpg.

isn't it also used in the latest X5 4.0d?


edit to answer my own question

yes

http://www.bmw.co.uk/bmwuk/pricesandspecifications...

Edited by y2blade on Wednesday 28th July 19:29

[phelix]

Welshbeef said:

The new BMW 8 Speed box (or is it seven?))... appears to dramatically increase mpg... fancy changing the gear box can this be done?

The improved fuel economy comes from a huge list of changes; the gearbox is just one of them. On top of that modern BMWs are very integrated - unless you have access to the control SW you're highly unlikely to be able to successfully install the gearbox.

[christer]

Dude - the aerodynamic resistance ias exponentially higher at 70 than it is at 56mph and so on. You cannot in ANY way try to calculate this based on the rpm only. Throttle opening, mass and aerodynamic efficiency are the key factors. it would be far more economical (ironically perhaps) to look at buying a more modern model than trying to make simplistic gearbox changed based on 3rd grade mathematical theories.

Welshbeef said:

gizlaroc said:

Changing the gearbox would cost far to much, you would need to be doing 150k miles a year to even justify it.
You could however change the rear diff and final drive.

Swap a 330d 3.91 final drive for one with a 3.4 ratio and this would drop the revs from something like 2800rpm at 90mph down to 1800rpm at 90mph and be saving you a decent amount of fuel, plus you can just go to a breakers and nick the diff off a car that had that ratio as standard.

I get 70mph = 1,750rpm and on a flat that as per instant readout with cruise on appears to be 50-60mpg.

90mph = 1,800rpm would equate to 1,400rpm at 70mph.... That currently is 56mph on my box in 6th which shows well into the 70's mpg. That would represent a significant fuel saving