Thinking outloud- could just the 5th gear ratio in an m47?

[Bugjam1999]

Hi all,

Edit: title should be 'thinking outloud- could just the 5th gear ratio in an m47 be changed?'

So here's a wonder I've been having over the last couple of days- would it be possible to rebuild an M47 gearbox with a different (taller) ratio for 5th gear? I should point out that don't know a whole great deal about gearboxes, so if this is an idea that has fewer legs than a school of salmon, then do point out the obvious flaws

My thinking is that the M47 ratios are fairly well spaced for the majority of driving and its only on the motorway that I feel like the 5th gear isn't tall enough- can't remember the exact rpm at 70mph but more modern cars definitely cruise at lower rpm than the 240. Lower rpm would be quieter and less 'buzzy', which is the main objective... Would possibly be some fuel saving too.

I understand the concept of peak torque and that Volvo did a lot of r and d to get to the ratios that they did... But I'm in the process of installing an engine with more power and torque than the car had originally and am willing to bet that a taller top gear would work for me - and anyone else with a bit more power on tap.

Of course there's the m90 route, or even the Borg Warner t5 route that I've read about, but they're both fairly involved and fairly pricy. In my idealistic, simplistic wonderings, the modification of the m47 would 'only' involve a rebuild and the change of two internal pinions, possibly using pinions from another Volvo gearbox since Volvo kept so many parts modular etc.

The result would be a fairly cheap gearbox with 5th gear that was more like an overdrive just for higher speed cruising. Of course there'd have to be some thought behind which ratio to pick, or the result would be a 5th gear that was too tall for anything other than a light load on the flat...

Alternatively I could just invest in some soundproofing and turn the stereo up.

I don't want to change the final drive in the axle and sacrifice acceleration in the first 4 gears.

Any thoughts? Like I said it's just a wonder...

S.

[classicswede]

The high ratio GLT axle in 5th is around 90MPH at 3000RPM. With the extra go of the turbo engine you would not loose much acceleration and might even gain on gear changes.


There are rebuild kits for the M47 in Sweden but normally a full gear set. It should be possible to just change 5th gear for a taller ratio.

[Bugjam1999]

Ok cool thanks- so sounds like I should look for a glt axle then, I'll be pulling the existing axle off the car to redo all the suspension bushes anyway. I'll also have a look for rebuild kits for the gearbox, out of curiosity...

If I changed the axle the speedo wouldn't read correctly I assume?

Thanks,

S.

[Clifford Pope]

Originally Posted by Bugjam1999

If I changed the axle the speedo wouldn't read correctly I assume?

.

If the speedo sensor is in the axle cover (rather than the older gearbox position) then it wouldn't make any difference, because the crown wheel is always turning at road wheel speed.

That's assuming that the GLT crown wheel and sensor are electrically compatible with your speedo. There are two kinds of sensor, to my knowledge, and I found even swapping from a 93 car to a 92 the polarity needed to be swapped over or the speedo only worked in reverse.

[CaptainAhab]

If you change the ratio of 5th you would need to change the layshaft as well surely?

[Angie]

I've often thought that, even with the 2ltr engine and on ordinary roads, there could be a sixth gear, perhaps an electrically operated overdrive. Is it possible to 'attach' an overdrive to a five-speed gearbox?

[classicswede]

An OD could be fitted to add an extra gear and is a viable option

[Bugjam1999]

Is adding an overdrive viable? I've seen a couple of threads about this before and the sum up seemed to be that it wasn't:

http://forums.turbobricks.com/archiv...p/t-55271.html

http://forums.turbobricks.com/showthread.php?t=55271

that said, I stick by my earlier comment of not knowing much about gearboxes, so more than happy to be corrected...

I think for simplicity's sake (ha!) just changing the 5th gear ratio would be easier and wouldn't add extra points of failure - but then volvo overdrive units might be pretty strong?

As an aside, I haven't managed to find any rebuild kits for the M47 online - does anyone have a link to one?

S.

[classicswede]

Doing it the way talked out in the TB threads is not viable. Needs to be tackled in another way

[CWXC70]

Originally Posted by Angie

I've often thought that, even with the 2ltr engine and on ordinary roads, there could be a sixth gear, perhaps an electrically operated overdrive. Is it possible to 'attach' an overdrive to a five-speed gearbox?

Depends on your threshold for pain but a Laycock type OD unit could be fitted in place of the centre prop bearing. I dread to think how much this would cost (Laycock unit plus two bespoke propshafts and bracketry) but mechanically it's a straightforward job