Firstfives.org

Hi guys,
I do quite a bit of highway driving at 100km/h + and the ol' 3 speed automatic isn't really cutting the mustard.

Obviously a 5 speed is ultimately what i'd replace it with, but finding a suitable candidate is proving difficult & costly.

I'm wondering if there's another option to make the car more of a highway cruiser - possibly a 4 speed auto from a later E12/28?

Ideas welcome,
Cheers

The main thing to take in to consideration is if/how you can keep the speedometer working.
I think you can fit any 5 speed gear box for an M30 on the car, but the newer (easier to find) ones do not have the mechanical speedometer connection needed for your E12 dash. I've red (recently also here) that you could be able to retrofit the parts on newer gear boxes. Search on the forum. You would need also a drive shaft, brackets, pedal box, etc.

Newer auto boxes might work, if they fit physically.. but with the same limitation regarding the speedometer. I don't know about drive shaft length though. Be aware that some of the early 4-speed 4hp22 boxes were weak (easy to destroy). And the newer ones need all kind of electrics.

Maybe, if acceleration is no requirement, you might just change the diff to something with a smaller ratio.

I am not sure if there would be a lot of fuel economy (if that is your target).

Thanks for your reply.

Main requirement is lower rpm's at highway speeds.
Acceleration is not an issue as i'm not looking for performance.

Can anyone elaborate on diff options?

The speedo drive is the hard part. A G265/6 is the way to go, but apparently parts aren't as easy to find there as here. The 4HP22 autos aren't bad transmissions, really pretty good for slushboxes, albeit with a sometimes catastrophic, but avoidable quirk, but still, no speedo drive. I have heard of adapters being made for the driveshaft to run an electronic speedo, but that's getting involved and I have some electronics background.

Ok, I just had a really wild thought. How common are E28s there? I've heard the entire rear subframe will bolt up, and those cars have a much wider range of diffs available, and would support an electronic speedo also. Or cut the mounting flange off an E28 subframe and have it welded on so you could use later diffs. Early E28s used a rear cover which I think fell in the same place as E12 diff mounts, though I've never measured.

One last detail, if you find a G265/6 5 speed, you can use your 3HP22 driveshaft with it, they are different, but nearly identical in length.

Actually I glossed right over your real question, what's available. While geographically challenged, it appears you have a 3.64 diff. Here at least 3.45s were much more common in sideloaders, which you have and E24s had thru 82 model year. E24's also had it appears 3.07 and 3.25 available, depending on the year and model. But your speedo will be off.

Thanks guys, appreciate your knowledge.
There's no cheap option, really. (not that I expected one honestly)

Mike W. wrote: How common are E28s there? I've heard the entire rear subframe will bolt up, and those cars have a much wider range of diffs available, and would support an electronic speedo also.

E28's are pretty common actually & i have done some reading re. the rear subframe - interesting idea...

Mike W. wrote:One last detail, if you find a G265/6 5 speed, you can use your 3HP22 driveshaft with it, they are different, but nearly identical in length.

Good to know. The 5 speed really would really complete this car, but it's not viable at the moment.
It all comes down to cost/effort & making sure I don't over capitalise! I am very suprised at what i've been able to achieve so far on a small budget.. now here's where things get expensive haha!

Quick update.

I bought a complete 1978 528i 5 speed locally for $650. yay!
(if anyone wants euro bits, let me know)

I will be stripping it back in the coming months.

I assume swapping the entire rear end w/ the side loader will be the best option? What sort of maintenance should I perform on the diff before reinstalling it?

Also, what sort of RPM can I expect with the 5 speed at say, 60mph?

Cheers

RPM depends on the 5-speed (dog-leg or overdrive) and also the ratio of the differential.

I believe it's a 5-Speed Overdrive 265/6, Diff I would assume is a 3.45

bizz wrote:Quick update.

I bought a complete 1978 528i 5 speed locally for $650. yay!
(if anyone wants euro bits, let me know)

I will be stripping it back in the coming months.

I assume swapping the entire rear end w/ the side loader will be the best option? What sort of maintenance should I perform on the diff before reinstalling it?

Also, what sort of RPM can I expect with the 5 speed at say, 60mph?

Cheers

65MPH would be about 2500 RPM. To keep the speedo accurate you will need to match it to the diff, so if you swap one, swap the other. If it was me I'd probably leave the 3.64 diff if that's what you have, you will gain a 20% reduction on revs with the 5 speed to start with and you can always change it later if it's not enough. But you get a little more performance with the 3.64.

Mike W. wrote:65MPH would be about 2500 RPM.

Perfect, thanks for that!

Mike W. wrote: If it was me I'd probably leave the 3.64 diff if that's what you have

This sounds logical, the thought did occur to me fleetingly. I'll give this try to begin with, I can always swap the 3.45 in if I feel the need.

Cheers