Transaxle Forensics

Post Reply
mj2k
Posts: 112
Joined: Mon Feb 18, 2019 10:46 pm
Location: Herts

Transaxle Forensics

Post by mj2k »

I was planning to do this as another 'comparing' thread, but it's turning more into a detective thriller, with lots of twists and turns...

As a followup to my fiddling about with the 12v Busso, I'm now starting work on a pair of transaxles I've got hold of, with the intention of making one really good one with decent 2nd gear synchros. It might take me some time to pull the boxes a apart (I have a fair bit of work to do on my Subaru first), but I thought I might as well start by comparing flywheels.

First - front flywheels. Both in the below picture are the normal 2.5 gtv6 type (which is ideal for my crankshaft choice), but one as been lightened to a quite extreme amount:

Image

That takes the front flywheel weight down from 10kg to 7kg according to my scales, though it certainly feels a lot more when you're lugging the things about. I'm definitely using the lightened one, just got to hope there's enough 'meat' left in it for balancing when I've got all my engine bits and bobs together.

Next comes the rear flywheels. This is a slightly bizarre one - the 'old' rear flywheel is nowhere near as dished as the 'new' one, and when you spin them the 'old' one feels much lighter and easier to stop with a thumb than the 'new' one, which suggests the rotational mass is much smaller. However, according to my scales (which I'm now having serious doubts about) both complete flywheel / casings weigh exactly the same?! makes no sense whatsoever to me, but presumably there will be a fair amount more meat which can be removed on the 'old' one vs the 'new' one to bring rotational mass down more.

Image
Image

Since as usual there are no serial numbers I've got no idea which 116 series they've come off however, one's 'old' and I assumed it had come off a gtv or gtv6, the other is 'new' and I thought it had come off some sort of 75 or another. The 'old' one has a pin in the gear linkage and appears to use a pull type clutch (though all parts of it are missing in action), the 'new' one has a splined linkage and a push clutch. Inspired by curiosity I looked at the various clutch / flywheel kits on OKP, it looks like the 'old' flywheel is from a gtv6 2 disc clutch, whereas the 'new' one is from a 75, and since it's got a 215mm diameter, that suggests it's likely to be fine for a v6.

Sounds like the twin disk should have less rotational mass and would theoretically be able to handle higher torque (which might be needed if I decide twin turbos are a necessity) but by the sounds of most discussions on other cars, a single disk is less grabby for everyday use, and grabbiness is definitely not something you want with an Alfa transaxle...! So I'll be sticking with the new one I reckon :)

It seems there's dubious value in lightening the rear flywheel since you can only safely lose about 1kg, so might as well just get it cleaned up and use it as-is.

And now for the interesting bit about the gear selector splines - the 'new' transaxle has a splined non-isostatic selector which means it's come from an earlier model. And the 'old' gearbox has the pinned isostatic selector which means it'll have either come from a 75 or really late model GTV. Which means neither gearbox had the original clutch mechanism?!

So I'm still none the wiser as to what I've actually got, except that I've got the one early type box (unless the selector's been changed) presumably from a 4 cylinder since it's a push type clutch and possibly a late type isostatic v6 since it's got the pull type. So next stage will have to be pulling the diffs when I've got a spare mo.

mj2k
Posts: 112
Joined: Mon Feb 18, 2019 10:46 pm
Location: Herts

Re: Transaxle Forensics

Post by mj2k »

Got the diffs out - the 'old' transaxle has 41/10 on the diff (4.1), which is open (unfortunately). Since it's isostatic that puts it as a late 2.5, either gtv6 or 75. Unless the diff has been changed too of course...

'new' one is 43x10 open which identifies it as a 4.3 ratio, probably from a 2.0 4 cylinder and since it's pre-isostatic I reckon it must have either come from a gtv 2.0 or an Alfetta 2.0.

Data from https://www.alfabb.com/bb/forums/gearbo ... ost5800145

So looks like I'll be using the 'old' diff, 'new' selector and bellhousing and no doubt a mixture of the gears once I have the boxes completely broken down, though chances are the internal ratios will be identical between the boxes.

But looking at the speeds per gear, the 4.3 diff might be a nicer option since it'd give a tiny boost in acceleration whilst not affecting top speed per gear much:

43x10 open
3000 rpm
18
30
41
50
65

6500 rpm
35
59
83
108
131

41x10 open
3000 rpm
19
31
43
56
69

6500 rpm
37
62
87
113
137

Post Reply