The simplest upgrade is to an HS/Kadett GTE axle, but try finding one!
The 'usual' alternative upgrade is to the Manta A axle. This is exactly the same construction as the Manta B but with narrower tubes and halfshafts.
The swap is relatively simple. You need to swap the front crossmember from the Chevette axle (which can be de-spot welded from the torque-tube bush housing and swapped very neatly). You also need to swap the panhard-rod mount to the opposite spring hanger. You will find the axle does not quite sit centrally - the best bet is to weld a threaded adjuster into the panhard rod so you can adjust it to sit perfectly in the middle.
With the Manta A axle, you can go to ET28 on 6" wheels with unmodified arch lips which would be ET40 on 7" wheels. The problems you will face on wheel choice is that the fronts get a lot of neg camber on droop and foul the shocks. To have no fouling at any suspension position, and unmodified lips, then max width is 6" with ET between 28 and 40.
Manta A pros:
Relatively easy fit
Perfect width
Can get LSD
Will take 200BHP if driven fairly gently!
Manta A cons:
Rebuild cost is very high - bearing set is over £300 alone - only FAG still do them.
Getting hard to come by now
LSD expensive
Will break with over 200BHP if used hard (ie. drag racing/drifting etc) either diff or torque tube.
Ok, so here is the big question - what to go for if you want big power and reliability. In my opinion the Salisbury 4HA.
The 4HA is an axle design that is available in a lot of different widths, and was fitted to a lot of different vehicles, from old Jags, to Astons, to Thwaite dumper trucks!! This has the bonus that a vast range of ratios are available, from around 2.5:1 to 7:1!
The 4HA's that are a useful width were fitted to the Reliant Scimitar. The Scimitar SE5 (1968-1975, pointy front) axle is approx 54" between drum faces, ideal for the Kadett/Chevette. The Scimitar SE6 axle (1975-1986, flat front) is around 56", ideal for Manta B/Ascona B.
As standard they have a 4-link and watts linkage setup, although the links are not in a useful position, so the standard brackets need removing.
The cool part is due to the fact the basic diff design was used in a lot of other vehicles. First off the watts linkage diff cover can be swapped for a non-watts linkage tin cover from a Jag axle. Secondly, and most importantly the diffs from almost any Jag axle (including the later IRS type diffs from Jag XJ/XJS) will fit, and Jag Powrlok LSD's are easy to come by and cheap - they were fitted to all V12 models, XJR's and an option on all XJ/XJS. Also the Jags have a good selection of ratios, 3.54 is very common, and 3.71 available (usually on the 2.9l XJ40). You should be able to get a powrlok diff in perfect order for £150.
As for fitting the axle, you will need to 5-link it. The axle tube is the same diameter as an Atlas, so a standard 5-link kit from Rally Design will be perfect. The standard panhard rod mount on the shell can be used, so you just need to weld in the link boxes and mounts. You can then use coilovers into the standard turrets.
The axle is very strong, I've never heard of one breaking. 500+ BHP should be no problem.
4HA pros:
VERY strong
Cheap to rebuild
JAG LSD fits and is cheap
Wide range of ratios available
Available in a suitable width for the Kadett
4HA cons:
Requires some fabrication work on the shell (link boxes/brackets) so no back seat!
Getting harder to find.
4HA fitted in Kadett Coupe:
(also converted to disc brakes)
And the result when coupled with 400BHP!
http://www.youtube.com/watch?v=D_Qs1gLiZXM
Grip off the line is the problem at the moment - 195 tyres a bit limiting! the 127mph terminal hints at the potential once we can get the power down
