Hi I was wondering if anyone knows if I could mate a 4.2 straight six to a 2.8 box and back end as I have. Series 1 with a 2.8 that’s locked and I need to rebuild the engine but Ive manged to track down a good 4.2 engine on about the same money as a rebuild will cost me and I was wondering if anyone here knows if the gear ratio would just get shredded by the 4.2 engine
4.2 engine with 2.8 litre rear end will give you stupendous acceleration and MASSIVE engine braking and even more abominable fuel economy.
Sorry if thats not what you want to hear but one of our club members did this years ago and this was the result, oh and a very high revs v speed 
Well would I just have to chance the crown wheel and it’s just cause my series 1s engine is seized but she has a manual box
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The drive train on all ‘series’ Jaguars should be interchangeable, Harry - if in ex-factory configuration…
Frank
xj6 85 Sov Europe (UK/NZ)
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So I should be able to put a series 4.2 engine onto a series 1 2.8 manual box and back end
As per Franks affirmation but my remarks still stand.
Keep the 2.8 flywheel, starter and bellhiusing together as a matched set on the back of the 4.2 and you’ll be fine. Theoretically, you may overpower the clutch, especially if you go up in gearing, but you should be OK in normal driving or you could fit a 4.2 clutch at swap time.
Harry,
I once had the chance to test drive an ex-2.8l MOD SWB SI car. I’m fairly sure the engine swap is fairly easy (remember the additional expansion tank though) and in my case I’m fairly sure that nobody had worried about changing the gearbox or rear axle.
Compared to my own SII LWB auto the SI SWB manual was an entirely different driving experience, much more of a sports car than a luxury sedan. The gearing wasn’t bad either as the manual gearbox was fitted with an overdrive. I’d guess that any losses in fuel ecoonomy caused by shorter gearing would be compensated by the use of a manual gearbox.
As for the clutch, my SII parts catalogue specifies different part nos. for the clutch housing of 2.8l cars as opposed to 3.4 and 4.2l cars and the clutch plate and the release bearing. Interestingly, 3.4 and 4.2l engines used identical parts.
I’d guess with some consideration in your right foot and no trailering you should be able to make do with a clutch from a 2.8l engine.
Good luck
Jochen
75 XJ6L 4.2 auto (UK spec)
the 2.8 has a 4.09 rear end for the auto I believe.
If you have a MOD, (5 speed), that would be ok, but not so good for a 4 speed, as you will be revving ~4000rpm at 100kmph/60mph
It would be possible to swap the whole IRS end, and use the 4.2 diff ratio ?
My favorite ratio was 3.31 LSD which came in all Oz V12, matched with a 5speed
1st gear was all done in 2secs, would not have wanted to go lower
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