Hello,
I have been looking at several Jaguars for sale. I keep coming across various Jaguars that have had the chevy conversions done. I really don't like the conversions done to these cars. I found a 1974 XJ12 that has a chevy conversion. The car is in amazing shape. What I am curious about is seeing how hard it would be to change the engine back to a V12? I have also found several 1980's XJS V12 motors for sale, so would one of those fit or not? Has anyone done this. If your like me it has stopped the sale and interest in several Jags, while on that note do you think it would be possible in a later XJ6, XJ12, or XJS. I have a 1998 XJ8 and its got me and my dad hooked on Jags.

 

Yes, it can be converted back to Jaguar power. How difficult the job will be depends on what was done during the original conversion to Chevy power....but it should be entirely doable.

There are various approaches to the job but personally I'd consider the job *only if I had a donor car alongside* so there was ready access to everything needed.

It wouldn't add to much more to the job to go with the later "HE" variant (1981-later) of the V12. Much better fuel economy with no loss in power. The fundamentals of the engine installation would be the same but you'd need the fuel injection ECU and harness, complete, from the donor car. An 80s-vintage XJS would be a logical choice.

Cheers
DD

 

Hey Doug Thanks for the super fast reply! Do you know how many or what electronics a 1974 XJ12 would require attention to in the change back? As far as donor car what years would be acceptable for the task? would a late 70's or early 80's work? Once again thanks for the super fast reply!

 

I had considered swapping the 6 cylinder engine in my driver to a 12 a while back... I got lots of good information from a friend who did the same conversion..

Aside the obvious things you are going to get into (motor mounts, radiators, etc), all of the electronics will be different too... So you'll have the speedometer and tach you'll need to change out, wiring for the ECU, engine wiring harness, etc.
Problem in going with the 74 XJ12 is going to be that was a carbed car. The XJS was fuel injected from the get go (I think).. So you're going to have to consider fuel return lines that are specific to the XJ12, wiring for the ECU, electronic (transducer) speedometer....
I have a 73 XJ12 that someone had started to changeover from the carbed engine to a later HE... The engine is in place, they had stared running the harness for the ECU, there was a pile of new fuel return lines, tanks that need to be swapped out... The project was stopped about mid way through.
In other words, with the 74 it would be an energetic project, where with a 75 or 76 that started out as a fuel injected car would have some of the ancillary stuff already in place...

Unless you were going to convert the car back to a carbed V12... that may take some of the difficulties out of the picture...

Cheers,
David
Everyday XJ

 

Assuming everything was removed for the V8 swap (fuel injection ECU, ECU harness, all engine bay wiring, etc) I'd say you'd need....everything.




Any of the 70s-80s-90s 5.3 V12s will physically fit and can be made to work.

The 70s vintage "pre-HE" v12s were terrible gas hogs and the fuel injection and ignition systems were not very good. But, if you found a complete (and running) mid/late 70s-vintage XJ12 or XJS you certainly COULD use it as a donor.

Circa mid-1981 the HE engine was released. Much better economy, much better ignition. much better fuel injection. PLUS...much better factory wiring diagrams and replacement parts availability. In USA model year terms you'd be looking for an '82-to-early '89 XJS as a donor.

(No sedans with the HE V12 were sent to the USA)

(I mention "early 89" XJS to avoid the V12s fitted with Mareilli ignition...which would add needless complication to the swap with no particular benefit)

IMHO if going thru all the trouble of a retro-conversion ...and it WILL be a lot of work...I'd go with the HE engine.

You'll also have to decide on what transmission to use.

Cheers
DD

 

Was it? (I'm asking, not arguing)

I thought all Ser II XJ12s for the USA market were fuel injected...with fuel injection becoming standardized for all markets in 1975.


Correct!


Cheers
DD

 

What about a series 3 HE from 1981 on wards as a donor car it would have the fuel tanks - fuel lines plus fuel pump and solenoids plus all wireing and ECU ' and possibly a Turbo 400 gear box .
cheers Ray

 

Great choice but as rare as hen's teeth in the USA as Jaguar didn't import them here. Ocassionally one migrates down from Canada, though, so there's always a chance of latching on to one if you wait long enough and keep your eyes peeled.

Cheers
DD

 

Thanks i didnot realise that over here they were a dime a dozen' I gues that cuts the opptions down to XJS HE .i all way thought putting a chevy motor into a Jaguar is like gold plateing a toilet brush

cheers Ray

 

Hey Doug...

Yeah, the 74 XJ12 kept the 72/73 XJ12 engine... Fuel Injection started in 75... One year of the Series 2 XJ12 with carbs...
It was also the only year that they had that skinny park light in the lower valance... After 74 they had to manufacture a cover for the valance, and put the parking light in the bumper (US Cars).
I've actually had two carbed 74 XJ12s here...

Here's one on YouTube: (Skip to about 3:50)

So for this conversion, the fuel return lines and tanks are a consideration too...

Cheers,
David
www.EverydayXJ.com

 

Thanks, David. I've learned something new for the day and it's only 8:00Am ! I can't take the rest of the day off!

You're right, then, going to the F.I. would need a few extra steps

Glad you chimed in on this.

Cheers
DD

 

The very few we see down here are usually rust buckets. A mint condition example will bring big money, though!



Plenty of those here in the 'States!




I generally agree, espeically some of the hatchet-jobs from the 70s-80s. Sometimes, though, you come across some really well executed conversions that are damn nice....even if the basic idea doesn't appeal.

Cheers
DD

 

You don't mention which state you are in. Depending on your emission rules you may have even more fun on your hands swapping engines out.

 

Good man! I've often had similar thoughts. I have zero interest in these cast iron pushrod chevy conversions and would extract the boat anchor in a shot!
I would try to get a later 6 litre V12. There were various upgrades- including fully electronic Denso fuel injection and mapped ignition, better gaskets and sealing and a lip seal crank rear main seal rather than the rope seal. Issues with the V12 were usually because the pld radiators would get silted up from not changing the coolant regularly and of course the Lucas electrics- but the V12 engine itself was a very sturdy unit- contrary to popular belief. I would tackle the known issues with an after market radiator upgrade and a modern loom upgrade with good connectors. That way you can keep an engine of pedigree

 

I don't understand all the V8 hate and it seems on this board few miss an opportunity to insult what is arguably the best and most versatile engine ever made. If a Jag owner finds satisfaction in the V12 good for them, if another finds satisfaction in swapping in a GM v8...thats fine too. But in the interest of equal opportunity I'll take a moment to to bash the V12... The numbers don't lie and there is nothing romantic about Jaguar's heavy and bloated V12 that leaves you broke and stranded waiting for a tow. The V12 is much heavier and less than powerful than a GM engine, it costs way more in parts and fuel to maintain. The V12 is less reliable and much more difficult to service than a GM V8. AFAIC there is nothing romantic about a vehicle that could leave you stranded. The BW trans behind the early v12's is a travesty and even Jaguar saw the light and used a GM product behind later V12's.

As for converting back...why...the v12 is inferior in just about every way. As a carb engine it's unreliable and fidley. As a fuel injected engine it will be a very difficult job not worthy of the effort IMHO. If it were my car I'd spend the time and money adding TPI and a 700R4 to the top and back of that chevy engine. Then you'd have something quick, nimble, smooth, reliable and fuel efficient. If you want a purist V12 save your money... they come up on eBay from time to time and it will be cheaper and more time efficient than converting back.

 

At the risk offending some, which I prefer not to do, I have a lump. So far just fine.
decently installed, though I've seen better. An LT1 is a very good engine, and the OD make the combo even better.

And, some use the LS1 and derivatives. All alloy, but till pushrods. OH's are romantic, but, kinda fluff for he average guy's road engine.

I like all engines and messing wth a V12 could be fun!!

I wrote a paper, "lump Thoughts". If asked PM, I'll share.

As to carbs vs FI. A lot there as a topic of it's own. But, providing a return line is not a large hurdle.

If one is in California, the largest hurdle by far is SMOG compliance. So, a retroversion legal in some states could well be illegal in CA. The conversion regs are stiff and badly outdated. So, thst might limit the market value of an oitherwise fine job. Can't put carb'd engine in a FI cae. Period, And the engine must be of the same year or newer. No 78engine in an 83 car!!!

Your car, your deal.

Carl

 

I don't see a problem with the Chevy engines.. plenty of power, more upgrade options, cheaper parts, easier to work on.. everything about it makes sense to me. I do understand some people are purists and would rather have the Jaguar engine that's cool too no point bashing people for having different ideas or opinions

 

For me the chevy engines are not worth it. I have had chevy's in the past and they just don't hold up for me. For example my dad's '88 GMC 4x4 which I had for many years, has had 4 transmissions, 2 engines and countless other issues. It only has about 160k miles on it. You can say that parts for Jaguars are more expensive than Chevy's and other vehicles, but parts for anything are expensive anymore. For me its simple, why not have a little jag luxury if it is going to cost about the same? As far as engine, there are a lot of collectors that will only buy a car that has a factory engine in it. Even more that will only buy a numbers matching car. For some people a different engine in these cars is ok, but for me, if I wanted a Chevy I would buy a Chevy.

Just my two cents.

Thanks for everyone's help and insight. I will have to keep it in mind for future, but as for now I think it would just be to big of a task for me.

 

Yeah I completely understand where you are coming from. Me, I didn't want a Chevy, but couldn't even dream of affording repairs on a Jaguar so I didn't have much choice haha but as for the re-sale value it doesn't concern me as I plan on keeping this car until the day I die so unless something really unfortunate comes up, this car won't be for sale (: if it wasn't already converted before I bought it I probably would have left the Jag engine in it.. at least until it gave out *shrugs* everyone has different opinions not gonna let that get to me as long as I like what I own then that's all that really matters, right? Good luck with your project and hope it's not too much of a headache for you!

 

Hey I am the dad here that he is talking about. In 1988 I bought a brand new special ordered GMC 2500 pickup. It has ALL the bells and whistles. This truck has about 160k on it now. Very lightly used and for part of its life not even driven for about a total of 1.5 to 2 years. This truck has been the worst investment I have ever made. The 350 engine has been over hauled and even replaced. The transmission was redone 4 times. The A/C has never worked. The breaks well they just are not there. Said all that to say I think the Jaguar power plant is a far superior setup. Yes they use the old old 400 turbo transmission from GM and it is a very good tranny (GM has not used them for years now and is very sad). I just bought my first Jaguar this weekend a 1999 XJ8 vanden plas lwb so far just love it. My son got his Jaguar about 8 or 10 months ago and its a dream to drive also. Next up is to sell my 1972 Buick Centurion Convertible and then plan to buy a XJS with a v12 and then the fun will continue on. Anyways I dont think I would ever trust a 350 motor again unless it was built back in the 1970's when GM still knew how to produce a real engine. The newer 350 motors are not made
as well. The repair guys and dealers tell me it’s not if but when the manifold gasket will break down on the newer 350 motors, they all do it. I just wished they still built them like back in the early 1970's. Sorry if I stepped on anyone’s toes this was not my intent, just my first hand experience over the past 40 plus years.