I still have the tendency to work against it. And probably will forever haha. After working on it, it feels like I've learned to dislike the v12 and transmission which wasn't necessarily the outcome we all wanted haha. But now I can at least make more informed decisions on what I'm doing with better options than just being stuck pushing the big reset button that is a drivetrain swap.

 

I’m a racer, not a restorer, but I know what an expensive trap an engine swap is. Plus if you want power please don’t be fooled. The V12 is more powerful than the Chevy small block or the Chevy big block. It’s just measured in a different way.
Plus there is so much additional power available in that V12 for very tiny money. The only real cost is it will sound louder. ( remove mufflers and replace with straight pipes leave the resonators at the back to deal with keeping it legal ).
Also remove the intake on the air cleaners and replace with a cold air intake. Those two will Yield an extra 50 hp
If you want power at high speed, swap to a pair of reground camshafts. ISKY or Crower. In the US and Piper and Kent in England. It takes about 1/2 the time required to swap Jaguars pair of camshafts than swapping one camshaft for a Chevy. That will yield between 80-100 extra horsepower at 6500rpm
If you want racy instead of luxury. Swap in a Chevy manual transmission. It’s not hard or horribly expensive here in America. Plus swap the rear end for the later outboard disk rear end. Now you can smoke the tires but it will only go about 125mph instead of 150.

 

I have a hunch that I have at least one cylinder not getting much if any compression, but that's an un-educated guess at this point. Need to actually take a good look at it but some of this will depend on that too.

The transmission and rear end have been things I'm looking at regardless of the state of the engine. 3.54 rear end is more alluring to me than the 2.88 I have now.... as well as outboard brakes .

You said something I don't quite understand though. "Plus if you want power please don’t be fooled. The V12 is more powerful than the Chevy small block or the Chevy big block. It’s just measured in a different way." How do you mean? I'm sure they have different power and torque curves and whatnot. But power/torque figures I get for a more modern LS3 vs. my 5.3L HE V12 on a quick google are pretty far apart. Unless you're saying power and torque was measured at the wheels back in 1989 instead of at the crank?

 

The fastest way to identify the lack of compression in one cylinder is with a leak down test. When troubleshooting it doesn’t need to be warm just on top dead center that cylinder with both valves closed.
If the leakage is heard in the crankcase the cylinder is the issue( rings or a hole in the piston). If it’s heard in the exhaust it’s a bad exhaust valve. Heard in the intake, intake valve. Heard in the radiator blown head gasket.
Next regarding power I had used GM’s numbers on both the small block (350) and big block (454)
If you choose to use a newer engine such as the LS series. You would do well to also consider the Jaguar V8 which is also available and often even cheaper than an LS.
In which case instead of called a Lumped Jaguar it’s considered an upgrade.
To be very honest other than casual reading I don’t care much for engines of this century. Actually I expect future performance to arrive in EV’s rather then our 4 stroke piston start stop things. That’s a pity. I spent my life working on them. Have some real favorites.

 

For better or for worse, if I do an engine swap I would want it to be fairly new. Partly because I'm more familiar with working on new and very old, less of the stuff kind of in the middle. I didn't bring this up but I did briefly consider an EV conversion because, in terms of build quality, the XJ-S is a decent car for it in my opinion. There's just a huge chance I would start and never finish though which would be such a waste of a car. I'd rather try it with a hunk of junk.

 

I’m coming from the opposite. I started working on cars in the mid 1950’s and paused as the new century came in.
When I left off EFI was still new and OBD2 hadn’t yet taken hold for the sake of the parts replacers.
To my mind the V12 is the strongest engine I’ve ever worked on and extremely suitable for racing without the need of a massive influx of expensive aftermarket band aids
There is beginning to be a bigger and bigger movement to original production. Stock Bottom ends the drag racers call it. Or price limited to endurance racing groups such as LeMons or Champ Car.
As a Vintage racer I see it as the only hope for real vintage racing and not just silhouette racing. I’ve heard tales of $20-30,000 spent on just engines for vintage racing.
I digress.
Jaguar V12’s forged steel crankshaft made with massive bearings suitable to engines 200 cubic inches bigger that the 326 of original. The ease it will go out to over 7 liters is a clear indication Sir William intended it to compete with the Cadillac’s 500 cu in V8
Then the quality of the fasteners, the brilliance of the engineering while still building it with a lot of the equipment used on the old iron block 3.4 dating from WW2
That engine influenced many much more modern engines. Look at the oil pump of the LS Chevy. That design was first used in the 1971 V12, However where Chevy cheapened out and went to Sintered Iron for cost reasons. Jaguar used forged EN40 40 steel and once machine work was done hardened the surface. Same treatment to the connecting rods.

 

Been a lot of these cars saved from the scrapper by engine swaps. The older carb'd swaps were pretty straight forward and I suspect they are where the idea of doing so being a way to simplify and increase reliability came from, and it's true enough, I guess.
A 70's 350 with a quadrajet and HEI is about as plain jane as you can get.

There's truth to the later model injected swaps being somewhat unserviceable by anyone other than the builder. Not just a jag swap thing either, swaps get a bad rep for such.
Generally that's from people hacking up factory harnesses or doing roll your own EFI and/or really custom performance stuff. Not much to be done for that
but for a stock-ish LS swap, use a harness from a big player like Painless Wiring and anyone with any sense can understand what's going on with it.
www.painlessperformance.com/wc/p.php?s=lscar

 

I completely understand that sentiment. But why go through all the effort to swap engine? The problem isn’t the engine which really is a beauty and extremely robust.
The problem is the overly complex EFI ( well, lacking OBD2 & Was totally novel for the time) So get rid of it and go back to carburators.
Jaguar used 4 lawn mower simple carbs from 1971- 1974. On both the XKE and the xJ12 Sedan.
Remove the EFI, put the retro carbs on. It’s a simple task. Much easier than swapping engines.
Yes you’ll lose 17 horsepower. And maybe some mileage but it will run plus the result will have a lot more power than a 350 small block with a Quadrajet. At least 50 more Net horsepower. ( Net, not Gross)
Ironically sometimes the problem isn’t the fuel injection it’s ignition. Under that Plastic Lucas cover is a Chevy ignition module from an 85 K1500 about $35 NAPA . ( Jaguar wants $1200 for it with the plastic Lucas cover)
Today just pull out the Lucas ECU and replace it with a Mega Squirt or another well known and understood ECU. The hardware is there all you need to do is change to domestic sensors.

PS I can appreciate your sentiments regarding fear of not finishing it. Thank you for those.

 

Hello again everyone, it's been a bit since I responded here. I've been making some decisions regarding the car. I stressed over it a lot actually

Like I said, I got the engine to turn over which was a relief. I went hunting to see what I could find and ended up driving around a V12 (XJ12 I think?) for awhile.
I know my XJ-S is going to be different, but I can see why these V12s have a bit of a cult following. It was a fun and unique sound and had a very
nice feel to it. I'm not really sure how to describe it haha.

However, I didn't really fall in love with it. So that put me in a weird spot. I realized I just don't feel the same way
about it as I do about some other classics I'm around, like my all time favorite Studebaker Rockne. I don't have the desire to preserve my XJ-S
like I do with the Rockne. I'm driven to make it what I want it to be, what I think it can/should be. So this may not be a popular decision, but I
am going to drive train swap it. It'll be expensive and time consuming, but I know it'll be fun and the end product will also be fun.

I appreciate everyone's time and effort in helping me figure all of this out. I learned a lot about how I need to think if I continue on with a hobby
of having a few older cars around. Each car is going to end up being wildly different, and more importantly what I want to get from each car
is going to be wildly different.

I also learned that turning the engine into a boat anchor out of frustration might be a tempting offer.... but it deserves much better than that

 

Very well said. The Jaguar V12 is completely hobbled by the transmission and the teny tiny exhaust system killed any aural thrill that remained. Most Americans make a Pavlovian association with performance and the cross plane V8 burble. Many Europeans, and some older American BMW / Porsche enthusiasts associate performance with the rhythmic sound of a flat or inline 6 of which the V12 Jag is that x 2. When uncorked they're amazing. The V12 with a manual swap is a different car. Even a the V12 with a 700R4/ 4l60e automatic is way better than what Jag built.

That said, no shame in Chevy swaps. I've done many. Let me warn you, the LS swap is expensive. It's fiscal death by a-thousand cuts. The temptation is to do a LQ4 / 4l80e swap from a truck as they are plentiful but you'll be right back in the same boat with the lazy 4l80e transmission. Yes the 4l80e got better over the years but a sports car is not served well by a balky and brutish truck transmission. You're best and most fiscaly prudent approach is a LT1 from a 95 Corvette / 95 /96 Camaro. The 6 speed manual version of the LT1 has a pull clutch that is extremely light and easy to drive and those LT1's can be massaged to about 330HP with minor bolt on. The light clutch driving experience should not be underappreciated. A early 2010's 5.3 with a 6 speed auto or manual is another reasonable choice but the SBC stuff is better looking. You have to spend buckets to get an LS to look nice with covers, intakes and serpentine systems. Of course there is an LS for every budget and so many ways forward that its daunting to pick one and they all can get expensive real fast.

The XJS with a manual and a V8 swap takes more than 400Lbs off the nose. This way they are an exceptionally thrilling and responsive car to drive yet remains very conformable. A true GT that Jag could have built had they sourced a v8 from someones parts bin.

 

Just don't cheap out, there are a lot of half finished or barely running swaps out there, and even more that give such a bad name.
Spend the money and be rewarded.
100K out of 240k total daily driven miles on my 90 that got an LT1 after the 12 fell apart for the 3rd time in 2008
or so.

 

I'm expecting with all the little things I want to do after putting a new drive train in it to end up in the neighborhood of $30k. But some of that will be long term things, not an immediate change.

Along with that price, "death by a thousand cuts" couldn't be more accurate. However, it's a lot better than it would be with a lot of other cars. The XJ-S has both the space and (mostly) the
structural integrity to swallow the extra power and torque. Plenty of cars out there that absolutely couldn't handle that type of power increase.

As far as half finished project cars.... sometimes I don't mind. There are plenty of cars where I think "Well, that was little Timmy's first project and it didn't go well but I bet he
learned something" and I'm fine. This isn't one of those cars for me. I'd honestly feel pretty bad if I ended up with something that never actually got done. Well, most project cars are
never actually "done" haha, but you know what I mean.

 

Yes, to a point. How much HP are you planning? The XJS structure is far above average for a platform that was developed in the 70's. The XJS chassis was developed with the lessons learned from all the structural problems that surfaced in the XJC Broadspeed racing program. There are 2 areas that need improving once you get north of 300HP if you also have traction.

1. The rear cage will want to roll upward causing wheel hop and general rear end unpredictability. In extreme cases 400 HP or so , the drive shaft will bang the floor pan and the rear sub-frame mounts will give up. There are many ways to solve this with some additional bracing that will pay for itself by saving the rear sub frame bushing. Do a search for rear sub-frame twist or cage twist. This phenomenon and solutions have been well covered here.

2. A strut rod that runs under car and links the two front jack point makes a tremendous difference in chassis rigidity and steering feel. More so with high performance tires. This is becasue the trans tunnel area is open, large and unsupported. You'll need to plan your exhaust droop accordingly. I've engine swapped SBC, LS and an AJ16SC using 2.5 and 2.25 pipe sizes with both automatics and manuals in all cases and always have been able to fit the strut rod. When using the 2.5" exhaust and a large trans like a T56 magnum the strut needs a slight bow to clear.

 

Semi off-topic but has anyone ever solid mounted the front and rear subframes? Seems like they would, especially up front, stiffen the chassis up a bit and remove some flex. At the cost of NVH of course but who cares at that point I assume lol...

 

Yeah I knew of the rear cage but I hadn't heard of doing the strut rod before. I'm already planning a sway bar upgrade though so maybe that'll get me where I want for steering feel.

Either way though, you mention "if you also have traction". I can't see right now that I could possibly have traction to be honest haha. Previous owner put 215 tires all around the car. Last car I had with 400+HP at the crank had 285s in the rear and still had the tendency to break loose if I coerced it. Obviously suspension geometry, weight, tire compound, blah blah blah all make a difference but 215s aren't going to cut it. Has anyone ever done anything to change the bolt pattern and/or center bore sizing so that I can get wheels from more common modern sizes on the car? Maybe this should just be its own topic.