Learn me-Jaguar XJS V12

Since I've last posted here, I've straightened out my daily driver issues(my Supra will be 7MGTE swapped and I will have a 94 Toyota Pickup as transportation), and I'm ready to plan my next build, which is a love older than my license: The Jaguar XJS V12. I really want to do a stripped/fender flared/manual XJS and I've done a lot of research on the V12 in particular, but I still need some thoughts and input from those of you that have raced/repaired these beauties.

So, what years should I look for? I would like to find a 6.0 coupe, but that appears to be almost impossible. I'm not trying to spend several tens of thousands of dollars on a car I'm going to, for the most part, strip inside and straight pipe.

What is reliability actually like on the V12s? I've heard they're hand grenades and I've heard they're bombproof. Which is it?

I plan on leaving it relatively stock. I don't need to megasquirt it or have custom headers made, though more power would be nice. I feel 400 at the crank is reasonable to expect in a stripped XJS. I have heard talk of people fooling the computers in the car to be able to run bigger injectors. How is that done, exactly? I have the ability to convert it to carb though it seems to be a step in the wrong direction if I intend to drive this thing with any sort of regularity. I know how much a rebuild costs so I'd like to try and get it right the first time. No forced induction, the N/A sound of one of these uncorked is unbelievable.

And lastly, are there specialists for performance parts? Not engine parts per se, but things like larger brakes, sway bars, shocks, springs. It's at the very least a 20 year old Jag now, so I wouldn't even know where to begin.

That's pretty much it, I'm not going to rush into this, I want to get started at some point after we find us a good house, so I've got plenty of time to read up on it and research stuff. I've always thought that V12 Jags were some of the coolest cars on the whole damn planet, and I think I can start planning.

Learn me something, GRM!

I don't know much about V12's, but I have a V8 swapped XJS coupe for sale...

There was a dude who I think got banned that was always talking up these cars. I believe he said that when you rip out a lot of the luxury stuff they were pretty reliable and corvette parts can be swapped on the wheels etc?

Motorcars inc. in ct are experts on all things jag and are super good people. Give them a call.

Search for posts by frenchd i think was his name, seems like every other post of his answered all your questions. Idk if he will be along to answer them here or not

Its so hard to find anyone that knows anything about these cars. I might sell my Stealth to get the one that's right near me in Raleigh, looks clean too. I'll search him up, thanks.

Excellent name, Chiodos, by the way. That band takes me back.

In reply to justthatguy:

Those who claim a JagV12 isn't stout simply haven't taken one apart. The bare block looks like one of those 10,000 horsepower Top Fuel engines with 4 extra holes. The head is held on by 29 studs each side!!!! The quality of parts is right out of mega buck NASCAR engines. The V12 is capable of making over 800 horsepower and lasting for over 24 hours at full tilt boogey on French pump gas. You can get out your wallet and order everything you want from Rob Beere (in England) or save a few bucks and find a lot of what you need here in the States. For example Wilwood brakes just bolt on and you can have some insane stopping power for a modest cost. If you want to modify the stock fuel injection contact AJ6 engineering (in England) 5 or 6 speed transmissions (American) can be easily adapted or if you like you can simply write a big check and have everything arrive in a box ready to bolt on. (do be cautious though there are kits out there that will ruin your engine)

Stone stock, a Jag V12 can make up to 318 horsepower in 6.0 form. There are more 6.0 engines in sedans then XJS's plus the later ones after Ford spent millions of dollars on them are more reliable and less quirky. (yes they will fit wherever a 5.3 was sitting) Or as little as 242 horsepower in early 5.3 carbureted form. Surprisingly a combination of both* is your best power. 400 horsepower can be achieved relatively easily and even though it's modified you will be shocked at how much torque (We're in 454 big block Chevy territory)and how flat the curve is..

The best combination is the early (pre 1980 {H.E.}) cylinder heads, called the flat head on top of the later 6.0 block. The problem is then the stock pistons will get you in diesel territory! Actually that is a very good thing too! The engine is begging to be bored out. It's a cast iron sleeve in an Aluminum block and as usual for Jaguar it's 100 times thicker than it needs to be.. so slide the sleeves out chuck them in a lathe and go bigger. There are several piston makers out there capable of giving you the piston you'll want But for the lightest, best power piston use the Cosworth (England) piston. Oh and while you are at it make the crankshaft lighter as well! The stock crank throws are 2.300 so if you offset grind them you can approach 7 liters {British for 427 cu inches} Use aftermarket Chevy Connecting rods with a little bit of modification) The stock camshafts run out of breath at about 5500 rpm although the factory took the engines up to 8300 RPM and had no issues with valve float. The trouble is most regrinds/aftermarket camshafts cost bottom end power loss in order to gain power at higher RPM. For a street driven car you won't improve on the stock but if racing is your goal then Isky, Crower and others can provide you with regrinds that are an improvement. (use Isky valve springs they are better than the stock ones and cheaper as well) However if you have a few extra dollars and really want a serious improvement Kent Cams (England) has the best billet cam out there.. while you're at it order a set of 1.5 inch lifters from Rob Beere.

There are some things you need to learn though. Quirky things like the fact that they have a distributor and it needs a few drops of oil every oil change.. The advance/retard rubber diaphragm needs to be checked to ensure old age hasn't caused it's rubber to crack or split. The early engines had the biggest fuel injectors although not quite big enough to use alcohol and their "computer(Not a microchip in it) is the easiest to trick into going richer.

The early XJS are also the lightest, 1975,76,77,& early 1978. But you need to put even them on a serious diet if racing is important.. The rear end is capable of living under anything you can put in front of it. But the engine compartment is crowded, work on air flow if you want it to stay cool at racing speeds.

Headers offer little improvement unlike most V8's. A V12 is in perfect balance and the firing order doesn't interfere with adjacent cylinders like a V8 does. Unless you are willing to go to a real equal length properly designed set there isn't much power to be gained. If you do want headers, there is something like 27 feet of tubing in just the primary pipes and I think it's 28 inch long collectors (4).. for the set I used in my XK-E race car. Stock cast Iron manifolds only weigh 4 pounds each (there are 4 of them) so they will be lighter than 27 feet of tubing. Not to mention the skill required to fit all that tubing in an extremely tight engine compartment.

Javelin wrote: I don't know much about V12's, but I have a V8 swapped XJS coupe for sale...

Among Jaguar people Chevy engine swaps are called lumps.. They reflect a lack of knowledge and frequently the swap displays it..

Little things like what about the tachometer? Those beautiful gauges are set for a V12 so one has to be yanked out and replaced by a V8 tachometer? What about the speedometer? Some transmissions need a different set up. Is the wiring butchered to make the temp gauge work? Air-conditioning? A V8 is a shaky engine compared to a properly running V12, has that issue been dealt with?

So called reliability issues are frequently a lack of knowledge. Can't get it running right? Did you check if the distributor has been properly oiled? Are all the hoses free of cracks? Are the ground wires tight? The throttle set up isn't for the ill-informed. That plus well intentioned mechanics will often lean out the engine for the guy who complains about the fuel mileage.. lean to the point of problems from the proper set-up Overheating? Did you bother to check all of the above? How about leaves, grass and other debris between the radiator and A/C?

In reply to frenchyd:

Wilwood makes a kit for this car? I did not know that, that's helpful. What about swaybars and shocks and what not? Would it be safe to assume that any place that has those brakes has these other parts?

I've also read that a lot of the late 6.0s actually had EDIS spark control on them, another reason for wanting a later model. I could tune better for less with a distributor, but I know how the ignition fails on these, one bank at a time. I'm not trying to go all out on one, just enough to give the Hondas/Subarus a run for their money, provided my poor driving skills don't fail me.

I'd be fine with the right 5.3 though, are there any specific year models that are ones to look for/ones to avoid?

I figured there has to be some sort of double or triple pass radiator that will fit in there, maybe two temp controlled fans and that should take care of the cooling. I hope, anyway.

I should also mention it will likely have a TKO500 put in it, along with 3.54s. I think these are basically Dana 44s, there has to be some way to get 4.10s or even 4.56s in there. Then I could use the T56.

frenchyd wrote:

Javelin wrote: I don't know much about V12's, but I have a V8 swapped XJS coupe for sale...

Among Jaguar people Chevy engine swaps are called lumps.. They reflect a lack of knowledge and frequently the swap displays it..

Or maybe, reflect a desire to try something else, reflect ingenuity, or the want for simplicity. Or maybe, someone bought them a jag with a motor that's blown or without a motor and just wanted to throw something simple or awesome under the hood. Hell maybe someone had a SBC laying around and found an XJS on CL with issues for $500 bucks. To me, a well executed swap shows actual skill and knowledge. It's harder than people think to get a swap done. Harder than maintaining whatever is already in the car from the factory

I do love V12's but oh do I love motor swapped cars just as much!

IIRC, a big part of their woes was underhood heat. They baked everything, which then became brittle/fragile, which then became broken.

frenchyd wrote:

Javelin wrote: I don't know much about V12's, but I have a V8 swapped XJS coupe for sale...

Among Jaguar people Chevy engine swaps are called lumps.. They reflect a lack of knowledge and frequently the swap displays it..

Knowledge life the SBC weighing in at 270Lbs less than the Jaguar V12? Or making more power and torque, even stock? And mine was done at a highly regarded shop with the correct kit and utilizes the GM multi-port EFI system, which is world's nore modern than the Bosch-based system Jaguar had. It also has all of the gauges wired up correctly and fully functioning, including the Smith's tach.

Considering most Jaguar people turn their noses up at a mid-80's XJS, why not make it lighter, faster, and more capable?

I've owned a properly-sorted V12 XJS and a V8-swapped one, and I would take the V8 every time now that I've sampled both.

Here's a link to a free 600+ page book about xjs - history, maintenance, modifications, etc.

Jag Lovers XJS book

I'm not saying that an I6 or a V8 doesn't sound good, God knows I've had plenty of both, but the howl that comes out of a V12 at full tilt is enough for me to warrant the headache of owning one. An LS powered XJS would be a potent mixture, maybe something for the future, and would definitely be the way to go if I were to actually make a pure track XJS. But almost every single XJS I see on the road is an I6 or V8 swapped. I can't remember the last time I actually saw a V12 powered Jag on the road. That's part of the charm, it might be a useless boat anchor, and to be completely honest, I don't care. This is that subjective part of the cars we choose to build. It's like the guys that build 289s. Would a 429 make more power? Yeah, but they're at 8000 rpm and are enjoying every second of it. I've already got a turbo Japanese 6 cylinder rocket and am soon to have another, I don't need neck snapping launches.

At any rate, thanks for the link schmidlap. I'll read that this week.

yupididit wrote:

frenchyd wrote:

Javelin wrote: I don't know much about V12's, but I have a V8 swapped XJS coupe for sale...

Among Jaguar people Chevy engine swaps are called lumps.. They reflect a lack of knowledge and frequently the swap displays it..

Or maybe, reflect a desire to try something else, reflect ingenuity, or the want for simplicity. Or maybe, someone bought them a jag with a motor that's blown or without a motor and just wanted to throw something simple or awesome under the hood. Hell maybe someone had a SBC laying around and found an XJS on CL with issues for $500 bucks. To me, a well executed swap shows actual skill and knowledge. It's harder than people think to get a swap done. Harder than maintaining whatever is already in the car from the factory I do love V12's but oh do I love motor swapped cars just as much!

There are three basic flaws with engine swaps into Jaguars. But before I mention them I'm not a Chevy Small block hater. The one in my pickup is approaching 400,000 trouble free miles and I've owned dozens of them as well as raced them.

The three flaws are First SBC's are like belly buttons, everybody has them..

Second They don't have anything pretty, pretty costs extra and racing quality costs a lot extra

Third All V8's suffer from a second order imbalance and an exhaust stumble due the variation in firing order that has adjacent cylinders firing from 90 to 270 degrees apart.

1st. V12's are Unique, some years Jaguar barely made 1000 of them and Ferrari even fewer.

2nd. Jag V12's are all aluminum and polish up beautifully. Plus once you examine the quality of parts in a stone stock engine you'll wonder how they ever sold so cheap!!!

3rd. A 60 degree V12 is in perfect balance and the firing order compliments adjacent cylinders. As a result the wildest full tilt boogie race cam sits and idles smoothly at 650 RPM and the exhaust note sounds like it's racing at that speed because you can hear all 12 cylinders unlike a V8 where you basically hear 6 and a stumble.

With regard to costs I've bought V12's for as little as $50 and don't pay over $300 often with transmissions still mounted. (like anything else buying at a low price requires knowledge) the advantage is the demand isn't there.. everybody knows a SBC but most are afraid of a 12 (Hint, if you run out of fingers take a shoe off)

Oh yes I do agree with you completely a properly done SBC swap is hard, and maintaining one is harder than maintaining the V12 The sole difference is knowledge. I don't know why except for the monkey see monkey do factor. A lot of people have experience with Chevy's while only a tiny few are either trained or can figure out a V12. The principles are the same all you need to do is count to 12 instead of 8 Jaguar even makes it simple you just count to 6 and know the left bank from the right.

In reply to Javelin: Regarding V8's you need to explain that only some SBC's have more horsepower and very few have more torque. (the advantage of 12 cylinders over 8) If you take late 70's - early 80's SBC's you'll note even Corvette motors of the era were weak as well. As far as weight goes, Yes you have the weight of 4 extra cylinders plus the engine was designed to be taken out to over 7 litres (Big block size) but if light weight is what you want look at the inline six which is all aluminum and 4 valves per cylinder. In fact GM thought so much of that engine they bought it and put it in their SUV's. OK they increased it from 4.0 to 4.2 and put Delco stuff on it rather than Lucas. On the other hand they didn't take the 5 speed Manual Jaguar was using but saddled it with a much weaker GM automatic.

If you take the time, money, and effort used in making the swap to modify the V12 from the engine in a luxury barge to making some respectable power you'll understand why Group 44 kicked Corvettes butt when they raced

That is the bible of V12 and thanks for the link I always need to hunt it down..

In reply to justthatguy: No Wilwood doesn't make a kit (that I know about) but the calipers bolt right on and you can use the stock jaguar vented rotors or use Wilwood's pieces to swap bigger thicker rotors (and they still cost less than the factory ones..)

As for the later EDIS ones if you don't intend to go to Megasquirt then any changes will need to be done by someone familiar with modifying the system (AJ6 in England) my suggestion is bite the bullet. While I'm a luddite who prefers to work with carburetors (and wish they still had a hand crank to start) even I appreciate the ease which computers make things simple.
The earlier distributors can be made as reliable as a clock, just read Kirby's book on how to. It's really simple.

Regarding the 5.3, Well what is your goal? street, racing, a cruiser? The early ones (pre H.E. 1971-80) had the best flowing heads and they are the ones all the racers want.. however those tend to be rare. Some years fewer than 1000 were made for world wide consumption. From 1971 to 1974 the engine had carburetors. The manifolds are the problem designed to fit under the XK-E bonnet the intake charge wanders around up and down like a drunken snake. Cut off the worst of it and you pick up 50 horsepower, modify the Fuel injection manifold to take 3 Strombergs (or SU's) and you've gain 100 horsepower with the potential for more.. Unless you have a wallet that is aching to be picked don't bother with Webers. While they are wonderful eye candy getting the jetting even close will cost you more than $1000 in jets etc. and a couple of days on the dyno..

Regarding the fuel injection, AJ6 has a sweet modification that appears stock from the outside but has a significant improvement so the potential is there. As usual all out power requires a total replacement.

The earliest (1971) had 9.0 compression but then they dropped down to 7.8 and stayed that way until the H.E. of 1981 when they popped up to 11.5 but with those terrible flowing H.E heads which they kept through end of production in 1996 (exception 1980 engines not bound for the U.S.)

Frenchy I'm not making the argument I'm just giving you a reason why. Yah, everybody does it. Yah, they don't sound as nice. But, I don't think people who swap them are just too un-knowledgeable to maintain them, I just think there's lots of reasons why they do. Plus, I don't believe in "why you shouldn't swap x into y" I love motor swaps.

With that said; since my recent x308 xjr purchase, I've come across a few xj12's that really make me tingle. Just hard when I find the S600 more affordable (as relative as an affordable cheap v12 can get) but slightly less beautiful.

I do appreciate your knowledge of the jag v12 though.

I got my first speeding ticket in a late 80's V12 XJS I remember it being like a small plane, I'd love to try that car out again. This is really interesting. In a bad way.

I owned an '88 for a number of years as a DD. We've talked about this before, do a search. The Kirby Palm book linked above will tell you everything you ever need to know and then some.

frenchyd's description of the V12 reminds me of the SU carb. Based on core design principles, both read like a recipe for the planet's greatest cake; cake that then gets covered with a generous lathering of fecal matter frosting on the way to production. "Quirky" is a kind way to describe the supporting cast of parts and quality that undermine a great design. The other commonality is that it is the paying customer who must adjust to dealing with these endearing quirks...perhaps something to ponder while waiting for the tow-truck?