Only thing I ever raced was a 59 BugEye Sprite. Had a perfectly good Pontiac Catalina that I sold for the Sprite, which was my first real teardown. Never could beat the Mini Coopers.
The key is not to advertise that it's GM powered.
I know I'm an old timer when I regale my kids and grandkids about all the cars I wish I had back.

 

Just FYI: The 94 Caprice 9C1 came with LT1 engines, not LS1 engines. The LS is superior in every way.

 

This debate will live on, lots of misinformation and opinions be spoken as facts in this thread.

I'll post a link in this forum to my upcoming XJS V12 to GM V8 swap when I get started, still recovering from an elbow surgery and need to sell my Porsche and XJR to have the space. I'll be sure to document my cost and I always do clean work. I will def need yall advice in the suspension and brakes department as I love to have more brakes than I need and I dont enjoy old worn suspension on cars that are high powered.

I do plan to turbo the car after it's swapped and run it at the Texas Mile to see what it'll do.

 

If swapping a Chevy where a Jaguar had been was so simple why would it cost so much to have it professionally done? Over the years I’ve seen countless half finished jobs offered for sale or in junkyards. So let’s not encourage anybody with over simplification please?

I am anything but a purist. I’m a racer. If something is better I use it in a heart beat. Further nearly every new car I’ve bought has been a Chevy. That’s about 20 cars over my lifetime. ( the sole exception is my current Ford F-150 ) Chevy’s are good transportation modules. Better than most.

As a Jaguar racer ( and I’ve raced Corvettes too) I know how much has to be upgraded to be race worthy. Every Chevy motor I’ve built for racing has cost me a small fortune. Usually the only thing suitable that is retained for racing is the block. Very serious racers (NASCAR, and most Professional ) even replace that with specialty high nickel content or aluminum if allowed)

In racing since 1964 I have never had a Jaguar engine fail me. ( and I’ve won some serious races with them ) even my highest powered engines use mostly stock parts.

Finally with regard newer versus older, yes you are right. Newer designed engines make more power than older vintage engines. Hot rodders have been doing it forever. The Model A Got the Flathead V8 and that got replaced with a Chevy V8. If you are going to replace aV12 engine designed in the 1960’s with a newer design why not install the Jaguar V8? It comes with power levels up to 580 horsepower. Is compact and considered an up grade not lumping. If 580 horsepower isn’t needed you can match horsepower with Virtually any LS version going all the way back to the early 4.0 Jag V8. Jaguar V12’s got OBD2 about the same time ( 1992) as everyone else.However Jaguar went to EFI when Chevy was still trying to cobble up carburetors to meet emission rules.
Finally the difference between SAE and DIN is trivial. However the high horsepower engines of the 1960’s to early 1970’s were all gross or Advertized horsepower not Net or as actually installed in a car. The old 350 horsepower Chevy 350 actually nets out about 160 horsepower. While the very earliest V12 nets out as 244 horsepower and 262 once fuel injected. Ultimately reaching 318 horsepower DIN net. Due to the intended buyer there is more than 80 horsepower left on the table to ensure the engine is so quiet as to be unheard. At the time it was the most powerful engine available.

 

Absolutely correct. That was a typo on my part. And yes, I wish I had the shekels for an LS motor.

 

If you like additional power in your XJS and the V12 frightens you, seriously consider buying a Jaguar V8. Search car-parts.com and you’ll find very competitive prices with Chevy’s. Don’t forget if you intend to put a Chevy in where a Jaguar was you will need the Chevy transmission. While some Chevy’s have the same transmission as the Jaguar does the case is different and won’t bolt up to the Chevy.

 

Good info, MG, but getting another car would be hazardous to my health. If my wife did not smother me in the night, my neighbors would certainly see to a lynching. Far too many projects on the drawing board, including a kitchen remodel. You know it's bad when you have to build a 12 X 12 two story shed because you've outgrown a 3 car garage.

 

Let me begin by saying if you own a car you are absolutely free to change or modify it any way you choose. Skill, taste and judgement is a matter for others to decide but you’re just as free to ignore them as you are to change your car.

With regard to power output of a V12 compared to a newer Chevy LS. Not all make more than 318 Net horsepower. ( the last iteration of the V12 ) So once we start down that rabbit hole, shouldn’t we modify the V12 a little to see what it’s capable of?
First the intake has a reverse cone to prevent any intake noise from bothering the occupants. If that air intake remains it costs 20 net horsepower. In addition in order to get the best fuel mileage the air intake if from heated air after the radiator. Another 20 horsepower lost. Since cold air intakes are part of most modern cars fabricating one is not rocket science and 318 + 40 is a real improvement.
In addition AJ6 engineering offers a stainless steel 2&1/2 in exhaust system which is worth 35 horsepower over the stock system. Finally if a sporty exhaust is your preference the front muffler delete option is reportedly good for 27 horsepower. ( you still retain the resonators to avoid the dreaded droning)
OK ow we are at 420 net horsepower. Again call up AJ 6 and see what they can do to upgrade the stock ECM. I hesitate to recite numbers because I’ve seen too many different ones to know which is correct. Let us say 30?

Replacing a camshaft on a pushrod V8 requires just about a complete tear down of the engine. On a Jaguar the camshafts are on top and Jaguar has a bracket on each side of the engine to hold the cam sprocket out of the way so no messing with timing chains is needed. Off with the valve cover, remove the cam bearing journals, pull the cams. Align the new cams with the factory provide cam alignment tool in each tool kit and about an hour later you’re finishing up.
Kent Cams, Piper cams, in England, Isky Cams, Crower Cams in America are very familiar with what works. But I have a local cam grinder 25 minutes away and he knows as well. ( and will be much cheaper than the others). Piper cams is the best IMHO
Unlike a V8 a better camshaft isn’t lumpy and rough idling. The biggest most radicle camshaft will sit there and smoothly idle at 650 rpm just like stock.
The factory took a stock engine up to 8300RPM before the valves started to float.

Drive a V12 and because of the conservative lift ( only .375 ) and duration power starts to fall off above 5500rpm ( factory red line is 6500 rpm).
changing the camshaft to something more serious but still conservative will net you 80-100+ horsepower.
That’s 500+ Net horsepower. The XJS is a heavy lump. 4200-4600 pounds. Depending on where it’s sold and what options it has. But 500 is more than 262. And will likely cost you less than 1/2 of a swap.

 

Not getting 75HP from exhaust and air box improvements. Doesn't work this way, its not going to be 318+40 becasue jag did all those changes just to get to 318HP. On the 6.0L V12 Jaguar got rid of the trumpets (reverse cone) on the air box in favor of a larger intake pipes and made a cold air intake that pulls air from the high pressure zone in front of the radiator and the 6.0L it struggles to make 320 HP.

Uprated exhaust wont add 35HP. I conducted 3 tests last year in preparation for a track day at Limerock park Average power was collected from a Gtech Pro performance meter at night with similar air temps. For the track this is a meaningful test because they are geared so that the driver will spend most of the time between 3500 to 5500 RPM.

1. 6 speed 6.0L XJS averaged 230 HP from 2000 to 6000 RPM
2. 6 Speed 6.6L SBC averaged 340HP 2k to 6k RPM.
3. Same car as test 1, uncorked V12 with custom cat-less 2.5 inch downpipes and straight through AM DB7 Exhaust averaged 240 HP from 2000 to 6000

The Exhaust in test 3 was an Aston Martin DB7 V12 exhaust system known to support 420 HP and purchased at great expense. The AM DB7 exhaust system is straight through with no baffles. I was convinced it would yield 30 to 40 HP but it only got 10. Disappointing HP improvement but wow does it sound and drive fantastic! I was hoping for more power but what did I learn? The bottle neck has to be at the intake and intake valve flow becasue the V12 pulls hard up to 4500 RPM and falls on it face so to speak. If it was exhaust valve limited, the engine would push through and still pull albeit at a low rate of change (delta).

In the SBC and Porsche V8 world its not unheard-of for intake changes alone to yeild a 100 HP gain with supporting mods. I think the V12 really needs a better intake design from the valve back.
Another data point... I also have GTO with a 6.0L LS engine backed with a T56 and up to 4500RPM it pulls hard and feels the same as the 6.0 Jag V12 but after 4500 RPM LS keeps pulling to 6000 RPM freely.

I also made a set of long tubes headers for the SBC Chevy in another Jag to replace and Sanderson shorties, GTEK testing showed No change in average power but the long tubes did trade some top end power for low end torque , exactly what I didnt want for the track but it was too late to go back. How could this be? I'm a mild cam guy, I don't like or appreciate rowdy motors so exhaust scavenging and pulse separation from long tubes isn't all that beneficial in this scenario. Same would go for the V12 with it's mild cam and weak exhaust pocket so I learned the ideal exhaust is only worth 10 HP average on an otherwise stock 6.0 XJS with a T56. Probably 5HP with the Auto.

 

Those are interesting numbers and I accept them as valid. For the newest XKS. It’s true I’ve never messed with the 6.0 V12 although I’ve actually built one bigger out of the 5.3
There I was handicapped by the use of Weber’s which while wonderful eye candy have too short an intake tract to make real power. Group 44 solved that on the XJS to a degree by lowering the engine and modifying the hood.Besides my whole engine was seriously modified for racing only. Including the use of Piper’s billet camshafts, ported heads. Racing header sets.
The intake ports on both the HE and the earlier ( and better ) Flathead are really ugly. Both castings have ports that vary in size. Flow numbers increase drastically with just smoothing out that variation. Beyond that Rob Beere racing sells a bigger intake ( and exhaust ) valve. It still allows you to retain the stock seat but increases flow significantly.
I digress.
Since the Jaguar is so heavy. Designed as a luxury cruiser rather than a commuting car ( with a nod to performance in certain models. ) Some of that feeling has to do with the weight disadvantage. TheXJS weighs between 4200-4600 pounds according to the manual depending on equipment, when it was built, and what market it was sold into.
In race configuration it can get down to 2700 pounds. But any semblance of luxury is gone.
However newer Jaguars are much lighter.
The main reason the V12 falls on its face above 5500 RPM is the engine is designed to last nearly forever. The camshaft only opens the valves .375 (3/8ths ) while many newer engines lift the valve 1/2 inch or more. In addition to achieve good fuel mileage in a really heavy car the duration is brief compared to newer engines.
Camshaft replacement
All that can be fixed easily on a V12. Changing a camshaft on a V8 requires near total disassembly.
On a V12 once the valve covers are off the factory provides a bracket to hold tension on the timing chain requiring simple removal of the cam bearing mounts and lifting off if the base circle of the replacement camshaft is known the whole process can be done in an hour or so. Admittedly by someone who knows what he’s doing.
Oh, and I had the biggest nastiest billet camshaft Piper made and it quietly sat there at 650 RPM and was smooth as stock. (I’ll discuss why if you’re interested )

Finally depending on where you live there is an additional power gain available by the use of E85. Racers and hot rodders have known for a long time the benefits of Alcohol in power production. Indy cars have used 100% ethanol since they switched from Methanol. NASCAR uses 15% ethanol. And in Drag racing, Sprint car racing, and Top speed racing. The use of alcohol is common for those who want to win.
In the HE E85 should be worth at least 12% more power due to the high compression the factory had to retard the ignition from 38degrees BTDC to only 17 degrees. Because alcohol allows so much more advance before preignition
Now a stock Jaguar is not equipped for 85% alcohol but can be converted easily enough. Most new car’s have optional flex fuel ability. For about $100 you can put whatever you want in your tank and the computer will figure out how to adjust everything.

 

Not every way. It is very hard to get a good exhaust note out of an LS. Make them louder than stock and they all sound like a crackling farting vacuum cleaner. The old SBC has one of the best exhaust notes known to man. Having said that Chrysler and Rover/Leyland/Buick alloy V8's all sound just as good as the old Chevy V8.

 

Exhaust note is subjective, to which I do not agree with you on that but it doesn't matter because its purely opinion. I meant in measurable ways.

 

I've been wrestling with the XJS/LT1 exhaust. Got smog legal cats but they left the mufflers behind them. Then straight thru. Though the younger crowd likes it, it's too loud for my taste. I would have liked to eliminate those mufflers and found something quiet enough that could fit in that skinny but tall space where the resonators used to go. If it wasn't necessary to allow the cage to drop, I would just run 2.5 pipes under it and eliminate the reducers and all the bends. I was under the impression that the pipe over the cage is 1 7/8". The exhaust guy says he can run 2" over the cage but not 2.5.
The PO put a very impressive sound system in the car so I want to be able to hear it untainted by exhaust rumble [a little throatiness at the rear is good].
How have other resolved the backpressure vs. performance vs. noise issue.

 

well about time , these problems have been cured, we can all rest in peace!

 

Wow, this guy obviously has never done a simple XJS conversion, much less driven one, a lightweight 400hp small block Chevy fits fine with many improvements over the over engineered v12, I even ran a big block Chevy (454) but didn't like the gas mileage, so went back to a 350 with vortec heads and a cam, haven't raised the hood in 3 years, other than to change oil. The car Jaguar should have built, they already use a GM transmission, power steering pump, ac compressor, just finish it out, even room for headers.

 

Gus, you are correct on two issues: First, here in CA the referees have made all conversions extremely difficult and have become rabid about how the ECM is installed. Wiring can certainly be an issue, though there are still a few SBC conversion kits that supply the necessary harnesses. These kits also contain the hoses necessary to mate the SBC to the stock ac.
However I have to forcefully disagre with your comments about the engine compartment. For reasons I cannot explain, the dimensions just don't tell the story. You have to see them side-by-side to appreciate the difference. The 12 is literally stuffed into a compartment designed for the straight 6. The amount of plumbing required to make that motor emissions compliant occupies every square inch of that bay. By contrast, SBC conversions open up a good deal of real estate making for much neater and more welcoming layout. Changing plugs is no problem, though changing plug wires on SBCs that have the Opti-Spark system is less enviable.
What most people overlook is the vast improvement in performance and drivability that comes with replacing the Jag/GM 3 speed trans with the GM 700r4. The Jag tach can be recalibrated just by soldering in a 50 cent resistor. The speedo cable does need a readily available adapter to the 700r4.
I would dearly love to read an article written by an objective author who road tested both iterations of the XJ6. These conversions were popular enough to have supported an industry providing pros and novices making kits to facilitate them. I'd have thought that after all these years a serious comparison would exist.

 

I suspect the lions share of v8 swaps, the ones that end up working well anyway, are cars like mine where the previous two owners had given up sinking small fortunes into the v12 and sold what was still a very nice-looking car for cheap.
Mine was done in 2008 and is at 238K daily driven miles, I think it was about 100K when we got it along with a laundry list of painful looking service history including rebuilding the heads and a cylinder sleeve, along with all the other
expected ageing euro car stuff. We build cars and my father is a retired pilot/mechanic so it's all well in our reach luckily.
I'm not saying the 12 is a bad motor, but I'm thinking it likely that many of them end up being "maintained" by shops that ought not to be and costing the owner a fortune and then they give up and spend the rest of their lives
badmouthing Jaguar and/or the V12, leaving a wonderful opportunity for people to re-power them with something relatively cheap, plentiful and easy, that any idiot can work on if need be. That versus these already relatively few in number old cars having just
gone direct to the salvage yard or sitting in someone's shed or back yard rotting away, I think it's a win. Most of the time when I see these debates it's the people that aren't a fan of them looking at it from the perspective of taking a perfectly good running V12 out, when in reality it's usually
some basket case that is half disassembled and hasn't run in five years and would in all likelihood need a bloody fortune to make it do so again while still being full of possibly very expensive unknowns.
I certainly wouldn't yank a good running v12 out to put something else in but based on my experience it's hard to argue with going that way after a serious failure. It is not a $500 swap or whatever overly optimistic jazz you may here sometimes, and I can't imagine paying or trusting anyone
else to do it (and adequately document it such that you can service it years later), but if you're smart and capable it can easily be a much safer option in the long run.
This ole car with a 96 iron head LT1 and a 4L60E has been just as reliable, if not more so, and a lot less cookie cutter a daily driver than any old Mercedes or BMW or such of the era because of it and I don't think anyone would argue that it was so with the V12. And the MPG is even almost tolerable.
I see a few but not many 200K+ mile regular driven v12 cars around and to be frank those folks must have deep pockets for not only fuel but maintenance and repairs. And yes, we ditched the optispark pretty early on the LT1, it's been as boringly reliable as you'd expect after that.
Power wasn't really a goal, with a pretty free flowing exhaust and remapped ECU the old chevy has more than ample torque for regular
use. We have a Jeep pickup with an LS1 in it as well, honestly, I like the LT1 better overall when power is out of the equation, but if I do another one it'll be an LS just for availability of parts and doners. And I concur on the exhaust note of the LS being less enjoyable
as mentioned above. I think if anyone is contemplating such and doubts either their technical ability, their potential installer, or budget, they should find a later six cylinder car. I suspect you'd be gaining more than you would lose over the v12 or the v8 for regular daily use.
I still see the later 6 cars needing paint and interior work but driving well in the $2-3K range if one wanted a project

 

Your point about "I'm thinking it likely that many of them end up being "maintained" by shops that ought not to be and costing the owner a fortune and then they give up and spend the rest of their lives" is dead on. Almost nine years ago I bought .90 Majestic, when it was delivered, I went through it looking for necessary repairs. The shoddy, incompetent work performed by "a well-known, Seattle Jag shop" I found was downright disgusting. I do my own work and my Majestic remains the most reliable, durable DD I've ever owned.

 

Same thing happens to a somewhat, sometimes, lesser degree with aged Mercedes or what have you. There is just a lot to know to properly service them.
Near as I can tell from reading and research the v12 is just more to know and more parts to replace and more money than even your run of the mill european stuff,
and they depreciated to the point where people that really ought not to have owned them did and that as they say, is that.
Until someone like Roger buys them and rehabilitates them or like me and swaps to something else.
Keeps them out of the junkyard either way I suppose.

I can't count how many people I've seen buy old cars, of all types, that were completely unprepared for owning them.
Both financially and ability-wise. You've just about got to be able to read/research and do anything up to and including
serious electrical and internal engine work, or be able to afford to pay someone competent to do so, to be able to own
such and not leave it worse than when you got it. Less so now but ten or fifteen years ago it was common to see really
nice cars in salvage yards that showed up with half the wiring pulled out, several of any given control module laying around
where someone had been trying to throw parts at it, engines half disassembled, suspension wear items twice past their
service limits, etc, etc. There are very few bad cars, but a lot of bad owners. I've been in the parts and salvage business
for a long time and it's a real pet peeve of mine. It's not like the supply of these things is endless.

I bought an Alfa Romeo 164 a few years ago and drove it for about a year, it was well going that way when I got it but not too
far gone. It was expensive and took a lot of digging to find good service info and experienced practical advice but despite it's near
universal reputation for complication and unreliability and being on fire, it was fine, I repaired everything that was neglected and not working.
No worse, just different, than any of its peers of the era.
I only sold it due to a flat-out inability to get parts at the time and I couldn't stand to have something not-fixed on it.
Lovely motor, great car, as usual the market price went up 4x two days after I sold it lol.....

 

Chevy’s are fine, I’ve bought 22 new ones in my life but the engines are weak compared to the Jaguar. Others have pointed out the weakness. Stock they have less power. A lot less.
The Jaguar can be cheaply modified to make more power. Without open the engine up you can easily add 50 hp at almost no cost. But if you want you can kick the power up over 600 hp again without opening the engine.
Change the camshaft on a 350 takes a lot!! To do it on a Jag is just a matter of removing the valve covers. Takes less than 1/2 the time of a Chevy to replace both.
Yes the a accessories are big and bulky, so replace them! Tidy up the engine compartment. The engine is really beautiful once you do.
Then there is the whole question of who’s going to work on the Lumped Jaguar? Chevy dealer won’t. Jag dealer won’t.
You’ll have to find someone and let him figure everything out-•••••• on your time, at your expense.
It’s like buying an old Cadillac and putting a Chevy in it. Why?
It’s not that hard to count to 12 instead of 8.
The only thing that is really unique is the EFI. That’s hard if you are used to carbs. So put carbs on it. The factory did for the first 3 years and they are simple, - lawn mower simple and bigger than a Holley Dominator.
Spend a lot more money ( but less than an engine swap will cost) and buy the the manifold that takes 2 Holley 4 barrels. Or you can buy a set of Webers, looks crazy beautiful.
The EFI is a bit scary but have someone help hou the first time you try to figure it out. Goes a lot easier. If you really want to make extra horsepower replace the Lucas ECU with a Megasquirt Pro 3 or there is a guy in England selling reasonable plug and play units.
Jag works like a Chevy. In fact the ignition module is a Chevy with a Lucas plastic cover on it. Comes from a 1976 K1500. Pop it out of the plastic case and bring it into your local parts store.
Rock Auto has a lot of the parts needed to keep it running. They typically aren’t the quality you’ll get from places like Moss Motors or Welch.
Just don’t go to the Jag dealer for parts.