So... a while back I picked up a 93 V12 XJS as a weekend car. My daily is an x308. What can I do to get some power out of this thing? It's one thing in my XJ8 when new Camrys are quicker but it's pretty bad in a sports car. I am looking to produce in the neighborhood of 400-425hp. Is this possible on a 6.0 HE on stock internals or am I dreaming? Also... what are the chances of hooking up a 5 speed auto like my XJ8 has?

 

First things first, the XJS was never a sports car, it was a luxury touring car, quite a difference there, But in saying that there are corporations out there that made it into a sports car, Lister comes to mind and I doubt that many of the modern cars would even in the same ballpark as one of those speed wise.
So what you have now is just like the rest of us, it is a car that was made for fine/refined smooth comfortable driving, relexing over distance and heavily detuned for comfort, forget the looks and the V12 statis, it has the breeding of a thoroughbred but is shaccled to donkey speeds.
400-425Hp is in my mind quite reasonable if not a lot more, if you where looking at 750+ well maybe that is a little more challenging but quite possible. The internals I think are fine for the HP you want, there is a lot to be made in the exhaust and heck of a lot more in the Inlet side. The big problem is there is no off the shelf fix. All this work you will have to do or get somebody to do it, it becomes expensive so nobody ever does it, the ones that do are few and far between but have magnificent results.
your gearbox is the big setback, there is plenty on the forums about exchanges, my preference is a T5 manual, but there is a discussion at pressent about the 5 speed auto. Do a little reasearch.

 

I love the Jag XJS but *personally* if I wanted a 400-425 hp car I be sorely tempted to take the easy route and simply buy one. Even the 6.0 version of the V12 leaves you 100-125 hp shy of your goal.

But, OTOH, I'm sure it's possible to do as you wish but, as mentioned, you'll be pretty much on your own. No off-the-shelf speed parts to bolt on. You might find 25-30 hp in DIY-ish breathing improvements but the next 75-100 hp might get very expensive.

Also agree that the transmission really anchors the car down....and doubly so when coupled to a 2.88 differential.

I also add this: are you sure you car is even running up-to-snuff? Many XJSs are not...and the owners don't realize it. But, even if it is, I'm sure it'll fall well short of what you're after in terms of power.


Cheers
DD

 

For the money, you will go faster by putting a 5 or 6 speed in it rather than trying to add 100hp to the valve train, which is what I am assuming you are trying to do since you said stock internals (which are forged btw)

If you are getting passed then you should just put your foot down more. The XJS is limited in its 0-60 because of the stupid 3 speed, but I would love to see a camry keep up after that. A lot of it is giving the car gears to do what it wants. I implore you to either put it in first and let the car rev til it shifts and see if you change your mind about its speed or drive it past 100 somewhere and see if that changes your tune.

You may want to mod your kick down switch, does it even work?? When you stomp on the XJS and it doesn't do much of anything, then the kickdown switch isn't doing anythinng


The v12 is an engine used in racing, but the heads were different, extremely costly to change. Even so, people do race the things today, again the biggest mod being the transmission. You can do forced induction, play with the valves or have custom cams ground, but like DD said, its all custom, which means expensive. Like Katoh said, it is a GT not a sports car. While the 3 speed hurts it a bit for this, it is mostly just in the noise and fuel department, otherwise this car will eat up miles like noones business if it is well sorted.

I used to take a fairly unused highway to see my girlfriend before I moved down, it just passes farms and has some semis on it every once in a while. Cruising between 90 and 130 makes good time.

 

Your location seems to be Tokyo, so you must have Japanese spec XJS, right?
Easy mod you can do is...

1. Remove the clutch fan and install an electric fan
2. Open up the restrictive air intake of the air cleaner box
3. Remove the mid exhaust pipe cats and change the midpipe from stock 2.125" to 2"
(Japanese spec cars have 2.125" mid pipes as stock, for the second cats are in front of the diff instead of silencers.
If you remove the downpipe cats you won't be able to pass the car inspection/Japanese MOT.
IMHO when removing the second cats 2.125" is too large. It might be good for high rev but around 2-3000rpm you will hear noisy drone noise and can't pull the torque out. Narrower pipes give you much better 2000-4500rpm response)

I did all of this, however, I guess the change is around 20ps, as Doug says.
In Japanese saying, "1ps gain equals 10000yen."
So if you want 100ps more, you will have to spend at least a million yen...
Plus, if you try such things in Japan, it will cost far more than you do it abroad.
They will charge you much more money than when they do a domestic cars just because it is a Jag.

Eventually, it might be cheaper to import a Lister 7 liter from England...

 

Gentlemen
I could not agree more with what all of you have written.
Doug you made a fantastic point about if the car is not running to snuff, love the thrase! Again I agree and I too was guilty of the same thing, untill I fixed a load of problems a mere timmed ***** then turned into a scowled cat.
Just something I would like to clear up on talking HP. Jaguar rated there out of the crate Euro spec 5.3 at 300hp, this figure I work on as a base, to squeeze another 30% to nearly 400hp should not be too difficult, Yes it is all custom work and expensive, as already stated simply because there are no off the shelf solutions. Exhaust, Inlet and heads should give you nearly all that if not more, also things like an after market ECU and even looking at better ignition such as oil packs is a big big plus.
Now I think the whole trick is getting that to the rear with minium loss, I will be the first one to say 400hp at the rear? I dont think so, without major mods, but plenty of them running around with 300hp, with drivetrain loss that puts you in the ballpark. On this subject a manual drivetrain will gobble up approx 15% but an Auto especially if it has bad gearing could be 30% +.

 

Here's how one XJS owner addressed the low power problem

Bradley Smith XJS in Detail




Cheers
DD

 

Oh, and that car is for sale.....

Jaguar 1985 XJS twin super charged


Cheers
DD

 

WOW Doug!
Far to much Hp for me, plus don't go much on the bonnet scoop, looks at little cacky.

 

I would LOVE to swap in a manual but my wife had to be able to drive it too and she has a license for auto only. >_>

Intake and exhaust is easy to take care of, and there seem to be various vendors for the ECU, but what sort of head work do you mean? From what I have read there isnt much we can do with the HE heads to get more out of them.

 

If you want substantially more power from the V12, and usable reliable power, I believe the best way is as follows:

• Change the HE heads for the pre-HE so called flathead version
• Lengthen the stroke.

The AJ6 website discusses doing this, up to 7 or more litres is quite possible and it is basically simple to do and makes very strong, useable and reliable power. Fuel consumption will be extremely poor, of course!


I completely agree that better gearboxes will make the most of what is there, but I believe stroking the flathead is the most cost effective way to extra actual BHP.
Greg

 

Hi Greg,

Is the old head really that much more capable? I thought the HE heads har larger inlet valves???

 

The old head is far easier to tune and to improve breathing. The problems of tuning the HE (which IMO is a very good road car engine) are:

• The exhaust valve cannot be enlarged significantly as it is in a deep pocket
• The inlet valve can be a little, but without better exhaust valve flow there is not much point. Also the design puts great thermal stress on the exhaust pocket, the exhaust valve and the plug, so preventing spontaneous detonation from hotspots in the combustion chamber at increased power levels is a serious problem.
• All this means that above about 5,500 RPM the 5.3 litre HE cannot flow any more air and cannot develop more power

By contrast the flathead, although not ideally shaped from the combustion chamber point of view, can have its inlet and exhaust valves very substantially enlarged. Thus it can take larger displacement as it can fill it at lower revs and also continue breathing at higher ones. The 6 litre XJS engine is a stroked 5.3 engine, and is at about the limit of what is possible with the HE head, while retaining decent (!) mileage and drivability, I believe; (always open to correction).

The TWR ETCC racing XJSs of the 1980s had flathead engines, in spite of the HE badging on the boot! All this is very clearly and interestingly explained in Allan Scott's fantastic book on the TWR racing XJSs. Really worth getting hold of a copy. Also Roger Bywater's book (the AJ6 Engineering guy) on engines is very informative and really interesting from a technical point of view. Most notably on exhaust pipe diameters and the myths surrounding same!

So the most cost effective way to get substantially more power from the V12 is by stroking the motor and by ensuring better breathing by using the pre-HE head suitably big-valved. If cost is not a problem (and sadly my last Euromillions ticket means it still is) there is a genius in Australia (another genius, the first is Grant Francis) who can put the inline 6 head, with its classic pent roof four valve combustion chamber, on the v12 and make it produce power like a Cosworth DFV. Fitting it into an XJS engine bay may be a problem mind! But AJ6 engineering's point is that about the same power can be achieved far more cheaply by a flathead and stroking strategy.
Greg

 

Cheers Greg. So the HE heads are as it seems fine for everyday driving and are good consumptionwise but are at high rpm nonideal... The pre-HE are good for tuning reasons but have a rubbish efficiency.

I think the engine has enough power. Best gain in my opinion would probably be getting better airflow to the engine... I mean best gain without altering much in the engine. I'd love to widen the intakepipes to the airfilter housings...

 

tests have shown with better inlet airflow(larger/cooler), you may get 3-4% hp more.

larger free breathing exhaust may get 10-12%(noisy tho). its an old car so may be MOT exempt.

like greg says; Allen Scott said a long stroke makes the most USABLE torque that can be done to the 5.3L engines. he also said that an 80-84mm stroke finally made good road usable torque. on the road driving nothing more annoying than to WAIT for the power to come on, by then the street race is over!

question; are you sure you have a 6L engine, not a 5.3L made in 1993

 

I haven't read the the book on TWR yet, but that's fascinating that they used the flathead in the HE cars. I remember reading once that while testing/qualifying in Bathurst they lost a motor, they found another straight out of a crate motor at a local Jag dealership, simply bolted their bits on and off they went again. Good testimonial how good these motors are.
There was a thread not so long ago where someone was re-shaping the HE head, I wonder how that ended up? It wouldn't be that hard with an NC Mill, compression loss would be my main concern.
Diam, I simply removed the front of my air boxes, drilled out the spot rivets and then they come off as a unit, just welded in a mesh to keep larger bits out and to make it keep its shape. Could be my imagination but it seems to go harder, one thing is the intake noise is load and music to my ears.
Amjin90, Why dont you simply teach your wife how to drive a manual, I think everone should have that knowledge, if used or not. The other thing is as Greg said the HE heads don't produce much power over 5500, but in a street car do you realy need that? I will love to have power from idle to 5000 that suits my why of driving, but also Im in diesel,s all day 365 days a year, the XJS is a toy driven only when time permits and that's not much.

 

katoh, thats what i mean, for street driving we need lotsa TORQUE, let the HP numbers fall were they may.

quick reference is a Formula 1 engine makes 800hp but only 250 torque.

now an Audi V12 DIESEL makes only 500hp but a real usable 700lb.ft. torque, so what would you like to drive on the street??

its all about useful-ness. HP is an arbitrary number, torque you can feel!

 

700lb lbs of torque on the street? Why would I like to drive that?
The usefulness of torque is completely limited to how much the car will stick in the back. If you can spin the wheels with 300ft/lbs the other 400 are just wasted.

The HE heads are cut for efficiency for street driving, hence the name. Changing them would require a good deal of work in machining and design, so it is easier to just slap the flat heads on and increase compression.

With the number of HE engines compared to pre-he I will probably eventually make some designs in CAD with the HE head. I have a spare HE engine, and number of people looking to mod with HE heads, it would be interesting to me. This isn't something I currently have time for, but I will definitely make posts whenever I do it. I'll upload the CAD models for free if anyone wants to take their heads to a CNC shop to get them cut.

I have never studied them, but I hear they are deeply cut, so optimizing overall efficiency while retaining enough strength and heat resistance may be hard. I don't think there isn't anything to work with, it is just beyond the scope of what most people can deal with. it is easier to just increase compression with some old heads than have someone come up with a design for HE heads. It isn't something you can mark up and just cut out with a grinder, it would be too much metal to accurately cut

 

Ron
I think its trying to get that balance of torque and Hp in the right rev range that's the challenge and yes keeping it on the ground is another. I read once and again I can not recall the author or the validity, they where talking about the 5.3 and mods and the question was posed just to increase bore size only to max, the answer back was it simply produced more torque with no gain in HP, Well this should be a good thing, No?

Its so Funny how things work out, as just this morning I was thinking how would you or what design would you put into place to alter these HE heads. I simply don't know, not my field and don't really have time to study it. But You have drawings of what to do? Could I ask Please would you send me a copy?
I should add I build CNC machines in my spare time, I have 3 of my own and I think that my Bridgeport would do a fine job of altering one of the 4 HE heads I have here.

 

I highly doubt increasing the bore gave zero increase in hp, as it is increasing displacement overall.

I haven't ever made any drawings or even though about it, I am studying mechanical engineering and imagine using it for a project or something after I take some higher level classes with thermodynamics, fluid mechanics, or some other applicable areas of study.

Like I said, I am not even sure the limitations of the head, i haven't looked at it much. I believe its efficiency is due to the swirled ports that force air into a vortex, allowing better combustion at high compression. It is designed specifically for fuel efficiency at normal speeds, which means it isn't designed for anything else. A port and polish would likely minutely increase low end torque, but hurt fuel efficiency and throttle response a good bit. I think something about the deeply cut exhaust valve limits flow high up and maxes the HE at 400-450....that is stock though. There is always a trade off with something, the V12 makes enough torque as it is that I would gear it towards making more high end power.

I am getting a decent welder soon too, so maybe I will get good enough to make so headers along with it. Another issue would be the intake, which can be limiting. If they could be updated to match any changes in the head design or valvetrain, it could all come together as a pretty good package.

Aj6 documented a tune, wider throttle bodies, and new plenums to take the 0-60 down by more than an entire second. No hp numbers, but thats a big gain.