Jaguar XJ-S. Manual - part 27

 
 

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96mm x 95mm -- almost an inch of additional stroke -- for a displacement of 8.4 liters!  This was done without 
significant modification to the block, and the stock H.E. heads were used.  At 3000 rpm, this engine produces 500 ft-lb 
of torque in naturally-aspirated form, and 820 ft-lb when supercharged as it is in the aircraft. 

According to White, that isn’t the limit by any means.  He reports that an outfit in the UK called Forward Engineering 
prepares Jaguar V12’s for use in offshore racing boats.  They install a spacer 3/4” thick between the block and the head, 
and use liners the same amount longer than stock to provide longer cylinders.  Longer studs hold the head on, and since 
3/4” is the length of a link of the timing chain the addition of four links allows an otherwise completely stock timing 
chain scheme to be used.  White says the engines so assembled are 9.3 liter, and are very successful at offshore boat 
racing. 

Changes to the engine displacement require modifying or replacing the EFI.  The stock EFI is hard to modify; it has a 
fixed (trimmable, but fixed) map for intake manifold vacuum and RPM versus fuel.  If the displacement is altered, this 
relationship changes.  One solution is an aftermarket EFI system that is completely programmable and re-
programmable (see page 309).  Most choose to ditch the EFI at this point for carburetors, introducing the problems 
outlined on page 308.  And the emissions inspector will not smile at them. 

 

LOTSA VALVES:  There have been experiments to adapt the 4-valve DOHC head from the Jaguar AJ6 engine to the 
V12.  The cylinder spacing and bolt patterns are the same -- according to Roger Bywater, not because the AJ6 was 
derived from the V12, but rather because there was a plan to put the V12’s H.E. head on some versions of the AJ6.  If 
the DOHC heads are just bolted onto the V12, on one side the intake ports will be on the outside and the exhaust ports 
will be toward the center -- not good.  So, one of the AJ6 heads must be turned around backwards -- and some 
complicated fabrication work is required to get such things as the cam sprockets rearranged.  Bill White (page 718) has 
worked with this idea, among others. 

Steve Averill reports that the Autumn 1988 issue of Jaguar Quarterly has an article on “a 60 valve DOHC V12 that was 
under development by Warrior Automotive Research.  They expected to achieve 100 bhp/litre in low tune with a 5.8l 
engine.  The head had 3 inlet & 2 exhaust valves per cylinder.”  No word on what’s happened since, but Warrior’s 
phone number was given as 061-928 3284 in Cheshire if anyone wants to try a call. 

 

BOOST:  There has been at least one experiment in Australia in turbocharging the XJ-S, but the results were apparently 
not good.  Officially the problems were blamed on the inability to assemble a drivetrain that would handle the 1000+ hp 
for more than a few seconds. 

Chris Sleeman (also in Australia) reports on a 1998 endeavor:  “When I picked up my Daimler Double Six yesterday 
from my local Jaguar specialist, he showed me an XJ-S he is working on.  It is a '76 model, with a 6.8 Litre Twin Turbo 
V12.  The motor was built by them, and the customer is apparently going to enter it into the Targa Tasmania in April.  
The motor is being dynoed next week, but they say it puts out around 700hp.  It is running 0.8 bar of boost at the 
moment, and will be using Motec injection.” 

Somewhere out there, Bradley Smith is driving around his XJ-S with twin belt-driven Whipple superchargers. 

 

NITROUS:  Martin R. Fooks has a nitrous system in his XJ-S:  “The Nitrous system was supplied by Trevor Langfield 
(page 716) and is a customized “High Power Nitrous” system.  Because of the size of the hit to the engine (150BHP) 
they installed a progressive controller, which fits neatly in my car where the trip computer used to be.  This enables me 
to control the way the extra power is delivered to the engine, such as starting power, ending power, time delay and time 
from starting level to ending level. 

“I am very happy with the Nitrous installation, which really seems to be very smooth on the V12.  Trevor’s people have 
the computer equipment to work out power and 0-60MPH times and that is where the figures came from.  As a side 
note, it ran 5.3 seconds 0-60 with a standard TH400 and only 75BHP jets in the NOS system instead of the 150 jets. 

 
 

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“Nitrous got a very bad press in the past, caused mostly by people adding too much power to their engines, or by not 
richening the fuel mixture when the NOS was injected.  The casualties in my case have been the torque converter and 
the rear IRS mounts, which were all bar 1 ripped off.”  (See page 401 regarding rear suspension mount weakness.) 

“The engine is stock except for the exhaust and intake, and I have had no problems at all with it (unlike the torque 
converter). 

“The builders of my system told me that they do not recommend putting any more than 50-75BHP into your engine 
without using a progressive system as avoidable damage to the engine could result. 

“Their main concerns with adding 150BHP were not with the engine as they believe it to be a very strong and reliable 
item.  The standard transmission and torque converter were their main objection to the increase which as it turns out 
was well founded.” 

“The amount of power provided in total by the NOS system can be changed by installing new jets at an English price of 
7 pounds per pair, so if 150BHP proves too much to handle, you can always fit a smaller pair taking very little time and 
expense.  200BHP is the maximum obtainable by my kit.  My suppliers stated to me that as a rule they do not fit 
systems with more than 50% of the original engine power (which is why I’m only using 150 Jets instead of 200).  
Obviously this is not the case if you bullet-proof your engine.” 

 

CAMSHAFT REPLACEMENT:  Most performance enthusiasts will agree that replacing the camshaft (or camshafts; 
the Jag V12 has two, some cars have four) is the most effective way to change the performance of an engine.  The 
entire personality of a car can be radically altered by merely changing the camshafts. 

Chad Bolles reports that Isky makes high performance camshafts for the Jaguar V12. 

Rob Beere Racing Services (see page 712) also offers hot cams, as well as tappet shims in extreme thicknesses that may 
be necessary for such installations. 

Note that any camshaft alterations should be accompanied by an EFI system modification; there will be increased 
airflow at wide open throttle, but since the feedback circuit is disabled under those conditions the fuel supply will 
remain at the original fixed map with no trim.  The engine will therefore run lean at full throttle, a situation that begs for 
burned pistons and valves. 

There has been some confusion regarding the difference between pre-H.E. and H.E. cams, since published valve timing 
data on the two engines seemed to differ.  However, Bywater points out that they actually use the same part number 
camshafts.  “When the V12 was launched the valve timing was quoted in Walter Hassan’s SAE paper 720163 as being 
17,59/59,17.  When the H.E. version was launched in 1981 the supplement to the manual quoted cam timing as 
13,55/55,13.  In fact, the same cam profile had been in use since the early 1970s and continued through into the 1990s 
under part numbers C42176/7, therefore all EFI V12s were produced with these cams.  Now it is not widely known that 
for some time the V12 was mildly plagued with excessive tappet noise and in the course of dealing with the problem 
the quietening ramps on the cams were altered at least once around 1972-3.  I was personally involved in an 
investigation into the causes of a spate of tappet noise around 1978 and am not aware of any cam change ever being 
made to the V12 in production for any other reason than to reduce valve gear noise. 

“Measurement of true cam timing is not a straightforward matter and it has been accepted practice to measure from the 
point where the quietening ramp ceases and the lift curve proper commences.  For those who are not familiar with the 
term, a quietening ramp is an area at the flank of the cam where the rate of lift is small, at around 0.0005” per cam 
degree, and to be fully effective must extend rather higher than the widest clearance likely to be encountered.  The idea 
is that any reasonable running clearance found in use it will always be taken up at a predetermined velocity which 
should not give rise to noise.  If the ramp geometry is changed, as we know happened on the V12, then the timing as 
measured at the top of the ramp could also vary and this accounts for the small difference of timing quoted at different 
times in its life.” 

Mike Cogswell elaborates:  “Duration itself can be misleading, since almost nobody measures duration from the instant 
the valve leaves the seat until the instant it returns.  Instead, duration is commonly measured at some point where there 

 
 

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is noticable flow, albeit typically a very low fit.  This is important, since different cam grinders measure duration at 
different lift, hence one man’s long duration might be less than another’s short duration.  Caveat emptor, as always.” 

Bywater again:  “It is perhaps of interest to note that over the years Jaguar used virtually identical valve timing on all 
their mainstream engines.  Consider the following: 

 

SS 2.5 & 3.5 litre   

16,56/56,16 

 XK 

3.4,3.8,4.2 

 15,57/57,15 

 V12 

  17,59/59,17 

(13,55/55,13) 

 

Clearly having found something that worked there was great reluctance to change from it, although the smaller 
displacement engines often had softer timing to beef-up the low speed torque as the following examples show: 

 

SS 1.5 litre 

 

10,50/50,10 

 

XK 240 saloon   

10,50/50,10 (5/16” lift) 

Note also that hot cams might not have the expected effect.  AJ6 Engineering (page 710) once offered a milder cam 
than stock, and the result was higher mid-range torque and a nearly one full second quicker 0-to-60 time.  Bywater 
explains: “...we introduced some short duration cams for the V12 back in about 1984.  They certainly boosted mid-
range performance, especially appreciated with the 3 speed BW and GM transmissions, but suffered a marketing 
problem because they gave slightly less peak power so we eventually discontinued them.  We sense that attitudes are 
changing and many drivers are realising that a performance gain around 40-70 m.p.h. is far more valuable than adding 
to a rarely seen top speed so we may well offer a modernised version of such a camshaft again in the near future.” 

 

TORQUE LINK:  When the engine/transmission turns the driveshaft, the reaction is a twisting force trying to tilt the 
engine/transmission assembly on its mounts.  There are only three mounts, two soft rubber mounts under the engine and 
the complicated spring assembly under the tranny.  The tranny mount really does little to counter this force; the torque 
is entirely taken by the two motor mounts. 

Since the mounts are soft, the torque can move the engine around quite a bit.  If the car is not stock and producing more 
torque than originally intended, the left-side mount may actually be damaged since it is put in tension under extreme 
conditions. 

A racing trick is to add a fourth connection between the engine and the chassis.  By adding a link, torque can be taken 
up before the engine moves very far or stresses the mounts too much.  Newer FWD cars are usually designed with such 
a link, but front-engine/rear-wheel-drive cars typically rely on rubber in tension. 

Under torque, the engine twists, which means the top moves to the right, the left side moves up, etc.  A link can be 
added anywhere that restricts this movement, but it is usually preferable to put it either on the left side of the engine 
connecting downward to the chassis, or from somewhere near the top of the engine connecting to the left side of the 
compartment.  Either of these locations puts the new link in tension (preferable for such parts) and helps keep the motor 
mounts in compression (protecting the rubber). 

Backyard mechanics have been known to accomplish this fix by bolting a length of chain between the left-side exhaust 
manifold and the chassis.  Under normal conditions, the chain is slack and does nothing but rattle.  When the engine 
tries to lift, the chain pulls tight and stops the motion.  This method does work, but it is hardly a suitable fix for an XJ-S; 
the chain makes too much noise, and the sudden jolt when the chain gets tight is not conducive to an impressive ride. 

To do a professional job, a better idea is to install a link made from threaded rod with some rubber bushings (available 
at any auto parts store), washers, nuts, and some fabricated brackets to provide holes for the bushings to fit into.  
Rubber bushings are essential, since a rigid connection would transmit vibration directly to the chassis.  If necessary, 
shield the rubber parts from radiant heat from the exhaust system. 

Alternatively, the ingenious mechanic may find a way to make a link from one of those FWD cars fit. 

 
 

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It should also be noted that the motor mounts on the XJ-S are not actually between the engine and the chassis, but 
between the engine and the front suspension subframe.  This provides two layers of isolation between engine vibrations 
and the chassis: the motor mounts and the subframe mounts.  Hence, adding a torque link from the engine directly to 
the chassis would defeat some of this isolation and perhaps expose the occupants to increased noise and vibration.  If 
possible, it would be preferable to connect the torque link to the subframe as well.  Or, use really soft bushings on it. 

 

Intake Upgrades

 

 

AIR FILTER REPLACEMENT:  K&N Engineering, Inc., makes permanent air filters consisting of special fabric 
sandwiched between aluminum mesh and treated with oil.  They have much less flow restriction than stock paper filters 
while providing improved filtration.  Tests on race engines show only a slight drop in power compared to no filters at 
all!  Since these filters are permanent and cleanable, they can even save money in the long run. 

The K&N part number is 33-2011 for any V12 XJ-S.  Note that stock air filters changed when the ABS brakes were 
added; since the housing shifted forward, the blank-off area over the throat had to move rearward.  The K&N filters 
have no blank-off area, so the same part number fits either application. 

Note that the 33-2011 filters will also fit most XJ12’s, but not the last couple of years.  The XJ40- and X300-based 
XJ12’s (1993-on) went to a remote filter housing similar to that used on the 6-cylinder XJ40 and X300, except that they 
have two of them -- one on each side of the car.  If your car has air filter housings that are mounted right on the 
butterfly housings and uses filters that are 16.5” x 5.75”, you need the 33-2011.  If your filters are 8.37” square, you 
need 33-2003.  The K&N catalog also lists a 33-2579 as fitting some V12’s, but this is apparently an error. 

IMPORTANT:  The K&N’s look like they can go in either way outward and either end forward, but you must install 
them with the flat side towards the butterfly.  In other words, install the filter into the cover so it sits flush with the bulk 
of the element within the cover, and then install the cover onto the engine.  Installing the other way, with the bulk of the 
filter nearer the butterfly, can cause the butterfly to get fouled with the filter element.  This would be seriously bad, 
possibly causing the throttle to jam wide open.  The concern is exacerbated with AJ6 Engineering’s enlarged butterflies, 
but there’s no reason not to be careful with the OEM butterflies. 

Needless to say, if you’ve been running these filters the wrong way around, you should clean them before reinstalling 
them correctly.  Otherwise you’ll be putting the dirty side of the filter facing the butterfly. 

Installed correctly, the K&N’s don’t seem to cause a problem with the OEM butterflies because they are pretty stiff.  If 
you want to be safe, it is a simple matter to fab a device to positively keep the filter element and the butterfly apart.  
This author used stainless steel wire 1/10” (2.5mm) thick from an old radio antenna.  Just fashion a loop in the wire to 
fit over one of the four bolts that hold the air filter housing to the butterfly housing, then a gentle arch over to the 
diagonally opposite bolt.  Make sure the butterfly moves freely underneath the arch before buttoning up the assembly.  
If the air filter element decides to distort, the arch will keep it out of the butterfly. 

The correct end of the K&N filter forward is not as critical, but you might as well get it right.  The flat edge is a bit 
longer on one end than on the other, and the longer edge goes towards the front of the car. 

Roger Bywater of AJ6 Engineering says, “...we gave up using K&N filters some time ago because they fall to bits 
around the edges where the air box clamps up.  We raised the matter with K&N Europe, for whom we do consultancy 
work on occasions, but they claim not to be aware of the problem...”  AJ6 Engineering now offers their own design 
permanent washable foam air filters. 

Glen E. MacDonald notes, “Roger Bywater gave up using K&N filters because "they fall to bits around the edges 
where the airbox clamps up".  True enough.  However, I was determined not to trash an otherwise perfectly acceptable 
(and expensive to replace) pair of filters.  The fix I found was to take four suitable lengths of windshield washer hose, 
split lengthwise.  These are then slid over the edges of the filter, after cleaning off whats left of the 'bits' Bywater refers 
to.  I made this alteration over 3 years ago and haven't had any problem since.”