Jaguar XJ-S. Manual — part 28

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Of course, K&N filters come with a “million mile warranty”, so another option would be to gripe to K&N about the
problem.

For some of us, performance is not the issue with air filters; the wear rate of an engine can be closely related to the
filtration of the air intake, and any risk of inferior filtration would not be worthwhile. K&N claims that a NASCAR
stock car fitted with its filters will outperform a car with no filters over a 500-mile event, since the deterioration of the
engine due to contaminated air over 500 miles will exceed the power loss due to the intake restriction of a K&N filter.
K&N claims excellent filtration -- although there have been some contradictory reports from other sources. K&N
filters contain a layer of cotton soaked with sticky oil; as the air passes through the cotton, particles should stick to the
oil. Such a system normally is not described as a particular size micron filter, since it really should stop all particles
regardless of size. The stock paper filter, on the other hand, is a porous media filter, and particles below a certain size
are free to pass right through.

Either K&N or the stock paper filter is probably acceptable from a filtration standpoint. However, speaking as an
engineer, I must express doubts about the filtration efficiency of most of the foam filters I have seen (I haven’t seen the
AJ6 Engineering foam filters). A foam filter works on a similar theory as the K&N, the foam providing a media for the
air to pass through that is coated with sticky oil. However, the passages between the cotton fibers on a K&N filter are
very tiny indeed, it’s hard to imagine how a speck of dust could get through without sticking, but some foam filters you
can see through. And even some of those you can’t see through appear to have passages that are quite large. And I’ve
also seen foam deteriorate, with crumbs of foam breaking off and going into the inlet. And even if the chunks of foam
don’t harm the engine, note that each chunk leaving leaves an opening for more dirt to pass through the filter
unimpeded.

OPENING UP THE AIR INTAKES: Each of the air cleaner housings has a long tapered tube with a relatively small
opening for the air intake. The purpose of the small intake opening is to accelerate the air to near Mach 1 at wide open
throttle. This prevents intake sound from coming forward through the intake. In other words, it makes the car quiet.

Unfortunately, the air is now moving fast, and there’s an air filter up ahead. If the air is allowed to simply run into the
filter, the energy associated with the speed will be wasted, and the result is a loss of pressure. Therefore, Jaguar
provides the tapered tube to gradually and efficiently slow the air down, recovering most of the energy and pressure.
Jaguar designed this tapered tube as long as they could fit under the hood.

Also unfortunately, once the air reaches Mach 1, the passage is “choked” and all the sucking the engine can manage
will not increase the amount of air flowing through it. The opening therefore forms an absolute control on the
maximum amount of air going into the engine -- and therefore the HP generated.

If you cut off this intake tube and form a large opening for the air intake into the air cleaner housing, you will eliminate
this restriction. If you interfere with the mounting of the temperature sensor in the left side intake tube, relocate it into
the housing itself (drill a hole and use a nut on the inside); it can sense the air temperature anywhere in there. Because
the EFI system detects manifold vacuum, the system will automatically compensate for the increased airflow; no tuning
modifications are required. There is no effect on emissions, so there should be no complaints from inspectors.

At part throttle (most of the time with an engine this powerful), the butterfly acts as an air-accelerating restriction, and
no sound gets out anyway. The car will sound like it did before. However, when you open it up, you will be greeted
with a sonorous growl from under the hood. This is a very sexy sound, but some Jag owners may not like it.

The performance improvement goes along with the noise. In any situation where there is still no sound, there is no
change in performance either. When the engine growls, there is more airflow than there was before.

This mod will have no effect on fuel economy, except when you hear the growl; at that point, increased fuel use
accompanies the increased airflow and increased power.

You can give this mod a trial run without much effort. Unscrew the air temperature sensor, then reconnect the wire to it
and tape it down anywhere convenient. Then, remove both air filter covers and tie the air filters in place with some
wire. This setup will provide the same performance (and a little more noise) than the intake tube removal described
above.

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OPENING UP THE AIR INTAKES - VERSION 2: AJ6 Engineering (page 710) has gone through several variations
on air filter housing intakes attempting to provide an optimum balance between unrestricted flow and noise. What they
now offer with their induction upgrade kits are replacement covers that appear largely unchanged from OEM except
that there is an additional intake opening on the bottom with a hose leading to another little bellmouth intake mounted
in the vicinity of the front anti-sway bar. Perhaps AJ6 Engineering will sell these covers separately, or perhaps one
could just adopt the idea and fab similar additional intakes.

OPENING UP THE AIR INTAKES - VERSION 3: An outfit called FasterJags offers a replacement air intake system
called the Growler. It replaces the entire air filter housing assembly with a pipe that curves forward and has a
cylindrical K&N filter mounted on the front end.

COLD AIR INTAKE: Any engineer will tell you that an internal combustion engine will run more efficiently (more
power and better fuel economy) on cooler intake air. Most automobiles nowadays (including some Jaguars) have hoses
directing cool outside air into the air cleaners rather than the hot air of the engine compartment. The XJ-S is a notable
exception; perhaps they felt that the hoses would make their engine compartment less attractive.

While opening up the intakes on the air filter housings as described above, a flange can be provided for an intake hose.
The author’s installation used 3” exhaust pipe, but 2½” pipe would work nearly as well (still four times the area of the
stock inlet) and would probably be easier to install. Intake hoses in many sizes can be found at most parts stores, and
some have built-in clamps. Remember to provide flexibility in the system, since the engine moves around on its
mounts.

All of that was easy. The hard part is routing the intake hoses somewhere. One possibility on the XJ-S is the back end
of the headlight compartments. By making a hole and providing a flange for the hose (2½” or 3” pipe again), cool air
can be routed from existing openings just inside the grille through the space behind the headlights and into the intakes.
An opening must be made in an unseen panel within the headlight compartment to permit air flow. The modification
will also require relocating the headlight relays. Be sure to clean up the intake path as well as possible when you’re
finished, and check your air filters for debris after driving a while.

On cars with ABS brakes, the air cleaner assemblies are essentially unchanged except that they are moved forward to
avoid the brake system. The same basic mod works; the intake hose just needs to be a bit shorter.

The good news here is that this mod will reduce the noise from the Version 1 mod. The intake hoses, as well as the
sheer length of the intake path, will help dampen the growl.

Since the EFI temperature sensor will detect the cooler intake air temperature, once again no tuning mods are needed.
The cooler air provides improvements at all operating conditions, and will improve fuel economy. There is even less
tendency to overheat. The only effect on emissions is a reduction in nitrogen oxide emissions.

Peyton Gill reports, “I used a Dremel to cut the horn out of the air filter cover and then rounded the “not so perfect
hole” with a grinding stone mounted in a drill press. I did not use the exhaust pipe Kirby mentioned, I went to Home
Depot and got 2 inch gray PVC couplings. The ones I got came from the electrical dept. The couplings I used have
threads on one side and open for a PVC pipe on the other. The conduit works good because you can get a “nut” that
fits the threaded side. This nut I’m talking about is more like a threaded ring, it is very common in the electrical conduit
world. The rounded hole you leave when you cut the horn out is close to the same size as the threaded side of the
coupling and the nut inside the air filter holds the coupling tightly in place. The hole I cut was not perfect so I sealed
any openings with black RTV (silicon). I did have to relocate the ballast resistor pack on the right hand side and
managed not to have to relocate the relays on the left hand side.

“I used a hole saw ( maybe 2 1/4 inch) or whatever size slightly larger than the conduit coupling threads to cut holes in
the panels behind the headlights. Since the hole was only slightly larger than the threads so they screw fairly tightly in
plus a little more RTV to be sure they hold. I got a flexible hose from Pep Boys (they have hoses made for air filter

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intakes). I took my conduit coupling with me and found a hose that fit snugly over the pipe side. The hoses have
clamps that snap into place and one of the hoses fit my conduit perfectly.

“All that was fairly easy; the good part comes when you attack the “hidden panel” described in Kirby’s book. This
panel is between the headlight opening in the front and where the hole was cut in the engine compartment. You can see
it easily enough when the headlight assembly is removed. Seeing the panel is one thing, cutting a hole in it’s another.
There is not much room to work or cut. I got a 12 inch extension for my drill and put the hole saw on this and cut a
couple 2 inch holes in the panel.

“The air temp sensor also has to be relocated. I drilled a hole in the air filter cover and placed the sensor in the direct air
path. I did not have a nut with the same size threads as the sensor so I cut the fitting off the old horn and used it as a
nut. I don’t think the threads are anything special, I just did not want to make a trip to the hardware store to get one. I
had to extend the wires a little.”

Pictures of both the author’s, Gill’s, and several other installations can be viewed at

http://www.jag-lovers.org/xj-s/book/ColdAirIntake.html

John Goodman reports that sometime in the early 90’s an access panel was provided within the front wheel wells to
enable changing the headlight bulbs without removing the assemblies from the front. On the earlier cars, such an
access wouldn’t have helped because that panel inside the compartment would have blocked access anyway. So, in
conjunction with providing the access panel, the unseen panel was also changed; it now has a “hand hole” in it. That
being the case, the task of providing this cool air intake scheme might have gotten easier, since the hand hole should
serve for an air passage.

COLD AIR INTAKE - VERSION 2: Scott Horner, apparently before he became aware of this book, designed his own
cold air intake system: “I based the cold air intake on my car on a friend’s XJ-S race car’s set-up. He has approx. 4”
tubing going into the front of the inner guards, with the air filters mounted within that panel (in front of the wheels).
But this is a bit rough for a road car as all you can hear is induction roar...

“For my car, I visited a panel beater friend to modify the guards, etc., as I wanted the mod to look as if it had come out
of the factory...

“I’ll try and describe this without pictures...We used 2 1/2” tubing which is bent to come out of the inner guard and face
up to the original air intakes, which have been cut down (with new tubing) to just a stub and attached via flexible
tubing.

“Within the guard, the tubing is bent down (to vertical) which then meets up with another piece of tubing (via flexible
hose) that is welded into the valance on the outer corners. These tube holes are cut just under the bumper, so are fairly
unobtrusive and with the angle of them look fairly sexy anyway...

“The great thing about them (apart from the cold air), is if you get someone to give the engine a rev while you’ve got
your hand over one of the intakes, you can really feel the engine sucking! There’ll be hell to pay the day I hit a
sparrow!”

COLD AIR INTAKE - BOLT-ON VERSION: John Goodman reports: “My (unmodified) ’89 XJR-S 6.0 has
different air cleaner boxes (part numbers SPE 1008 and SPE 1009 ) with large bore intakes. These take I believe the
standard air filters and fit the standard intake manifolds. In other words it’s all simple bolt-on improvements for any
V12.

“Even more interesting is a special very neatly designed radiator top crossbeam (part number SPD1164) which has
quite large smooth oval air intakes incorporated in it. The air intakes in this crossbeam line up perfectly with the air
boxes.

“It all looks very neat, but they are JaguarSport parts and won’t come cheap!!!”

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Regarding that top crossbeam SPD1164, Goodman adds: “I think the part no. has been superseded with SPD 1428.
This is the part no. listed for '93 XJR-S (American spec). The only difference AFAIK

2

is that this part has mounting

holes for the location of the Ign power amp, (necessitated because the manifolds were different) and rubber connecting
hose. There is a good picture of this component in Jaguar World vol.2 no.2; anybody handy with a welder could
modify a standard one.”

IMPROVED THROTTLE BORES -- HOME MACHINIST VERSION: If you have access to a machine shop, you
can make a minor improvement in intake airflow with little fuss. Remove the air filter housings, and remove the
butterfly housings. You will need a #30 Torx screwdriver to remove the butterfly housings. NOTE: The ports the
hoses connect to may look similar, but they may be different! Be sure to carefully record which hose connects to which
port prior to disconnecting the hoses.

Referring to Figure 8, machine a rounded edge on the inlet into the butterfly housing. This modification should not be
attempted by hand, as smoothness and consistency is important. It is important that the radius blend smoothly into the
inner surface of the passage, but it will form an edge with the air filter mounting surface.

Before starting, ensure that the machining will not interfere with the butterfly seat nor with any of the vacuum ports
near the butterfly seat; by avoiding machining more than 3/8” into the throat, problems should be averted. Also, avoid
cutting into the openings for the two bolts that hold the butterfly housing onto the manifold; while doing so wouldn’t
cause any leakage, it may cause a whistle as the air flows past the opening. Keeping the outer diameter at 2-13/16”
should be acceptable.

2

AFAIK is Internet shorthand for “As far as I know”.