Jaguar XJ-S. Service manual — part 81

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Unfortunately, this is not a properly-designed exhaust system. The general concensus is that the stock exhaust system
on the XJ-S is very restrictive, and the cats are a key part of that restriction. Simplest fix: take off the downpipes and
cats and bash all the innards out of them and reinstall.

Of course this will reduce the restrictions considerably, but for the optimal flow it’d be nice to have smooth pipes
instead. One possibility is to replace the US-spec downpipes with the European versions that have no catalytic
honeycombs inside. Unfortunately, they don’t have oxygen sensor ports either, and it’d be nice to maintain those.
However, oxygen sensor ports are commonly available from places like Jeg’s (page 714) or Summit (page 717) and can
be welded into the downpipes.

If you must face the emissions inspector, neither of those options will be acceptable. There are others that might be,
however. First, you could consider fitting some aftermarket catalytic convertors, preferably something broad and flat to
fit under the car and less restrictive than the originals. Or, you could consider retrofitting the later OEM cats to the
early car! According to John Napoli: “I happened to be at my local Jag dealer as someone was taking delivery of a new
pair of V12 head pipes. I noticed that these pipes did not have the corrugated metal inserts in each of the four down
tubes. These inserts are ostensibly part of the catalyst system, and are present on many V12s (including my ’82 XJ-S
H.E.). Anybody who has seen head pipes with these inserts in place would reasonably conclude that they add a lot of
restriction to the exhaust. The replacement pipes I saw today have a different shape to the cats -- more streamlined and
seemingly smaller than the cylindrical cats on the head pipes with the inserts. The parts man said that both types of
pipes are available (with the inserts and without), both are fully certified smog-wise, and that the design without the
inserts is considered a better performer because of the reduced back pressure. The downstream cats are required with
both designs. I did not inquire as to price, but it seems that anyone replacing their cats should consider these
replacements, as the Jag V12 rewards you for anything that helps it breathe better.”

EXHAUST PIPE TIPS: The XJ series Jaguars have a unique style of tip on the exhaust system, a sort of S-shaped
extension that places the outlets right out on the corners of the car. This is not a mere styling feature; these tips were
supposedly designed to correct a problem with exhaust fumes recirculating back into the car. The shape puts the outlets
out into the airstream coming around the car rather than into the dead air space behind the car. If you wish to replace
these extensions with some generic replacement tips, be prepared for fume problems when driving.

Of course, the aerodynamics are significantly different between the various XJ models and all of them have the S-
shaped tips. While the fume problem may be the case with some of them, it’s not likely to be the case with all of them.
Some people claim to have no problems, while others complain bitterly of the odors. Pay your own money, take your
own chances.

You might not really want to replace them anyway. The generic pipe tips available locally are usually cheap chrome-
plated steel, and the chrome comes off quickly and the steel underneath rusts away to nothing. The stock Jaguar tips
are made of stainless steel, so there is no plating to flake off. No matter how dirty they get, they can always be made to
look new again with a little work with some Comet cleanser.

J. C. Whitney offers stainless steel exhaust pipe tips, basically a 9” long straight piece of stainless steel pipe cut off
either straight or at an angle at the end and held in place with two setscrews. The ones to fit “1-5/8" to 1-7/8" OD pipe”
will fit the stock XJ-S muffler nicely; catalog number 12xx0949U for the straight cut end, 12xx0952Y for the angle cut
tip. Note that the outlets on the mufflers are turned down slightly, so these tips will not sit horizontally but will angle
downward a bit.

TAIL PIPES: Ernie Laprairie reports: “Just had two dual-tip Pacesetter resonators installed instead of the bulky Jag
style. Looks great and still quiet, only about 8 inches long plus chrome double tips on each side.”

ADDING A CROSSOVER PIPE: One typical performance enhancement to dual exhaust systems is to add a crossover
pipe: a connection between the two exhaust streams to permit each exhaust pulse to escape through both systems. This
idea has been used with dramatic results on V8’s, and many cars come with them from the factory.

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There may be precious little performance improvement from adding a crossover to a V12, though. First off, six
cylinders per bank results in smoother exhaust flow than four, so the pulse flow benefits of a crossover are smaller. The
exhaust pulses on a V12 alternate between banks while there are successive pulses on the same bank on a V8, which is
another reason the crossover helps the V8 so much.

There is another reason to consider a crossover, though: sound. Without a crossover, the note from each pipe is that of
a six-cylinder; with a crossover, each pipe sounds like a 12-cylinder. The Series III E-type was fitted with a crossover
pipe, and Roger Bywater claims it was strictly to make the car sound proper.

To avoid screwing up the EFI, a crossover should be installed after the oxygen sensors.

A crossover has two disadvantages: First, it can make the exhaust system, already a pain to work on, even more
difficult to assemble and disassemble. Second, being able to check the exhaust from each bank helps with engine fault
diagnosis. Both of these problems can be minimized by including a flat face flange in the crossover, and making a flat
blank-off plate that can be bolted into the connection when doing engine analysis.

Summit Racing (page 717) offers “balance tube kits” for less than $50. These require no welding to install; simply cut
sections out of each pipe, install tees with clamps, and install a connecting pipe with clamps.

MUFFLER REMOVAL: AJ6 Engineering (page 710) offers a pair of “Silencer Substitute Pipes” to eliminate the
mufflers forward of the IRS. They claim a 15-20 hp gain for this change; they also mention the inevitable change in
exhaust note.

Members of the xj-s@jag-lovers.org discussion list generally report that removing the mufflers forward of the IRS and
replacing them with straight pipes or less restrictive mufflers results in only a slight increase in sound, and the sound
that results is pleasant. However, removing the mufflers rearward of the IRS and replacing them with straight pipes or
less restrictive mufflers usually makes the car unacceptably loud, and the sound is not pleasant.

If your objective is the sound, consider modifying the intake system first as described on page 107. The sound that
results is wonderful, and only appears at serious throttle; at light throttle or idle, it makes no sound difference at all.

LARGER PIPES: AJ6 Engineering (page 710) offers a “Large Bore Exhaust System” and claims improvements of 20-
25 hp. They mention that this may only be justifiable if the entire system needs replacement anyway. Keisler (page
715) also offers a package. Both systems include downpipes that lack the catalytic convertor that’s integral with the
OEM downpipes on US-spec cars. Martin Karo says, “You can (and I did) section in a flat performance cat in the long
straight pipe section.”

Scott Horner, who has an extensively modified XJ-S, says, “I have 2-1/4" pipes the whole way from the exhaust
manifolds to the outlets. You do have to be careful adjusting these for clearance thru the cage, but I was able to bleed
the brakes at the weekend without any problems whatsoever...”

AJ6 notes the inherent difficulty with fitting anything really big through the IRS. If you’re that kinda guy, you can
forget about ground clearance and just run some big pipes under the IRS. This also eliminates several bends in the
piping, further reducing restriction. The Keisler system supposedly goes this route. Note that the IRS subframe is
mounted on rubber and moves around a bit, so pipes going under the IRS should not be attached to the IRS -- unless
attached with rubber mounts.

Roger Bywater of AJ6 Engineering adds that they also offer “a new V12 extractor exhaust system that is cheaper and
better than a conventional large bore system for road use.”

PIPE INSULATION: A performance trick popular with competitors is to thermally insulate the exhaust system. This
keeps the heat in the exhaust gases rather than permitting it to escape through the walls of the pipes and into the engine
compartment or under the car. The result is an improvement in exhaust flow, and thereby an improvement in
performance. In the case of a V12 Jaguar, there is another very significant benefit: lower underhood temperatures.

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There are a couple of ways to accomplish such thermal insulation, including wrapping the pipes, coating the pipes, and
coating the pipes on the inside. Wrapping the pipes is probably the cheapest option, but it can be difficult to
accomplish in a cramped engine compartment with convoluted manifolds. It’s also potentially dangerous. Brian
Schreurs says, “I must advise against traditional "header wraps". If oil gets on them somehow, they can catch fire. It's
happened in my F-body club.” Back in the old days, the wraps were made of asbestos. “It could be that the wrap itself
didn't catch fire; it just acted as a handy absorber for the leaking oil, and only the oil burned. I don't care to learn which
one it is on any of my cars.”

Wrapping also makes the manifolds themselves run hotter, because they’re not getting the air cooling on the outside.
This might be harder on heavy cast iron manifolds than on headers; Tom Bennett says, “The material I had thought of
using is Cool-It from Therma-tec, just read the back of the box, DO NOT USE ON CAST IRON MANIFOLDS!”

Ed Hyatt says, “According to my books on turbocharging, header wrapping is only acceptable on race cars. Because of
the additional heat stresses they place on the header, deterioration is greatly accelerated and cracks are common.
Distortion was also severe and common. It was discovered that after a header was removed, it could not be used again
because it was distorted too much.” Coating might not cause the same issues simply because it’s not as effective.
Coating on the inside has the opposite effect, helping the manifold run cooler.

As far as coatings go, the word is that there’s Jet-Hot and there are cheap imitators. Greg Peters reports from down
under: “I found a company, which is also in the US, called Jet-Hot Coatings (

www.jet-hot.com.au

). They have a

ceramic coating product that I had applied to the cast iron exhaust manifolds, engine pipes and the intermediate exhaust
pipe before the first mufflers. This coating has had a dramatic effect on the heat generated by the exhaust, because this
heat cannot escape through the metal of the pipes it is contained within the gases and exits out the back of the car. Now
the floor doesn't get hot, the AC doesn't have to work as hard and the under bonnet temperatures are now negligible.
Basically I am very happy with the result and Jet-Hot products.”

Schreurs provides the US website and additional details: “The website is

www.jet-hot.com

. Here is the idea. Instead of

wrapping headers in wraps, we get our headers coated with something (other than paint, which burns off in a couple of
minutes), generally a ceramic coating of some kind. The coating serves several purposes:

1) prevents rust; left bare, headers rust really fast.

2) controls heat by containing it, which is good in two ways:

a) by reducing the dissipated heat, underhood temps drop

b) by keeping the exhaust hot longer, it increases the velocity of the exhaust gases, the engine scavenges better.

This process has diminishing returns, there is a point where you want the exhaust to cool, but it is not in the
headers.

3) looks cool.

“Members of my F-body club have tried many different brands of coating, either by design (price sensitivity, usually)
or by happenstance (the headers occasionally come pre-coated). The only brand of coating that is consistently praised
for its quality is Jet-Hot.

“Jet-Hot takes your headers, cleans them for you (of anything -- rust, paint, previous coating, whatever), advises you if
they need repairs, then coats them inside and out. A lot of other places only coat the outside.

“Of course all this extra service, plus the quality of the product, makes it about the most expensive coating on the
market. But, where I know of several club members who had their headers recoated by Jet-Hot, I don't know of anyone
who has gone from Jet-Hot to another brand.

“It used to be that the only finish you could have was a chrome-like silver finish (except that the Jet-Hot coating doesn't
go blue like chrome does), but lately they've introduced several hues. The durability of the finish is excellent; a friend
put Jet-Hot coated headers on his daily-driver Mustang 18 months ago and they look as new.

“The cost is the only drawback. But I would strongly recommend Jet-Hot to anyone planning to keep their car a very
long time.

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“I am looking at a diagram of the H.E. exhaust system. I think I would consider coating the downpipes as well as the
manifolds, but of course the cats would have to be gutted or replaced with straight pipe -- the coating would destroy
them.”

Jeb Boyd says, “I agree with Bry in a big way. The Jet-Hot process, albeit expensive, is a proven HP gain and it drops
exhaust temperatures. It also helps everything cool off quicker upon shutdown. I've used the stuff in my shop and on
my own cars.

“It is always best done on new parts if you can, but it does work very well on used parts such as cast iron manifolds as
well.

“I have used Jet-Hot for almost ten years and have been very happy with their product and service. They turn the stuff
around in a guaranteed 72 hours, and you can get rush orders as well. I believe that they have a certain amount of
popular stock on the shelf as well.

“My MGB GT has the satin black coating on its exhaust manifold for about two and a half years and it still looks
stellar! It is expensive, but if you're an anal retentive wacko like me, there's nothing like it!

“I have had them Jet-Hot the caliper halves on a couple of cars as well. Boy, is that cool! They will also coat
suspension parts, alternator shells, and other bits.

“No interest, yadda yadda, but I love the stuff!”

Jet-Hot:

Heat management coatings
55 E. Front St. Bridgeport, PA. 19405
order: 1-800-432-3379

tech: 610-277-5646