hey guys I was looking through forums and cant find alot about
performace modifiactions for my xj12.
what would be some good basic mods to start off with?
all your help appreciated–
series 1 xj12
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What year is your car? I’m pretty sure we are talking about a 5.3, but is it a Pre-HE?
You can start by opening up your exhaust system, add bigger sway bars, and make sure that all of your suspension components are not worn out. Add larger wheels that will accept high performance tires. You can go as high as 18" wheels with low profile tires, and have no issues.
How much money are you prepared to spend? How fast do you want to go?
Swap you automatic transmission for a manual 5 speed.
What ratio diff is in the car now? You might want to change that to something shorter.
I don’t know what you are looking to do, but the archives are loaded with information. In fact, other than repair help, a lot of the discussions pertain to performance, upgrades, etc.
A.J.On Jul 10, 2011, at 4:03 AM, s1 xj12 wrote:
hey guys I was looking through forums and cant find alot about
performace modifiactions for my xj12.what would be some good basic mods to start off with?
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In reply to a message from A.J. Simpson sent Sun 10 Jul 2011:
thanks for your reply
its a 1973 so it is a pre HE. its just got the standard diff in it
i would assume. im prepared to spend a bit of money as im planning
on having it for awhile–
series 1 xj12
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Others may know for sure, but you probably already have one of the shorter diffs in your car already.
The heads on the Pre-He are the ones to have.
Tighten up your suspension, and make sure that your cooling system is up to snuff.
The biggest bang for your buck will be had by do something with that restrictive exhaust, and converting to a 5 speed
I’d say you have the makings of a good start.
Through
A.J.On Jul 10, 2011, at 8:05 PM, s1 xj12 wrote:
In reply to a message from A.J. Simpson sent Sun 10 Jul 2011:
thanks for your reply
its a 1973 so it is a pre HE. its just got the standard diff in it
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In reply to a message from s1 xj12 sent Sun 10 Jul 2011:
How much performance do you want? There are let’s go
racing sorts of changes and hey that would be cool… types
of changes…
To begin with that’s a very heavy car, Nearly 4200#'s…
so anything done to make it lighter is going to dramatically
increase performance. However decrease the elegant nature of
the car.
The 5 speed conversion is probably the best(if you don’t
want an automatic for some reason…
The automatic you have is really the worst! Heavy, slow,
and woefully weak. You can update to a later transmission
by simply buying a rust bucket for a few hundred dollars and
swap the turbo 400. While you’re at it take the fuel
injection system and install it in your car… that’s worth
over 30hp by itself. (and better mileage)
If you want to retain the carbs there is a modification
where you simply cut off the dog leg and move the carbs up
and pick up 25 hp.
Don’t waste money on headers… The stock cast Iron
manifolds flow beautifully and the so called ‘‘headers’’
won’t gain you anything…
Dump the rest of the exhaust system though… woefully heavy
and restrictive… No you don’t need to buy those expensive
ones. A decent muffler shop can greatly improve the system…
How are your brakes? It’s cheap and easy to go with
bigger, better, lighter, calipers. (at least in the front)
Stiffer, better, front sway bar is the biggest single
handling improvement you can make. (and won’t ruin the ride
notably). Stiffer front springs will be called for if
racing is in your future…
Don’t think for a moment that new, expensive, wheels are
your only choices… Your bolt pattern is the same as
Chevy/Corvette/Camaro/Pontiac Firebird, etc. With a little
modification some of those wheels will fit and improve the
cornering.
Do not change Camshafts… Not for street use… The cam you
have now is already too big for most uses. In fact double
serious full out racing camshafts may only have .069
additional lift, and 4 degrees more duration…
That’s a camshaft designed for over 7000 rpm…
1973 is the transition year. some had 9.0-1 compression
(will have a S in engine number) and some had 7.8 -1
compression (will have an L)… Obviously 9.0 will make more
power and get fractionally better mileage. Above that is
either race gas or octane boosters.–
The original message included these comments:
performace modifiactions for my xj12.
what would be some good basic mods to start off with?
all your help appreciated
–
MGuar
Wayzata Minnesota, United States
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In reply to a message from MGuar sent Mon 11 Jul 2011:
thanks guys for all your feed back
ive only had the car for a few months so still learning about it
but looks like there will be heaps to keep me busy. I like the idea
about buying a rusty 1 and swapping parts over. fuel economy is
pretty bad but didnt buy her for that lol–
series 1 xj12
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In reply to a message from MGuar sent Mon 11 Jul 2011:
the brakes on the car are pretty bad so they will need to be
upgraded sometime. I would love to lighten the car but as u said I
dont want to decrease the elegance of the car.–
series 1 xj12
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Do as Kirbster suggests,and up grade the rear brakes with vented rotors.
A.J.On Jul 11, 2011, at 7:44 PM, s1 xj12 wrote:
In reply to a message from MGuar sent Mon 11 Jul 2011:
the brakes on the car are pretty bad so they will need to be
upgraded sometime.
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In reply to a message from s1 xj12 sent Mon 11 Jul 2011:
To do a really decent job of upgrading the brakes look at
racing stuff. Wilwood JFZ etc. all sell stuff that either
fits or can be made to work…
For the rear end the best choice is to pull the whole rear
end out of a later model sedan, XJ-S. they have outboard
brakes So those tiny little solid rotors that will cook the
differential anytime you really use them…
I can pick up whole newer rear ends at junkyards in that
$300 range That saves you a lot of suspension work and the
expense of new shocks etc… … No it’s not a straight bolt
in but it fits and will work with a little creativity… If
your front suspension is toast do the same for the front
end. Unbolt the old one and buy a new one for about $200.
You’ll get better steering, etc…
OK
Here’s stuff that will waste money…
Headers… Won’t add any power and are expensive. Racing,
reground camshafts. Stock is more than you actually need. It
puts out good bottom end power and makes a nice power curve…
Unless racing is in your future…
There is a cheap way to get about 500hp from a stock
engine for about $500. But you will have to be a good
fabricator… Aftermarket EFI units… OK if you just like to
mess with stuff… but no real power gain over stock…
You can use aftermarket alternators, starters, and A/C
compressors. Not only will you save a lot of weight. and
bulk… but have greater reliability. Don’t poo-poo the
reduced bulk. one of the real short comings of a Jaguar V12
is it’s overly tight engine compartment… Those little
peanut alternators and small A/C compressors can really
clean up the engine compartment…
One area we haven’t spoken about is smog gear… The V12
will certainly clean up a lot if it’s OK to remove it… Plus
there is a little power in doing so…
Finally, Read about the Norman Lutz cooling modification…
almost free to do and results in better cooling… In fact
the whole cooling system could use a critical eye.
Aftermarket radiators are an improvement over stock and
there are even some truck radiators that can be adapted to
work well.–
MGuar
Wayzata Minnesota, United States
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In reply to a message from MGuar sent Mon 11 Jul 2011:
I wouldn’t change the brakes without changing the wheels/tyres, as
otherwise there’d be no improvement in ultimate stopping power,
only modulation or fade resistance.
However if the brakes are currently pretty poor, as described,
there’s something wrong as they should all be able to lock the
wheels (hence the above comment).
If they can’t lock the wheels then there’s something sub-standard
about them and the first step IMO would be to make them work like
they should, then see if that’s enough with decent tyres.
But like everyone has said, before deciding tactics, work out the
strategy. If you’re not sure where you want to get to, discussing
routes is a waste of time.
Pete–
1E33100 67FHC, 1R7977 69OTS, 1E75339 Zealia/Lynx D-type
Cambridge, United Kingdom
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PeterCrespin wrote:
I wouldn’t change the brakes without changing the wheels/tyres, as
otherwise there’d be no improvement in ultimate stopping power, only
modulation or fade resistance.
Tires are all-important, for sure. I recall many years ago a road
test in a magazine where they took a VW Scirocco (sp?) and tried
different tires on it, with the end result being a car that would out-
brake and out-corner a 911! In another test, a Volvo sedan of some
sort started out with a 60mph stopping distance of something like 160
feet and was tested with various different tires until it eventually
stopped in less than 120 feet.
As far as the XJ-S is concerned, I feel the OEM brakes and tires are
pretty good. The first area for significant improvement is weight
reduction. I always recommend the upgrade to vented rotors at the
rear, but that’s not for braking performance, it’s to address oil
leaks at the diff output shaft seals. Of course, if those seals are
already leaking, vented rotors will dramatically improve braking
performance, but theoretically if the seals are not leaking the
vented rotors won’t actually change stopping distances much.
– Kirbert
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In reply to a message from Kirbert sent Tue 12 Jul 2011:
Kirbert,
There are so many later outboard braked Jags around for
near nothing that swapping out the whole rear end makes
sense to me.
You not only wind up with much better brakes but also a
rear end with more acceleration than the stock rear end and
suitable for over drive transmiossions.
No it’s not a direct bolt in but it is doable for someone
with even moderate fabrication skills…
As for tires, yes they are important, however they
shouldn’t be among the first Things installed. Tires get
harder with age and if tires are put on before the rest of
the car is ready for them. When the car really can use them
the bloom will be off the rose…
Assuming like so many of us he’s doing this on as time and
budget allows basis…–
The original message included these comments:
sort started out with a 60mph stopping distance of something like 160
feet and was tested with various different tires until it eventually
stopped in less than 120 feet.
As far as the XJ-S is concerned, I feel the OEM brakes and tires are
reduction. I always recommend the upgrade to vented rotors at the
rear, but that’s not for braking performance, it’s to address oil
leaks at the diff output shaft seals. Of course, if those seals are
–
MGuar
Wayzata Minnesota, United States
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In reply to a message from MGuar sent Tue 12 Jul 2011:
I wasn’t suggesting tyres be one of the first things installed.
Probably Kirby wasn’t either?
I was just pointing out that if anyone thinks they are going to do
wonders for a car’s stopping distance by fitting fancy brakes
working through the exact same tyre as before, they are sadly
mistaken.
Not that there aren’t shed-loads of brake ‘upgrades’ sold to such
people every day, but still…
Pete–
The original message included these comments:
As for tires, yes they are important, however theyshouldn’t be among the first Things installed. Tires get
–
1E33100 67FHC, 1R7977 69OTS, 1E75339 Zealia/Lynx D-type
Cambridge, United Kingdom
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In reply to a message from PeterCrespin sent Tue 12 Jul 2011:
Pete;
Your point is certainly valid… Well except for us racer
types…
Modern brakes will shed heat far better than the old cast
Iron beasties that Jaguar fitted… Shedding heat is required
for extreme uses such as racing etc…
Aside from that It’s extremely doubtful that given the
weight of a Jaguar the stock brakes are capable of locking
up at top speed…
Since peak braking happens just prior to lockup there can
be a really significant difference in stopping distances
between The stock Jaguar brakes in average condition and
Racing calipers with racing pads. While even they cannot
lock the wheels up at 150MPH, they will lock up far sooner
than the stock caliper…
The changes in calipers and pad science in the past 40
years is astonishing…–
The original message included these comments:
I was just pointing out that if anyone thinks they are going to do
wonders for a car’s stopping distance by fitting fancy brakes
working through the exact same tyre as before, they are sadly
mistaken.
–
MGuar
Wayzata Minnesota, United States
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In reply to a message from s1 xj12 sent Mon 11 Jul 2011:
The easiest brake upgrade is to get the front uprights and 4 pot
calipers of a S2 XJ then consider ventilated for the rear, not
cheap but effective.–
The original message included these comments:
the brakes on the car are pretty bad so they will need to be
upgraded sometime. I would love to lighten the car but as u said I
–
850225/679,1E21003,2W2001BW,JNAEY3AC100218,SAJNV4841KC156072
HEIDELBERG HEIGHTS, Australia
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Norman LUTZ wrote:
The easiest brake upgrade is to get the front uprights and 4 pot
calipers of a S2 XJ
Does the S1 still have those 3-pot calipers? Yeah, I’d upgrade those
in a heartbeat. Should be dirt cheap to pick up a set from a
junkyard since they were used in all XJ6/12/-S for many, many years.
Should bolt in, too. Note that the 4-pot calipers have metric
fittings, so Jaguar used lines that switched from Brit threads to
metric threads somewhere between the master cylinder and the front
calipers.
then consider ventilated for the rear, not cheap
but effective.
Actually, I’d argue that it is cheap. Considering the cost of a
regular rear brake rebuild with new solid rotors and caliper R&R,
it’s only marginally more expense to convert to vented rotors while
you’re at it. It is more fiddly, though, requiring a bit of thought
rather than simply bolting in new parts.
– Kirbert
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In reply to a message from s1 xj12 sent Sun 10 Jul 2011:
The first thing to do is make sure the engine is running
correctly and sound. If its bad forget everything else.
The stick shift upgrade will boost performance and economy
far more than any other thing you do.
The early 5.3 liter flat heads were good for power but bad
for fuel economy. Conversion to injection will boost the
power again.
You should be close to 300hp at this moment.
Now go for a big bore exhaust and better larger inlet
manifold butterfly from say Roger Byewater or another tuning
outfit and you should have enough power for the present.
Now replace the brakes with later ones along with adjustable
shocks and all new bushings.
Replace the radiator and mechanical fan with aluminum and
electric. Shave weight from the vehicle and you would be
surprised how much better it is.
Alex P–
alex paterson
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In reply to a message from alex paterson sent Fri 15 Jul 2011:
Good Morning Alex;
I’m in agreement with most of what you wrote… maybe a
little bit of nuance though on a couple of items…
I do agree about the 5 speed conversion being the best
value for the dollar, if you want to change the nature of
the car. An automatic is more in keeping with the luxury
nature of the Jaguar 4 door sedan. IMHO(see what I mean
about nuance?)
The flat heads don’t hurt fuel mileage as much as commonly
thought. Don’t forget when they went to the HE they made
several other changes all directed at improving fuel mileage…
I’d love to be able to state exactly what sort of fuel
mileage difference there is. I could do so exactly if I
could get flow bench numbers from a HE head. I have reverse
engineered the heads by taking the power curve,peak
HP/Torque and matching it. Then compare the BSFC of the
flathead to the HE BSFC…
The trouble is it all becomes guesswork and that shouldn’t
be accurate enough for flat statements. My best estimate
considering everything is about 1.1 MPG.
Do you remember the gentleman who put a V12 into a MGB?
His Stromberg equipped stone stock(but wounded) V12 motor
made about 300 hp once he eliminated the dog legs in the
manifold.
Finally the need for the big bore fuel Injection manifold
on a relatively stock engine… The stock fuel injection
manifold flows 1200 CFM. To make 300 horsepower less than
500 CFM is required. (something the stock carbs can easily
flow (800CFM) While it can’t hurt, the need for a big bore
inlet hasn’t been shown on a stock(ish) motor.
OH I absolutely agree on the value of a big bore exhaust
system. That can be made for modest money from any decent
muffler shop (one that doesn’t crush the tubing as it bends
them) Someday someone will flow test various mufflers and
give definitive values for mufflers. I have absolutely no
knowledge in that regard. My data collection is done with
open exhaust systems.
There are several big aluminum radiators available for
American pickup trucks that will fit nicely into the
Jaguar… The current junkyard price around here is $100.
Finally selecting shock absorbers doesn’t require the use
of adjustablity… Jounce and rebound numbers can be
relatively easily calculated thanks to modern computers and
racers. My experience is that once a good rate is
selected there is no further need to make adjustments. (It
took buying 6 aluminum bodied double adjustable Koni’s to
discover that… (That was $2400 3 decades ago)…–
The original message included these comments:
The first thing to do is make sure the engine is running
correctly and sound. If its bad forget everything else.
The stick shift upgrade will boost performance and economy
The early 5.3 liter flat heads were good for power but bad
for fuel economy. Conversion to injection will boost the
Now go for a big bore exhaust and better larger inlet
manifold butterfly from say Roger Byewater or another tuning
Now replace the brakes with later ones along with adjustable
shocks and all new bushings.
Replace the radiator and mechanical fan with aluminum and
–
MGuar
Wayzata Minnesota, United States
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In reply to a message from MGuar sent Sat 16 Jul 2011:
Hey MG how’s it going. We agree on most things I think. I
suggest the exhaust and inlet manifold changes simply
because the work together. The stock stuff will produce
300hp it where you go from there.
Perhaps the most import thing to the average Joe when buying
performance upgrades is to go to a reputable tuner and buy
the kit as a piece.
That doesn’t mean to say you cannot do it yourself but the
reality is that to get real results requires a dyno
something most people do not have access to. If you do your
own thing it can still take several iterations although
computer modeling has helped a lot in these circumstances.
Some things that seem intuitive dont work and others do so
what you pay the tuner for is his development work and not
just the pipes.
Personally I prefer the flat head engine although I’ve had
both and I agree with you that the largest improvement with
the HE came from the diff ratio rather than the engine itself.
Best wishes Alex P–
alex paterson
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In reply to a message from alex paterson sent Sun 17 Jul 2011:
I’ve been working/playing on V12’s for about 35 years now
and most of what is bought is either intended for race cars
or not really effective on the street…
Headers/tubular exhaust for example… Ask anybody with them
on and at a minimum they will tell you they are worth 25-50
hp… The truth is they are worth zero…
I saw one set of equal length headers but unless they match
a particular camshaft they too won’t gain anything… Plus
while the tubing is lighter there is so much more of it that
I suspect the weight is actually lighter with cast Iron…
The Modification Mark did on his MGBV12 is probably the
best improvement for a Carbed V12 for the street… That and
9.0 pistons… Yes in 1980 Jaguar did a 10.00 for Europe,
but while it was slightly more powerful it required gas that
simply isn’t commonly available here in America… To use
10.00 on the street would require 99 octane fuel…
Now we can get there… If you buy pump grade premium 92-93
octane… and dump in about a gallon of ethanol you will
have the required octane. However that’s not the only mod
you would need to do. If Carbed you’d need to richen the
carbs up. If injected you need to fool the computer. You
could go for slightly larger injectors, you could modify the
manifold pressure sensor to read about 5% lower pressure
(making the fuel system 5% rich), adjust the fuel pressure
regulator to 5% richer, or you could put a resistor in the
line into the ECU. (not sure what resistor to use there…)
You could safely carry enough ethanol to fill the fuel
tank 4-5 times… Put it in gallon gas cans and put those in
a water cooler filled with water. The cooler would provide
added safety in the event of a collison, and if it’s really
a bad one that turns the whole cooler into mush and breaks
the gas cans, the water would dilute any spilled alcohol
enough so it couldn’t burn…
Then if you’re going on a really long trip that will
require more alcohol than you have with you, simply stop at
any hardware or big box store and pick up the required cans
of Denatured alcohol… Since that is methanol which is 116
octane instead of the 114 octane of ethanol you’d need
slightly less.
Not only would you get the added power of the higher
compression, since alcohol carries it’s own extra oxygen
you gain the added power of that as well!
Here’s the results… Take a stock de-smoged 5.3 flat-head
and the 9.0 will make 17more hp and 15 more ft. pounds of
torque than the 7.8 low compression motor…
If you treat the fuel as mentioned enough to run 10.0
you will gain 47 more hp and 11 foot pounds of torque over
the 9.0. Now that is larger than the European engine makes
but that is because you can run additional advance. IF you
were to run the stock advance you could go to 11.00 and pick
up another 47 hp and 11 more torque… plus the power curve
becomes much broader with torque completely flat from 3600
to 4800 RPM.–
MGuar
Wayzata Minnesota, United States
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