[v12-engine] efI conversion advice (73 v12)

V12-Engine
1

I am in the process of restoring my 1973 series 2 double six
which I intend on keeping and using for road trips not
racing.
The car is in original and complete condition throughout and
I want to faithfully restore the car.

I am canvasing advice which way to go with modifications
without devaluing the car. Mechanical work will be carried
out by my mechanic. The engine is tired with low vacuum and
pressure so it needs some work, and while Im at it try and
improve things.

Is there a consensus on EFI conversion for a pre HE engine?

From a purist point is it worth doing or will it devalue
the
car.
From a practical point is it less hassles balancing the
carbs, better running engine, improved economy/power or is
there more?
What is involved apart from rails, manifold, fuel pump and
computer.
Lastly are there any favoured combinations that are more or
less standards that tried and true?–
Ljo
vic, Australia
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2

In reply to a message from Ljo sent Wed 20 Apr 2011:

Ljo;
From 1976 through 1980 the early engines all had Non HE EFI…
To convert a carburated car to EFI you will need the following…
the distributor, (probably the hardest part to find*) the whole
system, all the external units suce as the computer (On early cars
it’s not really a computer), the amplifier, plus you will need the
fuel tank and fuel pump (High pressure one), fuel lines, and most
important of all a wiring harness.
While you can modify a later HE system to run your older V12 with
all those parts you will still need someone to richen the system
up since the HE runs much leaner at part throttle positions…

  • the distributor has to match the system… Non HE distributor
    with the early fuel injection system (no your’s won’t work) later
    HE’s with the distributor they were made with… (there are a
    serious bunch of variations)
    OK you will gain some power over the carbs. Minor better fuel
    mileage. (unless you make all the changes the factory did when they
    introduced the HE which include, the turbo 400 transmission 2.88
    rear end ratio, smaller injectors, (and three others That escape my
    memory right now)–
    The original message included these comments:

out by my mechanic. The engine is tired with low vacuum and
pressure so it needs some work, and while Im at it try and
improve things.
Is there a consensus on EFI conversion for a pre HE engine?
the
car.
carbs, better running engine, improved economy/power or is
there more?
What is involved apart from rails, manifold, fuel pump and
computer.

–
MGuar
Wayzata Minnesota, United States
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3

In reply to a message from Ljo sent Wed 20 Apr 2011:

Purist response:-
If you ask 20 purists for an answer, I suspect you’ll get 20
different answers, if not more.

If the modifications you make to the car are reversible and you
keep the original parts, then there is no reason to argue that the
car is devalued. For your car, it existed in both fuel injected and
non fuel injected guises, so one approach would be to source a
cheap (probably rusted out) donor, remove and renovate the ‘‘new’’
components and effectively transport your car several years into
the future. As Mguar points out the later cars had more
sophisticated fuel injection systems than the first.

Practical response:-
If you go down the route of custom fabricating elements of a newer
setup then a Jaguar bolt-in replacement, your mechanic will
probably send you a much bigger bill.

The fuel injected cars will always return better fuel economy than
carburettored cars because it is carefully metered out in all
circumstances, but people tend to understand their working less
rather than a simple carburettored setup. Have a look at the AJ6
website for a background reading as to how (and why) the v12
changed through time.

The next step then may be to look at a few more modern v12 saloons
to see where all of the bits fit. Also consider the photo albums
of various JL contributors have a wealth of detail, as does a parts
manual for a newer v12 car than yours.

kind regards
Marek–
MarekH
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4

In reply to a message from MarekH sent Wed 20 Apr 2011:

MareKH is right especially regarding purists.
One trival exception… the earliest fuel injected cars if driven
hard could return 8MPG or less. (typical was 10-12 MPG) Something
the Carbed cars if properly tuned could approach.
While later fuel injected cars cars did yield better fuel mileage
much of that was due to the modifications to a 2.88 rear end ratio,
Turbo 400 transmission instead of the cast iron old Borg Warner,
higher compression 11.0-1 compared with your 7.8-1 and smaller
injectors… (plus 2 more items the escape me)
Without the (fireball head, May head, HE head) whichever you
prefer to call it you cannot run as lean as the HE does without
some rather serious misfiring. Unless you can run that lean at
part throttle (plus the other modifications) the 18-22 miles per
gallon simply are not possible…–
The original message included these comments:

If you ask 20 purists for an answer, I suspect you’ll get 20
different answers, if not more.
If the modifications you make to the car are reversible and you
keep the original parts, then there is no reason to argue that the
car is devalued. For your car, it existed in both fuel injected and
non fuel injected guises, so one approach would be to source a
cheap (probably rusted out) donor, remove and renovate the ‘‘new’’
components and effectively transport your car several years into
probably send you a much bigger bill.
The fuel injected cars will always return better fuel economy than

–
MGuar
Wayzata Minnesota, United States
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5

Ljo wrote:

I am in the process of restoring my 1973 series 2 double six
which I intend on keeping and using for road trips not
racing.

Carbs?

Is there a consensus on EFI conversion for a pre HE engine?

You’re talking about converting from carbs to D Jetronic, the EFI
that came on the Double Six only a couple of years later? Or are you
speaking of converting to a modern EFI system such as megasquirt?

From a purist point is it worth doing or will it devalue
the
car.

You’ll probably find general agreement here that these cars are so
devalued already that it scarcely matters. The money you save in
fuel economy and maintenance alone over a couple of years is probably
more than the entire value of the car! Most Jaguar XJ owners have
already resigned themselves to being the last owner of the car, do
with it whatever makes you happy, drive it until the wheels fall off,
then have it towed to the junkyard.

From a practical point is it less hassles balancing the
carbs, better running engine, improved economy/power or is
there more?

The four carbs on the pre-EFI V12 are the very definition of hassles.
Going with EFI of any sort will leave you wondering what to do with
your weekends. However, I’d highly suggest you also consider an
ignition upgrade, at least to Crane Cams or Lucas CEI and possibly
all the way to Megasquirt or something similar. This might affect
your decision on what to do about the EFI, since going to D Jetronic
would require distributor work to install the trigger board while the
later EFI options do not.

What is involved apart from rails, manifold, fuel pump and
computer.

You probably should also consider whether you’re going to closed-loop
operation with oxygen sensors. They’re not difficult to install, any
exhaust shop can weld in bungs for the sensors themselves. The
biggest factor in that decision might be what sort of EFI you plan to
use and where do you plan to get the components. ECU’s that use
oxygen sensors might be easier/cheaper to find than ones that don’t.
I believe you’ll find that an ECU designed to run without oxygen
sensors would work just fine – good performance and fuel economy –
but using an ECU designed for use with oxygen sensors without
installing oxygen sensors would not, it’d run rich and get poor
performance and fuel economy and possibly even foul plugs.

Lastly are there any favoured combinations that are more or
less standards that tried and true?

A goodly number of people in your position would opt to swap out the
entire engine with an H.E., possibly including swapping the
transmission for a GM400. Might even go one better and install the
6.0 engine with the 4-speed auto. Either way, your fuel economy
would increase by an astounding amount, perhaps 50%, and in this day
and age of ever-increasing petrol prices we can all appreciate that.
The latter option, the 6.0, would also yield a significant
improvement in performance, although as MGuar would happily point out
you could improve the engine you’ve got if performance is what you’re
after.

As far as keeping the engine you’ve got, I think some might suggest
the simpler upgrade of swapping the Zenith-Stromberg carbs for SU’s.
I’m not entirely sure how much benefit there is to that upgrade, but
it seems to be popular.

– Kirbert

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6

In reply to a message from Kirbert sent Wed 20 Apr 2011:

Kirbert,
One issue I take exception with… The idea that carbs are fussy!
Hassles I believe you said. Even on my race car I ‘‘tune’’ the carbs
less than yearly… street cars almost never… tune them right once
and that’s really the end unless something changes.
Now I will give you a little leeway… In racing I measure air
density just prior to going out on the track and make a mixture
adjustment according to what the air desity is doing…
That ‘‘adjustment’’ takes me less than 3 minutes to perform… I
certainly could go out and race without that final ‘‘tune.’’ Most
racers don’t need to mess with their carbs if they are properly set
up… however it’s the little things that count.
Group 44 found a significant power increase when they went from
the four 1&3/4 Strombergs to the four 2inch SU’s. However in the
end Huffaker was able to make more power with the Strombergs than
group 44 did with the SU’s
At the time Group 44 was faster beacause they had the handling and
braking sorted better…
That said, with the price of fuel as a consideration the later
EFI’s make sense if you can find an inexpensive way to enrich the
fuel at part throttle… On all HE’s there is the potential for fuel
stratification which will result in lean cylinders… It’s something
Jaguar was aware of. It isn’t hypercritical on an HE engine with
it’s shrouded valves but would caused problems on the Pre HE.
Especially with those in Australia which I’m reasonably certain
were imported with 9.0-1 compression rather than the 7.8-1 here in
America…
I do agree with you that the potential for greater fuel economy is
with the later systems which controlled the fuel so much tighter…
However, realistically without the modifications that occured the
same time as the Fuel injection the numbers would be
disappointing. I would be extremely surprised if a non-HE engine
with the latest EFI could get 15 mpg.–
The original message included these comments:

You’re talking about converting from carbs to D Jetronic, the EFI
that came on the Double Six only a couple of years later? Or are you
speaking of converting to a modern EFI system such as megasquirt?
The four carbs on the pre-EFI V12 are the very definition of hassles.
Going with EFI of any sort will leave you wondering what to do with
your weekends. However, I’d highly suggest you also consider an
ignition upgrade, at least to Crane Cams or Lucas CEI and possibly
As far as keeping the engine you’ve got, I think some might suggest
the simpler upgrade of swapping the Zenith-Stromberg carbs for SU’s.
I’m not entirely sure how much benefit there is to that upgrade, but
it seems to be popular.

–
MGuar
Wayzata Minnesota, United States
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7

In reply to a message from MGuar sent Wed 20 Apr 2011:

Ljo, Have a look at my photo albums. I currently run a half way
house - with ECU controlled distributorless ignition combined with
SU carbs -

There are many different brands of aftermarket ECU out there. They
all have their strengths and weaknesses. My advice would be that if
you are not going to do the work yourself, rather than getting your
regular mechanic to do it, as competent as he/she may be, I would
go to somebody who specialises in this kind of conversion. Possibly
recommended by an ECU supplier (there are a few in Australia) and
ask to speak to a couple of existing customers for reference.

You need to be aware that you will spend more than the car is
worth, especially as you are combining this with an engine rebuild.

Under touring conditions I enjoy around 17 - 18 mpg all the time,
and will top twenty if the conditions are right. This is no doubt
helped in part by 10:1 compression. Your engine is probably 9:1,
not 7.8:1 as has been suggested. See if your engine number has the
suffix SA or SB. If so this is 9:1, if it starts with L that’s 7.8:1–
Neville S1 XJ12, Megasquirt ECU
Christchurch, New Zealand
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8

In reply to a message from Nev.W sent Wed 20 Apr 2011:

Thank you,
lots of valuable information to consider.

Having all your input is making me realise that I may not be
yet fully informed to make an educated decision.

At the moment I am leaning towards a purist approach with
the line drawn well towards no modifications and just get
the car going with an engine rebuild so I can enjoy it while
gathering more knowledge.
You have made me more aware there are many factors to
consider , and it seems not a simple decision to make in
haste.

I have read that the pre HE engine head has some advantages
over the HE but do not really understand why. There is some
suggestion that the benefits attributed to the HE engined
cars (power, efficiency etc) may in fact be due to other
factors that were mated with this engine like transmission
and gear ratios.

The factory EFI options sound like they all have their
problems and implementation difficulties and the after
market systems with computer engine management are the best
though may cost more.

I understand why the fuel pump and fuel line needs upgrading
with EFI but why do the fuel tanks need changing?
As of costs, I got the car for throw away money and a decent
service will cost more than the car so I am resigned to what
seems to be the first rule of Old Jag Ownership - spending
multiple amounts on what the car can be sold for. I am doing
it for the love of these machines and to enjoy driving them
having admiring them in my youth and only dreaming of ever
having any.

Finally Tony’s engine bay is a dream I would love to
realise, but that would have to be my 2nd project!
http://www.tonysv12jag.com/picturespage.htm--
Ljo
vic, Australia
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9

In reply to a message from Ljo sent Thu 21 Apr 2011:

Ljo,
I think you’re approaching this the correct way… Let me explain
why your heads are better than the HE heads and why they are worse.
If I may…
Look at a HE head sitting on the bench. The first thing you will
note is it has a combustion chamber… If you looked at your head
you will not see a combustion chamber your head would be completely
flat. That’s why it’s called the flathead.
OK the flaw in the HE head is the combustion chamber shrouds the
valves… It’s harder for the fuel and air to flow than the flathead
which once a valve pops open nothing is in the way…
That shrouding limits peak horsepower… max with an all out
Racing motor is around 450HP… The flathead is already past 700
Now the good things about the HE,
Because of it’s combustion shape fuel is pulled past the the
exhaust valve heating it up. As it swirls around the chamber fuel
is heavier than air and so tends to flow towards the outside of the
chamber… and right across the sparkplug… Heated fuel concentrated
over the sparkplug and you have the perfect situation for
ignition. Thus lower than normal levels of fuel will ignite. Once
lite it’s easy to burn all the fuel. However with the flat head to
ensure complete combustion it has to run richer.
So the HE will operate in a leaner condition than the flat head
will.
Next the compression, the higher the compression the more efficent
the engine… Due to it’s retard ignition, and lean burn properties
an HE can operate safely at a higher compression than the flathead
can…If you retarded the ignition of a flat head you could run with
11.0-1 compression like the HE and have similar power.
If you put the latest fuel injection system on an early flathead,
ran 11.0-1 compression ratio and did all the other mileage
improvements the flathead would get within one or two MPG of the
best HE… (and the potential for a whole lot more power)
The real reason for the HE motor was California’s smog laws.
Because it is leaner the HE is cleaner… the old flat head couldn’t
be made to pass the new smog laws coming…
Oh let me restate what has already beeen said. The best way to
update an older car is have a complete newer car to remove the
parts from. (accident damage, rust, etc)…
Because it tends to get poor fuel mileage Jaguars tend to be
junked long before they are worn out… That’s a good thing… makes
parts really cheap!
I get offered several complete cars a year and likely 5-6 engines
trans etc… Free. Hual it away, the transmission, computer,rust,
whatever is broken and it doesn’t pay to fix it. Often the owner
doesn’t want to mess with scrapping out the car… he just wants it
gone. If others know of your interest you’ll find Jaguars
multiply like rabbits. Right now if you scrap the aluminum, wiring
and catilitic converters of a Jagaur it’s worth $1000 scrap value
but it will take work and the ability to haul scrap around…–
The original message included these comments:

At the moment I am leaning towards a purist approach with
the line drawn well towards no modifications and just get
the car going with an engine rebuild so I can enjoy it while
I have read that the pre HE engine head has some advantages
over the HE but do not really understand why. There is some
suggestion that the benefits attributed to the HE engined
cars (power, efficiency etc) may in fact be due to other
factors that were mated with this engine like transmission
and gear ratios.
with EFI but why do the fuel tanks need changing?

–
MGuar
Wayzata Minnesota, United States
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10

Ljo wrote:

I have read that the pre HE engine head has some advantages
over the HE but do not really understand why. There is some
suggestion that the benefits attributed to the HE engined
cars (power, efficiency etc) may in fact be due to other
factors that were mated with this engine like transmission
and gear ratios.

The primary difference there is in compression ratio. In general,
higher compression ratio yields better efficiency, but you run into
detonation problems and have to run higher octane fuel which is
expensive. Or you retard the timing, which kills all the efficiency
gains you hoped to have. But you can get away with more advance at
part throttle. So, the H.E. runs high compression for more
efficiency, retards the timing at WOT to avoid detonation which ruins
the efficiency at WOT, but pulls that advance back in at light
throttle, yielding excellent fuel economy in normal street operation.

The higher compression also gives a hair better performance, just
enough that Jaguar could get away with installing a taller final
drive ratio which brings the performance right back down. I suspect
most owners would rather they hadn’t. Still, that’s a relatively
minor factor in the fuel efficiency; it helps, but it’s not huge.

The H.E. head primarily helps ignition and flame propagation at the
relatively lean mixtures required for good fuel economy and
emissions. Without it – for example, if a flathead were constructed
with the same 11.5:1 compression as the H.E. – it’d probably have to
run richer to avoid misfires.

– Kirbert

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11

In reply to a message from Kirbert sent Thu 21 Apr 2011:

Kerbert;
Exactly right regarding the need for the flathead to run richer to
ensure the misture would fire properly (compared to the HE)
The piston and head surface of a V12 is significantly larger than
that of a V8 Fuel on the surface does not burn and thus the V12
emits more hydrocarbons(?) then Calfornia was going to allow. Since
California is Jaguars largest sales base, failure to meet
California’s emmisson laws would have cut sales large enough to put
Jaguar’s economic viability in question…
It’s important to remember That to that point Jaguars investment
in the V12 had not paid off. Sir Lyons was approaching retirement.
Plus there was the whole issue of BLMH.
Somehow the V12 had to work and when May rejuventated the old
Chevy fireball head for use in the V12 it had to be sold not as a
way to meet emisions but as a perfornace enhancement.–
The original message included these comments:

The H.E. head primarily helps ignition and flame propagation at the
relatively lean mixtures required for good fuel economy and
emissions. Without it – for example, if a flathead were constructed
with the same 11.5:1 compression as the H.E. – it’d probably have to
run richer to avoid misfires.

–
MGuar
Wayzata Minnesota, United States
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12

In reply to a message from MGuar sent Thu 21 Apr 2011:

‘‘but why do the fuel tanks need changing’’

I converted my V12 E-type to EFI using Megasquirt2 (detail
on my web site below). The fuel tank itself does not require
changing, only the fuel pump. You have a choice between
mounting an in-line EFI fuel pump somewhere or mounting a
submersible EFI fuel pump in the tank. I would always
recommend the latter only because they are quieter than the
in-line pumps. You can see how I did it on my website below.

However, it is on my agenda to replace the current fuel pump
assembly with that from a Land Rover Discovery (95-99).
When my tank gets towards empty there are moments during
pull-away and cornering when the engine suffers fuel
starvation due to the fuel washing to one side in the tank.

BR
Philip–
My website: www.jaguardiy.net
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13

In reply to a message from jagwit sent Sat 23 Apr 2011:

Philip:
When you installed the submersible pump in the tank did you
locate it in any sort of well or baffled area?

I know most modern EFI cars with submersible pumps have the
pump sitting in a cylindrical can that sits down inside the
larger fuel tank. I used an external swirl pot or surge tank
that the EFI pump (salvaged from a XJS) pulls from. Right
now that 1qt tank is supplied by the original low pressure
fuel pump. I plan on getting rid of the low pressure pump
soon and gravity feeding the small surge tank.

You are correct about the noise from the external pump. My
Jensen-Healey with the Megasquirt EFI conversion has an
external pump and I can clearly hear it when I am sitting at
a stop light.–
The original message included these comments:

When my tank gets towards empty there are moments during
pull-away and cornering when the engine suffers fuel
starvation due to the fuel washing to one side in the tank.
Philip

–
Steven A. DuChene - XJ6VDP XJ6C XJ12C XK120SE 3.8S E-Types
Phoenix, AZ Atlanta GA, NE Oh, United States
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14

In reply to a message from StevenD57 sent Sat 23 Apr 2011:

Steven,

No well or baffled area - I was rushing the project in that
regard thinking I’ll tend to that later.

I did consider building the swirl pot from the XJS into the
car (with submersible in the swirl pot) and using the
original fuel pump to feed that but it seemed simpler to
just go with the submersible in the main tank.

It only becomes a real problem when the tank reaches halfway
between empty and 1/4.

BR
Philip–
My website: www.jaguardiy.net
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