[xj-s] V12 5.3-litre HE from US specs to EU/UK specs

XJ-S
1

hello
Im a NYC resident and my daily car is a xjs coupe 1985,
which I love. Since this car, is already free to do the
yearly emission test in NY State, and probably in most of
the other states too, I was wondering what it should be
necessary to be done in order to bring the engine from the
actual US specs to the less restrictive and happier British
specs(almost 30hp brighter). Out of the States the v12
5.3-litre HE was working at a higher compression. How did
Jaguar decreased compression on the emission engines
versions (US specs). What mechanical elements should be
modified or replaced to match EU/UK specifications?
Best regards–
volido
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2

In reply to a message from volido sent Sat 18 Jul 2015:

The green book mentions that the lower compression was by
a slight dishing in the top of the piston to reduce
compression. If you could find a engine breaker in the UK
who could find you a set of piston/liners to get it up to
12.5:1 from 11:1 should make it euro specks. The 6.0 and
5.3 have different Pistons.

Dan–
The original message included these comments:

Jaguar decreased compression on the emission engines
versions (US specs). What mechanical elements should be

–
DanS
columbus ga. usa, United States
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3

…I was wondering what it should be
necessary to be done in order to bring the engine from the
actual US specs to the less restrictive and happier British
specs(almost 30hp brighter).

Different pistons, different ECU, different downpipes, probably a few
other items. No way it’s worth it. There are better ways to spend
one’s money on this car.

– Kirbert

// please trim quoted text to context onlyOn 18 Jul 2015 at 6:24, volido wrote:

4

The green book mentions that the lower compression was by
a slight dishing in the top of the piston to reduce
compression.

I think ALL pre-H.E. pistons were dished. The differences in
compression resulted from the dimensions of the dishing.

– Kirbert

// please trim quoted text to context onlyOn 18 Jul 2015 at 16:45, DanS wrote:

5

I think ALL pre-H.E. pistons were dished. The differences in
compression resulted from the dimensions of the dishing.

Isn’t that obvious? I mean, if they weren’t the compression would be infinite.

Ed Sowell
'76 XJ-S coupe, red
http://www.efsowell.us

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6

In reply to a message from Ed Sowell sent Sun 19 Jul 2015:

Yes the pre HE were all dished due to the flat heads, the
HE all have the recessed exhaust and intake valves that
ment the piston tops were much less dished and even a flat
piston top would not produce an infinite number. But it
would be higher than the 11.5 the U.S. Engines had, also
there is a chip from Rodger Baywater that would enrichen
it. Clean out the cats and the cool air mod and you could
add thirty horses!

Dan–
The original message included these comments:

I think ALL pre-H.E. pistons were dished. The differences in
compression resulted from the dimensions of the dishing.
Isn’t that obvious? I mean, if they weren’t the compression would be infinite.

–
DanS
columbus ga. usa, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
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7

In reply to a message from DanS sent Sun 19 Jul 2015:

Dear Dan,

If you are determined to increase the compression ratio of your HE,
then you could simply skim the heads.

kind regards
Marek–
v12 E-type running MS3/3X sequential lpg and petrol
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8

I think ALL pre-H.E. pistons were dished. The differences in
compression resulted from the dimensions of the dishing.

Isn’t that obvious? I mean, if they weren’t the compression would be
infinite.

No, it wouldn’t. Norman is rebuilding engines using flattopped Chevy
305 pistons and attaining reasonable compression ratios. It all
depends on the compression height of the piston. Jaguar dished the
pistons not to obtain a particular CR but rather to achieve a desired
combustion chamber shape.

– Kirbert

// please trim quoted text to context onlyOn 19 Jul 2015 at 8:08, Ed Sowell wrote:

9

In reply to a message from Kirbert sent Mon 20 Jul 2015:

They called the dished pistons ‘‘reversed Hemi’’ with the
flat heads. (Marketing Dept.)
Wouldn’t milling the heads require a lot of valve stem work
also, and there is no going back!

Dan–
The original message included these comments:

pistons not to obtain a particular CR but rather to achieve a desired
combustion chamber shape.

–
DanS
columbus ga. usa, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
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10

Wouldn’t milling the heads require a lot of valve stem work
also, and there is no going back!

Why would it require valve stem work?

As far as going back is concerned, you could always just stack two
head gaskets in there to lower the CR. Still, I wouldn’t recommend
trying to go back; rather, just adjust the ignition timing and/or use
higher octane fuel to operate with the higher CR.

– Kirbert

// please trim quoted text to context onlyOn 20 Jul 2015 at 6:45, DanS wrote:

11

In reply to a message from volido sent Sat 18 Jul 2015:

I’m not convinced this will be a very productive route for Volido
to go down as any change in air means you’d need a change in
fuelling. Aside from an oem solution, the only cost effective way
to do that would be to fit a Megasquirt and gain full control of
the ignition and fuelling outright. You can then tune the car how
you like without having to pay a third party.

kind regards
Marek–
v12 E-type running MS3/3X sequential lpg and petrol
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12

In reply to a message from MarekH sent Tue 21 Jul 2015:

At the risk of putting my 2 cents in where it’s not needed
or wanted, Just to recap, the discussion is on the lines
of raising the compression ratio on a pre-HE V12.

I hang out at the local Jaguar shop quite a bit, and what
I’ve learned from watching is. Milling the head(s) is 1 a
point of no return, and 2 some valve / piston work will be
required. I don’t know this first hand, but from what I’ve
read, and witnessed, Jag engines (all of them) are
interference engines. (just in case you don’t know what
that means - the tolerences between valves (timing and
clearances) and pistons are, well lets just say tight.

I witnessed a XJS come in after a ‘‘valve job’’ on a flat bed
because the owner wound the engine up (under redline) and
the valves kissed the pistons. The owner reported the car
went like stink, until the unfortunate event that occurred
which ultimately landed him and the Local Jag Service Cntr.

That being said,
I would think long and hard before I’d mill a head. If it’s
too late, I would definitely revisit the valve and piston
tolerances and most importantly factor in enough tolerance
for valve float.

In my previous life, I built high performance small block
chevy engines for some local racer wanna be’s. I’ve
addressed the tolerance issues with flat top pistons a
couple of ways.

  1. change the cam to a cam with less lift, or
  2. machine space in the top of the piston for the valve.
    See the manufacturers specs as to how much can be hog’d out
    of the piston top without jeopardizing the structural
    integrity of the piston.
  3. Two head gaskets. Not recommended; almost guaranteed
    head gasket leak.

If it were me, just to save money (in the long run) and
countless hours of frustration, start saving for a
head/valve and piston assembly that already been tested.

As we all know, these engines aren’t cheap, and (in the US)
hard to come by. So breaking an internal engine part, can
be catastrophically expensive.

The milled head guy mentioned above, his total bill was
about $8,000.00 to fix and bring back to stock.

For what ever it’s worth
M–
2002 XJR-100, 1988 XJS Coupe, 1985 XJ6, 1974 XJ6
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