[xk] 54 Engine rebuild

XK
21

Roar,

I was a little unsure when making my comment, and too lazy to check, but in
back of my mind I seem to recall that USA uses a different convention/method
of determining octane rating of fuel than is used in both England and
Australia (and other “British Colony” countries).

Equally, I believe what was standard in England in 1950s is different
methodology (thus ocatane number) than is used today, so makes it necessary
to compare apples with apples to make any sense out of this.

In the 1970s and later, in UK “FIVE-STAR” fuel was available and is what is
required/recommended for a 9:1 cr XK engine - with FIVE STAR being 100 RON
octane. I think (and our UK listers can correct me), but 4-STAR fuel was
97/98 RON and only useable without “pinging” if you retarded ignition from
optimum (with resultant overheating/fuel-consumption risk) ( I purchased my
E-Type 9:1 cr in UK in 1975 and drove it around for 9 months happily on
FIVE-STAR, but NOT on FOUR-STAR).

In today’s terms, in Australia at least, we have three main fuel qualities
available 91 RON unleaded, 95/96 RON “Premium” unleaded and 97/98 RON
“Ultimate” unleaded with anything more only available through special
racing-fuel/aviation fuel outlets or octane-enhancing additives to be added
to your fuel.

A 9:1 cr engine will operate satisfactorily on 97/98 RON “Ultimate” if
engine is carefully tuned and in good condition, or if driver just ambles
around not using full performance capability. 95/96 RON is unsuitable.

A 8:1 cr engine will operate perfectly OK on 95/96 RON under all
circumstances, or again if driver just ambles around, can get away with
using 91/92 RON.

A 7:1 cr engine totally happy with 91/92 RON.

In the real world (in Australia), all outlets tend to offer both 91/92RON
and 95/96 RON, but only selected outlets offer 97/98 RON and in country
areas (rather than major cities), 97/98 can be difficult to find.

So practical everyday usage 8:1 cr engines have no trouble sourcing 95/96
RON or if caught out can get by OK with the occasional 91/92 RON fuel.
A 9:1 cr car is constantly aware of need to access 97/98 RON fuel.

And of course there is a price premium for 97/98 RON fuel compared with the
lower-octane fuels.

If you live in cities and don’t use XK engine often, then accessing 97/98
RON is not overly taxing/expensive so liveable, but if you drive/tour in
country/remoter areas it most certainly can be a problem.

If you are saying in US that your “Premium” unleaded is 93/94 R+M/2 octane,
would suggest that your octane rating methodology is different to ours and
UK’s, and of course I have no idea what that means in practice in US re
availability of higher-octane fuel - some 20 years since I last drove
extensively in USA and that was just in “cooking” cars and not an XK.

Will have to look and see if there is some direct correlation between
different rating methods, but one thing that is not relevant to todays fuel
availability is the 1950s “Research” method ratings as quoted in Autocar
magazine.

But as before - the practical reality in Australia is 8:1 cr presents no
problems/issues with 95/96 RON fuel availability, but 9:1 cr does create
ongoing difficulties/problems, and in UK (been a few years since I was
driving/buying fuel in UK - so don’t know current situation) FIVE STAR fuel
in either no-longer available, or has limited/restricted availability, with
FOUR-STAR being the “Premium” fuel available. Our UK listers may care to
bring this comment up-to-date.

AND while talking about all of this XK engines do not in any way care
whether fuel is leaded or unleaded - it’s a matter of octane rating ONLY,
despite many vendors (in Australia at least) making money suggesting a need
to modify valves/valve seats or use additives to be “unleaded” acceptable,
although these days, problems with added 10% ethanol to 91/92 RON “unleaded”
is more of a concern than the old leaded/unleaded concerns.

Roger Payne - XK140MC OTS; E-Type 4.2 S.1 OTS; DSV8.
Canberra.-----Original Message-----
From: owner-xk@jag-lovers.org [mailto:owner-xk@jag-lovers.org] On Behalf Of
Roar Sand
Sent: Wednesday, 25 August 2010 9:00 AM
To: xk@jag-lovers.org
Subject: RE: [xk] 54 Engine rebuild

Roger,
I am a little confused by your statement:

The theoretical 5-10 hp gain is unlikely to be

achieved, if in fact a driver could actually tell anyway,
unless all other
aspects of engine and driveline condition were in 100%
as-new condition and
state-of-tune, but they would constantly have issues with
obtaining the 100
octane fuel required to optimise their 9:1 cr needs.

The report in Autocar, previously mentioned, claimed that 85 to 90 octane
fuel was needed for 9:1 compression ratio XK engimes, and you say 100.
Granted, neither are specific as to whether the numbers represent “Motor” or
“Research” method, however, my engine with 9:1 and “B” head seems to run
nicely on our unleaded premium fuel, which runs at 93 or 94 R+M/2 octane.
Admittedly I have not checked to see what basic spark advance is set at by
the shop, but I do know that the carbies are jetted on the rich side, based
on the fact that I have yet to achieve significantly over 13 MPG, or 18
liters per 100 km.
Regards,
Roar

22

Roger,
“Premium Unleaded” is universally available here in the U. S., and “Racing fuel” can be found at a few select stations.
There is no direct relationship between Research and Motor octane numbers, but, and don’t quote me on this because I am relying on my memory, always a shaky proposition, but the difference used to be around five numbers, I think.
That may be different today with very different fuel blends.
Roar— On Tue, 8/24/10, Roger Payne rogerpayne@bigblue.net.au wrote:

From: Roger Payne rogerpayne@bigblue.net.au
Subject: RE: [xk] 54 Engine rebuild
To: xk@jag-lovers.org
Date: Tuesday, August 24, 2010, 9:05 PM
Roar,

I was a little unsure when making my comment, and too lazy
to check, but in
back of my mind I seem to recall that USA uses a different
convention/method
of determining octane rating of fuel than is used in both
England and
Australia (and other “British Colony” countries).

Equally, I believe what was standard in England in 1950s is
different
methodology (thus ocatane number) than is used today, so
makes it necessary
to compare apples with apples to make any sense out of
this.

In the 1970s and later, in UK “FIVE-STAR” fuel was
available and is what is
required/recommended for a 9:1 cr XK engine - with FIVE
STAR being 100 RON
octane. I think (and our UK listers can
correct me), but 4-STAR fuel was
97/98 RON and only useable without “pinging” if you
retarded ignition from
optimum (with resultant overheating/fuel-consumption risk)
( I purchased my
E-Type 9:1 cr in UK in 1975 and drove it around for 9
months happily on
FIVE-STAR, but NOT on FOUR-STAR).

In today’s terms, in Australia at least, we have three main
fuel qualities
available 91 RON unleaded, 95/96 RON “Premium” unleaded and
97/98 RON
“Ultimate” unleaded with anything more only available
through special
racing-fuel/aviation fuel outlets or octane-enhancing
additives to be added
to your fuel.

A 9:1 cr engine will operate satisfactorily on 97/98 RON
“Ultimate” if
engine is carefully tuned and in good condition, or if
driver just ambles
around not using full performance capability. 95/96
RON is unsuitable.

A 8:1 cr engine will operate perfectly OK on 95/96 RON
under all
circumstances, or again if driver just ambles around, can
get away with
using 91/92 RON.

A 7:1 cr engine totally happy with 91/92 RON.

In the real world (in Australia), all outlets tend to offer
both 91/92RON
and 95/96 RON, but only selected outlets offer 97/98 RON
and in country
areas (rather than major cities), 97/98 can be difficult to
find.

So practical everyday usage 8:1 cr engines have no trouble
sourcing 95/96
RON or if caught out can get by OK with the occasional
91/92 RON fuel.
A 9:1 cr car is constantly aware of need to access 97/98
RON fuel.

And of course there is a price premium for 97/98 RON fuel
compared with the
lower-octane fuels.

If you live in cities and don’t use XK engine often, then
accessing 97/98
RON is not overly taxing/expensive so liveable, but if you
drive/tour in
country/remoter areas it most certainly can be a problem.

If you are saying in US that your “Premium” unleaded is
93/94 R+M/2 octane,
would suggest that your octane rating methodology is
different to ours and
UK’s, and of course I have no idea what that means in
practice in US re
availability of higher-octane fuel - some 20 years since I
last drove
extensively in USA and that was just in “cooking” cars and
not an XK.

Will have to look and see if there is some direct
correlation between
different rating methods, but one thing that is not
relevant to todays fuel
availability is the 1950s “Research” method ratings as
quoted in Autocar
magazine.

But as before - the practical reality in Australia is 8:1
cr presents no
problems/issues with 95/96 RON fuel availability, but 9:1
cr does create
ongoing difficulties/problems, and in UK (been a few years
since I was
driving/buying fuel in UK - so don’t know current
situation) FIVE STAR fuel
in either no-longer available, or has limited/restricted
availability, with
FOUR-STAR being the “Premium” fuel
available. Our UK listers may care to
bring this comment up-to-date.

AND while talking about all of this XK engines do not in
any way care
whether fuel is leaded or unleaded - it’s a matter of
octane rating ONLY,
despite many vendors (in Australia at least) making money
suggesting a need
to modify valves/valve seats or use additives to be
“unleaded” acceptable,
although these days, problems with added 10% ethanol to
91/92 RON “unleaded”
is more of a concern than the old leaded/unleaded
concerns.

Roger Payne - XK140MC OTS; E-Type 4.2 S.1 OTS; DSV8.
Canberra.

-----Original Message-----
From: owner-xk@jag-lovers.org
[mailto:owner-xk@jag-lovers.org]
On Behalf Of
Roar Sand
Sent: Wednesday, 25 August 2010 9:00 AM
To: xk@jag-lovers.org
Subject: RE: [xk] 54 Engine rebuild

Roger,
I am a little confused by your statement:

The theoretical 5-10 hp gain is unlikely to be

achieved, if in fact a driver could actually tell
anyway,
unless all other
aspects of engine and driveline condition were in
100%
as-new condition and
state-of-tune, but they would constantly have issues
with
obtaining the 100
octane fuel required to optimise their 9:1 cr needs.

The report in Autocar, previously mentioned, claimed
that 85 to 90 octane
fuel was needed for 9:1 compression ratio XK engimes, and
you say 100.
Granted, neither are specific as to whether the numbers
represent “Motor” or
“Research” method, however, my engine with 9:1 and “B” head
seems to run
nicely on our unleaded premium fuel, which runs at 93 or
94 R+M/2 octane.
Admittedly I have not checked to see what basic spark
advance is set at by
the shop, but I do know that the carbies are jetted on the
rich side, based
on the fact that I have yet to achieve significantly over
13 MPG, or 18
liters per 100 km.
Regards,
Roar

23

I cannot tell you how thrilled I am to see John Heywood cited for this
principle. Dr. Heywood was gracious enough to serve as an expert
witness in a patent infringement case for me, and I cannot say enough
about what a knowledgeable and gracious person he is. And I think you
will agree he writes clearly and persuasively.
Bill Rooklidge
etc, etc, etc

Gary Grant wrote:> In reply to a message from karl sent Tue 24 Aug 2010:

If anyone is interested, J.B. Heywood explains it a little more in
his textbook ‘‘IC Engine Fundamentals’’, page 703.

He also agrees that above 10:1, increasing compression ratio
transfers more heat to the coolant. Below 10:1, increasing
compression ratio decreases heat transfer to the coolant.

Apparently increasing compression ratio (at constant throttle
opening) changes the gas properties of the intake charge such that:

a) cylinder gas pressures and peak burned gas pressure both
increase. This would be due to the gas being compressed more
before, and during, burning, as compared to a lower compression
ratio. The more a gas is compressed, the more its temperature goes
up.
b)gas motion increases, because of increased turbulence, causing
better atomization and mixing and more complete combustion
c) combustion is faster, because of increased charge density. The
charge molecules are compressed closer to one another thus
increasing flame speed
d)the surface area/volume ratio of the combustion chamber, when the
piston is at TDC, increases, thus allowing more heat of combustion
to transfer to the cylinder head and coolant
e) the gas temperature late in the expansion stroke and during the
exhaust stroke is reduced. This is because, the more a gas expands,
the more its temperature decreases. With a higher compression
ratio, the gas expands more than with a lower compressuion ratio,
during the expansion and exhaust strokes.

The above effects a,b,c and d tend to increase heat transfer to the
coolant, while effect e tends to decrease heat transfer to the
coolant.

Apparently for compression ratios above 10:1, when you increase the
compression ratio, effects a,b,c and d are more important and
overwhelm the effect e) and heat transfer to the coolant increases.

And for compression ratios below 10:1, when compression ratio is
increased, the gas expansion cooling effect e) is more important
and overwhelms the other effects, and heat transfer decreases.

He even explains that generally head and exhaust valve temps
decrease with increasing compression ratio, due to lower expansion
stroke and exhaust stroke temperatures which cool off the valves
better. Piston and sparkplug temps increase, due to higher peak
combustion temperatures at higher compression ratios.

Once the compression ratio is increased enough to cause knocking,
the very high gas pressures and temps result in very much localized
huge increase in heat transfer to the piston and head, eventually
causing much damage, because the heat cannot be transferred away
fast enough.

–
Gary Grant S818919DN
Ottawa, Canada
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php –

24

See following copied from Wikipedia:-

Research Octane Number (RON)
The most common type of octane rating worldwide is the Research Octane
Number (RON). RON is determined by running the fuel in a test engine with a
variable compression ratio under controlled conditions, and comparing the
results with those for mixtures of iso-octane and n-heptane.

Motor Octane Number (MON)
There is another type of octane rating, called Motor Octane Number (MON), or
the aviation lean octane rating, which is a better measure of how the fuel
behaves when under load as it is done at 900 rpm instead of the 600 rpm of
the RON[2][3]. MON testing uses a similar test engine to that used in RON
testing, but with a preheated fuel mixture, a higher engine speed, and
variable ignition timing to further stress the fuel’s knock resistance.
Depending on the composition of the fuel, the MON of a modern gasoline will
be about 8 to 10 points lower than the RON. Normally, fuel specifications
require both a minimum RON and a minimum MON.

Anti-Knock Index (AKI)
In most countries, including all of those of Australia and Europe the
“headline” octane rating shown on the pump is the RON, but in Canada, the
United States and some other countries, like Brazil[4], the headline number
is the average of the RON and the MON, called the Anti-Knock Index (AKI, and
often written on pumps as (R+M)/2). It may also sometimes be called the Road
Octane Number (RdON), Pump Octane Number (PON), or (R+M)/2.

Difference between RON and AKI
Because of the 8 to 10 point difference noted above, the octane rating shown
in the United States is 4 to 5 points lower than the rating shown elsewhere
in the world for the same fuel. See the table in the following section for a
comparison.

So Roars advice of his “engine with 9:1 and “B” head seems to run nicely on
our unleaded premium fuel, which runs at 93 or 94 R+M/2 octane” would appear
to equate to our (Australian) 97/98 RON, which as advised, I find marginal,
but OK for 9:1 cr engines designed to best run on FIVE-STAR (100 RON)

Roger Payne - XK140MC OTS; E-Type 4.2 S.1 OTS; DSV8.
Canberra.-----Original Message-----
From: owner-xk@jag-lovers.org [mailto:owner-xk@jag-lovers.org] On Behalf Of
Roar Sand
Sent: Wednesday, 25 August 2010 11:23 AM
To: xk@jag-lovers.org
Subject: RE: [xk] 54 Engine rebuild

Roger,
“Premium Unleaded” is universally available here in the U. S., and “Racing
fuel” can be found at a few select stations.
There is no direct relationship between Research and Motor octane numbers,
but, and don’t quote me on this because I am relying on my memory, always a
shaky proposition, but the difference used to be around five numbers, I
think.
That may be different today with very different fuel blends.
Roar

— On Tue, 8/24/10, Roger Payne <@Roger_Payne2> wrote:

From: Roger Payne <@Roger_Payne2>
Subject: RE: [xk] 54 Engine rebuild
To: xk@jag-lovers.org
Date: Tuesday, August 24, 2010, 9:05 PM
Roar,

I was a little unsure when making my comment, and too lazy
to check, but in
back of my mind I seem to recall that USA uses a different
convention/method
of determining octane rating of fuel than is used in both
England and
Australia (and other “British Colony” countries).

Equally, I believe what was standard in England in 1950s is
different
methodology (thus ocatane number) than is used today, so
makes it necessary
to compare apples with apples to make any sense out of
this.

In the 1970s and later, in UK “FIVE-STAR” fuel was
available and is what is
required/recommended for a 9:1 cr XK engine - with FIVE
STAR being 100 RON
octane. I think (and our UK listers can
correct me), but 4-STAR fuel was
97/98 RON and only useable without “pinging” if you
retarded ignition from
optimum (with resultant overheating/fuel-consumption risk)
( I purchased my
E-Type 9:1 cr in UK in 1975 and drove it around for 9
months happily on
FIVE-STAR, but NOT on FOUR-STAR).

In today’s terms, in Australia at least, we have three main
fuel qualities
available 91 RON unleaded, 95/96 RON “Premium” unleaded and
97/98 RON
“Ultimate” unleaded with anything more only available
through special
racing-fuel/aviation fuel outlets or octane-enhancing
additives to be added
to your fuel.

A 9:1 cr engine will operate satisfactorily on 97/98 RON
“Ultimate” if
engine is carefully tuned and in good condition, or if
driver just ambles
around not using full performance capability. 95/96
RON is unsuitable.

A 8:1 cr engine will operate perfectly OK on 95/96 RON
under all
circumstances, or again if driver just ambles around, can
get away with
using 91/92 RON.

A 7:1 cr engine totally happy with 91/92 RON.

In the real world (in Australia), all outlets tend to offer
both 91/92RON
and 95/96 RON, but only selected outlets offer 97/98 RON
and in country
areas (rather than major cities), 97/98 can be difficult to
find.

So practical everyday usage 8:1 cr engines have no trouble
sourcing 95/96
RON or if caught out can get by OK with the occasional
91/92 RON fuel.
A 9:1 cr car is constantly aware of need to access 97/98
RON fuel.

And of course there is a price premium for 97/98 RON fuel
compared with the
lower-octane fuels.

If you live in cities and don’t use XK engine often, then
accessing 97/98
RON is not overly taxing/expensive so liveable, but if you
drive/tour in
country/remoter areas it most certainly can be a problem.

If you are saying in US that your “Premium” unleaded is
93/94 R+M/2 octane,
would suggest that your octane rating methodology is
different to ours and
UK’s, and of course I have no idea what that means in
practice in US re
availability of higher-octane fuel - some 20 years since I
last drove
extensively in USA and that was just in “cooking” cars and
not an XK.

Will have to look and see if there is some direct
correlation between
different rating methods, but one thing that is not
relevant to todays fuel
availability is the 1950s “Research” method ratings as
quoted in Autocar
magazine.

But as before - the practical reality in Australia is 8:1
cr presents no
problems/issues with 95/96 RON fuel availability, but 9:1
cr does create
ongoing difficulties/problems, and in UK (been a few years
since I was
driving/buying fuel in UK - so don’t know current
situation) FIVE STAR fuel
in either no-longer available, or has limited/restricted
availability, with
FOUR-STAR being the “Premium” fuel
available. Our UK listers may care to
bring this comment up-to-date.

AND while talking about all of this XK engines do not in
any way care
whether fuel is leaded or unleaded - it’s a matter of
octane rating ONLY,
despite many vendors (in Australia at least) making money
suggesting a need
to modify valves/valve seats or use additives to be
“unleaded” acceptable,
although these days, problems with added 10% ethanol to
91/92 RON “unleaded”
is more of a concern than the old leaded/unleaded
concerns.

Roger Payne - XK140MC OTS; E-Type 4.2 S.1 OTS; DSV8.
Canberra.

-----Original Message-----
From: owner-xk@jag-lovers.org
[mailto:owner-xk@jag-lovers.org]
On Behalf Of
Roar Sand
Sent: Wednesday, 25 August 2010 9:00 AM
To: xk@jag-lovers.org
Subject: RE: [xk] 54 Engine rebuild

Roger,
I am a little confused by your statement:

The theoretical 5-10 hp gain is unlikely to be

achieved, if in fact a driver could actually tell
anyway,
unless all other
aspects of engine and driveline condition were in
100%
as-new condition and
state-of-tune, but they would constantly have issues
with
obtaining the 100
octane fuel required to optimise their 9:1 cr needs.

The report in Autocar, previously mentioned, claimed
that 85 to 90 octane
fuel was needed for 9:1 compression ratio XK engimes, and
you say 100.
Granted, neither are specific as to whether the numbers
represent “Motor” or
“Research” method, however, my engine with 9:1 and “B” head
seems to run
nicely on our unleaded premium fuel, which runs at 93 or
94 R+M/2 octane.
Admittedly I have not checked to see what basic spark
advance is set at by
the shop, but I do know that the carbies are jetted on the
rich side, based
on the fact that I have yet to achieve significantly over
13 MPG, or 18
liters per 100 km.
Regards,
Roar

25

Another thing to consider, and I’m not exactly sure why this is, is that as
you move west across the US, the R+M/2 rating drops. I see 93 octane back
East, whereas here in Arizona, premium is only 91. That means that RON will
only be 95-96 here, which is well below the requirements for a 9:1 engine.
As mentioned elsewhere, every time a block is decked or a head is milled,
you increase compression. So it is very likely that compression in an
original XK head/block with a couple hundred thousand miles on it, might
already be running close to 9:1. IOW, the cheapest way to run 9:1
compression, might be to just use the stock metal head gasket.

It would be a complex calculation and perhaps someone has done it already,
but I wonder what the increase in compression is for every mm (.04" approx.)
shaved off the head. Or, perhaps you can deduce the compression ratio from a
compression test.

So Roars advice of his “engine with 9:1 and “B” head seems to run nicely on
our unleaded premium fuel, which runs at 93 or 94 R+M/2 octane” would appear
to equate to our (Australian) 97/98 RON, which as advised, I find marginal,
but OK for 9:1 cr engines designed to best run on FIVE-STAR (100 RON)

Roger Payne - XK140MC OTS; E-Type 4.2 S.1 OTS; DSV8.
Canberra.

26

Roger,

As a UK lister I agree with your summary of the UK fuel supply then & now.

I’m shortly going to rebuild a 3.8 which left the factory with 8:1 comp pistons.
I considered upping the spec to 9.1 pistons but the problem as you and others correctly identify is the availability of modern low octane fuels.

I suspect that any gain from the increase in compression ration results in an overall reduction in power once the timing is sufficiently retarded to prevent pinking.
It is not unknown for pistons to melt due to poor fuel and over advanced timing producing pre-detonation.
Anyone who has seen an engine ‘running on’ should be familiar with the problem and dangers.

DJOn 25 Aug 2010, at 02:05, Roger Payne wrote:

Roar,

I was a little unsure when making my comment, and too lazy to check, but in
back of my mind I seem to recall that USA uses a different convention/method
of determining octane rating of fuel than is used in both England and
Australia (and other “British Colony” countries).

Equally, I believe what was standard in England in 1950s is different
methodology (thus ocatane number) than is used today, so makes it necessary
to compare apples with apples to make any sense out of this.

In the 1970s and later, in UK “FIVE-STAR” fuel was available and is what is
required/recommended for a 9:1 cr XK engine - with FIVE STAR being 100 RON
octane. I think (and our UK listers can correct me), but 4-STAR fuel was
97/98 RON and only useable without “pinging” if you retarded ignition from
optimum (with resultant overheating/fuel-consumption risk) ( I purchased my
E-Type 9:1 cr in UK in 1975 and drove it around for 9 months happily on
FIVE-STAR, but NOT on FOUR-STAR).

In today’s terms, in Australia at least, we have three main fuel qualities
available 91 RON unleaded, 95/96 RON “Premium” unleaded and 97/98 RON
“Ultimate” unleaded with anything more only available through special
racing-fuel/aviation fuel outlets or octane-enhancing additives to be added
to your fuel.

A 9:1 cr engine will operate satisfactorily on 97/98 RON “Ultimate” if
engine is carefully tuned and in good condition, or if driver just ambles
around not using full performance capability. 95/96 RON is unsuitable.

A 8:1 cr engine will operate perfectly OK on 95/96 RON under all
circumstances, or again if driver just ambles around, can get away with
using 91/92 RON.

A 7:1 cr engine totally happy with 91/92 RON.

In the real world (in Australia), all outlets tend to offer both 91/92RON
and 95/96 RON, but only selected outlets offer 97/98 RON and in country
areas (rather than major cities), 97/98 can be difficult to find.

So practical everyday usage 8:1 cr engines have no trouble sourcing 95/96
RON or if caught out can get by OK with the occasional 91/92 RON fuel.
A 9:1 cr car is constantly aware of need to access 97/98 RON fuel.

And of course there is a price premium for 97/98 RON fuel compared with the
lower-octane fuels.

If you live in cities and don’t use XK engine often, then accessing 97/98
RON is not overly taxing/expensive so liveable, but if you drive/tour in
country/remoter areas it most certainly can be a problem.

If you are saying in US that your “Premium” unleaded is 93/94 R+M/2 octane,
would suggest that your octane rating methodology is different to ours and
UK’s, and of course I have no idea what that means in practice in US re
availability of higher-octane fuel - some 20 years since I last drove
extensively in USA and that was just in “cooking” cars and not an XK.

Will have to look and see if there is some direct correlation between
different rating methods, but one thing that is not relevant to todays fuel
availability is the 1950s “Research” method ratings as quoted in Autocar
magazine.

But as before - the practical reality in Australia is 8:1 cr presents no
problems/issues with 95/96 RON fuel availability, but 9:1 cr does create
ongoing difficulties/problems, and in UK (been a few years since I was
driving/buying fuel in UK - so don’t know current situation) FIVE STAR fuel
in either no-longer available, or has limited/restricted availability, with
FOUR-STAR being the “Premium” fuel available. Our UK listers may care to
bring this comment up-to-date.

AND while talking about all of this XK engines do not in any way care
whether fuel is leaded or unleaded - it’s a matter of octane rating ONLY,
despite many vendors (in Australia at least) making money suggesting a need
to modify valves/valve seats or use additives to be “unleaded” acceptable,
although these days, problems with added 10% ethanol to 91/92 RON “unleaded”
is more of a concern than the old leaded/unleaded concerns.

Roger Payne - XK140MC OTS; E-Type 4.2 S.1 OTS; DSV8.
Canberra.

-----Original Message-----
From: owner-xk@jag-lovers.org [mailto:owner-xk@jag-lovers.org] On Behalf Of
Roar Sand
Sent: Wednesday, 25 August 2010 9:00 AM
To: xk@jag-lovers.org
Subject: RE: [xk] 54 Engine rebuild

Roger,
I am a little confused by your statement:

The theoretical 5-10 hp gain is unlikely to be

achieved, if in fact a driver could actually tell anyway,
unless all other
aspects of engine and driveline condition were in 100%
as-new condition and
state-of-tune, but they would constantly have issues with
obtaining the 100
octane fuel required to optimise their 9:1 cr needs.

The report in Autocar, previously mentioned, claimed that 85 to 90 octane
fuel was needed for 9:1 compression ratio XK engimes, and you say 100.
Granted, neither are specific as to whether the numbers represent “Motor” or
“Research” method, however, my engine with 9:1 and “B” head seems to run
nicely on our unleaded premium fuel, which runs at 93 or 94 R+M/2 octane.
Admittedly I have not checked to see what basic spark advance is set at by
the shop, but I do know that the carbies are jetted on the rich side, based
on the fact that I have yet to achieve significantly over 13 MPG, or 18
liters per 100 km.
Regards,
Roar

27

When the 9:1 engine of my 3.4L 150 S needed to be rebuilt, I got advice to
put 8:1 pistons in.

Some years ago, when we drove our 9:1 4.2L Jaguar 420 with 4 persons and
holiday luggage (incl. 3 ladies), I noticed “pinging”.
I was told that this was due to a combination of 9:1 pistons, modern fuels
and a very hard working engine.
Pinging is said to be very bad for an engine.

For this reason the 150 got 8:1 pistons during the rebuild, although I had
never noticed pinging with the XK.
After 5 years and many miles of happy motoring, I can say that I have not
noticed reduced performance.
There certainly are enough HP avalaible for spirited driving on the open
roads.

On the continent in Europe, Ron 95 unleaded petrol is used the most. Some
oil companies sell higher octane fuel.
Shell sells V-power Ron 97 petrol (unleaded). V- power is around 5% more
expensive at a modest EUR 1,53 per liter (in Holland).
Shell explains that 2/3 of the petrol price is for taxes anyway, so I
support our country by putting lots of V-power into the petrol tank of our
150!

Wim Bakker
T831746DN

28

In a message dated 8/27/2010 12:41 Eastern Daylight Time, wb.bakker@wxs.nl
writes:
Some years ago, when we drove our 9:1 4.2L Jaguar 420 with 4 persons and
holiday luggage (incl. 3 ladies), I noticed “pinging”.
I was told that this was due to a combination of 9:1 pistons, modern fuels
and a very hard working engine.
Pinging is said to be very bad for an engine.

Hi Wim,
I shouldn’t say anything because the engineers on this list will probably
correct me but the pinging you experienced was due to a number of other
factors, i.e. too low of fuel octane for the 9.1 pistons, too much carbon
buildup in the cyls. or too much advance timing or poor cooling.
High octane gas has a higher flashpoint, ( the point at which the gas
ignites ), so if you’re using anything below 89/90 octane the gas will ignite
before the spark does it & will cause the knocking.

If you have too much carbon buildup, a piece of carbon may start to glow &
ignite the gas as it enters the cyl. & that flame front will collide with
the one created by the spark plug, causing the pinging.

Too much timing will create the same problem, the gas will ignite before
it’s supposed to. Excessive heat will also contribute to pre-ignition, it may
not have been necessary to go through all that work & expense when some
investigative work might have solved the problem.

Regards, Otto M.

29

In reply to a message from Mark Stephenson (J-L) sent Wed 25 Aug 2010:

The only way really to predict the CR is measure the volume of
the head chamber. Then use one of the different thickness head
gaskets to dial in what you want. You can get it to some
extent from a compression test but there are other variables
(ring condition for example) which leaves a lot of room for
doubt.

Regards

Keith–
The original message included these comments:

It would be a complex calculation and perhaps someone has done it already,
but I wonder what the increase in compression is for every mm (.04’’ approx.)
shaved off the head. Or, perhaps you can deduce the compression ratio from a
compression test.

–
Keith Bertenshaw
Rockaway, NJ, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php –

30

In reply to a message from KeithB sent Fri 27 Aug 2010:

It would seem that if you know the piston diameter, and the stroke,
and the current compression ratio, it is straightforward to compute
the unswept volume.

Then by using different trial values for the change in unswept
volume with change in gasket thickness or head shaving , the new
compression ratio is directly calculable with no physical
measurements needed except for the various gaskets.

Is there something I am missing ?

Karl
karl–
karl
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
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31

Karl and Keith,

The compression ratio given by manufacturers is a purely theoretical number, as compression of the combustible mixture does not start until the intake valve closes.
Real compression therefore also depends on cam duration and timing.
Roar— On Fri, 8/27/10, karl klkirkman@aol.com wrote:

From: karl klkirkman@aol.com
Subject: RE: [xk] 54 Engine rebuild
To: xk@jag-lovers.org
Date: Friday, August 27, 2010, 6:41 PM
In reply to a message from KeithB
sent Fri 27 Aug 2010:

It would seem that if you know the piston diameter, and the
stroke,
and the current compression ratio, it is straightforward to
compute
the unswept volume.

Then by using different trial values for the change in
unswept
volume with change in gasket thickness or head shaving ,
the new
compression ratio is directly calculable with no physical
measurements needed except for the various gaskets.

Is there something I am missing ?

Karl
karl

karl
–Posted using Jag-lovers JagFORUM
[forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php –

32

However, if you really want to be accurate, you have to factor any engine
work that decreases the size of the combustion chamber (as in head milling
or block decking). Because of the hemi head, successive mms shaved off the
head will increase the compression at a greater rate. Assuming an engine in
good nick, wouldn’t a compression test be indicative of compression? I just
checked my Mark 1 and XK120 Service Manuals. They list the following
pressures for the available compression ratios in psi:
Head----------------
CR B type A type
7:1 125 105
8:1 155 120
9:1 180
There’s no listing for the C-type head. Oddly, the compression for the Mark
VII with the 7:1 CR is listed as 110 psi, but the 8:1 is 120. Go figure…

You can see that there’s quite a discrepancy, which, at cranking speeds, is
probably due to cams. It would be interesting to know the compression
figures for the 120 S models with high lift cams. Don’t forget that altitude
would also be a factor. However, I don’t think that’s all of the story. At
speed, carbs and the inlet manifold would be factor. Larger carbs and a
ported and polished manifold would allow easier breathing and higher
compression. Nevertheless, these represent stock figures for common
configurations and could be assumed to be a starting point.

If you just had your XK120 engine rebuilt with 8:1 pistons and compression
was up around 135, you could conclude that your real compression ratio was
probably closer to 9:1. Now that begs a completely different question. I
wouldn’t think that compression ratio means as much when it comes to fuel
requirements and pinking as does the measured cylinder compression. After
all, a supercharged engine doesn’t have a very high compression ratio, but
the forced injection raises the cylinder head pressure to the point that it
is equal to or greater than a normally aspirated engine.

Given that an XK120 with the stock head at 9:1 compression probably has less
head pressure than an XK150 with 8:1 compression, can we conclude that that
XK120 can run on the same octane as an 8:1 XK150 or is there a flaw in my
logic?

“Mark 1” Mark Stephenson Phoenix, AZ
52 XK120 S673129; 59 Mark 1; 84, 85, 86, 95 XJ6

33

And how do you get the compression ratio from a compression test?

DJOn 27 Aug 2010, at 23:23, KeithB wrote:

In reply to a message from Mark Stephenson (J-L) sent Wed 25 Aug 2010:

The only way really to predict the CR is measure the volume of
the head chamber. Then use one of the different thickness head
gaskets to dial in what you want. You can get it to some
extent from a compression test but there are other variables
(ring condition for example) which leaves a lot of room for
doubt.

Regards

Keith

The original message included these comments:

It would be a complex calculation and perhaps someone has done it already,
but I wonder what the increase in compression is for every mm (.04’’ approx.)
shaved off the head. Or, perhaps you can deduce the compression ratio from a
compression test.

–
Keith Bertenshaw
Rockaway, NJ, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php –

34

In reply to a message from Roar Sand sent Fri 27 Aug 2010:

Roar,

I believe the definition at issue back when these engines were
designed was not a theoretical value that depended on valve
opening, etc. When I was taught engine design just a few years
later, compression ratio was the ratio of the maximum volume of the
combustion volume to the minimum volume of the combustion volume.

If you think about it, and especially recall that old science class
topic ‘‘significant figures’’ , it makes sense that they would use a
simplified metric that could be easily measured or calculated with
tools of that day.

Karl–
The original message included these comments:

The compression ratio given by manufacturers is a purely theoretical number, as compression of the combustible mixture does not start until the intake valve closes.
Real compression therefore also depends on cam duration and timing.

–
karl
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php –

35

DJ,

By reading the manual, seeing what the compression pressure is supposed to
be for stock compression ratios, then extrapolating CR based on the measured
compression. If you are running 8:1 pistons, but the compression as
indicated by a compression test is greater than the stated compression
should be, then you are probably running a higher actual CR, all other
things being stock.

“Mark 1” Mark Stephenson Phoenix, AZ
52 XK120 S673129; 59 Mark 1; 84, 85, 86, 95 XJ6-----Original Message-----
From: owner-xk@jag-lovers.org [mailto:owner-xk@jag-lovers.org] On Behalf Of
D J @ EF
Sent: Saturday, August 28, 2010 3:15 AM
To: xk@jag-lovers.org
Subject: Re: [xk] 54 Engine rebuild

And how do you get the compression ratio from a compression test?

DJ

36

Karl,
That’s pretty much what I said, or at least what I meant, - it is theoretical.
Roar— On Sat, 8/28/10, karl klkirkman@aol.com wrote:

From: karl klkirkman@aol.com
Subject: RE: [xk] 54 Engine rebuild
To: xk@jag-lovers.org
Date: Saturday, August 28, 2010, 7:21 AM
In reply to a message from Roar Sand
sent Fri 27 Aug 2010:

Roar,

I believe the definition at issue back when these engines
were
designed was not a theoretical value that depended on valve

opening, etc. When I was taught engine design just a
few years
later, compression ratio was the ratio of the maximum
volume of the
combustion volume to the minimum volume of the combustion
volume.

If you think about it, and especially recall that old
science class
topic ‘‘significant figures’’ , it makes sense that they
would use a
simplified metric that could be easily measured or
calculated with
tools of that day.

Karl

The original message included these comments:

The compression ratio given by manufacturers is a
purely theoretical number, as compression of the combustible
mixture does not start until the intake valve closes.
Real compression therefore also depends on cam
duration and timing.

–
karl
–Posted using Jag-lovers JagFORUM
[forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php –

37

In a message dated 8/28/2010 07:47 Eastern Daylight Time,
mark@jag-lovers.org writes:
By reading the manual, seeing what the compression pressure is supposed to
be for stock compression ratios, then extrapolating CR based on the
measured
compression

Hi Mark,
What manual are you looking at for the compression pressure? They’re not
in the owners manual, or service manual. In the engine overhaul book, all it
mentions is that the compression readings should be within 10% from cyl.
to cyl. If you have them available, what are the numbers for the 9:1 pistons
when checking the comp. pressure both cold & hot.
The shop told me that he had to take .020 off the head due to pitting.
I’ve heard of shaving .005 & .010 but .020 is unusual. Currently I’m using 93
octane, that’s the highest around here but the engine appears to run
without problems except for a tiny leak here & there which can be fixed.

Regards, Otto M.

38

Otto,

It’s there, just not where you’d expect it. For the Mark 1, they are in a
section entitled “Compression Pressures” on page B.31. I couldn’t find
anything for the 140 or 150 in their respective supplements. The XK120 data
is under Engine Tuning, Compression Pressures, page B.63. In there, they
state that their compression tests were performed with the water outlet
temperature at 70C. They also warn that compression may be different with
different oil viscosities.

While I was looking that up, I realized that another book I had might have
some information. It wasn’t exactly what I was looking for, but “How to
Power Tune Jaguar XK Engines” by Des Hammill validates a lot of what I was
theorizing. He does indicate that while 8:1 pistons in an unmodified engine,
yields compression “a bit low … as it happens, XK engines are not very
compression sensitive…” He notes that with “block deck and cylinder head
planing … the compression ratio of XK engines can be increased to 8.5:1
through 9:1, and more, even if the pistons fitted are essentially 8:1 CR
units.”

“…You would be safe to assume that on the low side, 8:1 compression is
almost always suitable for 90 to 97 octane and that 8.5:1 compression is
almost always suitable for use with 95-97 octane fuels [RON as this book was
published in England].” Hammill gets more specific about various CRs and
what octane “should” work, e.g. “9.5:1 compression should be okay with 97
octane fuel…” In the end though, he concludes, “The reason why XK engines
can run well on what might appear to be quite low CRs, is that they have
very long strokes, very long connecting rods and, in modified cylinder form,
very good volumetric efficiency (cylinder filling). They don’t need to have
a lot of compression to generate good engine torque and produce good engine
power. With these engines volumetric efficiency is more important than CR.”-----Original Message-----
From: owner-xk@jag-lovers.org [mailto:owner-xk@jag-lovers.org] On Behalf Of
Ottman0401@aol.com
Sent: Sunday, August 29, 2010 1:29 PM
To: xk@jag-lovers.org
Subject: Re: [xk] 54 Engine rebuild

In a message dated 8/28/2010 07:47 Eastern Daylight Time,
@Mark_Stephenson3 writes:
By reading the manual, seeing what the compression pressure is supposed to
be for stock compression ratios, then extrapolating CR based on the
measured compression

Hi Mark,
What manual are you looking at for the compression pressure? They’re not in
the owners manual, or service manual. In the engine overhaul book, all it
mentions is that the compression readings should be within 10% from cyl.
to cyl. If you have them available, what are the numbers for the 9:1
pistons when checking the comp. pressure both cold & hot.
The shop told me that he had to take .020 off the head due to pitting.
I’ve heard of shaving .005 & .010 but .020 is unusual. Currently I’m using
93 octane, that’s the highest around here but the engine appears to run
without problems except for a tiny leak here & there which can be fixed.

Regards, Otto M.

39

Otto,

I should have added that they don’t list the 9:1 compression, I presume
because those pistons were not listed as an option. I would guess that all
other things being equal, the compression pressure for 9:1 pistons would be
130-135 psi.

.020 is unusual due to pitting, but I doubt it’s unusual due to warping,
especially in the old days before they put the head in an oven to unwarping
it. Planing a warped head flattened the face, but left the cam journals
misaligned. When those heads are rebuilt today, they heat them to remove the
warp, then plane them again. Double the material is gone. If you flip a head
over, you’ll see ribs that are machined flush with the mating surface of the
head. I don’t know much they are raised above the adjacent cast areas on a
new head, maybe a quarter of an inch? Dick Maury, who runs the machine shop
at Coventry West, says he regularly gets heads in where those are almost
flush. So there are heads with .15-.20" machined off. If anyone has an
unmolested head that they can measure, it would give us a starting point to
determine how much has been milled off our heads.

Assuming that the .020" is the only amount ever taken off the head, you
should be able to calculate the increase in compression ratio by comparing
the stock volume at BDC and TDC with the volumes after removing the .020".

Someone double-check my logic on this.

CR=V(BDC)/V(TDC) However, applying the formula for volume, that breaks out
to
CR=h(BDC) x pi x r^2/ h(TDC) x pi x r^2 because the radius doesn’t change we
have
CR=h(BDC)/h(TDC) where h=the distance to the top of the cylinder. Note that
this assumes a pure cylinder top to bottom.

We know the CR is 8:1, so the CR in our pure cylinder is 8. We also know
that the stroke is 4.17" Plugging those values into the equation, we get
this:

8=4.17"+h/h
8h=4.17"+h
7h=4.17"
h=.5956" distance from top of cylinder at TDC
8h=4.7648 distance from top of cylinder at BDC

Isn’t algebra fun? As I said, this assumes a perfect cylinder, however we
have a hemi which throws things off. The difference is that as you take more
material off the head, the rate at which the compression ratio rises,
increases. For estimating purposes, this should be OK, but it will read low
if a lot of material is removed.

To compare that to the head with .020" removed, we are looking at this:

CR=4.7648-.020/.5956-.020
CR=4.7448/.5756
CR=8.24:1

Wow! That shocks the daylights out of me. So much so, that I rechecked my
numbers three times. I had no idea that little bit of material would
increase the CR that much. I recalculated the CR using .20 taken off the
head/block, and the CR is 11.5:1! A tenth off and the CR is 9.5:1. Working
backwards, 75 thou off the head and block would yield 9:1 compression with
8:1 pistons.

No wonder they make the thicker composite head gaskets. I think the stock
composite gasket is .065, but they have them up to .125", I believe.

“Mark 1” Mark Stephenson Phoenix, AZ
52 XK120 S673129; 59 Mark 1; 84, 85, 86, 95 XJ6

Hi Mark,
What manual are you looking at for the compression pressure? They’re not in
the owners manual, or service manual. In the engine overhaul book, all it
mentions is that the compression readings should be within 10% from cyl.
to cyl. If you have them available, what are the numbers for the 9:1
pistons when checking the comp. pressure both cold & hot.
The shop told me that he had to take .020 off the head due to pitting.
I’ve heard of shaving .005 & .010 but .020 is unusual. Currently I’m using
93 octane, that’s the highest around here but the engine appears to run
without problems except for a tiny leak here & there which can be fixed.

Regards, Otto M.

40

In reply to a message from D J @ EF sent Sat 28 Aug 2010:

DJ,

It is indicative at best, certainly not absolute. Mark posted
some numbers with some caveats - my 4.2 is stated to be a CR
of 9.4:1 - calculated with 9:1 pistons and the volume of the
head (which is engraved on the head face) and I guess anything
milled off the block. Compression test is around 185psi. I
seem to remember that 9:1 is is around 175.–
The original message included these comments:

And how do you get the compression ratio from a compression test?

–
Keith Bertenshaw
Rockaway, NJ, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php –