Hi, I am a newbie here seeking advice on boring out a ‘A’
series 3.4 later in a mark 7 to 87mm (3.8 ).
I am told to use steel liners. What else do I need to change
besides, pistons and rings?
Also do advice if this whole effort is worth the power
output I am looking for?
Regards,
Dev–
Devendra
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In reply to a message from Devendra sent Wed 18 Nov 2015:
Why not just buy an early 4.2 and be done with it? Unless you get
your machine work for free, that would have to be cost competitive
with the boring and liners for a 3.4.–
The original message included these comments:
series 3.4 later in a mark 7 to 87mm (3.8 ).
I am told to use steel liners. What else do I need to change
besides, pistons and rings?
–
Mike Spoelker
Louisville,Kentucky, United States
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In reply to a message from Devendra sent Wed 18 Nov 2015:
If it is improved performance that you are looking for, the
most efficient thing to do is to install a later 3.4 engine
with the B type head, as fitted to mark VIII, and mark 2 3.4
cars ( or just using a B type head on your present block ).
This will produce better results than boring to 3.8 but using
an early head , and almost as good results as using a normal
3.8/mark IX engine - certainly in terms of cost/benefit ratio–
christopher storey
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In reply to a message from Devendra sent Wed 18 Nov 2015:
More HP is gained in head design than CID. Get a B head.
Joel–
ex jag, '66 E-type S1 4.2, '56 XK140dhc, '97 XJ-6
Denison, TX, United States
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In reply to a message from ex jag sent Fri 20 Nov 2015:
That is sound advice.–
The original message included these comments:
In reply to a message from Devendra sent Wed 18 Nov 2015:
More HP is gained in head design than CID. Get a B head.
Joel
–
Mike Spoelker
Louisville,Kentucky, United States
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In reply to a message from Mike S sent Fri 20 Nov 2015:
I think the redline for maximum rpm is higher for the 3.4
than for the 3.8, so you get more cubic inches per minute at
full throttle with the 3.4 than with a 3.8.
Keep the nice smooth cam covers from your original engine
and put them on the new head to preserve the original
appearance.
P.–
Peter J. Smith, 1966 3.8S MOD
Carson City Nevada, United States
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In reply to a message from ex jag sent Fri 20 Nov 2015:
If he doesn’t want a revver, more capacity always provides
more torque.–
1E75339 66 D, 1R27190 70 FHC, 79 S2 XJ12L
Gaithersburg, Maryland, United States
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In reply to a message from PeterCrespin sent Sat 28 Nov 2015:
Also, they say it’s the way to go if you’re in any doubt.–
The original message included these comments:
If he doesn’t want a revver, more capacity always provides
more torque.
–
Bob Wilkinson, 73 XJ6
Saint Louis, MO, United States
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In reply to a message from Devendra sent Wed 18 Nov 2015:
Buy an early 4.2L, toss the straight port head, fit a late B-head,
which will accept your original carbs and manifolds, rejet the
carbs to suit the new displacement. Max torque, minimal change in
appearance. Buying the 4.2L short-block would likely still be
cheaper than boring and sleeving a 3.4.–
Mike Spoelker
Louisville,Kentucky, United States
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In reply to a message from Devendra sent Wed 18 Nov 2015:
Buy an early 4.2L, toss the straight port head, fit a late B-head,
which will accept your original carbs and manifolds, rejet the
carbs to suit the new displacement. Max torque, minimal change in
appearance. Buying the 4.2L short-block would likely still be
cheaper than boring and sleeving a 3.4.–
Mike Spoelker
Louisville,Kentucky, United States
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In reply to a message from Mike S sent Mon 30 Nov 2015:
Mike,
Could you clarify why you would toss the straight port head?
Jaguar thought it was better, did they not? A choice I have
to make soon.
Cheers,
David Hadley–
The original message included these comments:
Buy an early 4.2L, toss the straight port head, fit a late B-head,
–
luxman
brisbane, Australia
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In reply to a message from luxman sent Wed 2 Dec 2015:
If you don’t mind me butting in…toss the head so you can keep
all of the original inlet system including carbs and linkages–
The original message included these comments:
Could you clarify why you would toss the straight port head?
Jaguar thought it was better, did they not? A choice I have
to make soon.
–
Phil.D 3.8 etype, XKR,XK150 FHC, XK120 FH2.2 diesel Xtype
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In reply to a message from Phil.Dobson sent Wed 2 Dec 2015:
Exactly what Phil says.
If you are talking about improving the from-rest acceleration,
but not necessarily the top speed, of a heavy saloon, and are
restricted to naturally aspirated induction, then the objective
is maximum torque, not increased RPM and gross horsepower. In
that context, cubic inches are king. Two ways to increase the
displacement, either increase the stroke or increase the bore.
Easiest way to do that is increase the bore, and a 4.2 has a
bigger bore than a 3.8. All three have the same relatively
long stroke.
Straight Port heads will not accomodate your existing intake
manifold and linkages etc, but the B head will. The B head
will also deliver a usable increase in power over the original
A head. You would have way more torque than an overbored 3.4,
but it would still (mostly) look ‘‘original’’ Fit the heavy,
high-inertia 4.2 flywheel and you could drive around in top
gear all day.–
The original message included these comments:
In reply to a message from luxman sent Wed 2 Dec 2015:
If you don’t mind me butting in…toss the head so you can keep
all of the original inlet system including carbs and linkages
–
Mike Spoelker
Louisville,Kentucky, United States
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In reply to a message from Mike S sent Wed 2 Dec 2015:
How do you tell an A head from a B?
I have an early unumbered head with the non-straight port
intake studs so I know it’s either A or B but don’t know
which. It has the full number of cam cover fixings so I
know it’s not VERY early before they addrd the teo front
inboard studs. All valve seats are 45 degrees but the
seats have been replaced so the inlets may have originally
been 30 degreez
Pete–
The original message included these comments:
manifold and linkages etc, but the B head will. The B head
will also deliver a usable increase in power over the original
A head. You would have way more torque than an overbored 3.4,
–
1E75339 66 D, 1R27190 70 FHC, 79 S2 XJ12L
Gaithersburg, Maryland, United States
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In reply to a message from PeterCrespin sent Wed 2 Dec 2015:
How do you tell an A head from a B?
Pete,
Smaller exhaust valves and lower lift cams. My experience
and testing show that the big valve 4.2 S3 straight port
head makes more power everywhere, even on the smaller
engines, than the ‘‘B’’ Type head.
Paul–
The original message included these comments:
How do you tell an A head from a B?
I have an early unumbered head with the non-straight port
intake studs so I know it’s either A or B but don’t know
–
PS
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In reply to a message from PeterCrespin sent Wed 2 Dec 2015:
On the underneath side there will be a casting number
beginning with the letter C, and in the plug valley there
will be a hand stamped number either in the middle by the
hex socket plug or at the back end, usually one letter and
up to 3 numerals. Let us know those numbers and we can
figure out what it is and approximately when it was made.–
The original message included these comments:
How do you tell an A head from a B?
I have an early unumbered head with the non-straight port
–
XK120 FHC, Mark V saloon, XJ12L Series II, S-Type 3.0
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In reply to a message from PS sent Thu 3 Dec 2015:
There should be a part number difference as well.
I believe there is some small difference in the port design as
well. I believe B head was developed using data Jaguar had
acquired in their early LeMans efforts, so the B head is
considered by some to be a performance step up from the over
the counter and XK140 ‘‘Type C’’ head. There have been lengthy
discussions about this over on the XK forum.
I actually have a 3.4L with an early (doesn’t have the extra
water jacket holes at the rear) straight port head on it that I
built when I intended to vintage race the 120. Finding a
suitable intake manifold and making the 120’s mechanical
tachometer drive work are the biggest hurdles.–
The original message included these comments:
How do you tell an A head from a B?
Pete,
Smaller exhaust valves and lower lift cams. My experience
–
Mike Spoelker
Louisville,Kentucky, United States
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In reply to a message from PS sent Thu 3 Dec 2015:
I agree, but sadly there’s no such thing as a correct
slanted-engine (& no thermostat) D-Type manifold for the
straight port head, unless you go for Lucas Fuel Injection
AFAIK?
Certainly my sources have the correct manifold (in two
versions - either DCO or DCOE) only for B ports. In fact
even those have always needed an adapter plate to move the
front-most and rear-most stud pairs further forward and
backward to make room for the #1 and #6 Weber runners. Not
sure if C-Type is the same on studs (I know the manifold is
different).–
The original message included these comments:
Smaller exhaust valves and lower lift cams. My experience
and testing show that the big valve 4.2 S3 straight port
head makes more power everywhere, even on the smaller
engines, than the ‘‘B’’ Type head.
–
1E75339 66 D, 1R27190 70 FHC, 79 S2 XJ12L
Gaithersburg, Maryland, United States
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In reply to a message from Rob Reilly sent Thu 3 Dec 2015:
I know that from your excellent table Rob, but I’m away
from the car (which in any case just uses a mule seized
engine during build-up).
The refurbed head I’ll probably buy is about 50 miles away
so I can’t check numbers.
However it is cam-less / valve-less so I don’t care about
those specs, but does have 1 5/8’’ exhausts & 1 3/4’’ inlets
judging by the new seats so it must be B not A which was my
concern. Of course the swirl ports aren’t quite as good as
the straight ports at higher revs and I especially like the
big valve/big port which I have used on E-types. My D copy
needs to look correct with the B-head plug positions, cable
tach and certainly no long head/big-valve overhang at the
back.
I also think it should have Allen head plugs, not the hex-
head type. Who is the guru of B head porting? Bob Grossman?
Paul Salt?
Pete–
The original message included these comments:
On the underneath side there will be a casting number
beginning with the letter C, and in the plug valley there
will be a hand stamped number either in the middle by the
hex socket plug or at the back end, usually one letter and
up to 3 numerals. Let us know those numbers and we can
figure out what it is and approximately when it was made.
–
1E75339 66 D, 1R27190 70 FHC, 79 S2 XJ12L
Gaithersburg, Maryland, United States
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For what it is worth, I wish to correct an oversimplification in the
discussion on boring the 3.4.
If the objective is to improve acceleration from rest with no regard for top
speed, then you might just want to change the diff ratio. (The force that
accelerates the car is the torque at the wheel.) Given the difficulty of
achieving anything over the speed limit these days, this might well be a
good option, but it will not likely aid your fuel economy.
The objective of maximum acceleration is not simply achieved by maximising
engine torque; rather by maximising the area under the torque curve, between
the usable rpm (shift points). That is, you want more torque for more of the
time, or to say it another way, you need to maximise the useful power. This
might result in quite a different target if you have a 3 speed auto or 4, 5
or 6 speed manual fitted. (You actually want to shift on either side of the
max power)
Ironically, an engine that is popularly described as “being very torquey”
will most likely have a wide power band, which allows lazy shifting, because
you have a fatter power curve between shift points.
An engine that produces more torque at lower revs will win the day only if
the driver shifts at those low revs … which may well often happen driving
around town, but will likely be less effective during “spirited driving” on
a back country road or race track.
If we accept that “the big valve 4.2 S3 straight port head makes more power
everywhere, even on the smaller engines”, then some analysis would show that
this would also produce better acceleration, even on the smaller bore block.
Cheers
Mark-----Original Message-----
If you are talking about improving the from-rest acceleration,
but not necessarily the top speed, of a heavy saloon, and are
restricted to naturally aspirated induction, then the objective
is maximum torque, not increased RPM and gross horsepower …
-----Original Message-----
My experience and testing show that the big valve 4.2 S3 straight port head
makes more power everywhere, even on the smaller engines, than the ‘‘B’’
Type head.
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