[xk-engine] Replacing strombergs with 3 su carbs

XK-Engine
21

In reply to a message from PIPERJACK sent Tue 15 Nov 2011:

Hello Jack

I don’t know who the ‘we’ refers to but if you are working
with a pro he ought to at least be able to figure out if
it’s an induction or ignition problem. I sure hope you are
getting to keep as spares all the good spare parts
fruitlessly replaced. 2800 rpm is a pretty gross misfire
and not exactly subtle high-end mystery stuff.

Has anyone tried a plug chop to see if it’s rich or weak? A
colortune or gas analyser would be better still but the
effect (if any) of juggling the choke lever is a strong
diagnostic pointer.

A rolling road or Sun tune-up would get it right but this
sort of issue was being solved by deduction long before
either of those were commonplace. Good luck and let us know
how you get on. Don’t discount casting/corrosion flaws in
old manifolds that may have had plain water in them for
decades. I have a brand new-looking (actually,
overpolished) 4.2 tri carb manifold refurbished by Burlen
themselves that seeps water into cylinder #2 IIRC.–
1E75339 66 Zealia D-type, 1E33100 66 FHC, 1R7977 69 OTS
Cambridge, United Kingdom
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22

In reply to a message from PeterCrespin sent Tue 15 Nov 2011:

Peter,
A couple of points. I’ll try to keep this as polite as
possible because maybe we’re saying the same thing but in a
different manner?
The factory rated 3 carb engines more powerful than 2 carb
engines…Is the factory wrong?
The intake manifold, carbs and distributor are the only
real difference between similar spec engines… (well
assuming the later straight port head) but if the
carbs/manifold bolt on the head is the same…
Second time to change intake manifolds. Air conditions
can change in as little as a few minutes… However to be
fair let’s assume it’s a nice day without weather fronts or
quickly changing conditions…
I don’t have a flat rate manual… What sort of time is
called for to remove the intake manifold and replace it? Am
I far off the base at saying 4 hours? I suppose it really
depends on which car we’re talking about. Some of the
sedans are a pain in the posterior to work on.
Plus don’t forget that a change in linkage and fuel lines
etc. would be needed… Yes I can pop out a distributor and
replace it in say 10 minutes? Add another 5 minutes to check
the timing? Plus don’t forget to add some time to adjust
everything properly.
So is 4 hours too slow to be reasonable? If so what’s
fair? I’ll assume I don’t know everyplace but when I dyno’d
in San Diego and dyno here in Minnesota they all change in 4
hours… 4 hours will change (temps, humidity, and
pressures) are likely to change as well. I suppose it
depends on exactly which 4 hours we’re talking about. Early
morning? Yep by say noon temps etc will normally be up.
Noon? by 4:00 temps etc. would be different…–
The original message included these comments:

simply swapping carbs and manifold many of those are
irrelevant because the only thing chaned between dyno runs
would be the induction system and, possibly, air pressure
and temp. It doesnt take THAT long to swap carbs over and
there’s every chance air conditions would be similar before

–
MGuar
Wayzata Minnesota, United States
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23

In reply to a message from MGuar sent Fri 18 Nov 2011:

I totally agree but don’t see your point. In the absence of any
numbers from anyone, Paul is asking for some numbers from someone
on the 2 for 3 swap. Objective info, not opinion. Not exactly an
outrageous or impractical request.

Everybody aghrees the 3 carb engine makes more power than the twin
Stranglebergs - that has never been at issue. It would just be nice
to see some before and after numbers.

If we’re now on the same page and are ignoring all the other
variables except atmospheric conditions, the potential for error is
narrowed down to that. Such changes are measurable and their
resultant power ‘error margin’ either directly or notionally able
to be factored in when comparing the before and after numbers.

Not only that, it’s entirely possible to arrange that the lower
power twin carb run be done first. That way, by the time the
weather has heated up or whatever and the 3-carb run is done, all
the ambient change will mean is that the increase is a conservative
number. I.e. the power hike would have been higher still if the
triple carb run had been in the cool of the day like the twins.
That would be fine, and would let us say that for a given engine on
X day in Y weather, the swap released Z extra ponies, at least.

As of now, without those numbers we are all just blowing hot air
and have no solid answer to the OPs question and nothing to work
from except Jaguar’s highly suspect numbers with old tech fuel and
instrumentation.–
1E75339 66 D, 1E33100 66 FHC, 1R7977 69 OTS, 65 Mk2 3.8 MOD
Cambridge, United Kingdom
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24

Peter,

Thank you for expressing my thoughts exactly.

In G*d I trust, everyone else please send the data on the before and

after performance on an XK engine first with dual Zenith Stromberg’s and
then with the Triple SU. Excuses or rationalization of why this is not
needed or difficult to do are not welcome.

Absent any real data I will continue to believe that the net

increase is marginal and the cost to implement not worth it unless you are
doing it for esthetics.

Regards,

Paul M. Novak

1990 Series III V12 Vanden Plas
1990 XJ-S Classic Collection convertible
1987 XJ6 Vanden Plas
1985 XJ6 Vanden Plas (parts)
1984 XJ6 Vanden Plas
1969 E-Type FHC
1957 MK VIII Saloon
Ramona, CA
P.M.Novak7@gmail.com-----Original Message-----
From: owner-xk-engine@jag-lovers.org [mailto:owner-xk-engine@jag-lovers.org]
On Behalf Of PeterCrespin
Sent: Friday, November 18, 2011 2:45 PM
To: xk-engine@jag-lovers.org
Subject: RE: [xk-engine] Replacing strombergs with 3 su carbs

In reply to a message from MGuar sent Fri 18 Nov 2011:

I totally agree but don’t see your point. In the absence of any
numbers from anyone, Paul is asking for some numbers from someone
on the 2 for 3 swap. Objective info, not opinion. Not exactly an
outrageous or impractical request.

Everybody aghrees the 3 carb engine makes more power than the twin
Stranglebergs - that has never been at issue. It would just be nice
to see some before and after numbers.

If we’re now on the same page and are ignoring all the other
variables except atmospheric conditions, the potential for error is
narrowed down to that. Such changes are measurable and their
resultant power ‘error margin’ either directly or notionally able
to be factored in when comparing the before and after numbers.

Not only that, it’s entirely possible to arrange that the lower
power twin carb run be done first. That way, by the time the
weather has heated up or whatever and the 3-carb run is done, all
the ambient change will mean is that the increase is a conservative
number. I.e. the power hike would have been higher still if the
triple carb run had been in the cool of the day like the twins.
That would be fine, and would let us say that for a given engine on
X day in Y weather, the swap released Z extra ponies, at least.

As of now, without those numbers we are all just blowing hot air
and have no solid answer to the OPs question and nothing to work
from except Jaguar’s highly suspect numbers with old tech fuel and
instrumentation.

1E75339 66 D, 1E33100 66 FHC, 1R7977 69 OTS, 65 Mk2 3.8 MOD
Cambridge, United Kingdom

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25

Now that I’ve just poured a fresh cup of coffee I’ll chime in with a few
unsolicited remarks and opinions, randomly expressed :slight_smile:

First, I don’t see how anyone can argue with the logic of objective
measurement of actual results. A dyno test makes perfect sense no matter how
you slice it.

Always wanting more power but never having loads of excess cash I’d be
reluctant to spend $2000-$3000 (which I think we’re talking about here) for
a power upgrade unless I was assured that there’d be quite an
improvement…enough improvement that even seat-of-the-pants feel would be
undeniable even if the actual numbers remain unknown. And if the
anticipated improvement would be negated or even substantially reduced by a
4 hour difference in atmospheric conditions on the same day, well, count me
out :slight_smile:

There are some non-technical, non-precise ways to verify a power
improvement. This may sound a bit silly and amateurish but I have some
favorite test routes and routines. With a bit of focus towards consistency
in driving you can check your before and after speeds a certain points. A 2
or 3 mph difference could be down to any number of variables. A consistent
8-10 mph improvement is probably good proof that the power really has
increased and it’s not simply wishful thinking.

I installed a “Superenhanced” ECU in my old XJS. I really felt a SOTP
improvement. And…don’t laugh…had to consciously feather the throttle
to keep traction on wet roads where before I didn’t have to. Not just on one
occasion or one road, but consistently. I can’t say what the numeric change
was but I do know that I had to alter my driving habits…so “something”
changed :-).

I say all this as a veteran of many modifications that produced no SOTP
improvement all. Oh, there might have been some fractional increase but
if I can’t undeniably feel it or see it, it doesn’t exist…and I’ve failed
:-).

Cheers
Doug Dwyer
Longview Washington USA
1995 XJR

I totally agree but don’t see your point. In the absence of any
numbers from anyone, Paul is asking for some numbers from someone
on the 2 for 3 swap. Objective info, not opinion. Not exactly an
outrageous or impractical request.

//please trim quoted text to context onlyFrom: “PeterCrespin” jag@thewritersbureau.com

26

In reply to a message from PIPERJACK sent Tue 15 Nov 2011:

piperjack;
I wish I knew how many 3 carb XK engines there were out
there… off the top of my head I’d say a lot… they
don’t have problems…
So let’s begin at the beginning. what could the problem be?
Fuel or ignition… Don’t assume it’s the carbs nor that
new parts are the solution… Test and verify…
I assume you have the filters off and are looking inside
each carb at the piston as it rises. What are they acting
like? How much advance is in your engine at 2800 RPM (use a
dial back timing light to find out) what is your fuel
pressure at 2800 rpm?
One final question, how old is the gas?
It will likely be something basic or simple…–
The original message included these comments:

Great thread but I wish I could get my converted SU’s to work well
enough to even have the car tested. We have done everything and
the engine continues to start missing over 2800 rpm during
distributor, new fuel lines, plugs, wires, gas cap, etc. etc. Any
suggestions would be appreciated.

–
MGuar
Wayzata Minnesota, United States
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27

In reply to a message from MGuar sent Tue 22 Nov 2011:

To the gent who has a big misfire, I dont know what manifold
you are using, but the 420G/HD8 has an odd spring loaded
setup that will leak under duress if not correctly installed
between carb and mani.

As the others said, a good mechanic with modern equipment
should be able to diagnose that sort of problem within a
short time??..especially if its consistent

Sometimes its cheaper to get them on a towtruck, even though
most mechanics arent all that keen on failed home mods ( I
am assuming you have not had this work carried out by a
mechanic with Jaguar pedigree ?)

Re the triple vs 2 carb power debate, no doubt at the top
end, but unless you are going to spend time at high revs, I
would be interested to see a power curve at the full RPM
range with the various carbs. Fuel consumption as well I suppose

In Des Hammills book, he suggests some similar procedures to
McGuar for real road test.

One I liked that may? be legal on an appropriate public
road, was along the lines of crossing a marked point at a
given RPM in 2nd or 3rd gear, and stopwatch to another
marked point.
Note terminal RPM, speed and time elapsed after each adjustment–
The original message included these comments:

So let’s begin at the beginning. what could the problem be?
Fuel or ignition… Don’t assume it’s the carbs nor that

–
awg
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28

In reply to a message from awg sent Tue 22 Nov 2011:

The manifold choice is a good point for performance too.

Today I collected my new Mk2 3.8 and drove it 150 miles home
at speed. It only has two small SUs but because it was
designed that way it was very peppy. Despite being heavier
it was much peppier than the 4.2 E-type I just sold that had
two Stanglebergs the same size but on the awful emissions
manifold…–
The original message included these comments:

To the gent who has a big misfire, I dont know what manifold
you are using, but the 420G/HD8 has an odd spring loaded
setup that will leak under duress if not correctly installed
between carb and mani.

–
1E75339 66 D, 1E33100 66 FHC, 1R7977 69 OTS, 65 Mk2 3.8 MOD
Cambridge, United Kingdom
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29

In reply to a message from PeterCrespin sent Fri 25 Nov 2011:

I agree about the manifold. The Canadians received a nice
twin SU manifold on many of their XJs. I have one, I
think–at least I know I bought it and I know I never sold it.

I never figured out the reason for the spring-loaded O-ring
attachments on the Mark X. It might be because the air box
is a bit on the heavy side, cantilevered outboard of the
carbies. Perhaps Jaguar thought the weight might break
studs or castings if not shock-isolated. Anyway, I replaced
my springs etc. with the much simpler gasket/insulators
found on the E-type.–
The original message included these comments:

The manifold choice is a good point for performance too.

you are using, but the 420G/HD8 has an odd spring loaded
setup that will leak under duress if not correctly installed
between carb and mani.

–
Bob Wilkinson, 73 XJ6
Saint Louis, MO, United States
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30

In reply to a message from Kailua boy sent Sat 5 Nov 2011:

I have harped on this before, but you need to make sure your
Stombergs are set up right in the first place. Jaguar quit
selling the correct carb spacers for the SII around 1974
and substituted the SIII spacer. Unfortunately there is a
difference in performance because of this and it has to do
with the secondary throttle arrangement.

See:
http://www.xkebooks.com/images/Stromberg%20set%20up.pdf
for details. However if you have removed the secondary
throttle plates, this is moot.

Richard Liggitt–
'70 E Roadster 1R11998, '98 XK8 Roadster, www.XKEBooks.com
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31

In reply to a message from Robert Wilkinson sent Fri 25 Nov 2011:

Yes, it’s an odd difference although in my case I went the other
way and have used the MkX o-ring and alloy spacer system on my E-
type because it offers better thermal insulation.

Richard, pardon me but once an editor, always an editor (not that
you’d know from my unchecked spelling…): in your next version you
might want to change the first sentence. It currently
mentions ‘butterfiles’. Unless, of course butter files are similar
to butter knives? :-)–
1E75339 66 D, 1E33100 66 FHC, 1R7977 69 OTS, 65 Mk2 3.8 MOD
Cambridge, United Kingdom
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32

In reply to a message from Doug Dwyer sent Sat 19 Nov 2011:

Doug;
I used to ‘‘dyno’’ my engines on a steep hill near my
house… Stop watch started from a chalk line part way up the
hill 2nd gear 3500 rpm to the curve sign near the top If
it took less time consistently it was more powerful. Once I
got my air density meter the variations started to make
sense…
Now I can spend as little as $50. with a friendly chassis
dyno owner to confirm I’m properly tuned… Tuning a car to
optimum is much more time consuming… especially if variable
such as different plugs or parts are tried for back to back
comparisons… It’s easy to spend hundreds of dollars
confirming small changes…–
The original message included these comments:

anticipated improvement would be negated or even substantially reduced by a
4 hour difference in atmospheric conditions on the same day, well, count me
out :slight_smile:
There are some non-technical, non-precise ways to verify a power
improvement. This may sound a bit silly and amateurish but I have some
favorite test routes and routines. With a bit of focus towards consistency

–
MGuar
Wayzata Minnesota, United States
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33

In reply to a message from Paul Novak sent Fri 18 Nov 2011:

Paul;
There already are before and after numbers… Done by the
factory. Off the top of my head 265 HP and 220 are the
differences. 45 horsepower…
Assuming those numbers are right is 45 horsepower enough
for you? That’s the real question you are asking*.
Anything else would have to have conditions and tuning
factors taken into consideration… Which spark plugs, what
atmospheric conditions, how much overbore, how much
surfacing done to block and heads affecting compression,
which camshafts?
The Chance of two or more 40-50+ year old engines being
identical to another is just silly…
On a worn out sloppy old engine triple carbs may not
improve performance. Nor would a set of Webers… Or fuel
injection etc… Or maybe they will.

  • let’s be honest here… few Jaguar owners race their cars
    and both cars are plenty fast. If you can’t get at least 2
    tickets each time you drive them you’re just not trying

So the next question is do you want to change an original
car into a modified one?
I suspect based on the fervor some are posting here that
you don’t and you simply want the question to hang in the
air as if it seriously was the reason to not change…
No need to go to that length… simply say you want to keep
the engine original… That’s certainly valid.–
The original message included these comments:

In G*d I trust, everyone else please send the data on the before and
after performance on an XK engine first with dual Zenith Stromberg’s and
then with the Triple SU. Excuses or rationalization of why this is not
Absent any real data I will continue to believe that the net
increase is marginal and the cost to implement not worth it unless you are
doing it for esthetics.

–
MGuar
Wayzata Minnesota, United States
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34

In reply to a message from MGuar sent Sat 3 Dec 2011:

There already are before and after numbers… Done by the
factory. Off the top of my head 265 HP and 220 are the
differences. 45 horsepower…

MGuar,

The factory claimed output change from twin to triple carbs is 20
hp, not 45 hp.

Since there were no normal production 3.8’s with ‘‘B’’ type heads and
triples, the only direct comparison is a 4.2 twin carb 420 engine
and a triple carb 4.2 E or 420G engine. The factory claimed 245 for
the 420 and 265 for the E, a difference of 20 hp, which is somewhat
believable. The exact same engine with triples, made the exact
same torque 283 ft/lbs, but at 250 rpm higher, accounting for the
20 hp difference at similar peaks (5400 vs 5500 rpm). This also
implies that the triple SU carbs are not the limiting factor to
output, which I think is true. The factory also claimed a 10 hp
increase for a point of compression (3.9%), which is also
credible. These are gross numbers and are inflated by at least 20%
compared to net or DIN figures.

Personally, these numbers do not reflect the change in throttle
response the driver feels which makes the conversion worth it.
Much like the 70-80’s Porsche 911s with CIS injection. A swap to
carbs only nets between 10-20hp on the dyno, but on the street it
feels like 50 hp. Usually this is all that matters on the street.

In regard to your misunderstanding on XK tappets, they are flat by
design and never had any radius or taper. They are based on the
Morin 1916 French patent cup type cam follower. This was copied by
Henri on the 1919 Ballot, by Miller in 1921, Bugatti in 1930,
Jaguar and Aston in 1948, Coventry Climax in 1960 and by Ferrari,
Weslake, Cosworth and Fiat in 1967. Now you can find them
everywhere. The only engine I know of that used Morin type cup
followers with tapered tops was Miller in the 1920’s. This was
eventually taken to the extreme on one version of the Offenhauser
which used cup tappets that were drilled all over with 40 holes and
the tops were domed ‘‘strips’’ with cut aways to either side, and
they were pinned in place, so that could not rotate. XK tappets do
rotate somewhat because of the offset position of the cam lobe, but
apparently not enough for the British military who had the XK
engine fitted with positive valve rotators which were never fitted
to a production road car engine.

There were some American pushrod engines with tapered tapets
(around 0.002’') and a machine to restore the taper (Storm Vulcan
902 Grinder), but none of this has anything to do with the XK
engine.

Paul–
The original message included these comments:

Paul;
There already are before and after numbers… Done by the
factory. Off the top of my head 265 HP and 220 are the
differences. 45 horsepower…
Assuming those numbers are right is 45 horsepower enough
for you? That’s the real question you are asking*.
Anything else would have to have conditions and tuning
factors taken into consideration… Which spark plugs, what
atmospheric conditions, how much overbore, how much
surfacing done to block and heads affecting compression,
which camshafts?

–
PS
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35

In reply to a message from PS sent Sat 3 Dec 2011:

Paul,

Thanks for your usual very informative post. Might I ask
that you elaborate on a couple of your points?

If I understand correctly, you are comparing a twin-SU
configuration with non-smog manifold to a similar engine
except with triples. It seems that the torque peak is
shifted up a bit, accounting for the difference in specified
HP. Then, you seem to side with those whose
seat-of-the-pants driving experience votes for the triples.
This experience is largely at other than 5400-5500 RPM.
So, finally to my question: is the torque curve broadened in
addition to having its peak shifted upward? How else would
you explain the subjective difference?

Next question: Is your comparison between a 420 and a 420G a
fair approximation to comparing a 420G or S1 E to a S2 E or
S1/S2 XJ, both with the smog double manifold? As I mentioned
earlier in the thread, I would think that the smog manifold
versus either twin- or triple SU earlier manifolds would
make more of a difference than the 20 HP in your example.–
The original message included these comments:

The factory claimed output change from twin to triple carbs is 20
hp, not 45 hp.
Personally, these numbers do not reflect the change in throttle
response the driver feels which makes the conversion worth it.

–
Bob Wilkinson, 73 XJ6
Saint Louis, MO, United States
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36

In reply to a message from Robert Wilkinson sent Sat 3 Dec 2011:

Bob,

There is only one direct comparison of factory hp figures for the
same engine (same head, ports, cams, pistons) with twin SU and
triple SU, the 420 vs 420G or 4.2 E. The 3.8’s cloud the issue
with different heads, ‘‘B’’ on the twins and straight port on the
triples. The ‘‘B’’ type head shifts the torque curve down between
750 and 1000 rpm. A 265hp 3.8 in a E or MK10 is not a direct
comparison to a 220 hp 3.8 in a MK2 or S Type.

The subjective feel of the car’s performance on the road can be
altered by many things other than output. Some OEM’s use offset
cams and linkage, especially on 4 cylinder models, that give non
linear throttle opening making the engine feel more powerful in
normal driving. Having driven one, a Porsche 944, with and without
the offset cam, the ‘‘butt dyno’’ effect is remarkable. Some OEM
turbo cars use an overboost feature that allows higher manifold
pressure for a few seconds for a similar effect. The XK twin carb
log manifolds are all far from ideal and the triples give a much
better arrangement. The paired cylinder manifold in theory allows
each cylinder to draw from a 50mm throttle, but in practice most
XK’s cannot inhale enough to fully open the pistons. Some of the
highest output XK’s ever built, claiming over 350 net hp, still ran
2 inch SU’s, showing that bigger carbs are not needed on a road car.

I am not making any ‘‘fair’’ approximations, only comparing factory
figures on identical engines. The post 68’ smog motors have several
other differences and there is no direct comparison for twins and
triples. I think the US rating dropped all the way down to 162 hp
under the ‘‘net’’ hp ratings mandated in 1972.

Paul–
The original message included these comments:

Paul,
Thanks for your usual very informative post. Might I ask
that you elaborate on a couple of your points?
If I understand correctly, you are comparing a twin-SU
configuration with non-smog manifold to a similar engine
except with triples. It seems that the torque peak is

–
PS
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37

In reply to a message from PS sent Sat 3 Dec 2011:

The XK150S used a straight port head I guess, versus B for twin
carb 3.8s so no comparison there either…

Your 420/E-type comparison uses 2’’ carbs both times, unlike an
emissions-strangled E versus any ROW E-type six cylinder from any
year. Thus one might hope for a fraction more than a 20 bhp uplift
by fitting 3 x 2’’ triples where there were 2 x 1 3/4’’ carbs even if
as has been mentioned, some of it comes at revs the average driver
is not going to explore.

I have no data but suspect a siamesed inlet manifold gives a slight
ram effect in mid range compared to the twin manifold with short
convoluted passages, even if it’s not as good as a choke-per-
cylinder Weber or Dellorto set-up. But as Paul Novak has said, we
need real-world numbers on a swap with today’s engine and fuel, not
catalogue comparisons from a different era.

Were there significant ignition changes between the twin and triple
versions? I guess any difference would only be a horsepower or two.

My week-old Mk2 3.8 has been a revelation. OK, it has a 3.77 Powr-
Lok as opposed to the 3.54 in the E, but second gear is similar
overall gearing for the Getrag E and the Moss Mk2. The 4-door is a
fun car with twin 1.75’’ SU and pulls very well indeed. It is the
most fun I’ve had in an XK-engined saloon by a long way and the car
wants to hurry along on backroads or motorways. I guess the little
SUs suit the B head better than the Stranglebergs suit the straight
port but still, the Mks is noticeably more peppy than the emissions
2+2 I sold a few weeks ago. The manual box is part of it but they
are chalk and cheese to drive.

MGuar did say his eyesight is no longer good enough to see the
taper on his XK cams and tappets, so he’s on the same page as the
rest of us ‘forever flatties’ now - if only for medical reasons. :-)–
1E75339 66 D, 1E33100 66 FHC, 1R7977 69 OTS, 65 Mk2 3.8 MOD
Cambridge, United Kingdom
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38

In reply to a message from PS sent Sat 3 Dec 2011:

Paul;
We totally agree on at least 2 points.
With regard the 1&3/4 Strombergs versus the 2 Inch SU’s
I’ve had enough of each on my flow bench to confirm that
the 1& 3/4 inch Strombergs flow right around 200 CFM and the
2 inch SU’s flow right around 315 CFM.
We are in total agreement that the SU’s at 945 CFM are not
the limiting factor in power output.
What is relevant is the effect of Webers on an engine…
Please bare with me. If you put a 45DCOE on the flow bench
depending on which chokes you are running you will be right
around 155-160 CFM per throat. Close enough to the 315 CFM
SU that I won’t dispute the difference. Yet dyno experience
has proven that in similar relatively stock engines,
properly tuned the Webers are worth 15-20 hp. more than the
SU’s.
There can be only one logical reason for that gain…
Better airflow. A straight path. Not a curved one or one
with actual bends like the 2 carb Stromberg.
The reverse has to also be true.
Yes it’s likely that the Strombergs flow enough air at
400CFM to feed a little 3.8, or 4.2 engine of relatively
mild state of tune…
However the flow patterns is much worse. Part of that
problem of flow is the different mass of fuel and air…At
every corner/curve the fuel will go to the outside of the
curve relative to the air… if the fuel should land on the
walls of the manifold there is every likely-hood that it
will return from a vapor state to a liquid state… Liquids
don’t burn (hence flooding)
If the slight curve of an XK-E triple SU manifold costs
15-20 horsepower over an SU. The much sharper curves of the
the duel Stromberg manifolds is likely responsible for the
majority of the difference… Not the size of the Carbs…
It would be an interesting experiment to put three 1&3/4
Strombergs on a Triple SU manifold and see if indeed makes
the same power… (I suspect that’s likely)
Well except we know otherwise, almost… 1&3/4 SU’s on 140
engines replaced with a pair of Sand cast 2 inch SU’s on the
140MC How much of that gain is from the bigger carbs and how
much from the other factors?
I digress,
Now to deal with cams and lifters… All lifters are called
flat tappet… Yet anyone who has seen a worn camshaft knows
that the wear starts on one side of the lobe and results in
hollow tops on the lifters.
Have you never seen a lifter guide in a Jaguar come up
and be hit by the camshaft? I see two gouges of
approximately equal amount indicating the lobe of the cam is
hitting the guide in the center. If the lobe was offset from
the center of the lifter one would be deep and one shallower.
The example you pointed out, the Miller was copied by Fred
Offenhauser in his engines built on Miller machinery.
Yes I’ve seen pictures of the tappets drilled with many
many holes to lighten them… and also the ‘‘pinned’’ one
(I’ve got the Offenhauser book myself) Yet even the later
short block turbo’d engines had that same radius lifter,
tapered lobe arrangement.–
The original message included these comments:

20 hp difference at similar peaks (5400 vs 5500 rpm). This also
implies that the triple SU carbs are not the limiting factor to
output, which I think is true. The factory also claimed a 10 hp
design and never had any radius or taper. They are based on the
Henri on the 1919 Ballot, by Miller in 1921, Bugatti in 1930,
There were some American pushrod engines with tapered tapets
(around 0.002’') and a machine to restore the taper (Storm Vulcan
902 Grinder), but none of this has anything to do with the XK

–
MGuar
Wayzata Minnesota, United States
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39

In reply to a message from MGuar sent Thu 8 Dec 2011:

‘‘Please bare with me.’’

I don’t know Paul personally, MGuar, but from what I’ve
heard I don’t think he’s that sort of guy.

:slight_smile:

I may be wrong, but it just seems obvious to me that a
single choke per cylinder is the best because the vacuum
pulses are evenly spaced, so that flow is smoothest. The
mixture’s momentum changes least as a function of time.
With two cylinders per choke, the pulses are uneven and the
momentum changes because both direction and timing of flow
changes (momentum is a vector). Three cylinders per choke
is even worse. As you say, curvature is also minimized with
one choke per cylinder. That also minimizes change in
momentum. Change in momentum requires an impulse of force,
whose energy requirement is provided by a decrease in the
energy of flow into the cylinders. This sort of analysis is
of course most relevant at the valves, where velocities get
very high, and momentum (Reynold’s number) gets very high
concomitantly. I would think, however, that it would have
some bearing (baring?) on flow through the manifold as well,
although as I said I could easily be wrong–it’s a
theoretical argument and I have no numbers.–
The original message included these comments:

Please bare with me. If you put a 45DCOE on the flow bench
Better airflow. A straight path. Not a curved one or one

–
Bob Wilkinson, 73 XJ6
Saint Louis, MO, United States
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40

In reply to a message from PS sent Sat 3 Dec 2011:

Believe it or not I FINALLY sorted out my 68 etype. A few years
ago I converted my Stormbergs to 3 SU’s and tried everything to get
them to perform properly. New ignition parts (all) rebuilt carbs
of course (twice), new exhaust, new distributor, fuel pump,
filters, lines gas and changed the plugs to a new heat range.
Found the problem after my mechanic got new carb needles from Joe
Curto in Queens, New York. He diagnosed the break up at high RPM
problem immediatly. Wow, cars runs better than it ever has and is
now the great sports car it was meant to be.–
E-Type New York State Reg
Islip New York, United States
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