Well at first I thought of course that Ed must have tweaked
this one, just a bit. But no, apparently this happened
somewhere in the US of A. Is that Lompoc, California where
the speed trial took place?
PS. I have now completed 1600 miles on my MKV DHC, but not
much faster than 70 mph. I will post some pics once I get
a chance to scale them down and also some regarding Rob’s
question about the exhaust bracket that was able to trace
down on paper.–
MKV 3.5L DHC, E V12 OTS, XJ6C MOD, XJ8 Executive
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In reply to a message from ptelivuo sent Tue 20 Sep 2016:
Cecil DeMille was a highly-regarded film maker. He lived in
Southern California. Southern California Timing Association is
a well-regarded speed trials organization still going strong,
see http://www.scta-bni.org/ .
117.5 mph in a Mark IV? Well, at 29 inch tire diameter and 4.27
rear end ratio that would need 5,817 rpm at the engine, just
for however long was needed for timing certification.
In terms of power needed, if the drag coefficient was 0.4 and
the car was five feet wide by five feet high, a quick power
estimate is 91 kW = 122 HP needed to go 117.5 mph. With a 3 1/2
engine, that HP was within reach, although I am uncertain about
achieving that HP at 5800 rpm.
If the engine could hold together long enough at that rpm, then
it seems quite possible. I’m not volunteering my engine, anyone
have any experience in rpms over 5000 on a 3 1/2 liter engine?
Norman Dewis told me he had tried his Mark V at 90 mph once and
was not interested in trying again. I note DeMille was not the
driver on that plaque, it lists Ron Cameron. DeMille was 67 at
the time of the SCTA run, maybe Cameron had the huevos to run
someone else’s car at that speed.–
Roger McWilliams
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In reply to a message from Roger McWilliams sent Tue 20 Sep 2016:
I’m rather more pessimistic about the the drag factor for
the MkIV.
About 20 years ago I drew a few graphs for the SS 100, XK120
and the SS Saloon.
Back then I figured you’d need about 250 bhp to run the
saloon at 117 mph. www.nostalgiatech.co.uk/new_page_4.htm
Peter–
The original message included these comments:
In terms of power needed, if the drag coefficient was 0.4 and
the car was five feet wide by five feet high, a quick power
estimate is 91 kW = 122 HP needed to go 117.5 mph. With a 3 1/2
engine, that HP was within reach, although I am uncertain about
achieving that HP at 5800 rpm.
In reply to a message from Darryl sent Wed 21 Sep 2016:
Hi,
Side-trekking a bit, but I think I remember a Top Gear
episode where a Porsche beat in acceleration a VW Beetle
falling vertically…
My MKV is quite happy at 70mph but I have to run it for a
while afterwards to cool it down or all the heat in the head
will cause local boiling (ask me how I know, having replaced
three core plugs in two years) but I have not revved the 3
1/2 Litre engine much past 4000rpm, as it has a whopping
110mm stroke (XK is long stroke at 106mm) I don’t think it
likes to rev too much.
Cheers,
Pekka T. - 647194
Fin.–
The original message included these comments:
Back then I figured you’d need about 250 bhp to run the
saloon at 117 mph.
Unless it is falling vertically!
Darryl Grey
1948 3.5 litre (comfortable at 60 mph)
In reply to a message from ptelivuo sent Wed 21 Sep 2016:
The ‘‘Autocar’’ road tested a 3.5 saloon and got 91 mph out of it.
For the record the chassis number is 637196. Knowing the
California hot rodding mindset, they probably bored and
stroked the engine, removed the headlights, air cleaner,
bumpers, muffler, seats, spare tire, and maybe the top and
windshield and came up with some sort of tonneau cover and
grille cover. It has a red block, which I have heard adds to
the zoominess.
I see it now has a one gallon fuel capacity.
I will admit to once getting my Mark V up to an indicated
100 mph, but it was probably really about 90.–
XK120 FHC, Mark V saloon, XJ12L Series II, S-Type 3.0
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In reply to a message from ptelivuo sent Tue 20 Sep 2016:
I would say very wishful thinking.
My MKIV has and Auscrank crankshaft,with 12 counterweights
and they tell me it’s good for over 6000rpm.
So with Chev mains, 3/8’’ valve lift , 1 3/4’’ carb bodies,
and a bit more comp and matched ports, it runs very well
but it just isn’t going to get enough mixture in to get
anything like that. Bearing in mind 93 mph was the factory
claim.
And an XK120 needed 160 bhp to achieve 120 mph with a
vastly more aerodynamic shape. Even if you took the
headlamps off the MKIV.
The new car will have a 3 1/2 litre engine with an
Auscarnk shaft, 3/8’’ lift etc and a blower. And a 3.54
diff. But I think I’d very very lucky to get 117 mph from
that. And that will have only cycle guards.
Although my engraver could make me up a plate …–
The original message included these comments:
In reply to a message from Rob Reilly sent Wed 21 Sep 2016:
With all the will in the world , I doubt the 3 1/2
pushrod block can be stroked. There is just not enough
room to swing longer throws.
And you come up with issues like the cut outs in the
bottom of the bores to clear the conrods.
One could bore out a little but not heroic amounts.
And the inherent limiting factor is the inlet tract
inside the head. Which can result in the end cylinders
starving.
Which is where a supercharger provides a good solution. It
doesn’t have to even be running positive pressure to see a
vast improvement, just getting better flow.
In reply to a message from Ed Nantes sent Thu 22 Sep 2016:
Of course at this late date we will never know what may have
been done to squeeze more top speed out of this car, nor the
accuracy of the timing by SCTA in those early days. But I
would think it was within 1 percent considering the
importance they attached to it.
For stroking I was thinking more along the lines of common
hot rodder practice in the 1940s with V8 Fords. They used to
regrind the crank throws off center and put in undersize
bearings; thus bringing the piston down lower on the intake
stroke but also allowing it to protrude above the deck a
little bit at the top of compression stroke. Sounds odd, but
apparently you could get away with this on Fords, so maybe
it would work on our 3.5 heads?
Along with milling the head, lower diff ratio, maybe bigger
valves, and of course they would not have a decompression
plate in the US. This car would have had the manual
distributor advance, right? I suppose one could experiment
with that to get more top end if that was the only goal.
Just idly imagining what one might have done back in 1949 to
tweak what was just a late model car to them.
It is no longer in the inventory of Gullwing, so if the new
owner joins us maybe we will learn more.–
XK120 FHC, Mark V saloon, XJ12L Series II, S-Type 3.0
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I notice one of the rules is no glass windshields.
Some sort of engine cover is required, but only on top, not
the sides. Fenders are not required, and a lot of the cars
seem to have oversize rear tires.–
XK120 FHC, Mark V saloon, XJ12L Series II, S-Type 3.0
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In reply to a message from Rob Reilly sent Thu 22 Sep 2016:
I guess you could remove the wings and lamps and either
remove or just fully open the windscreen and improve the
drag factor a bit. Whilst rather lower Old Number 8 with 169
bhp on 14:1 compression ratio achieved a lap speed of 125
mph around Brooklands.
In reply to a message from Peter Scott sent Thu 22 Sep 2016:
Don’t think my top end ever has exceeded 1/2 of which
y’all are trying to figure. Being a non engineer or
physics expert I hope none of you lay down the challenge
to duplicate. My buy high sell low philosophy would only
get worse. Fun to read your opines boys.
In reply to a message from Rob Reilly sent Thu 22 Sep 2016:
Interestingly , looking at the entrants none , bar one ,
is listed as a brand of car. Listed by parts in the engine
but little else.
The factory with 18008 had access to a variety of diff
ratios. Mainly because spiral bevel c
CW&Ps are realtively easy to make one offs
And they used some innovative approaches… Hillclimbs
used about 5.3:1 but didn’t use 1st gear. But in a MKIV
with a Salusbury diff making a one off CW&P would be a
giant project. The only available replacement a 3 .77:1
was later used in o/d XK120 but this was later.
This would also have been with the old alloy conrods.
I think Peter did a comprehensive article about the
relationship between power and top speed. I may be wrong
but I Think torque was also a factor in being able to
increase speed.
Looking at pics of entrants, it may be that the body was
removed.
I wonder if stroking would help as the longer the stroke
the less revs. I recall that David Barber was said to have
destroked a 3 1/2 SS engine in 18008 to get more revs and
hence more power.
In reply to a message from Ed Nantes sent Fri 23 Sep 2016:
Well, Salisbury or Dana axles were common in the US, and if
they took off the fenders, top, windshield and seats, put on
a tonneau cover and oversize 18’’ tires on the rear, it would
be fun to know what it really could do. But it sure is funny
to imagine a Mark IV DHC stripped down as a Lakester.
The main competition at this event was for American engines
only, so the Jag may have been run after the main event was
done, perhaps like a ‘‘run what you brung’’ fundraiser where
you pay your fee and get to run, collecting nothing but your
time and bragging rights. If so, the SCTA might have a
record of it. Rod Cameron was an actor known mostly for
cowboy pictures. Perhaps Cecil or Rod had the dash plaque
made by a local jeweler.
The main competitors’ car makes or brands are listed under
‘‘body’’, where nearly all of them are Fords so they just say
the year and Model T or A or V8. A few of them are belly
tanks or specials. Most of the engines are Ford or Mercury
V8s with a few exceptions like Cadillac, LaSalle or Chevy,
even a Duesenberg. With the Fords and Mercs it would seem
the choice of heads, manifold, cam and ignition were
important to these guys.–
XK120 FHC, Mark V saloon, XJ12L Series II, S-Type 3.0
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In reply to a message from Rob Reilly sent Fri 23 Sep 2016:
Rob
I think the Dana’s were all 4HA and the Mk4&5 and XK120
ran a 2HA
But if I’ m wrong and there are other ratios available it
would be good to know.
Especially for MKIV owners who are inclined to want lower
revs for cruising.–
The original message included these comments:
Well, Salisbury or Dana axles were common in the US, and if
In reply to a message from Ed Nantes sent Fri 23 Sep 2016:
I would begin a search of Ford, Jeep, Willys, Studebaker,
Kaiser, Frazier and Dodge trucks of the era. The Dana 41
appears to be the same as the 2HA.–
XK120 FHC, Mark V saloon, XJ12L Series II, S-Type 3.0
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In reply to a message from Rob Reilly sent Fri 23 Sep 2016:
Lompoc is 200 miles from El Mirage dry lake. Lompoc has an air
force base, Vandenberg AFB now. There was an army base, Camp
Cooke there previously, that closed in 1946. The air base came
in the mid-1950s. Maybe there was high speed trials on the
straight long beaches on the base or one of the several mile
long straight roads in the interim.
Note the date of the run in Lompoc is 9 May 1949, that was a
Monday, not likely part of the El Mirage activities so far
away.
Of course, De Mille could have the plate made as a joke, but we
do know he filmed movies on the beaches in the area, such as
the 1923 film ‘‘The Ten Commandments’’.
My impression is the 2HA was a licensed production of Dana rear
ends, some of the English production had British threads and
some did not. Not sure what was used in 1949.
Guess we would need some corroborating evidence to think his
car, modified or not, went 117.5 mph even for one second.–
Roger McWilliams
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I will post some pics once I get
