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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