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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