Bill,
Soldering the poppet valves has been done by many drivers. The idea of
the springs was to allow excess air to enter the engine during the
over-run or coast down period of operation. Over time, though, the
springs got weak and the engines didn't run as well, so almost everyone
has soldered them shut.
Re-shaping the butterfly spindle (to make the air flow over it
smoother) is a handy idea. Question, though, is at what shaft angle one
wants to have it streamlined? The racers want it at full open throttle.
A street driver would want it at a different angle. What effect is there
at lower air velocities for street use? Hard to measure, but we know
that the air flow over a round object is far smoother at most velocities
than it is over an airfoil that is operating at a different angle from
that for which it is designed.
For a better description of air flow over a round object, hunt up
CJMarchaj's books on aerodyamics, or Manfred Curry's books. These show
patterns of turbulence that develop behind objects in a wind tunnel. Me?
I would leave the shafts alone in a street car.
The butterfly screws should be smooth, flush with the shafts. They are
in the carbs on my MGs. The smoother they are, the less turbulence is
created in the air flow past them. One could trim and smooth if they
project.
The locking compound, or an equivalent way to keep the screws in place,
is important, as screws mixing in with the air and gas to the engine have
adverse mechanical effects. One does not want bits of brass bouncing
around in the cylinders.
Bob
On Thu, 15 Jun 2000 09:05:15 -0400 Bill Saidel <saidel@crab.rutgers.edu>
writes:
> Morning,
>
> I was reading Burgess's book, How to Power Tune MGB
> 4-cylinder Engines and
> in his section on carburation, he gives numerous tips about 1.5inch
> SU's
> regarding the butterfly and spindles.
> 1) solder poppet valve shut (BTDT)
> 2) reshape the butterfly spindle
> 3) trim the two screws holding the butterfly to its spindle and lock
> them
> with locking compound.
>
> I am wondering if any lister has done those mods for a street
> machine and,
> in real street performance, they make any difference.
>
> TIA,
>
> Bill
> 76B
>
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