Jay Laifman wrote:
> Also in the Horn is an article called "Technical Tips: Not Going Holbay."
> He makes an argument why "bolting on 2 40 DCOE Webers" on an otherwise
> standard engine will not produce any startling results" based upon carbs
> being too big so that velocity of air will be too low. While this may be
> true for any carb being too big at low RPM's, Webers are infinitely
> adjustable and can be made to be the correct size. Also, it is not true at
> high RPMs and in fact the better breathing Weber is better at high RPMs.
> Instead of Webers, superchargers or wider cams (which he also dismisses
> because of low rpm power loss), he suggests other means of getting more air
> velocity into the carbs by creating an air ram that uses a 4" tube to
> collect air in the front of the car and ram it into the air filters. He
> uses this with a 28/36 Weber down draft. He guesstimates that the ram
> alone will boost his hp by 12 bhp.
>
> Jarrid, your thoughts on this side of the pond? It seems to me that the
> air ram is also useless until the car gains momentum and air is being
> rammed into the tube. Maybe the better idea is to have both Webers and the
> ram.
> Jay
[beware long and techy]
Jay,
Ooooh Ooooh!
A Tech posting!
Interesting theories, but I agree with Jay.
I hope the author is not with us, as I would hate to offend, but I disagree in
all respects,
save perhaps that webers alone will not fix all wrongs.
Point #1, that a horn when installed will with a 32/36 DGV, will enhance
performance
on the order on 13 to 15%?
Not likely, the problem here is that the airflow flowing through each choke
will be always
at different velocities, and aproaching equal at higher RPMs, the flow going
into the ram tube
will always be turbulent, and will never laminarize. Smoothing the airflow
into the carbs is always
a good idea, but the SCCA tested such airhorns for the 32/36 DGV, and found
their improvement
marginal, and is now allowing the use of such a horn in spec Formula Ford.
High air velocity is not the reason that an engine makes good power, it is a
side effect of the
compromise in attaining to proper pressure differential across the chokes, and
thus head to
pull gasoline through the mains. In general lower velocities will mean lower
pressure
diff across the chokes, and a weaker head on the gasoline. Main jet sizes will
tell you if
your velocity is too low, as when the jets get too big, to make it run run
right, lower load
opperations will suffer, and need to be compensated with other mechanisms like
bleed
and pilot enrichments.
Real race engines will sacrifice low end fuel control, for high RPM control
linearity and
lower induction restriction.
Point #2, that the low RPM air velocity is too low with the webers, would
entirely depend how
the setup is tuned. The difference between running 1.7 liters (every 720
degrees) or air through
4 chokes "venturies" like dual DCOE, or 2 chokes like twin 36 zeniths, when
both setups have
the same choke diameter, simple logic would tell you that more chokes will give
higher flow
or less restriction. Unfortunately, its not a linear correlation in cross
sectional area, but actually
in how the flow is multiplexed in shared runners, in relation to single choke
to runner systems "dcoe".
The magic factor here would be about 1.3, where dual 1bbl zeniths with 2 28mm
chokes will flow
about 70% as well and dual DCOEs with 4 28mm chokes. The air velocity would be
more or less
the same if the cross sectional area of the DCOEs were reduce by the
reciprocal. This would yield
similar levels of performance, all things being the same.
This says that putting in 23 or 24mm chokes in the DCOEs will be about the same
velocity wise
as the stock carb setup.
In actuality, DCOEs are somewhat more efficient in design than conventional
carbs, and
opperate very well at substantially reduced choke velocities.
So, in a nutshell, the weber DCOEs, will at the very least be configurable to
the same
performance\drivability compromise offered from the factory, with an immense
supply
of jets and chokes and parts to allow them to opperate forever.
Now for velocity stacks on DCOE equiped engines.
As I said before, smoothing the flow is always a good idea. DCOE's have a
sharp
flow transition as air enters the carbs. The velocity stack allows the air to
enter the horn
at a lower velocity, accelerate and to enter the carb at max velocity, without
flowing across that
sharp edge. While ram is nice to have, it requires induction resonance, which
you just
dont get across the rev range of the engine.
Q. Since you cant use ram at all RPMs, why do we really fuss over using the
velocity stacks?
A. Because the velocity stack increases our runner length, which lowers the RPM
point of
resonance, increasing torque at more usable RPMs, where and when we are in
resonance.
I think i calc'd a desired 13 inch runner length for a 1.4 inch id alpine
runner, to create
resonace a 5000 RPM. The velocity stack gives us some "resonant
supercharging", while
getting us closer to a more realistic resonant point.
Stepping of the podium now....
Jarrid Gross
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