Larry,
Thanks for the info on the A.
I have re-read my posts on SU operation. It is frustrating for me because
I have a very clear picture in my mind of what I am trying to say but the
words don't do a very good job of painting the same picture. I could
explain it in 30 seconds with a sketch on the back of a napkin. I will
give it one more try.
The typical gasoline engine uses adjustments of air pressure (the opposite
of vacuum) at the intake valve to control power output. If that pressure
is high (low vacuum), more air (by weight) flows into the cylinder and the
engine makes more power. if that pressure is low (high vacuum), a lower
mass of air flows into the engine and less power is created. For most gas
engines, intake valve pressure is about the same as manifold pressure.
Mannifold pressue is adjusted by restricting the flow into the manifold
using a "throttle valve." An open throttle valve allows the pressure in
the intake manifold to equalize with the surrounding atmospheric pressure
(low vacuum) and the engine develops max power (ignoring superchargers, ram
effect, air cleaner loss, price of tea in China, etc). A closed throttle
valve restricts air flow to below what the engine would pull in without
this restriction so the pressure in the manifold reduces (vacuum
increases).
So manifold pressure is a product of how fast the engine wants air and how
fast the throttle allows it to flow. For a given engine rpm, high
manifold pressure (low vacuum) will result in a high mass volume of air
flow and low manifold pressure (high vacuum) will result in a lower mass
volume of air flow. Common sense says that and 1800cc engine would always
use 1800cc of air every two revolutions but remember, air mass is
proportional to air pressure so when the throttle is closed, that 1800cc of
air contains far fewer molecules of oxygen (and nitrogen and cow farts) so
the mass of airflow is lower. Remember that when you here an air / fuel
mixture of 14.7:1 they are talking 14.7 pounds of air to one pound of fuel,
not 14.7 gallons of air to one gallon of fuel. So, manifold pressure
regulates how much power an engine can produce.
The SU carburator dashpot is controled be the difference of pressure
between the entrance of the carb and the bottom of the slide. Both of
these are outside of the throttle valve so they are not directly effected
by manifold vacuum. In fact, if you think about it, piping manifold vacuum
(the difference of pressure between the manifold and the atmosphere) to the
top of the dashpot would result in the opposite behavior than what would be
desired. At low load the vacuum is high so it would pull the dashpot up
and dump loads of fuel into the engine (or else destroy the venturi effect
and stop fuel flow completely). At high load the vacuum is low and the
dashpot would drop effectively cutting off airflow.
So as you can see, using manifold vacuum to help control an SU would not
work. The dashpot is designed to keep a constant pressure difference
accross the jet. It does this by raising and lowering the slide in
relation to the mass volume of air flowing into the engine. Since the
pressure at the front of the carb and the bottom of the dashpot slide are
both, theoretically, constant, density of the air remains constant in this
area regardless of throttle position. Since the power output level is
controlled by the throttle body controling the density of air in the
manifold (as measured by manifold vacuum) the jet control is completely
unrelated to the loading level of the engine. It is controlled only by the
mass volume of air the engine requires.
I hope this helps. If not, it is the best I can do without pictures. I am
sorry.
Regards,
Bill Eastman
61 MGA
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