Hi Gang,
Today I was asked why dirty air cleaners hurt the
performance of our LBCs so badly. After a little thought,
here is the answer I founded. As usual for me, it is TMI.
The solution to this problem lies in understanding
pressure. First, let's understand the ranges of pressure
within the system.
On the inlet manifold-side of the carb's butterfly valve,
the pressure ranges from a theoretical maximum of +1 bar,
gauge, [for example, when one lifts at redline at the end
of a straight] to near atmospheric at WOT [right before
the lift...].
On the air cleaner side of the butterfly valve, the pressure
is almost always atmospheric! The "almost" is where the
flow-rate of the air cleaner comes into play. To understand
this side of the solution, we first must understand how the
SU carb works.
Air flowing over the venturi [the little restriction in the
bottom of the carb where the jet sets] has to speed up or
there would be an imbalance between what goes in and what
goes out. Faster moving air exhibits lower pressure.
Therefore, when we open the butterfly with the throttle pedal,
the atmospheric pressure forces air into the carb throat.
That air accelerates over the venturi, dropping it's pressure
and causing fuel to flow up out of the jet (and around the
needle) to be atomized into the air flow. Once this 'charge'
passes the butterfly valve, the pressure drops even further
and any fuel droplets left in the liquid state flash into the
gaseous state -- or at least that is how it works ideally.
But what happens when the air cleaner is restrictive
because of design or dirt, not that any of our LBCs would
ever have dirty air cleaners. First of all, the flow rate is
reduced and the pressure on the air cleaner side of the carb
drops BELOW atmospheric. Therefore, to get more charge
into the cylinders, we have to open the throttle (butterfly
valve) further. Economy goes down, not that that is a key
issue here. The pressure difference between the carb bowl
and the jet, where the pressure is supposed to be at
atmospheric, other than for the venturi effect, is also
increased. Effect? The mixture is FURTHER enriched because
of an imbalance in the fuel level in the jet!
So what does this translate to in terms of daily life? A
number of things:
(1) Rick should not be allowed at his keyboard after lunch
on Friday.
(2) Dirty or poorly designed air filters restrict air flow
causing overly rich running for obvious and not-so-
obvious reasons. Hopefully the not-so-obvious
ones are now better understood.
(3) Overly rich running conditions cause carbon build-
up, fouled plugs and hard-to-start, stinky cars. Yet,
these conditions are avoidable by simply making sure
your LBC can breathe!
Happy Friday!
Rick K&N-filers-on-all-performance-cars Lindsay
(no K&N connection, just a happy customer)
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