Simon Matthews wrote:
> Seriously, I was told or read that this is what really happens: as you move
> to high flow rates, the petrol tends to stick to the sides of the pipes
> instead of being mixed with the air.
Depends to a great extent on the design.
Side draft carb setups are victimized by this, of course, as gravity is always
working against it. The SU carb design does not improve the situation as the gas
dribbles out from a bridge positioned along the bottom of the damper piston.
The "venturi effect" tries to counteract this. This is where the narrowest
diameter along the intake is where the fuel is introduced to the air flow. This
causes and increase in air speed at this point. That is the point of the
restrictive-looking bridge along the bottom of the SU carb mouth.
But, yes, gas is heavier than air and, as it is introduced along the bottom of
the intake passage, will have a propensity to stay there without some turbulence
to boast it into the air stream. That is the reason the bridge across the intake
for the gas jet is so abrupt -- try to make the fuel droplets "hang glide" into
the air stream off the back of the bridge. (Sorry for the technical jargon.) In
any case, never, ever, take your Dremel to this part of the carb.
Down draft carbs drop the mixture into a manifold plenum and, at decent flow
rates, the intake vacuum immediately sucks it towards the open intake valve.
Hopefully, the intake runner is short enough to get the fresh mixture all the
way
to the cylinder before the valve slams shut.
The cardinal rules for "wet" intake flow is try to go downhill all the time and
the more the better. Even the "side draft" carbs are supposed to have a slight
down angle. This is the reasons that the Weber DGV setup for Triumph sixes seems
like such a bad idea. They have an adapter that looks for all the world like a
sewer trap. The mixture leaves the bottom of the DGV then must get twisted and
sucked up into the manifold runners. I'm surprised the motors don't choke on
every hard right turn as the puddled gas gets splashed up into the engine.
Say what you want about the Weber DCOE (side draft) carb but it introduces the
fuel into the middle of the air stream and inside its own, small venturi. This
promotes better mixing at medium to high air flow rates.
And for those extrovert, masochistic, demented mini perverts, the hot setup for
the sideways 'A' engine is to have a lot of down draft into those motors. That's
why you can find the velocity stacks pointing up towards where the cowl vent
would be. It would seem that such a manifold would be cool on a Midget once you
bought one of the newly offered plastic bonnets and Dremeled out a hole to fit
the carbs. Probably better for RHD cars, though.
> This is why carbs have accellerator pumps (or in the case of SUs, the damper)
> -- to compensate for the temporary lack of fuel as it starts running down the
> walls of the pipes.
Now you lost me.
Accelerator pumps and the SU damper system tries to correct for large changes in
throttle position. Without the SU damper system, sudden throttle plate openings
would cause vacuum to plummet. Dumping a stream of gas into the intakes is an
effort to counteract this problem which is the only purpose of an accelerator
pump.
The SU damper system takes another tact. It allows you to only suggest the flow
rates with the throttle plate. The movement of the damper piston controls the
actual air/fuel ratio. When you first nail the throttle, vacuum drops and the
piston momentarily drops. This tries to maintain "constant depression" (vacuum)
by limiting air intake and momentarily increases fuel flow as the throttle
needle
plunges into the gas pool of the jet. Then the engine recovers and creates more
vacuum, the float bowls bring the fuel level in the jet back into equilibrium,
and the air piston and gas needle rise in unison and, perhaps, the motor
throttles cleanly.
--
Bob Allen, Kansas City, '69CGT, '75TR6, '61Elva(?)
"I got lost in thought. It was an unfamiliar territory."
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