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RE: [FlyRotary] Returnless Fuel System - Fuel Cooler
Hi Todd,
Over the years, I've done a lot of
carburettor tuning. IDA Webers were my favourite device until digital EFI
came along.
Now from practical experience, I know beyond
all reasonable doubt that the size of the hole in any Weber jet
controls the flow capacity of either fuel or air. The bigger the
hole, bigger the flow, and vice versa. I also know from my
engineering studies that I can calculate the actual flow rate, given the
viscosity of the fluid, the shape of the jet, and the pressure drop
across the jet. (I don't bother "number running" because I have a box full
of different sized jets and just keep changing them until I get the correct
Air/Fuel ratio out the pipe. All else being equal, double the
diameter, increase the flow rate 4 times.
So I just can't see what the size of an
attached pipe has to do with the flow rate through a "vena contracta"
("vena constricta"??). You also say "but much of this pressure is
recovered downstream" Hmmmmmm, ... I say "so what"??. We aren't
interested in pressure drops or pressure recovery, ... it's flow rates that
are the SOLE issue, and the capacity of an open flow 1/4 inch
line to get rid of the fuel dumped by the regulator.
Somehow the sizes have got all muddled up (better
give Wally the muddle headed Wombat the message to move on!!). If the
5/16" supply is restricted by a 3/16" "vena constricta", then
the flow rate is controlled by the 3/16" constriction, the pressure drop
across it, and the viscosity of the fuel. Doesn't matter if it's
flowing into a 12" sewer pipe or a 5/16" fuel line, the flow limiter is
the smallest cross sectional diameter and the pressure drop across it.
Conversely, the fuel return line is open flow without any back pressure
except ambient barometric.
My contention, backed up by LOTS of practical
observation is that the 1/4' open flow return line will flow more than the 3/16"
restriction will allow under the controlled pressure of the regulator. So
please feel free to correct me if I'm wrong in my analysis.
Cheers mate,
Leon
----- Original Message -----
Sent: Saturday, February 19, 2005 6:12
AM
Subject: [FlyRotary] Re: Returnless Fuel
System - Fuel Cooler
Hi Bob;
SNIP>>
I should point out that while it has been mentioned that
since the outlet of the pumps is often less than the ID of the fuel line this
is your limiting factor in flow. This isn't really the case. A small
restriction such as this creates a pressure drop through the "vena contracta",
but much of this pressure is recovered downstream. If anyone actually cares
about this I can pass along the formula for calculation of the permanent
pressure drop through an orifice. This means that a 1/4" line with a 3/16"
restriction at some point will not have the same lower flow as a 3/16" line,
but will still be slightly less than a 1/4" line with no
restriction.
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