In a message dated 7/19/2007 2:47:34 A.M. Eastern Daylight Time,
lendich@optusnet.com.au writes:
You want to build a flow bench, or rent time on one and see the possible
outcomes of different schemes that people propose as the very best there is.
There is only one for each situation, and millions are spent each year trying to
sneak something by mother nature, but she always wins. She cannot be fooled by
the likes of us.
Likely a larger carb with a uniform tapered runner to the port. The end of
the runner flattened just to match the port shape at the gasket. Straight runs
if there is room. A "D" shaped curve if one is required. The flat of the "D" to
the inside of the curve. Maintain the same rate of change in velocity all
through the runner.
For a good sized periphery port 12A, you would see 310 HP at 10,500-700
RPM. Nice, but not useful.
The choke size for this is 44MM in a 48MM Weber. That is as big as you can
go with that carb without major modifications. The closer the choke size
(smallest part of the carb) gets to the butterfly size, the smaller the vacuum
signal used by the booster venturis to supply fuel. The aptly named choke
(removable main venturi)
supplies a restriction so Mr. Bernnoulli's principal can produce a low
pressure to dip the booster venturi into like a straw in your mouth pulling on a
Slurpy, the booster is pulling on a bowel of fuel. Not much of a pull
(vacuum) and the carb runs lean at top RPM.
The advantage of fuel injection, is that the fuel is pumped into the air
stream, and no venturi is required. So the tube has no restriction to slow flow.
The top flow rate of the tube the same size as a carb will generally be very
much higher.
Carbs are sized based on their butterfly size, and that is the biggest part
of the hole where all of the air goes through. It is only a vague indicator of
maximum flow through the carb. Internal shapes and choke size have a much bigger
affect on total flow.
The two major styles of carbs have either a booster venturi in the choke
area, or for slide valve carbs like the Mikuni, Stromberg or SU, a
flat ramp forms the choke area, and a single jet sits on that ramp, and a
tapered needle hangs through that jet. As the carb is opened a slide is pulled
up by vacuum or a cable, and the taper rate on the needle controls the amount of
fuel pulled into the air stream through the single jet.
The advantage of this slide valve carb is simplicity. Very few parts. Good
mixture control in various flow rates. Poor overall but good at a specific rate.
(single RPM).
The disadvantage in this carb is essentially its internal shape (the
intersection of two big drilled holes.
Or a combination of poorly shaped intersections. The flat ramp for the
slide to seat against, forms the choke.The vertical edges of the hole the slide
operates in, and production inattention to removing sharp edges. The float-less
Posa is the most simple iteration of the slide valve carb, and the last years of
the Stromberg with hundreds of widgets clamped on it to help agree with
pollution rules, the most complex.
So, in the slide valve carb, the butterfly size is a bit less of an
indication of total flow, than the booster style carb. But I run on as
usual.
Looking at a dyno sheet for a 12A with 36MM chokes in a 48MM Weber, I see
205 HP at 7,200 RPM and 218 HP at 7,500 RPM. This is a side port engine, with
runners that are too short to produce best power below about 8,200 RPM or 237.0
HP. Peak power for this engine was 244.8 HP at 9,400 RPM.
You get a big boost for the bigger displacement of the 13B housings, and
bigger boost from the Pport, and you will probably lengthen the intake runner a
bit to peak just before 7,200 RPM. The 44MM Mikuni sounds like it might flow
more than a Weber 48 with 36 MM chokes, but that may not be the case. You get a
bit from the better bearing loads from a stiffer crank and two counter weights
per rotor.
So let us pretend that the Pport on a 13B housing does no better than 20%
over the side ported smaller engine, so that the 205 number becomes 102.5 X 20%
or 123 HP. And the 218 number becomes 109 X 20% or 130.8 HP.
How about cruise power at 6,600 RPM? The side ported 12A (bridge port) is
well off the tune, but has 178 HP. So, that becomes 89 X 20% or 106.8 HP.
Not too shabby at all. You would want the 2.78:1 reduction to avoid using a
short prop.(smaller diameter). So you have a prop at 2,374 RPM, which is quite
reasonable.
This works for a very large number of aircraft designs.
I have drawings for a simple dyno and thrust measuring system if you like.
Also a simple flow bench, if anyone wants them.
If it didn't happen on the dyno, it didn't happen.
Lynn E. Hanover