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Subject: Re: [FlyRotary] Re: Mikuni
Date: Thu, 19 Jul 2007 12:24:39 -0400
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George,
  Something to remember is that a 44mm Webber venturi is the equivilent=20
of about a 40 mm motorcycle slide throttle carb. The Webber has=20
considerable obstruction. The Webber has a secondary venturi, (the=20
little bullseye one), in the bore AND a butterfly valve after the=20
venturi. The slide throttle motorcycle carb has no obstructions at WOT=20
except for the jet needle. (which isn't much)
Bill Jepson
PS, as Lynn mentioned you would have to use a flow bench to determine=20
which configuration flowed better.
WRJ


-----Original Message-----
From: Lehanover@aol.com
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Sent: Thu, 19 Jul 2007 8:09 am
Subject: [FlyRotary] Re: Mikuni


In a message dated 7/19/2007 2:47:34 A.M. Eastern Daylight Time,=20
lendich@optusnet.com.au writes:


Lynn,

Talking to David McC on carbies. I believe you advised him that 2x 42mm=20
Mikuni's are sufficient for a 13B to 300 hp.

I'm working on the single and for the demonstrator I will stick to=20
Carby, I'm looking at going for a 44mm inlet and am hoping for at least=20
125hp at a max RPM of 7,500, if I can get it at 7,200 all the better.

=A0

If I go for a 44mm tube what size Mikuni should be sufficient?

=A0

The tube I have is 41mm or 44mm, should I go for the 41mm ID for=20
increased velocity and a 42mm carby or a 45mm carby?

George ( down under)


You want to build a flow bench, or rent time on one and see the=20
possible outcomes of different schemes that people propose as the very=20
best there is. There is only one for each situation, and millions are=20
spent each year trying to sneak something by mother nature, but she=20
always wins. She cannot be fooled by the likes of us.

=A0

Likely a larger carb with a uniform tapered runner to the port. The end=20
of the runner flattened just to match the port shape at the gasket.=20
Straight runs if there is room. A "D" shaped curve if one is required.=20
The flat of the "D" to the inside of the curve. Maintain the same rate=20
of change in velocity all through the runner. =A0

=A0


For a good sized periphery port 12A, you would see 310 HP at 10,500-700=20
RPM. Nice, but not useful.

=A0

The choke size for this is 44MM in a 48MM Weber. That is as big as you=20
can go with that carb without major modifications. The closer the choke=20
size (smallest part of the carb) gets to the butterfly size, the=20
smaller the vacuum signal used by the booster venturis to supply fuel.=20
The aptly named choke (removable main venturi)

supplies a restriction so Mr. Bernnoulli's principal can produce a low=20
pressure to dip the booster venturi into like a straw in your mouth=20
pulling on a Slurpy, the booster is pulling on a bowel of fuel. Not=20
much of a pull

(vacuum) and the carb runs lean at top RPM.

=A0

The advantage of fuel injection, is that the fuel is pumped into the=20
air stream, and no venturi is required. So the tube has no restriction=20
to slow flow. The top flow rate of the tube the same size as a carb=20
will generally be very much higher.

=A0

Carbs are sized based on their butterfly size, and that is the biggest=20
part of the hole where all of the air goes through. It is only a vague=20
indicator of maximum flow through the carb. Internal shapes and choke=20
size have a much bigger affect on total flow.

=A0

The two major styles of carbs have either a booster venturi in the=20
choke area, or for slide valve carbs like the Mikuni,=A0Stromberg=A0or SU,=20
a flat ramp forms the choke area, and a single jet sits on that ramp,=20
and a tapered needle hangs through that jet. As the carb is opened a=20
slide is pulled up by vacuum or a cable, and the taper rate on the=20
needle controls the amount of fuel pulled into the air stream through=20
the single jet.

=A0

The advantage of this slide valve carb is simplicity. Very few parts.=20
Good mixture control in various flow rates. Poor overall but good at a=20
specific rate. (single RPM).

=A0

The disadvantage in this=A0carb is essentially its internal shape (the=20
intersection of two big drilled holes.

Or a combination of poorly shaped intersections. The flat ramp for the=20
slide to seat against, forms the choke.The vertical edges of the hole=20
the slide operates in, and production inattention to removing sharp=20
edges. The float-less Posa is the most simple iteration of the slide=20
valve carb, and the last years of the Stromberg with hundreds of=20
widgets clamped on it to help agree with pollution rules, the most=20
complex.

=A0

So, in the slide valve carb, the butterfly size is a bit less of an=20
indication of total flow, than the booster style carb. But I run on as=20
usual.

=A0

Looking at a dyno sheet for a 12A with 36MM chokes in a 48MM Weber, I=20
see 205 HP at 7,200 RPM and 218 HP at 7,500 RPM. This is a side port=20
engine, with runners that are too short to produce best power below=20
about 8,200 RPM or 237.0 HP. Peak power for this engine was 244.8 HP at=20
9,400 RPM.

=A0

You get a big boost for the bigger displacement of the 13B housings,=20
and bigger boost from the Pport, and you will probably lengthen the=20
intake runner a bit to peak just before 7,200 RPM. The 44MM Mikuni=20
sounds like it might flow more than a Weber 48 with 36 MM chokes, but=20
that may not be the case. You get a bit from the better bearing loads=20
from a stiffer crank and two counter weights per rotor.

=A0

So let us pretend that the Pport on a 13B housing does no better than=20
20% over the side ported smaller engine, so that the 205 number becomes=20
102.5 X 20% or 123 HP. And the 218 number becomes 109 X 20% or 130.8 HP.

=A0

How about cruise power at 6,600 RPM? The side ported 12A (bridge port)=20
is well off the tune, but has 178 HP. So, that becomes 89 X 20%=A0or=20
106.8 HP. Not too shabby at all. You would want the 2.78:1 reduction to=20
avoid using a short prop.(smaller diameter). So you have a prop at=20
2,374 RPM, which is quite reasonable.

This works for a very large number of aircraft designs.

=A0

I have drawings for a simple dyno and thrust measuring system if you=20
like. Also a simple flow bench, if anyone wants them.=A0

=A0

If it didn't happen on the dyno, it didn't happen.

=A0

Lynn E. Hanover=A0=A0

=A0

=A0


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