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George,
Something to remember is that a 44mm Webber venturi is the equivilent of about a 40 mm motorcycle slide throttle carb. The Webber has considerable obstruction. The Webber has a secondary venturi, (the little bullseye one), in the bore AND a butterfly valve after the venturi. The slide throttle motorcycle carb has no obstructions at WOT 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 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, lendich@optusnet.com.au writes:
Lynn,
Talking to David McC on carbies. I believe you advised him that 2x 42mm Mikuni's are sufficient for a 13B to 300 hp.
I'm working on the single and for the demonstrator I will stick to Carby, I'm looking at going for a 44mm inlet and am hoping for at least 125hp at a max RPM of 7,500, if I can get it at 7,200 all the better.
If I go for a 44mm tube what size Mikuni should be sufficient?
The tube I have is 41mm or 44mm, should I go for the 41mm ID for 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 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
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