Carb Tuning

[MTrat]

Someone expound on carburetor vanes and 'UFO'...

[Bullfrog]

If you do a search on "vanes" you'll find the information you are looking for.

Ditto on "UFO".

Ed

[olddogs]

I have both installed in the 34 mm carb on my Super Combat. The UFO fills the void area under the carb slide. It attaches right to the bottom of the slide and is meant to improve airflow by smoothing the air out as it passes under the slide and through the carb.
The vanes divide the airflow into layers and are also meant to smooth and speed the air as it flows through the carb. It basically divides the carb into layers. At 1/4 throttle the vanes block the air that is bumping into the front of the slide and tumbling around waiting to get sucked in. It allows for crisper jetting and throttle response. The divider's are both in front of the slide and behind. The more layers, the better.

[ossa95d]

Back in the late '60's when we were all building our own engines for short track we would argue incessantly about turbulent vs. laminar flow in ports. I wonder if any of the same arguments would apply in carburetors, although I realize there are more specific flow requirements in the venturi and jets so maybe not. I understand the benefits of vanes and UFO's. Do you think that turbulence plays any beneficial role in a carburetor? If so what do you think is the right balance? Maybe laminar flow is the right dynamic in a venturi to get a consistent pressure drop. It's all theory until used in a practical application. I know there are guys here that have the practical experience.

[MTrat]

Thanks for the useful info. Some pictures show crossed vanes, others show horizontal. Does vertical work at all, or does it seem to matter. On the UFO, is something like JBs likely used?

[Bullfrog]

It seems to me that the "jury is in" on the question of smooth airflow vs. turbulence as a desirable feature . . . and smooth air flow wins. Note that both the UFO and the vanes are methods of smoothing air flow through the carb.

While I'm not a dyno operator, I did stay at a Holiday Inn once . . . and it is clear that smoothing the airflow through the carb results in 1) more air getting into the engine with each revolution, which allows(actually, requires) 2) more fuel to be introduced to the engine, which 3) results in more power being produced. These effects appear to be significantly larger than those of improving the mixing of the fuel mist with the incoming air due to turbulence.

From my own seat-of-the-pants experience, I know that installing a vane on each side of the slide at the half throttle position seems to have completely eliminated the tendency of a stock, piston-port, Combat Wombat engine with stock 28mm carb to "load up" if the rider allows engine speed to drop too much and then grabs a handful of throttle. I have no reportable direct experience with more vanes. To the extent that they might reduce turbulence more and allow more to air flow, they would almost certainly be good. Though there is surely a "ceiling" on how many vanes to use, since at some point (more than 6 or 7?????) the vanes start to become an effective "plug" (they fill part of the throat of the carb and there is air drag to be considered).

Ed

[olddogs]

Answer to a couple questions. The vanes are horizontal. I have 6 vanes, 3 on each side of the slide. 2 will work as it essentially cuts the carb in half allowing you to more accurately jet low end and top end. The wisdom is 2 are good, 4 are better, but 6 is best.

As for the UFO, it is sculptured for flow across the bottom of the slide. It is held in with JB Weld to fill any gaps and has a screw coming down from the inside of the slide to keep it in place. If the UFO ever came off the reed cage would catch it. If the retaining screw were to come out it would just rattle around in the slide cup until you removed it.

Getting consistent idle rpms are the only bugs I have ever had to work out. With a ported cylinder and a HT pipe, throttle response is unbelievable.

[admin]

Speaking of the HT pipes. I have just spoken to my friends at Starting Line Products and they are almost ready to start another production run if we have the demand. I will be sending out notices shortly for a "buy in" offering again. We have to receive at the very least thirteen pipes to be pre-sold if we are going to start and complete the production this spring.
Paul

[racerclam]

Here are examples of the venture dividers that I build . I have many many carbs running out there in bikes , snow mobiles , go karts quads , yes they do work . Give me a call if you want to know more.

Rich

[racerclam]

Here is the other side that didn't load up.

[ossa95d]

Venturi dividers seem to make a lot of sense and it's great to hear how well they work in application.

Ed thanks for the response regarding laminar flow. I agree and now believe that for different reasons laminar or smooth flow makes a great deal of sense in both the carburetor and the exhaust port. However, it appears that the argument continues 50 years later regarding the benefit of turbulence in the intake and transfer ports. This quote is from Smitty's Cylinder Porting Guide,

"For better atomization of the fuel mixture, intake and transfer ports need to be “roughened”. This aids in breaking-up fuel bubbles, and ideally will turn wet fuel into a vapor, as the more atomized the fuel mixture is, the better combustion will be, and the better the fuel mixture will expand."

Gordon Jennings seems to be somewhat silent on the subject although he appears not to advocate over polishing of the ports. He also does not appear to support too much turbulence caused by uneven port walls or ripples. Of course no one disputes the performance robbing nature of mismatched ports or abrupt obstructions.

I know, this has nothing to do with vanes in the carburetor, but it was the basis for my original question about turbulence. I accept and agree with your explanation. Thanks for your patience with this old timer. I will shut up and listen now.

[Bullfrog]

I wonder if Smitty's recommendation for roughened ports might have come up with an incorrect theory to support something which was indeed proven to work.

A more modern theory is that a "roughened" surface aids in keeping the boundary layer of air attached to the port walls (actually reduces turbulence). The attached boundary layer is a thin "coating" of air between the port wall and the moving air column passing through the port. This thin slower moving layer (sort of, kind of) provides a thin "slick" layer for the remaining air to flow past. When the boundary layer detaches from the port wall, turbulence increases by orders of magnitude . . . and flow through the port passage decreases. Again, it seems that delivering MORE trumps delivering "well mixed".

Harry Taylor once told me a story of doing a base-line dyno check on an engine - then he went in and ground "golf ball dimples" in the walls of the transfer ports . . . and confirmed a 4% increase in power for the engine on the dyno. He didn't really worry about "how" the dimples did their work . . . he just had proof that they did work. My own reading on boundary layers, aircraft, the roughness of shark skin . . . and many other things relating to drag reduction/improving flow . . . suggests that control of the boundary layer to reduce turbulence is the performance enhancer.

Ed

[Bullfrog]

PS: Harry's golf ball dimple experiment was performed many (many) years after the demise of Hodaka.

Ed

[racerclam]

THe venturi dividers arnt about just smooth air flow and mixing. One is that carbs actually like to breathe from the side since air likes to accelerate around corners , the dividers that are at 1/ and 3/4 positions grab the air and direct it under the slide and the 1/2 position that I scallop cut allows air entry from the side , ( proven on a flow bench) after the air passes the slide the dividers on the engine side keep air straight and velocity high by not expanding until it reaches the manifold or reed cage. The end result is a more linear power application and stronger pull every where. One example is a customer with a shifter cart said now he can sin the tires out of the corners and drift where as without dividers would . That carb was a 40mm Kehin flat slide. Also note worthy is that the addition of a UFO will cause a round slide to out flow a flat slide carb up to 13/16 throttle opening and at wot is almost equal. All in all a large carb with dividers behaves small with big gains.

Rich

[racerclam]

Funny Myth busters built a car where the whole body of the car was dimpled and proved less drag and better mileage

Rich

[MTrat]

Wow, this post has brought out a lot of good information. I presume that the 'slide side' of the vanes, (both upstream and downstream in the airflow) have a curved edge to nearly reach the slide. Or do they just end away from the slide with a straight edge?

[racerclam]

When I make them I conform them to the slide with no more than 1mm clearance. If you get on my web site richstaylordporting.com and go to the venture divider page you can see a view from the top down .

Rich