hey TT, I dont think that its purpose to seal off the slide if thats what your saying, the mikunis are sealed at the top as well, the point is to stop the circular turbulence cause by the recess in the bottom of the slide. when i tuned the machine in i had to almost triple the size of the pilot jet and only up the main a few sizes, i was skeptical at first but once dialed in i saw awesome gains. here is a better diagram from the company,
http://www.thunderproducts.com/u_f_o.htm
(The following turned out to be a bit of a ramble. Ignore if you like.
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That's a better pointer to its function. I think I understand it better, now. I agree, it is not to seal the sllde bore.
Again, I must state that I don't now details of the Mikuni carb physical layout. I'm going to have to assume it is similar to the Keihin for this discussion. If this isn't true then this entire analysis is likely invalid.
The Keihin slide forward edge facing the engine is placed just over the pilot circuit delivery hole. When the slides are near closed (at idle), that exit hole sees maximum vacuum from the engine AND the highest air flow for venturi (pressure drop) effect. These two factors combine for a cumulative low pressure draw on the pilot circuit. The UFO (stupid marketing name) moves the high velocity low pressure area over the throttle valve. Thus losing some draw on the pilot delivery hole, since now the only suction is from the intake stoke of the engine. This might explain why you had to make the pilot jet larger, as with the UFO in place, vacuum draw on the jet was less than stock configuration.
I can see where the UFO might improve airflow matters at midrange to WOT throttle. Both the mains circuit and the throttle valve are now where max airspeed through the carb float is the highest (where mac restrict is). And, the concept would help to keep the airflow laminar, which could be helpful for max breathing in the engine at high air velocities.
However, I disagree with their atomization depiction. Laminar flow of the air does NOT lead to even atomization or smaller droplets (despite what their artist envisions). It's actually the reverse it operation. Turbulent flow increases atomization distribution and smaller droplet size within the airflow and promotes more even distribution pairing of the fuel with the available oxygen molecules.
I just noticed the artist depiction of the UFO shape does not match the actual picture of the UFO. While it has a straight ramp from front to rear. (rather than ramp and flat portions the artist draws, It still has depressions on either side of the ramp, so the entire bottom of the slide cutaway is NOT filled (which is implied in the drawing showing laminar flow).
I expect that with re-jetting the UFOs will still allow the engine to operate properly. But, I'd be skeptical of actual power or efficiency gains. I'd have to see comparative dyno and A/F ratio charts, with the inserted devices as the only mechanical variable (besides jetting). If the A/F ratio charts are the same, then power gains can be attributed to the UFOs.
Lastly, I'd have to wonder why both Mikuni and Keihin didn't offer a such slide bottom configuration in it's original offering. Being an engineer, I know that lot's of things are tried in the lab that never make it to production. Certainly carb engineers are aware of air flow issues through their carbs. So, there could be a real reason that the slide bottoms are the shape they are. And, why would they offer a shape that was known to produce less power in a highly competitive sales arena that favored high performance engines? The only valid reason I can think of might be a cost/performance trade off. But, is machining the slide bottoms to a different shape a significant cost issue? The people who sell the UFO would have you believe they are smarter than the people who make and sell carburetors successfully on a mass scale. I'd afraid their artist renditions have not convinced me this is true. So, I ask myself, do I want to pay for a product that requires me to go through all the work of properly re-jetting carburetors, based on an unproven claim of increased horsepower? And, for me personally, I don't think I'm sufficiently bored or inclined to add a claimed (unproven) 3 HP to a thirty year old engine.
I CAN tell you that the piece part cost for that plastic shape is well below $3, and probably below $1. So, it is a high profit margin product. Maybe I'd like it more if I were a dealer?
Cheers,
