Zenoah Velocity Stack

Can you tell me your experience without and without the velocity stack installed on a Zenoah. I have a G20 I'm going to mount on a Hangar 9 P-47 ARF 60 size that I'm putting together. I'm not sure that I want to cut a hole in the cowl for the stack. Anyone have this combination? It's recommended by Hoizon as a good combo, but I'm wondering. The engine with muffler and carb with velocity stack installed is 7 inches. The fuse at it's widest part is just a tad over 6 3/4 inches. Cutting holes all over the cowl for muffler, velocity stack, choke lever, spark plug, adjustment screws just kills the scale looks. This is my first gas engine. I'm not experienced with these. How necessary is the choke in getting it started? I'm in Central Florida so the cold days are rare. Thanks for your 2 cents. Howard

I have a Zenoah G-45 with a velocity stack. A velocity stack is not a necessity. I used one mainly to be able to choke the engine without having a choke lever rigged. I just cover the stack and flip a couple of times.
Yes, you must choke a gas engine.
John VB
Howard wrote:
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So long as the intake of the carb is inside the cowling, you don't need the stack. The stack is used to prevent fuel from being 'sucked out' of the venturi of the carb by the air moving past the carburetor.
"Howard" <howardh1951-at-hotmail.com> wrote in message
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"Jim" <> wrote
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That is one reason.
Others are that it keeps fuel from burbling out all over the place while the engine is at idle, and that it gives a little more HP at high power settings by keeping air flow going at better speeds and pressure, by acting somewhat as ram air; that is the air column gets moving, then the intake cycle stops, stopping the air for an instant, and a pipe around the intake contains the moving air until the intake cycle begins again.
I think there is a noticeable improvement in performance with a velocity stack. I have no data, just "I thinks." <g>

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I have the same lack of data and the same thoughts.
I think your idea of how it modifies and improves the pulses of air in the venturi is correct.
This page suggests that sucking back the gas that would have been lost is part of the big picture:
http://en.wikipedia.org/wiki/Velocity_stack
I have one (1) US Engine with a nice velocity stack on it. It looks great. Haven't flown the engine yet. Still searching for the right airframe. Ran it just once on a test stand. Its chainsaw heritage came through LOUD and clear!
It seems to me that some of the racing cars from the 60s or 70s had velocity stacks on them--I can't imagine what else they would be:
http://www.mlodeent.com/CANAM/07acar68.jpg
                    Marty

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I have the same lack of data and the same thoughts.
I think your idea of how it modifies and improves the pulses of air in the venturi is correct.
This page suggests that sucking back the gas that would have been lost is part of the big picture:
http://en.wikipedia.org/wiki/Velocity_stack
I have one (1) US Engine with a nice velocity stack on it. It looks great. Haven't flown the engine yet. Still searching for the right airframe. Ran it just once on a test stand. Its chainsaw heritage came through LOUD and clear!
It seems to me that some of the racing cars from the 60s or 70s had velocity stacks on them--I can't imagine what else they would be:
http://www.mlodeent.com/CANAM/07acar68.jpg
                    Marty

I have just installed a velocity stack.. It has a 45 deg slanted opening.
I am having proglems adjusting the engine with the 45 deg pointing toward the prop..
Is it designed to adjust by turning it so the prop blast does not enter ?
Floyd in Panama

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No

Velocity stacks are very tricky on fixed wing aircraft. Unless you installed it for a very specific reason, you will likely get better overall performance without it installed. At least one of the problems is that if you have a velocity stack with a angled cut on the inlet, pointing it into the airflow creates extra pressure which varies with both air speed and engine RPM. Turning the angle away for the prop blast/air speed creates a vacuum which also varies with airspeed and RPM. Further compounding the problem is that the reference pressure for the Walbro carb regulator is mostly fixed and does not see the variation that the carb inlet sees with a velocity stack installed. In fact, many people will solder a tube from the carb regulator plate and route a tube to the inside of the fuse so that a constant reference pressure will be maintained. Finally, changes in the air due to humidity and temperature are amplified with a velocity stack. All of this combined makes mixture adjustment problematic.
Helicopter and boat guys can get better performance from velocity stacks because the carbs are normally located in a place that does not have varying air velocity around it, and they run at fairly constant RPM. Similarly, airplane racing guys get better performance because they are tuning their engines for max RPM with a fairly constant air speed and don't much care about idle, acceleration, deceleration, and the like. Some of the rear carb engines, or engines that are fully enclosed in a cowl may also improve with a velocity stack, but that is a crap shoot.
But, if you are looking for good overall performance from a G-62 a velocity stack, especially one hanging out in the slipstream, may not be your answer.
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In most cases the purpose of what they sell as "Velocity" stacks are simply methods to keep excess fuel from spraying out of the carburetor rather than a definitive boost to the performance by gathering more air.
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