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Old Nov 20, 2012, 07:26 AM
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velocity stacks

What is the advantage or purpose of velocity stacks?

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Old Nov 20, 2012, 08:11 AM
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At model sizes they only serve to reduce the fuel blowback from the carb. In full scale they can take part in intake tuning.

Greg
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Old Nov 20, 2012, 08:50 AM
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So, if I am getting allot of fuel exiting the carb throat making a mess inside the cowl, would I get less mess with a stack?
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Old Nov 20, 2012, 09:21 AM
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Yes. Easy to do for a four stroke or gas engine. Not always practical on a front intake two stroke.

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Old Nov 20, 2012, 09:29 AM
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Velocity stack

Here is a very large one on my 3W 130cc twin. The engine is in the nose of my Wilga
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Old Nov 20, 2012, 09:32 AM
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You will also benefit in a better idle too. Not sure why they don't come with the engine new. They are cheap enough.
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Old Nov 20, 2012, 10:44 AM
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The intake stack reduces the amount of fuel the carb/engine spits out when it is running, especially at low speed. Also with Walbro carbs the main jet is near the top of the carb, about 1/4 inch down inside the bore. So when flying the air flow can disrupt the air flowing into the carb to some extent, which usually results in the engine tending to run more rough at speed during manuevers. So a intake stack helps to smooth out the air flow and reduce the effects at speed. In some cases, during really radical aerobatics, you could get the engine to miss, burble or even stall out from the rough air across the face of the carb.
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Old Nov 20, 2012, 08:43 PM
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I have a Saito 100 inverted in a Spitfire.

A stack would work for me?

If so, can I make a stack or does it have to have a particular profile?
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Old Nov 20, 2012, 09:10 PM
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It doesn't need any particular profile. The ones you see for sale vary in their designs. Short ones with a tapered intake, longer ones, and straight ones with or without an angled cut in them. Usually the lengths vary from around 1/2 inch to 3/4 inch long ones to ones about two inches long. You can take a look at some of the Saito engines to get an idea as to about how long one could be. But you need to ensure the intake stack will clear your other stuff under it inside of the plane.
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Old Nov 21, 2012, 02:47 AM
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Some people say that a tapered or trumpet (bell mouth) shaped stack increases intake airflow at full throttle, by eliminating turbulence that would happen at the edges of a straight-cut intake. The curve makes the air enter the venturi straight, which I personally could believe, I'm just not convinced that the difference is noticeable on small engines in flight. Maybe on a test stand....

Others say that reducing this turbulence has a negative effect on the mixing of fuel and air. I can imagine that in a 2-stroke where the mixture is passing the crankcase with its rotating bits 'n parts, makes a difference compared to a 4-stroke where the mixture is going straight into the combustion space.

Fact is that bell mouth intakes are sold for gassers as a tuning part, and some people claim to notice the power increase, or a reduction in fuel consumption.

I have no experience with shaped stacks myself, as I usually fit an air filter anyway, and that tends to do more or less the same (catch the spitback).
The fuel saving can be considerable though, if you are operating the engine in an otherwise unfavourable RPM range for the induction timing...

Brgds, Bert
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Old Nov 21, 2012, 07:03 AM
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To get a performance boost, you have to make it longer and "tune" the length. Unfortunately the length winds up being around 24 inches to 36 inches in length, depending on the RPM tuning point. You tune it the same way you would tune a tuned pipe, but for the intake instead.

When people notice a performance improvement, it is because the engine was running somewhat poorly without an intake stack. It is especially more noticeable with gas engines using a carburetor where the main jet is real close to the top of the carb where it is affected by the air stream flowing by at speed. Now just about all of the carbs tend to develop a fuel vapor cloud right at the mouth of the carb. A intake stack tends to preserve this vapor cloud which results in a smoother running engine.

I use the the intake stacks to reduce the fuel spitting from those engines with more aggressive port timing (or valve timing). In some cases it can make for a noticeable improvement in fuel consumption, but usually it reduces the extra fuel residue mess inside the engine compartment.
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Old Nov 21, 2012, 10:04 AM
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Quote:
Originally Posted by earlwb View Post
To get a performance boost, you have to make it longer and "tune" the length. Unfortunately the length winds up being around 24 inches to 36 inches in length, depending on the RPM tuning point. You tune it the same way you would tune a tuned pipe, but for the intake instead.
Not to be disrespectful, but I doubt that figure of 24 to 36 inches....

Japanese motorcycle manufacturers use stacks to optimize their engines for a certain RPM range (midrange for cruisers, toprange for sportsbikes) and they typically are working with lengths in the area of 2 to 5 inch. They even have developed adaptive intake stacks that can vary their length somewhere between those numbers. I have no clue if the total length of the intake tract has to be taken into account, but even then, on a typical japanese sportsbike, that length from carburettor intake mouth to valve dish would be somewhere inbetween 8 and 12 inch

As to the why of these figures I do not know for sure, but I am told it has to do with the mass of air in that tract and its inertia, not with pressure waves reflecting as is the case in tuned exhausts.

Brgds, Bert
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Old Nov 21, 2012, 10:15 AM
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Basically, I'm just trying to reduce the spit out from the carb, and keep the mess to a minimum. Not looking for increased performance, but if that comes along with installing a stack, that would be a plus. I hate having the cowl dripping fuel when I bring the plane home. Wifes not happy with it either. LOL
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Old Nov 21, 2012, 10:23 AM
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Quote:
Originally Posted by tacx View Post
Basically, I'm just trying to reduce the spit out from the carb, and keep the mess to a minimum. Not looking for increased performance, but if that comes along with installing a stack, that would be a plus. I hate having the cowl dripping fuel when I bring the plane home. Wifes not happy with it either. LOL
For that you can fit anything, from a stack to an induction box to an airfilter, and you will minimize spitback.

Brgds, Bert
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Old Nov 21, 2012, 10:30 AM
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Tuning length depends on the RPM and size of the intake tract. Japanese sport bikes which run 17kRPM will have a much shorter intake than an American V8. It's a simple concept, but there is no improvement in cylinder filling at model sizes. I think high RPM two stroke gas engines could take advantage of tuned intakes, but I've not seen any. I think any improvement one sees by adding an intake stack on the carb or trumpet is due to improved mixture either because the mixture was not ideal prior to readjusting the needles after the addition or because the carb was designed with the idea that there would be no loss of fuel due to blow back. Virtually any Walbro carb made would have an air filter or air box on it.

I added an inch long stack to my FS-52 and this resulted in a clean firewall. A little bit of needle tweaking was required because more fuel is getting into the engine and less onto the airplane or blown away. The length is not critical., I have one, I think almost two inches long on my FS-48 diesel conversion. I made it that long mostly to allow easy access for choking as the original choke mechanism can't be used with a stack.



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