Meschmidt

My Feedback: (15)

I have a US 41 that came with a short velocity stack from the factory. It is cut at an angle on the opening. Which is the best way to turn it for flying or does it matter? Thanks, Paul

earlwb

It doesn't really matter. But most people have it facing forward though.
I once tried different angles on a engine and I didn't see any differences when flying the plane around.
Of course I was running a plane with the engine and carb out in the open like shown in the pic.

av8tor1977

My Feedback: (6)

Technically, it should be installed with the angle to the rear to prevent pressurizing the carb with ram air. Doing so causes mixture problems as the diaphragm in the regulator in the carb is sensing static air, (hopefully), and the carb throat itself would be under a slight pressure. Not good.

Many people think you can get free supercharging through ram air entering a carb. Not really so. Studies have indicated that ram air only offers gains of 1 to 2 % at speeds of 150 mph, while causing carburetion difficulties. Race cars have hood scoops mostly to admit only cold, outside air to the carb(s), which does offer a useable benefit.

The stack is cut at an angle to prevent a venturi effect whereby the passing air would try to suck the air fuel mixture out of the carb in flight. Angle should go to the rear.

AV8TOR

a70eliminator

Ram air or cowl inducted, I choose Ram---->air

Tony Hallo

The straight pipe has little effect on performance. Note the inlet shape of the new 60CC FI engine offer by Horizon

Rodney

+1 very true

ahicks

My Feedback: (2)

No proof or scientific studies to back me up, but I think the reason a lot of guys used to run those venturis is for a more practical reason? It's to reduce the goop buildup due to a piston ported engine's tendency to blow back through the open intake port just before it closes. If it were not for the prop scattering it, on an engine running without a venturi, you would be able to see a fog or haze consisting of fuel and oil droplets just outside the carb throat when it's running at speed. Adding a venturi extends the engine's intake tract in an attempt to contain or capture that "fog" so the prop wash doesn't blow it down the side of the plane? -Al

a70eliminator

Yes that's exactly why i have one on my us41 engine, at that moment the throttle is slammed open I swear raw fuel sputters out the carb, the velocity stack contains it.

earlwb

Well they do have documented evidence of the vapor cloud that forms just at the entrance to the carburetors. It goes way back too. The spitting is called reversion. You can look it up, lots of information about it too. Basically the more aggressive the port timing or cam and valve timing you have on a engine, the more reversion you get or spitting. With more reversion the vapor cloud that forms at the entrance to the carb gets larger. With our model airplanes, zipping through the air at speeds 60mph or so greater, there is more of a tendency to blow that vapor cloud away which also affects the carburetor and how it draws fuel too. Using a short intake stack does help a lot with the spitting from reversion and helps keep the vapor cloud intact too. Thus the carb works better and the engine engine runs better. You don't normally see a performance increase, but the fuel consumption of the engine can improve some though, as less fuel is lost due to the spitting.

Since cars usually have air filter boxes and plenum chambers etc. you don't normally see the reversion effects with them. But it is still there though. A racing engine with a hot cam in it can spit a lot too.

Granpooba

My Feedback: (5)

Ditto !

Rodney

Very correct in most cases. However, when the venture is inside a cowl, things can change due to the way the air flow is directed with the internal baffles and how close the cowl skins are to the venture so you may have to experiment a bit there to get the best angle. If out in the open it is usually best to have the slant to the rear or have no slant on the venture at all (square cut ).

av8tor1977

My Feedback: (6)

I just finished up a Wild Hare Planes 50cc Extra with a Stihl 62cc chain that I converted. There wasn't enough room for a velocity stack within the cowl, and I didn't want a hole in the cowl so I just set it up without a stack. However, the airflow through the cowl would rush by the carb, and not only blow away the reversion previously mentioned, but also suck some of the air/fuel mixture right out of the carb throat. As a result, I could not get the engine to run rich enough, even with the high speed needle turned way out. I finally made an air dam in the cowl in front of the carb to shield it from the air blast. That solved the problem and the engine then tuned normally with the cowl on.

The problem had me cussing until I figured out what was going on.... At first I thought my (just rebuilt) carb was malfunctioning.


AV8TOR

Flyfast1

My Feedback: (1)

I have a First Place Engines (FPE) 3.4 (55cc) engine on a Spitfire. The recommendation of the engine manufacturer was to orient the angled cut forward so that air is forced into the carb and that has worked great for several hundred flights. The engine runs great at all throttle settings.

-Ed B.

Rodney

This is contrary to most experimental results done by many. Is your engine and port inside a cowl? If so that may explain it as the airflow beneath a cowl can certainly change how things work. If out in the breeze have you tried other positions, you may get an improvement if you do.

Meschmidt

My Feedback: (15)

Thanks to all that replied. I have flown the plane several times and the best running position is toward the rear. Any slant toward the front caused it to run rich and blubbered in the air even though it ran good on the ground. Haven't tried it with it off yet. I'll let you know if I try it. Paul

av8tor1977

My Feedback: (6)

It is always better to run a velocity stack if you have room for it. It helps contain the reversion, or "stand off" that comes from the carb. This will make for better fuel metering, less fuel use, and less fuel spatter all over the plane.

AV8TOR

Truckracer

My Feedback: (19)

Ya know, discussing velocity stacks is kinda like discussing which oil or radio is best! While we know there can be benefits from using one, especially on piston ported engines, their performance is not always predictable.

I've seen just about every case imaginable regarding stacks. Sometimes adding a stack will cure or cause a problem, sometimes removing one will do the same. Likewise, sometimes a slant forward will resolve or create a problem and in other cases, facing the slant to the rear will do the same. Sometimes you need a straight cut stack, sometimes a flared one. Sometimes you just have to get rid of the stack completely when you think you need one, etc. You get the picture.

The point of all this is we're dealing with strange airflow and pressure here and nothing is 100% predictable. You just have to be willing to experiment to see what works .... and what works in all airplane attitudes. And that's all I have to say about that.

av8tor1977

My Feedback: (6)

Like Bill from CH ignitions says, "It's black magic."

Some people are down on fuel injection, but they were when it first become common place on cars too. It will be fun if/when it actually becomes affordable for our little engines. It's getting close.... I just hope they make it "tuneable" with a laptop so I can play with it!

AV8TOR

Flyfast1

My Feedback: (1)

Hi Rodney,

The engine is inside the cowl, but the velocity stack is outside the cowl. Without a velocity stack the opening of the carb is just slightly past the cowl, if that makes any sense. I can certainly turn the velocity stack around and try it with the cut facing rearward, so that the air is not forced into the opening.

-Ed B.