frets24

My Feedback: (15)

More info please pics and specifications would be nice for those of us who may like to copy your plans( if you will allow of course)

Kostas1

Guys please confirm or deny if the velocity stack does provide a better mix of the fuel-air intake charge,
so leads to a more efficient burn!

hsukaria

I know that in automotive engines, a longer intake runner gives better low-end torque. Maybe that can apply here (if done properly)?

earlwb

It does not improve engine performance. it improves fuel consumption and how the engine behaves in the air when flying.

The short intake stack on the Saito engines, improves fuel consumption. When a engine is running there is a backflow occuring where a small plume or cloud of fuel droplets form just outside of the carb's intake. On a test bench you can hold your hand behind the engine running and feel little tiny cold droplets of fuel hitting your hand. the intake stack helps to keep that fuel inside where it can be run through the engine. So your fuel economy goes up with the engine and it gets more fuel. Sometimes you even have to lean the engine out a little bit more too. Also if the engine is out in the open on a airplane the airflow and turbulence around the engine and carb upsets that cloud of fuel just outside the carb and aggravates the situation. The intake stack helps prevent that too. 2 stroke engines have it more pronounced as well. They sometimes call the intake stack "anti-reversion" flow device. The 2 cycle engines with a short intake tube tend to spit fuel like crazy too, they call that "reversion" flow. The high performance 2 strokes are really bad at it with their more radicical port timing. the newer control line engines all have long intake stacks on the intake venturi nowadays as well. This helps prevent the fuel spitting and also the effects of the airflow in and around the intake causing interference too. The air flowing around the carb can actually cause momentary reverses where it actually causes intake suction to go away, this causes misfiring or momentary loss of power in the air in radical aerobatics. So the intake stack helps to prevent that from occurring.

Probably a good example of the effect is the HarleyDavidson V-twin engine with carb in between. Due to the uneven flow characteristics of the intake, a large plume or cloud of raw fuel forms just outside of the carb. If you ride a HD motorcycle at slower speeds up a long uphill mountain road, you'll see raw fuel dripping off the air filter box, on the engines with carburetors on them. it is something of a fire hazard too, as I have seen guys smoking cigarettes with their right hand, holding the lit cigarette right down next to the air filter box like that before. yes a few bikes have caught fire in that situation too.

For performance the intake stack needs to be designed as a velocity stack. It is not a velocity stack at this time.

Hobbsy

My Feedback: (102)

A tuned intake stack could also make the engine peaky by increasing torque at a certain rpm but fall flat on its face above or below that rpm.

Rudolph Hart

Trust you to be sensible and spoil the show earl i think frets asks a good question mate.If costus all keeps writing his posts in capital green red and orange colours i'm going to give that bugger a black eye or two,howdy mate..

hsukaria

The Saito 1.8 that I purchased used came with a velocity stack already installed.

hsukaria

I thought he does the red and green colors to give the Christmas Spirit effect.

Hobbsy

My Feedback: (102)

Here is the reason for the "dancing fuel" at the entrance to the carb.
Picture #1==intake valve opening
Picture #2==exhaust valve closed, both are open between these pictures.
Picture #3==intake valve closed, as you can see the intake valve is open for a long time, picture #3 is also the beginning of the compression stroke.

earlwb

I remember years ago seeing guys set the fiberglass insulation on the inside of their car's hood on fire, when they ran them without an air filter too.
They would put in a hotter camshaft, bigger carb, et cetera. But now the engine tended to spit fuel out of the carb. You can to put on a good air filter intake system to keep the gasoline vapor cloud just outside of the carb in control.


I think they had to do that on some engines as the carb was affected a lot by the air flow patterns around it during flight and aerobatics. You could perform something like a high speed snap roll and the engine would stumble, hesitate, and even stall out and quit on you. The quick burst of air across the mouth of the carb would negate the suction and the engine would stop as it couldn't draw fuel momentarily.


The gasoline engines with pumper carbs were even more affected, as the pumper carb had a air pressure equalization orifice that was affected by the rapid air flow of a airplane in flight. especially with the side mounted carb on the engines. So the intake stack was a big improvement in having the engine run reliably and well.


You know in thinking about it, I suppose it does improve performance, not in speed or RPMs or horsepower per se, but in reliability and consistency and good running.

clytle374

The intake valve stays open past Bottom Dead Center because the intake runner has velocity, this velocity continues to push air into the cylinder after BDC. This changes a lot depending of the rpm of the engine, and length of the intake runner.

The reversion happens because of this velocity too. When the intake valve closes the air compresses on the intake valve, then bounces back creating a revers flow.

As mentioned earlier runner length can be used to tun an engine by setting up a resonance at a desired rpm, but the lengths required are prohibitive. around 8" for an engine at 10K, exhaust figures differently. From something I read from a Chrysler engineer.

Intake
Obviously there might be better numbers now days, but this was muscle car era.

Velocity stack design is very tricky for optional performance. Here is a little run down on the art. http://en.wikipedia.org/wiki/Velocity_stack there is a link to a file named bellmouth.zip


Cory

Kostas1

I am quoteing the following information from wikipedia.



A velocity stack is a trumpet-shaped device of differing lengths which is fitted to the air entry of an engine's intake system, carburetor or fuel injection. It is designed to:

1. Allow smooth and even entry of air at high velocities into the intake tract with the flow stream adhering to the pipe walls.[citation needed]
2. Modify the dynamic tuning range of the intake tract by functioning as a resonating pipe which can adjust the frequency of pressure pulses, based on its length, within the tract.

In the real world, on high-rpm IR IC engine, using a minimum amount of inlet radius gives the best wave strength and a power boost of 2% to 4% over a 3000 to 3500 rpm range. Using a larger radius, like 3/4", broadens out the resonant pressure wave rpm range, but the compression boosting pressure wave is greatly diminished and almost unnoticed by the engine.

blw

My Feedback: (3)

You can appreciate the potential effect of the velocity stack for small Saitos by putting one on a 125 and seeing the engine lose performance.

clytle374

I thought they were smaller than the 1.25's carb bore? If so chocking the engine is a different subject.

Cory

hsukaria

Hmmm!!! I will keep that in mind for my 1.8 Saito. I only ran WITH the stack installed. I might try to remove it, but not sure if I will get to it this flying season, it is already getting rainy and windy.[&o]

hsukaria

The valve opening overlap plus longer intake runners cause a supercharging effect of the fuel/air mixture entering the cylinder.

In modern automobiles, the tube upstream of the throttle body is a complex design (involves a whole bunch of engineering). The plastic tube upstream of the throttle body is called the "zip tube" and is designed to improve airflow and dampen out noise. So, the backflow of fuel is not a concern due to port or direct fuel injection, but intake air hissing noise is a big concern. Intake air hissing noise is not something we are concerned about in this hobby.

Nikolas K

An example of one model engine with a real velocity stack is the OS 49PI Type II Rotary. It gave the engine the ability to swing a larger prop than the previous model.

Hobbsy

My Feedback: (102)

H, the reason the 1.25 loses about 150 rpm is that the velocity stack for the rest of the smaller Saitos has a smaller id than the 1.25s venturi. The 1.00 on down are fine, the .40a and the little .30 have there's cast in to the carb. The .40a stack is about 7/16" deep & convergent.

earlwb

Yes it looks like they have a pretty good one on the carb there.



I noticed that the control line engines all started putting on a longer venturi intake too. But it isn't a velocity stack though.

hsukaria

The minute somebody says ROTARY or RADIAL, I know I can't afford it!!!

hsukaria

Hey Earl, what is that wild looking single cylinder engine? Check out the massive front bearing on it!!! Too bad you can't get an RC version for it.

earlwb

Yes they have a RC carb version too. yes they put in a big crank and it needs a big pair of bearings too.

hsukaria

what size displacement is it?

AmishWarlord

My Feedback: (5)

I saw this 150 basket case on ebay.
http://www.ebay.com/itm/Saito-150-RC...item2a13e033dc


"not yet broken in" Oh this engine looks really "Broken in" to me. Why would someone strip down an engine that wasn't even broken in? Crash?

"Saito 150 RC engine", No this is used parts for a Saito 150 engine, not even all the parts for a Saito 150 engine.

Hobbsy

My Feedback: (102)

The backplate appears to have been violated some way, like maybe the crank being driven into it while under power.