At a recent event,  I asked a fellow modeler how his Vector 40 flew and he said that "it ws just a bit slow".
Since we were at "Fantacy Island"  (five flying circles with shop attached and free beer), I asked if I could
help improve the lap time by a tenth of a second or two.  He has a Brodak 40, which uses the "Double Star"
type spray bar asembly with 2 holes in line (or one cross hole if you like that better).

I filed a couple of flats across the holes to "dog-bone" the bar and it did improve the lap time by about
one or two tenths of a second (4.92 upright and 4.68 inverted).

All you did was increase the airflow across the needlevalve bar by decreasing the size of said nv bar. The extra flow of air resulted in more fuel being drawn thus increasing the engine speed a tad, yet could cause a problem in maneuvering due to the decrease of static pressure.

I would rather have filed the flats acoss the solid portion of the bar (parallel to the holes), but several folks
indicate that a thru-hole bar cannot be installed with "one hole up".

What is the advantage of the thru-hole system anyways ?

Rex

"one hole up" is definitely NOT a problem SOLVER!  Fuel draw in a standard NV is accomplished by increasing Dynamic pressure, thus reducing Static pressure which allows  fuel to flow out of the tank, still under Total (static) Pressure That is why combat and speed engines with the large venturis need pressure systems. There is simply inadequate pressure differential with the larger amount of air being drawn through the large venturi and thus static pressure having less diffenterial than static atmospheric.

The double hole spray bar serves a purpose of allowing more fuel to flow, especially when under pressure. If the bar is turned so that the front hole is forward and the rear hole is just out of sight, my thoughts are that the mixture of air and fuel begins right there. When I was a stunt competitor, all my Fox engines used OS/Enya needle assemblies with the rear hole just out of sight. My stock Fox .40, after a good break-in, was equal to my George Aldrich ST .40, Both were excellent engines.

As I am preparing to again try some CL stunt I will use the same set-up. Actually I have a Fox .25 and a Fox .35, many years old, that run very well using the set-up. I am working with an Evolution 60 at this time.

BTW, a single hole NV should have the hole just out of sight rearward. That is the lowest static pressure point thus the best fuel flow.

As better evidence, some years ago when Great Planes (pains) took over Super Tigre and moved it to China, they removed the restrictors from the RC .40 and .45s. The venturi is too large for good flow, especially during throtle application when the tank has little or no pressure in it. Idle muffler pressure is inadequate for a needle setting based on full throttle, therefore comes the ST gurgles and dying. I built some restrictors for those wanting them and a number of club members went from being POed to good pleasure with their ST .40s.
Back in the 1965- 68 days I used a Johnson Stunt Supreme.  I restricted it by some 30% and ran a 10-4 on a highly modified Thunderbird. Put a lot of trophies on the shelf.

Decreasing the spray bar size is NOT the way to assist a stunt engine using atmospheric air for fuel feed.

Back in the 1965- 68 days I used a Johnson Stunt Supreme.  I restricted it by some 30% and ran a 10-4 on a highly modified Thunderbird. Put a lot of trophies on the shelf.

Decreasing the spray bar size is NOT the way to assist a stunt engine using atmospheric air for fuel feed.

  Sounds like it assisted it exactly like it was expected to. He increased the choke area and got more power, so it went a little faster. Just like a tiny increase in the venturi diameter does.

    Adjusting the choke area is a classic engine adjustment. Just because you successfully adjusted a specific engine to have less choke area 50 years ago does not make it a universal rule.

     Brett

Last I heard,  B.40's come with a .305" venturi and .156" spraybar. That's pretty darned big choke area for a .40, so it would make more sense to add nitro and/or prop pitch. Reducing the prop diameter should also increase the speed, by allowing the engine to unload more. What prop was the guy running? What nitro content? Which version of the B.40, and which muffler?

Regarding Johnson engines, somebody recently posted a chart of their various bore/strokes. IIRC, at least one of the .35's and the .36 were the same bore and stroke. I had a .36BB, a POS, and the only Johnson engine I ever had or wanted to have. Thankfully, Super Tigre came along to save the day.   Steve

Doc,

I notice Mr Garf seems to question the two hole approach as well.

The spraybar I reworked yesterday had the hole straight across, not off-center.

ref :  http://hangarflying.proboards.com/index.cgi?board=wire&action=display&thread=2458

Rex

PS :  I think his name is Phil Brown.

Johnson.

What is the advantage of the thru-hole system anyways ?

Rex

I am of the opinion that the purpose of any multi-oriface system is to provide a finer droplet size for any given flow.

In theory a twin hole spraybar should produce a finer mist than a single hole bar, but in practice all the spray system does is provide the correct air/fuel ratio and the great majority of atomisation is done further down the food chain by the transfer ports.