I don't know the stock parts measurement. But if they run well with enough power, it doesn't matter imo.
Typically, the smaller the choke area, the less power, but more fuel draw you'll have, all other things being equal.

Dave, there is varying data in circulation regarding the OS stock venturi ID.

However, all that I bought from Tower—and there were quite a few—measured 6.55mm.

The OS factory spray bar is 3.5mm OD while the Enya part is 4mm.

There is more information and some charts that can be downloaded here—

http://www.flyinglines.org/dg.venturi.html

   The stock venturi diameter is not a mystery, it is .257x" (nominally 6.55 mm), measured from a bunch of different ones with my split-ball gauge and micrometer. Any differences are manufacturing tolerances. I am not sure how there is a debate about it, it hasn't changed in almost 30 years (20/25FP, 25LA) and all that I have measured are the same.

   I would note that it is not that easy to measure it to .001" using normal equipment. If you use calipers, for instance, you will get about .256+, just because the blades aren't sharp enough to be unaffected by the curvature of the surface. I use a Starrett split-ball hole gauge and a micrometer. Just not cleaning off the oil might be good for .0005 or so.

    The stock spraybar is .137x" (nominally 3.5 mm). The equivalent venturi choke area using a 4mm/.157x" spraybar is about .272 (Letter I drill). This should get you very close on area. Whether it runs the same is open to experiment.

   It doesn't matter too much if you are trying to experiment with it, but it matters a lot if you are trying to replicate the results without expert help. The stock system has long since passed the "experimental" stage, and most people would be best served by running it dead stock out of the box, because the unit-to-unit variation is negligible, and works better than almost anything anyone else is running no matter how much money is spent, and vastly better than most.

   Given that many people attempting to use this system probably have never used or witnessed anything of equivalent run quality, it makes a lot of sense to me to first try to establish a baseline performance, then experiment later.

   Brett

So there are bigger fish to fry in getting good stunt performance. I'll file the venturi and NVA measurements away for the time being. Nice to know in any event.

   The choke area of the stock system 0.0184 square inches, for reference. With the 4mm spraybar, the actual diameter you want is .274 to replicate it, and if you just drill through it with a .272 drill (Letter I), that's about what you get.

    Brett

   The choke area of the stock system 0.0184 square inches, for reference. With the 4mm spraybar, the actual diameter you want is .274 to replicate it, and if you just drill through it with a .272 drill (Letter I), that's about what you get.

    Brett

That's awesome info.
Brett do you remember how much choke area was lost to the 4mm nva vice the smaller OS stock nva?

If you don't recall, please don't waste time recalculating it. I'm just curious how much the loss is.

Thanks!

Regards,
Target

With a .257" venturi, if you increase the spraybar diameter from .135" to .155" (~4mm), then you have reduced the area from about .0172 sq. inches down to .0120 sq. inches. That is a reduction of 30%, give or take.

Dave

What plane will you fly? What weight? What muffler? What fuel? What prop? All this factors in when figuring Venturi diameter. Jim Lee and others make venturis of different inside diameter to fit these engines. Venturis from FP/LA 40s fit this engine. Try running engine the way it is with 10% 22% (50/50) fuel. Should pull a Ringmaster or Skyray if planes aren’t porkers. Ones I’ve used were steady and reliable running 2 stroke fairly fast. I use steel dial calipers. There are name brand quality instruments available on eBay. Excellent quality for a very reasonable price. Hard to get a good reading unless Venturi is pulled from engine. Easy enough to do. I like Lee venturis. They are longer than stock. Probably an illusion. But I think they straighten the airflow better than stock shorter venturis helping to make a more consistent run. Stock inside large FP 40 Venturi might flow similarly to a stock FP 20 venturi with a stock OS needle valve. It would compensate for the fatter Enya needle valve.


Sent from my iPad using Tapatalk

Randy,

You are correct. The question posed was what was the effect on passage area of putting an oversized spraybar into an OS engine. I culled the data and just plotted the typical OS venturi sizes. To use the chart, you can find your venturi diameter anywhere along the curve, and then go vertically down from the .135" (stock) spraybar to the .155" retrofit spraybar and read the new, reduced "choke area." Or, if you liked the original performance on the stock choke area, go horizontally to the right to where it intercepts the .155" curve and find the required new venturi diameter for the new spraybar. To replace the stock needle, you would need to open up the venturi to about .277" to get the performance back. The reason it is not completely exact is the effect of the boundary layer on the surface. As the passage area becomes smaller and smaller, the effect will increase somewhat. For what we are doing here, you can ignore that and start testing actual parts.

Dave