Hi.

 After the latest combustion chamber experiments, our .77 stunt engine runs so well that I have started to prepare the next (and hopefully last) version of it. I would like to have the new versions ready for world champs.
Our engine is very energy efficient, it only uses about 2,6oz of fuel / flight.
For me it's not a big problem, but the high efficiency has a price; the engine is slightly more sensitive to variations in atmospheric conditions. As I don't play with nitro, I must be able to change venturi size rather quickly.
I use "true venturi", made as per Randy's instructions, and they work very well indeed. The only addition to Randy's instructions is the use of fuel post(s), my venturi's use 2 posts at 60 degr. spacing. Both posts have a 0,7mm (1/36") hole for fuel flow. The posts extend 0,8mm (1/32") into the venturi bore. Average venturi size is 4,5mm (0,18") and I must be able to go up and down in diameter +/- 0,2mm, in 0,1mm steps.
So far my venturi's are like in the picture, plug-in type where 2 O-rings take care of leaks and keeping them in place.
They work well but sometimes they get stuck and in the end they are not so easy to change.
For the new engines, I've thought about making screw-in type venturi's, with a lenght of fine thread between the venturi shroud and first O-ring. In that case I must naturally have a sprinkler type venturi with several radial holes, and this brings us to my question:
How many holes and of what size?

Cheers, Lauri

Do you use any lubricant on the O-rings? Petrolatum is a common lube used on aircraft fuel system O-rings. It might ease installation and removal.

Bill,

The 20% oil in fuel keeps things slippery.

Tim,

The only control of rotation I need is to be sure that the position is every time the same. Being able to change the post direction was just one attempt to understand the issues I had with running symmetry. It's no longer necessary.
The BNC type bayonet is for sure worth thinking but at the moment I think the fine-ish thread is the best way.
But back to the original question. My biggest worries about the ST type radial holes/peripheral groove is how easily it gets blocked by dirt and how quickly the engine stops after I actuate the shut-off device. I think I'll make such a venturi to my current engine to find out. L

Hi Lauri  many years ago I made all types of sprinklers, and modified ST venturies, you know some of the downfalls of the ST, the burping, the tiny holes stopping up, bursting lean/rich, etc.. We made a piece that blocked off all but 2 of the holes, then opened the holes diameter up. This works much better with only 2 holes, I tried it with almost all number of holes, 1 or 2 is best. You can make a cylinder to slide the ventuire in and use JB Weld to block off as many as you want to try, or make a 3/4 circle that will block off holes.
I used the red silicone O-Rings top/bottom to seal, they take a set and stay, but a slight twist will pop the orings loose for removal.

Randy

     I also note Chris Wilson's suggestion about the Berringer-style venturi. I haven't tried it but it appears to solve the problem of being able to easily vary the choke area and also doesn't have the problems you get with flush fuel outlets.

    Brett

Thanks for taking note of that Brett, in hindsight I was wondering if plastically deforming the venturi side with an external adjustment bolt would be possible rather than having the bolt intrude internally might work? (The mount of tuning and therefore distance of deformation needed should be small enough surely.)

A thin walled Delrin section around the bolt maybe able to cope with it and perhaps give a better and cleaner section.

 ...polyethylene tubing...clamp...to squeeze it to size.

    Brett

That sounds like a solution that would work perfectly in perfect world or computer screen, but I'm a little cynical of its user-friendliness in reality.
Actually, the thing is that I like systems I can duplicate 100% after full dismantling, I hope you understand what I'm after. A flexible venturi is not  so such.
But, I dug an old proto backplate from junk box, and it may be worth further development.
It has a simple restrictor/plenum chamber/fixed fuel post, just like in team race engines.
I think I can duplicate that geometry to my current engine, as well as the ST style sprinkler, go testing and see if I find any differences from my std setup.

L

That sounds like a solution that would work perfectly in perfect world or computer screen, but I'm a little cynical of its user-friendliness in reality.
Actually, the thing is that I like systems I can duplicate 100% after full dismantling, I hope you understand what I'm after. A flexible venturi is not  so such.
But, I dug an old proto backplate from junk box, and it may be worth further development.
It has a simple restrictor/plenum chamber/fixed fuel post, just like in team race engines.
I think I can duplicate that geometry to my current engine, as well as the ST style sprinkler, go testing and see if I find any differences from my std setup.

L

So when you change the internal diameter of the venturi you change the distance of your fuel posts jets half of that distance from the side walls as a consequence?

That and it looks like that if you go for the old type of screw in insert you will have quite a step after the threads stop.

Would be feasible to stick with your current twin post set up, use a straight tube to mount them in as a fixed carrier and use a range of inserts that are off centre bored in such a way as to keep to the jets or the posts exits in the same relative place every time?

In other words changing the ID of the venturi will introduce a secondary change of moving the jets position relative to the center of the air flow (because they are not centred upon it in the first place) and could get into a boundary layer problem.

I envisage your inserts to be bored so that the jets end up the same distance from the side walls regardless of  insert diameter, hence the off set on any insert bar the master and having two slots cut into them from the bottom to allow fitment around the jets once slid in place.

This way could minimise the changes to purely throat diameter and be easier to make conclusions with.

Cheers.