Gents,

Getting ready to convert a ST S90 to CL use for one of our club members AFTER i have hip replacement surgery next week.

Was wondering what is a good (read suggested) venturi hole to start out with. 

thansk in advance folks.

p.s.  any one want to come over and rake up the leaves in the front and back yard for me?   

What size prop atre you going to run on that thing /
One of the Schneurles !

Are you dropping the liner & the Compression ? .

What type of Airplane will it haul .

Maybe try the same area as the stock carb at 1/2 throttle , if its stock. 

Hello Brett,
Interesting comments there on venturi sizes. Does the reverse hold? That is, can you keep that venturi area for smaller engines? I appreciate that at some point, suction will decrease and lack of fuel feed will require a reduction in venturi area.

  Sort of, yes. Since you only need so much HP in a static sense over a range of usable model sizes, the venturi size stays pretty close to the same. I think David Fitzgerald's PA75 is set up to have about the same, or slightly smaller, choke area than the stock venturi on a 20FP. I haven't tracked it recently, but I know it was that way when he won the WC with it. It's certainly putting out more power overall (and sucking enough extra air to also suck 2.5x as much fuel although it is being used much less efficiently), but this is a fundamental concept. Since we had enough power (i.e. since we had piped 40's) "getting more power" has not been an issue.

    I think I posted in another thread, but the range for most common stunt airplanes of normal size is something like 0.0175 to 0.0190 square inches of choke area. It matters a lot how you get the area ("true" venturi as Randy calls it, "restrictor" as Randy refers to it, or spigot/"fuel post" as Frank Williams put it) but it usually winds up there, and if you are too far off that, you are probably messing up.

   Of course, if you were to push the PA75 to the same level of fuel draw you get with a 20FP, you would wind up with a HUGE venturi and massively more power. I think it would be fairly easy to get upwards of 5 hp out of it, more than most lawnmowers. Power that you would have no way of using or dealing with. Recall that David's airplane is about the size of the ST46 airplanes from 30 years ago.

    Cue up the counter-examples and all the people lecturing about "can't have too much power". We know pretty well how much power it takes to fly a stunt airplane around in level flight at 5.25 second laps, if you have more than that you will be too fast, period, and it's not an opinion or a debatable point.  What it does *in the maneuvers* is another story but you have to figure out how to deal with it in level flight first.

    Brett

 

Not really a one size fits all answer, you WILL   have to tailor it to fit,,  Scot Bair used a .200 true venturie in his 88, this is about the same size as a 340 restrictor, and his ran perfectly at 5.9 to 6.2 laptimes  with a near perfect 4-2 break.
I used a 195 true venturie in 2 homemade 74 engines, they run about 5.5 laptimes with a very nice 4/2 break...just beeping 2 cycle at tops of maneuvers... this is equivalent to a .325 restrictor-venturie.
the stock OS small is .252 but has a spray bar across it, which makes it effectively  way smaller.  then you also have to take these factors in your calculations:
venturie height
spraybar thickness
fuel post (Randy) or spigot thickness, diameter, and how much it sticks out into the venturie
shape of the venturie

This is not simple, and no simple answer will work across the board

randy

  Actually, it *is* rather simple. It comes out about the same depending on the base power requirements, and the limitations of burning air and methanol. Almost all the successful systems, and all the current successful systems, fall in the range expected. It can certainly be made to work at other sizes but to start, the range noted is the place to be.

   It certainly IS NOT a function of the displacement, like the "venturi chart" that gets regurgitated periodically. And most people end up trying to use excess venturi size on their larger engines because of this, and then get the usual ill effects.

   Of course, this means you aren't remotely approaching the capability of the larger engines, but you don't need to because even a piped 40 can produce far more power than anyone can effectively use.

   Brett

In addition I will add this  : a stock PA venturie is a .194 size true venturie, which is in the .320 range the way OS does their venturies (restrictors)  the OS 20 is a .252
252 vs 320 ?? same size ?  don't think so

Also keep in mind , almost noone runs a .252 venturie on a OS 40 or 46 VF, or a OPS or a PA or RJ

Ted ran a 295 in his OS VFs.. I know I still have one here, plus he used the very thin SF CL needle which is only .130 diameter, this makes the are much larger than one with the Old  NVAs or the ST or PA. which are all .157. Si in effect Ted was running what was effectively way larger than most everyone, and much larger than an OS 20

Randy

and to just add one more thing to the pile is that
the venturi will of the style that the NVA actually holds the venturi in place
and the fuel is let in thru the hole drilled thru the side (if you will) of the actual
venturi where the NVA passes to hold it in place. The original draw bar hold the carb in place will be
replaced with the NVA.

I can never remember what style this is called.

Thanks Steve very much. Being that I speak more in metric than SAE standard, I will start out at 5mm i.d. which is super close
to where you suggested. Then if it needs a bit more umph, then I can bore the venturi in .005mm stages to get what he needs/wants.

Yeah, had the same thing happen with my Hip replacement at Walter Reed. They had me call the DAY BEFORE the surgery
at 3PM for the reporting time to the surgery clinic the following morning.
go figure.  LOL

Carl