This is all pretty much what is referred to as a "spigot" type NVA?? It can be achieved by taking the NVA out of an old OS R/C carb, drill and tap the venturi at a convenient place, screw in the NVA and lock in place at the correct depth with the original lock nut. And some where along the line through the years, didn't someone discover that the typical true venturi set up, like what is common of a lot of Super Tigre engines, works better if you put a tiny eyelet through the hole so it carries out into the center of the venturi? I think this goes back to the early days of the tuned pipe programs with the OS .40/.46 VF. Brett, something you and Dave F figured out?
It was definitely NOT something David and I figured out. The original idea was from a very interesting and useful article by the usual source of actual engineering information, Frank Williams, called the "fuel post venturi".
The original idea was to see if you could get larger venturis/more power with the same fuel suction, or the same power with better fuel suction, which it does in many practical situations, over a smooth-bore venturi with a flush inlet. We all read it, said it was interesting, and them moved on.
Not long after, the WC in Germany (and before they left), David had a problem with his engine frequently breaking into a hard two-stroke on the outside part of the square 8. I had previously had exactly the same issue with a different engines, and so had Paul, with Paul and I concluding independently that we just couldn't make that engine work, and returning to the 40VF. I actually switched the night before qualifying at the 96 NATs. And all the runners of the same engine at that NATs had either the same issue, or, they had detuned the engine so much to prevent it that they were obviously way down on overall power, usually with more head clearance or less nitro, which sort of papered over the 4-2 break power difference, and made it livable. David had gone to the WC with this issue, and had problems the entire time.
Then he noticed that Billy's engine seemed to run a lot better in this regard. After it was over, he somehow discovered that the difference was that Billy was running the Frank Williams "fuel post" system. He came back, made one, put it in, immediately vastly better. Instead of breaking hard in the outside part of the square 8 90% of the time, the incidence went down to about 5% - very occasionally. About the same time, David discovered the value of sealing the spraybar to the case, which made it run much steadier on the ground, and between those two things, he was off to the races, and has been one of the favorites ever since.
I came into this largely to tell people about it, since it was such a common issue (and still is among those who are unwilling to try it). I even helpfully pointed it out to one Top 5 competitor, because his engine was surging up and down on the ground to a remarkable degree (no spraybar seal) and also went screeching lean on the outside part of every square 8 (flush inlet venturi). I got a rather histrionic response telling me it was supposed to do that - just like all the Fox guys, who considered the burp/quit to be normal and desireable behavior, even when it quit outright.
Since then, I got the RO-Jett 61 BSE "mistake" version, which also had a flush-outlet venturi (of very large diameter, guaranteeing it would never be full of fuel) and it had the very desirable characteristics of running the same way on insides and outsides, except for sometimes going dead rich on insides. The solution was to get Jett to make a longer venturi with a smaller hole, into which one could insert the smallest SIG eyelet to make a fuel post, just like Frank's original. This cleaned up all the odd run characteristics and it's far an away the most symmetrical engine any of us have ever used. And the extra power you could get by running a larger diameter/more choke area was also welcome.
That's the story. Frank Williams did the seminal work, Billy found that it solved the "hard lean outsides" issue, David adopted it to tremendous success, and I told people about it and adopted the same system for the RO-Jett after diagnosing the occasional issues I had with it.
Since then (2003), and by paying attention to what other people were doing with 4-strokes during the purported "revolution", David and I have further refined the understanding by discovered that the fuel flow restrictions also have an effect, particularly on larger engines, and cause some of the "funnies" we were never able to get rid of. This from experimenting with fuel viscosity and fuel tubing, and spraybars.
In this case, I have no idea what is meant by "hypodermic". I envisioned something like a fuel post/"spigot" as I called it on SSW, but how that relates to a hypodermic needle, I have no idea at all, so I assume I am not seeing it right. It would take one damn big "hypodermic" needle to pass enough fuel for a PA75, horse-sized.
Brett
Topic: Terry Rollins nva. Anybody use them? (Read 2719 times)
