Does that really work, adding the pantyhose to make a drilled-out venturi hole effectively smaller? Never heard of that technique, but interested to hear about it. Presumably, the pantyhose would be secured with a tie-wire wrap, or similar.
It sort of works. The original use was as a filter and diffuser, the diffuser part being more important than the filtering. I first heard about it from Richard Oliver (for filtering purposes, which is a moderately effective thing - keeps your $125 piston/liner alive longer), and it works very nicely without requiring extra clearance required for something like a Bru-Line filter. It's not terrible effective as a filter, certainly not as good about excluding dirt/dust as a BruLine filter, but keeps out the big chunks.
You take a square of panty-hose material about 1 1/4" square, stretch it over the intake, and hold it on with an o-ring (same one you use for the venturi seal). It has a small effect on the fuel draw, slightly increasing the draw, but only a little bit. It's much less effect than a .005" change in the venturi bore. But, it also ensures that there is no laminar flow, turbulating the air flowing in, and seemingly reducing the sensitivity to the direction the air hits the end of the venturi. It's a similar effect to making the intake longer.
Of course, both effects are pooh-poohed by various people (with one pronouncement as recenty as last week), along with the even more critical spigot venturi* - fine with me, that just removes some of the competitors without me having to practice more. Unfortunately, not the ones I am habitually plagued with...
Later, people realized that you could stack multiple layers to adjust the fuel draw by increasing the restriction. I have no idea how to map multiple layers to an equivalent venturi diameter change, so it's entirely cut and try. Conceptually, it seems OK to me for fine-tuning, and I have tried about 3 layers and it does about what you would expect. But, for other reasons, I haven't needed to chang anything about the venturi diameter or restrictions/filters since about 2004, Nitro is easier and more predictable for adjustments.
Some of the lack of needing to change the venturi/restriction also comes from considering the other half of the fuel draw equation - how hard it is to get the fuel through the fuel system (flow restrictions/choke points/viscosity) - which I am predictably told *also* doesn't matter. David and I think otherwise, but why would you listen to us? Again, to each his own.
So, if you think you need more fuel draw, it's usually a quick change to try extra layers, and see what happens. My gut feel is that you are generally better off with the right size venturi and one diffuser layer than a too-big venturi and lots of restriction, but I have no particularly good evidence of that, and if you only have one venturi, you can solve your problem with a trip to any Safeway, hard to argue against it.
Brett
*for almost anything we (we meaning anybody) currently do, successfully, you can find people who are sure beyond metaphysical certitude that it is wrong. I read a solemn description in a major model airplane magazine, in the year 2010, as to how "tuned pipes have been tried in stunt, but have proven to give no advantage". This after *having dominated the competition in overwhelming numbers since about 1990*; that is *for the previous 30 years*! In the last few days, we have had a "if it wasn't on a Fox 35 it must not be important" sighting.
Again, no one is compelled to do anything they don't believe in, people on the internet give advice, lots of it conflicts, you have to look at the record to figure out who you want to follow. Or do it based on your personal beliefs. There's no such thing as the engine setup police, the judges have the only opinion that matters.
Topic: Insert into an FP 40 venturi (Read 2103 times)