The Evil Twin

[DocsMachine]

Try opening the bolt at a 15 degree angle - not sure how, without expensive tooling or some tricky compound work - like the tail end of a venturi meter.

-I've been wanting to try exactly this for years. That's actually what I thought a "venturi" bolt was back in the day, 'til I bought one, saw it and thought "that's not a venturi, that's a showerhead!"

That is, we make the port narrower at the ball end than at the valve port. The venturi effect says the gas charge will speed up as the passage gets narrower. Will that have any effect on velocity or gas efficiency? No idea. At this point we can only guess.

The old AKA "Lightning" bolts went the wrong way- the gas column heading toward the ball expanded and slowed, losing energy before it actually reaches the ball. The narrow-at-the-tip port won't gain energy, that of course is basic laws of thermodynamics- if it gained energy, where did that energy come from?

What I'm thinking is that if the taper to the port more or less matched the rate the gas column would have otherwise expanded in that distance (distance over time, and I have absolutely no idea how you'd calculate that) it could at least maintain more of the energy of the original gas charge.

On the other hand, I can see that also being very fiddly- that the best effect is closely dependent on the chamber pressure and valve dwell times. And, in any case, I'd wager the effect is very, very small. Like maybe an extra ten or twenty shots per tank.

I would, at some point, still love to try it out, and I'm still kind of champing at the bit to try the "tiny hole" bolt on a conventional 'Cocker.

Totally doable with a 3D printed bolt though.

-It would indeed. If anyone tries it, I'd suggest having at least the tip end set at a very fine resolution. I suspect that any benefit to this mod will be so slight, that a rough printed surface could add enough boundary-layer drag to offset the gains.

Anybody got a resin printer and a couple hours to spare?

Doc.

[superman]

This thread is what i miss of the old MCB.

Doc first thought came to mind was to try threading the screw through to the gap between the tubes and do the small hole. 2nd thought why not make the hole in the bottom of the top bolt smaller to restrict it. now i know you likely don't want to make bolt after bolt after bolt, but maybe you make 1 bolt and slowly increase the hole. another thought is to do a restricted hole through the lower bolt. what about using a venturi bolt that has the outside open like the freeflow bolts. seems like there are more than a few ways to tackle this one.

I think i would get a couple bolts that have removable tips like the shocktech bolts and play around with tip styles.

[DocsMachine]

Doc first thought came to mind was to try threading the screw through to the gap between the tubes[...]

-That's what I mentioned on the previous page. Something like the choke screw that came on the very early, first-generation Spyders, before they went to the RVA.

[...]and do the small hole. 2nd thought why not make the hole in the bottom of the top bolt smaller to restrict it.

-That's another option. When I made the 3/16" test bolt, the port on the underside, up from the body, was the normal .272". So an interesting test might be to make it 3/16" all the way through- possibly even reducing the though-body bore, too, though maybe only down to 1/4".

[...]now i know you likely don't want to make bolt after bolt after bolt, but maybe you make 1 bolt and slowly increase the hole.

-That was the original idea. I figured the 3/16" bolt would be significantly low, and so I could try bumping it up to .200, then maybe .225, then .250, and test it, of course, at each stage and see what happens to the velocity.

Now, we have to remember, that the result I got- that the tiny bore may have shot faster- was one result, with zero A/B testing, and only with maybe 15-20 rounds, no chrony.

It's entirely possible I got an erroneous result. What I need to do, is finish a second, proper body (no screwed up detent holes, a little more tube separation to the feeds don't break through, correct valve chamber dimensions, etc.) and after I have the timing and springing in a fair ballpark, then burn up some Delrin making a few test bolts. (Including one or two for my standard 'Cockers.)

And this time, try getting some solid chrono readings (an old athletic sock over one barrel, lets you chrono the other barrel, since you can't just leave the paint out of the one) and write down some results. It's an interesting enough topic it deserves a little proper analysis.

another thought is to do a restricted hole through the lower bolt. what about using a venturi bolt that has the outside open like the freeflow bolts. seems like there are more than a few ways to tackle this one.

-Yep. As I said earlier, I could try a six-hole "venturi" in both bored, or just in the top bore, to see if the mild "baffle" helps the air distribution, smaller ports in either bolt (the top of the middle one or the bottom of the top one, etc.) maybe even smaller ports through the body.

Regardless, I tend to like the idea of an adjustable choke screw, since, again, the conditions I set my gun, up here, with my paint, for, may not be the same conditions you end up with in hotter/higher/humid states.

I think i would get a couple bolts that have removable tips like the shocktech bolts and play around with tip styles.

-I have several of my own version, like that, typically with an aluminum body and Delrin tip. Was kind of thinking exactly hat, making a set of all-Delrin ones for this test, since I have a small basketful of short bits of the stuff, too short to make a bolt- even a halfblock one- from. Be a good use for some of that.

Doc.

[cfos00]

Welp, just got some more delrin to make another bolt with a tiny air passage to see what I can get for a FPS jump. Worst case scenario is I make a bolt that I don't use. Hopefully we see an increase. Back in a few to let you know results...

Thinking I'll bore it to 3/16ths to start.

[DocsMachine]

I'd start even smaller than that. The test one I made for the 'Twin had a 3/16" bore. Which I thought would be ludicrously tiny, but actually worked better than the "open" bolt.

And you have to remember that the area increases faster than the diameter.

1/8", or .125", has an area of about 0.0123"
.150" would be about 0.0177"
3/16", or .187" jumps that all the way up to 0.0276"
.210 takes that up to 0.346"

And so on. Small changes in diameter mean bigger changes in area.

AND... I think this experiment should be done three times- once (at least) with a modern big-chamber 11/16" marker, once with a conventional 5/8" valve marker, but with a good quality valve, and again with an older small-volume body and early style valve.

IE, low pressure, moderate pressure, and high pressure.

AND... in the interests of true scientific... er, interest, ideally, the BODY should be drilled to match. Possibly even the valve.

That is, a 3/16" bolt works well (probably) when fed by a high-flow valve with large connecting passages. What would happen if the entire channel, from the valve all the way to the face of the ball, was 3/16"? Or even .150"?

That's another thing I'd love to try, when I get some spare time.

Doc.

[flyweightnate]

Well... you need to remember the gas is going to be transsonic.

Somewhere along the line, you have gas moving across a choked port, where it'll reach the speed of sound. If supersonic, expansion leads to increased velocity, the opposite of subsonic. But the shock wave needs to be controlled.

The reason to go for 15 degrees expansion is that, regardless of velocity, it tends to be the widest angle without pretty significant eddy formation on the edges. You hold more energy than exiting a square- edged hole.

[Seajay]

Weird thought, did anyone ever try drilling multiple small holes in the body instead of a single hole for air travel?
I imagine you could do a lot of different things that way, restriction, turbulence, different affect on the pressure wave.

[cfos00]

I'd start even smaller than that. The test one I made for the 'Twin had a 3/16" bore. Which I thought would be ludicrously tiny, but actually worked better than the "open" bolt.

And you have to remember that the area increases faster than the diameter.

1/8", or .125", has an area of about 0.0123"
.150" would be about 0.0177"
3/16", or .187" jumps that all the way up to 0.0276"
.210 takes that up to 0.346"

And so on. Small changes in diameter mean bigger changes in area.

AND... I think this experiment should be done three times- once (at least) with a modern big-chamber 11/16" marker, once with a conventional 5/8" valve marker, but with a good quality valve, and again with an older small-volume body and early style valve.

IE, low pressure, moderate pressure, and high pressure.

AND... in the interests of true scientific... er, interest, ideally, the BODY should be drilled to match. Possibly even the valve.

That is, a 3/16" bolt works well (probably) when fed by a high-flow valve with large connecting passages. What would happen if the entire channel, from the valve all the way to the face of the ball, was 3/16"? Or even .150"?

That's another thing I'd love to try, when I get some spare time.

Doc.

I'll be using one of my valves in a 9/16ths body, which means a valve with low dwell on it, as they slam shut extremely fast. I'll give it a shot with 1/8ths to start and go from there.