Anyway, I am thinking I want to shoot for 100% porting, unless any of you think that would be a bad idea. Jason, you mentioned an oblong barrel port of .130" x .168" would achieve 75% bore area, so I am thinking I will have to go with an even longer (and maybe ever so slightly wider) port to achieve 100%.
As I understand your use of the word "blend," I need to make sure that the transfer port matches both the barrel port and the exhaust port, seeing as how the exhaust port is already .150" x .161" ... They are going to be different shapes, as a .150" barrel port will be too wide for a .177" pellet, creating potential loading issues (right?).
I turned the probe down to .025" on my drill press using a file and some 500grit sandpaper. ... If it breaks it breaks, and I will get a new one.
It was also mentioned that there is no real point in making things over 100%, so I am thinking there is no good reason to open up the valve throat any further, since I will already be at 466% there with the MRod valve stem... This is logical, right?
Another thought is that for optimal efficiency, areas from the throat to the pellet should be the same percentage all the way through to the pellet. They should at least be progressively smaller, without any larger pockets following smaller ones, else we run into the 50 gallon drum analogy. So to optimize efficiency, I need to make sure that I am at least 98% all the way back through the exhaust port. Do I have this right?
I added a box at the top of the spreadsheet for calculating the area of an oblong port so you can play with the numbers. Copy attached.
Or you can drill it out and glue in the shank of a drill bit.
I don't see any point in making it smaller, it just gets too fragile, and can damage the base of the pellet on loading....
I calculate the valve throat area like Jason does in his spreadsheet (hole area minus stem area) and make sure that is at least as large as the rest of the porting.... I have found that making that area slightly larger usually doesn't help the maximum power, but does help the efficiency slightly, probably because of the smaller area during the opening and closing parts of the shot cycle.... ......when I try for bore-size porting to get the absolute last bit of power from a given caliber.... ie I want 100% everywhere.... In that case, again my choices are simple.... I make the transfer and exhaust ports the same diameter as the caliber.... and the valve throat area large enough that it's area is about 10% larger (equivalent diameter about 105% of caliber)....
The bolt, in this case, I make retractable, using a "J-slot" for the bolt handle (or locking lug).... The nose of the bolt is flat, and long enough to load the base of the pellet just past the barrel port.... Then there is a second slot for the bolt handle, which after turning it to the locking position, you use to retract the entire bolt so that the nose of the bolt is flush with the back of the barrel port.... Voila, no restriction....
While we are on this subject, I will pass on my observations about pellets vs. bullets, and their affect on how big you need to make the ports.... Although this may change now that pellet manufacturers are starting to make really heavy pellets (for the caliber, ie high SD).... with most of the pellets used today I find NO point in going past 75% of the bore diameter for the barrel, transfer, and exhaust ports.... and equivalent areas elsewhere.... You just don't need the extra flow.... Bullets are different.... They are heavier (higher SD), so they can utilize the extra flow possible with bore-size porting.... So, for me, its pretty simple.... When I build a gun to shoot pellets, I use a round, 75% barrel port, and work back from there and use a probed bolt of the sizes I quoted above.... When I build a flat-out bullet shooter, I use transfer and exhaust ports that are caliber diameter, an oblong barrel port of equivalent area (and with bullets, you can go 80% of width, because they are longer and less prone to damage on loading), and a retractable bolt.... I make sure the valve throat is slightly larger area than the rest of the porting, and that the inlet side of the valve is about twice the area....
Also, what are the calculations on the bottom-right for? I am imagining they have something to do with how you are figuring I would need 1800psi with existing porting and 1400psi with better breathing for the 13.4gr's at 900fps? These guys here:
Is epoxy good? Or is there something more preferable? I was thinking of just turning down a finishing nail for the actual piece.
That's right, I was doing some extrapolation from other tunes I have done to look at your scenarios.
Seems to me that last bit about the machining skills is the most important factor in this. Hot glue, melted shopping bags, friendly plastic, epoxy... none of these is going to be good for more than a prototype part, if even that. Adhesion is the biggest issue with the epoxy on poppet stuff, which I'm not clear on what you're attempting anyway. Why not just make a more aerodynamic poppet? Out of one piece of delrin or nylon would seem the most sensible. Epoxies with which I'm familiar don't tolerate a lot of pressure along thin edges, tending to lift off, peel, crack, flake. A high grade construction adhesive might be better, the sort they use to glue cracks in bridge concrete... but you'd be paying $40+ for a massive tube of the stuff and using less than a dime worth of it. And it probably wouldn't work anyway. Seems what you really need is at least a rudimentary lathe. Even a drill mounted firmly in a vise can provide the basic spinning thingy, then a few files and a block of wood for a tool rest would let you shape plastic rod to some extent. I've found 95 durometer polyurethane rod to make the most durable and well-sealing seals for transfer ports, Foster nipple check valves, and bolt probe seals. Very challenging to machine nearly though. Delrin is easy to machine, but sometimes too firm for seals. Still, if the surfaces are shaped correctly it makes an excellent poppet. As does nylon. Considering your obvious enthusiasm for modding, I'd suggest firstly that you consider buying a cheap used lathe. A small bench top model of some sort. Most critical is that it run true, not wobbling or vibrating weirdly, so the bearings have to be good. After that, most things can be smoothed out with a bit of filing, polishing, common sense. A lathe would open up a world of possibilities for modding airguns. You'd love it. And you'd soon understand just how silly it is to consider glomming on bits of glue in a high pressure system.