MEGGA VALVE, 458 PROJECT– === 790 FPE ! =================
I BEEN BUILDING THIS .458 CAL. AIRGUN
WITH A 44 INCH LONG BARREL.
GOT 903 fps WITH A 300 GRAIN HARDCAST BULLET.
THAT WASNT ENOUGH, SO I BUILT A NEW VALVE THIS WEEKEND.
I’AM STILL IN THE TESTING STAGES WITH THE NEW VALVE.
THE OLD VALVE HAD A .437 DIAMETER STEM.
THE NEW VALVE HAS A .520 INCH DIAMETER STEM.
I.D. OF THE HOLE IN THE STEM IS .397 INCH.
ORIFICE DIAMETER (SO FAR) IS .305 INCH.
PORTS IN THE STEM WERE CUT WITH A .25 ENDMILL, THEN FILED OPEN.
THE SCREW FOR THE VALVE SEAT IS 6-32.
SOME OF THE STRENGTH OF THE VALVE IS LESSENED BY THE INCREASED VALVE STEM DIAMETER, SO THIS VALVE IS BUILT FROM 85,000 PSI TINSLE STRENGTH STEEL.
I GOT 543 fpe WITH THE OLD VALVE AND 300 GRAIN BULLETS.
HOPING TO ACHIEVE NEAR 600 fpe, MAYBE NOT! , BUT I’AM GONNA TRY.
THERE IS A TALON ss VALVE IN THE PICTURES AS A SIZE COMPAIRSON.
THE “PELLETS” ARE 21, 300 AND 500 GRAIN.
Mark
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NICE BAZOOKA!!!! 😀 😀
What type of barrel are you using?
hammer weight?
His is a home built gun. Im betting the bottle he is using is shorter than the ones we use. I think he has a 4500 psi bottle? Maybe that extra length is just for comfortable LOP?
Gorilla sized! Nice stuff, Mark. Any reason why the center section is super long?
aloha,
walt
airbrain
“Y” = .305 for my gun.
You will be using 800 PSI, so it may be larger.
My gun has a very strong hammer spring to open the .305 orifice against the 3000 tank pressure.
You will be opening aginst 800 PSI – a much easier job.
“X” doesnt matter, except it must be larger than “Y”, and not so big that it inhibits air flow.
Start with a small orifice opening and see if you can open it easily.
If it opens with an over pressure (3300 psi , in my case), then you can enlarge the orifice. Keep testing it with an overpressure and enlarging, until it will not open with an overpressure.
Theres a LOT of fooling around to getting one of these valves to work right. I have 40 hours into two valves, and its still not up to the performance level I want. I have made 100’s of test shots and as many tank fills.
Getting the valve to close at the correct time is another big tuning issue.
You gotta fool around with the strength and preload of the return spring.
If you are using 800 PSI, then your valve WILL be different from mine.
Mark
very nice!
I’m curious what dimensions you chose for X and Y, as X affects the force that is pushing the stem closed. For the Y dimension, choked flow plays a very major role in overall valve performance I think.
The main question is, how small a value Y could be in order not to adversely affect performance too much?
Nice job and great pictures! And thanks for telling the dimensions, helps those a lot those who want to build their own high-powered valves (like me 8) ).
I especially like your idea of using a hardened screw. The stem looks a bit thin near those three holes, hopefully it will last after hundreds and hundreds of hammer strikes. May I ask what steel are you exactly using?
Thanks Yellow and blodnob!
I think that I just found where the power was hiding!
Filed the 3 stem ports open and test shot, got an incerese in velocity.
Did it again, and got another increase.
300 grain bullet, 925 FPS, for 570 FPE. 3000 psi fill
I know theres more power available.
My test shots use a small 22 cubic inch tank (regular AF tank is 32), because it is easier to fill and drain.
The tank pressure drops by 1600 pounds for the shot.
That means that when you need the most volume (when the pellet is in the last 1/3 of the barrel and going very fast), your not getting it.
If I subsitute my 68 cubic inch tank, the pressure will drop less due to the increased tank volume available.
Watch out , Cyngus, I’am getting close!
Mark
What he said 😯
Thats a beast !
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nice . what the hell r u going to be shooting with that buig boy . wilder beast