GTA
All Springer/NP/PCP Air Gun Discussion General => PCP/CO2/HPA Air Gun Gates "The Darkside" => Topic started by: GadgetX on March 03, 2013, 09:46:39 AM
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Good Morning all !! I have put together some data to help myself understand my MROD somewhat better. Thanks to everyone here I believe I just about have the "InVision Tune" ;) My interest were in maximizing the energy on target with JSB Exact 25.4 gr pellets which shoot the most accurate in my MROD. So i started by trying a couple of different hammer springs, #10 & #15. I have suspected that I am going to have to go for hopped up valve kit in order to reach my goal of ~24 shots at ~900 fps. Originally I was going to purchase a Daystate Air Ranger XL which I was willing to lay out the bread for, but the support equipment to air it up was as much as the AG. :-[
So I tinker with the MROD and I must say I am having some fun !! I really dislike the looks of the "old fashion heavy stock", but I figure I will purchase a couple of optional aftermarket stocks to play with.
Here are the results I have come up with.
Any comments and advise is most welcome !!
Mike
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No expert, but what I see from others, with heavier springs, going to a higher fill value of 3200. A valve change won't help shot count, but can get velocity higher. Want more shot count, need a res ext (more air).
BTW, what are you trying to kill at what range?
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Beavers are the toughest for me, 20-30yards. I am happy with ~24 shot count. I am in no way an expert, but the last tune I listed, if I added a valve hopup kit seems that would be my cat's meow.
Mike
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A more free flowing valve, will change that shot curve. The more air per shot, the quicker the curve will peak and drop off.
PS: I have some beavers to dispatch this year as well. I need DEC approval, but easy to get given their high numbers in our area. Looking at Chairgun, 42FPE at muzzle will still have 34FPE at 30 yds. I'd think that is plenty for a beaver, especially since it will have to be a head shot (swimming). I have seldem been able to sneek up on them while on land. At that range a .25 Mrod can put the pellet in the eye socket. :)
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You won't need a "kit". You can do what needs to be done yourself with very little fab skill. The port going to the barrel can be easily increased to .156". The transfer port tube also needs to be increased to .156 as well as the port in the barrel. .156" is the largest that I feel safe going with when it comes to the port tube. If you have access to lathe, you can bore the valve body out to the same size of the threads to give a bit more available volume. Make sure that you remove the transfer port adjustment screw before you go cutting metal. At the ranges to stated, a larger ES will have little effect on shot placement. I think that you should be able to get 880-900+ and back to 880 over 16 shots with just a 10# spring. That is typical with stock M-rods +10# spring.
Bill
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Thanks Bill. I think I am more of a bolt on replacement type person. Boring and lathes may be beyond my skills. I have improved with just a spring replacement. May buy a new MROD when they come out, and send this MROD to an expert to fine tune, de-ping, drill, lathe, cut and any other mods which make sense...
Mike
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I understand. I guess i can over estimate things some times. I am Toolmaker by trade and work as a master craftsman for a Siemens co. I would recommend increasing the fill psi to 3200 and increasing the spring tension a bit. Should be around 900-910 around 2500psi If you have a tank that is down to 2500, tether it and tune. Once you are happy, disconnect, Fill and shoot a string. Fine tune from there.
Bill
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i too would increase my fill pressure with the 10# spring. maybe 3200. i would even try 3400 to see what kind of curve i got. ;D
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I understand. I guess i can over estimate things some times. I am Toolmaker by trade and work as a master craftsman for a Siemens co. I would recommend increasing the fill psi to 3200 and increasing the spring tension a bit. Should be around 900-910 around 2500psi If you have a tank that is down to 2500, tether it and tune. Once you are happy, disconnect, Fill and shoot a string. Fine tune from there.
Bill
Thanks Bill, I tried the tether at 2600 on the same day. The best I could get was 880 fps. That was at 6.5 turns preload. H=0, V=4.5
Mike
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Your results with the 15# spring are as I expected.... Your first shot is the fastest, and by shot #5 you are down 5% in velocity.... You therefore have a usuable shot string of only 4 shots.... If you back off on the preload, you can make that spring work (maybe) like the 10# spring....
The 10# spring at max preload, likewise has the first shot as the fastest, but this tme you have 11 shots with 4% ES.... You correctly reduced the preload by a turn for your next string, but you went a bit too far, as now the velocity is down 4% at 2800 psi, and drops below the 4% level again at about 2100 psi after an 18 shot string.... I would suggest increasing the preload to ~ 6.5 turns (assuming you want to use a 3000 psi fill), and you should be able to achieve a string of about 16 shots peaking at about 880 fps (44 FPE) between 3000 psi and about 2300.... Incidently, your current tune (P=6) works out to 18 shots @ 42 FPE = 751 FPE total from 700 / 14.5 = 48 bar x 13 CI = 627 CI of air, for an efficiency of 751 / 627 = 1.20 FPE/CI.... which is a good value....
If you improve the flow by hogging out the valve and the ports, I can tell you what to expect with no other changes from the above tune.... The peak velocity will increase, and the peak of the velocity curve will move to a lower pressure.... However, the number of shots with a 4% ES will likely decrease (not increase) as the bell-curve will become "peakier".... Just like putting a hot cam in an engine, you will get more power, over a narrower range.... If you are after increasing the peak power, you will likely achieve that.... You will then be able to increase the preload again (say, back to your P=7 level), and gain a bit more velocity.... However, your shot count is likely to shrink below the 16 shot (2 mag) level....
Bob
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Bill,
Check out the area of the throat; IFF you take the exhaust port up in area, it is also useful to get after the throat. Care avoiding the seat is required, as is making sure the expanded port is stull centered. OE the port size is only about .190, which with the .125 valve stem leaves you with an area quite comparable to the OE exhaust port with its .140 hole.
I got a pretty reasonable performance with the Staley o-rings and larger flow area. The o-rings do clip off the peak, which for a big valve is a useful thing to keep a tighter spread on start, peak and end string velocity spread. It also allows use of the valve at its wide-open efficient flow configuration v. running the velocity screw/needle partly closed.
cheers,
Douglas
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Thanks Bob for taking the time to explain all that. I believe in a nutshell what you are saying is even with hopped up valve expect less shots.
I wish every time that a manufacture list the FPS for a AG, they would specify what pellet they used. It is too easy to say this is a 1000 fps AG, and they are using PBA pellets or something light.
MIke
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Crosman rates the .25 Mrod at "upto 900fps", which is pretty close compared to how most of their guns are rated. ;)
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Hatsan rate their AT44 S-10 Long in .25 cal at something like 970 fps, 42 FPE, and 60 shots.... All are true, but not at the same time.... high velocity with light pellets, high energy with heavy pellets, and lots of shots if you include shooting until they only dribble out the barrel.... In reality, 24 shots within 4% ES at 42 FPE is more like it.... Typical advertising hype....
Bob
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Crosman rates the .25 Mrod at "upto 900fps", which is preety close compared to how most of their guns are rated. ;)
The Marauder will hit 900 fps with a Crosman pellet, which BTW is not the lightest available by a long shot. It'll actually deliver a useful tune at that speed for at least a clip worth if you pick the 20 grain FTT.
cheers,
Douglas
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Douglas I agree. I have made mine have the same area cross section from the poppet to the pellet so to speak. getting 60fpe for 12 shots. That tells me to up the power until I get 8shots with a small ES. That would be a great hunter for those beaver to yotes and can take smaller game at greater distances. IMO 24shot from the .25cal is good for beaver and coons at closer ranges and will limit taking the smaller game to around 65-70yrds max, unless you have a LOT of experience with your rig. With 24 shots, the ES seems to get too great for consistent accuracy over the whole string, beyond the 65yrd marker. I tend to be an ES snob I'll admit. So I don't mind filling more than most. Fortunately, I also like the higher power and they go hand in hand. My next change should take me to the 65fpe+ for 8 shots and that will leave me room for further improvement in the future. I see no reason that the marauder can't deliver 80fpe for one mag. My only concern is whether it can do it with the same level of accuracy that it has at 45fpe. To me, the extra FPE is worthless if it can't be delivered on the mark. The path to get there sure is a heck of a lot of fun though. I'm liking it as much as shooting.
Bill
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hey-Hey!!!,
Yah, power is ony good if you can connect it. I am going to get some of the 36 gr E-J pellets and see how they fly...but that is going to be for after I get the bottle rigged up...:) High power plinker. If you're going to send it, better know where it goes. I do doubt I'll go looking for anything out past 100 yards, though the new BSA barrel will probably suprise me, just like it did in 22 cal. 60-65 FPE with a domed pellet running 900 fps will do quite well enough for squirrels an' such.
cheers,
Douglas