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 Post subject: Re: 2240 Velocities
PostPosted: Tue Jan 14, 2014 12:45 am 
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Joined: Sat Nov 03, 2007 4:19 pm
Posts: 8976
Location: Coalmont BC
Cool Gizmo, Al.... :wink:

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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 Post subject: Re: 2240 Velocities
PostPosted: Tue Jan 14, 2014 1:10 am 
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Joined: Sun Jul 15, 2012 10:49 am
Posts: 6029
Location: Victoria, BC and Clarkston, WA
Gippeto wrote:
EHF....Something along these lines perhaps? Unfortunately the fellow who makes them refuses to ship to Canada. :roll:

Called a "Pro top"

Image

Al



What's the guys website Al?

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-Rick
Pardini K10
Brocock Concept .22
3 Custom Crosman 2260
Benjamin Discovery .22 w/Joe Hickey stock!
Crosman 150 pistol .22
4 Crosman MK 1 pistol .22
Mrodair CP-1M .22 and .177


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 Post subject: Re: 2240 Velocities
PostPosted: Tue Jan 14, 2014 8:29 am 
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Joined: Fri Aug 20, 2010 4:30 pm
Posts: 403
Location: SW, Ont
Copy of the "Pro Top" design

Part 1
http://anotherairgunblog.blogspot.ca/20 ... ckers.html

Part2
http://anotherairgunblog.blogspot.ca/20 ... -part.html


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 Post subject: Re: 2240 Velocities
PostPosted: Tue Jan 14, 2014 9:14 am 
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Joined: Sun Feb 10, 2013 1:16 pm
Posts: 1787
That would be perfect. More complex than I had imagined, but looks like a well engineered solution.


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 Post subject: Re: 2240 Velocities
PostPosted: Wed Jan 15, 2014 10:16 pm 
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Joined: Sat Nov 03, 2007 4:19 pm
Posts: 8976
Location: Coalmont BC
After measuring the lift in stock form, and finding out it was TINY, I thought about what was happening inside a stock 2240.... The hammer is VERY heavy for the FPE level of the gun, and the hammer spring is very weak, leading to small lift and long dwell.... In a PCP, this tends to be inefficient, so I decided to make an extreme change as an experiment, and lightened the hammer by 50%, from 60 gr. to 30 gr.... I turned down the middle of the hammer to 3/8", leaving only a small portion at each end that was full diameter.... You have to grind through the case-hardening to get even a carbide bit to bite, but once you are down about 0.030" you can turn it down OK, as long as you are working from the inside out (ie softer core to the harder outside layer)....

I tested the velocity and lift once more with the stock hammer, graphing both vs. the preload over a range that spanned the "knee" of the curve, to show both the plateau on the left and the downslope on the right.... I then swapped out the hammer, and repeated the test with the light one.... The results were as expected, a big drop in velocity for any given preload because the lighter hammer, while having the same energy, has only 71% of the momentum (square root of the weight at 0.50), so that means only 71% of the dwell, while maintaining the same lift.... This means that to reach the plateau you have to increase the preload, adding energy until that balances the loss in dwell.... It turned out that even at maximum preload (ie coil bind) I could not quite reach the original velocity, although you could just see the beginning of the plateau.... I then tried a 1377 hammer spring, and then settled on one from a Disco for the next test.... I repeated the test, achieving the highest velocity to date in .25 cal (528 fps) with a clear plateau, knee, and downslope.... Here are the results....

Image

You can see that with the light hammer and the Disco spring, the plateau (maximum velocity reached) was about 10 fps more than the previous best.... I think that is because the valve is opening more quickly, so that the initial "pulse" of CO2 arrives quicker, and the pellet gets a bit more acceleration in that critical first part of its travel.... The lift was much greater with the Disco spring, and that alarmed me that it might mean a lot more CO2 might be used, so I set the gun up for the same velocity as the string I shot yesterday (starting at 500 fps, averaging about 465), and I actually picked up 4 shots, so the efficiency is actually higher.... The report sounded different today, it was louder, but more of a sharp POP and less of a BURRRP, so I think the gain in efficiency may well be less hammer bounce due to the shorter, stiffer spring.... Whatever the cause, I'll take it !!!

This gun is already over PAL velocities, and if I were planning a rifle, I could use a 2260 tube and clip the hammer spring to keep it just under 500 fps.... In other words, I have already achieved a .25 cal CO2 non-PAL rifle, with basically just the valve extension (the lighter hammer was just icing on the cake).... However, I want to use a pistol length barrel, and my calculations show that with half the barrel length, I would only get about 400 fps.... Therefore, I'm going to modify the porting in the valve to (hopefully) increase the power so that when I step down in barrel length I will be in the high 400s.... I figure I need to get closer to 600 fps with the 24" barrel to achieve that goal....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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 Post subject: Re: 2240 Velocities
PostPosted: Thu Jan 16, 2014 6:47 am 
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Joined: Sun Feb 10, 2013 1:16 pm
Posts: 1787
Ah, I'd been wondering what the implications of different hammer weights are. Thanks again Bob!


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 Post subject: Re: 2240 Velocities
PostPosted: Thu Jan 16, 2014 4:59 pm 
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Joined: Sat Nov 03, 2007 4:19 pm
Posts: 8976
Location: Coalmont BC
I pulled the valve out today and drilled out the ports.... The throat is now 0.250" (still a stock valve stem, so 0.195" equivalent), the exhaust port is 0.211" (done with a 3/16" mill angled 20*), the transfer port is 0.219", and the barrel port is 0.190 x 0.250" (0.220" equivalent).... The transfer port is 5/16" OD Teflon rod, so I had to drill the breech and main tube to that diameter.... Otherwise, the gun is the way it was yesterday, lightweight hammer and Disco spring, Extended Valve with about triple the original volume, and the power adjuster from a Challenger, all mounted in a 2250 tube.... The barrel is a 23.8" Lothar Walther with a 12mm OD, turned down to fit the Crosman steel breech.... The first cartridge was used to test the velocity vs. the preload, and then I installed a new cartridge, set the preload to 6 turns out, and shot a string using 25.4 gr. JSB Kings.... Here are the results....

Image

The red line is the results from yesterday, same gun with stock ports, and the black line is today with the bigger ports.... As you can see, I picked up about 75 fps, reaching 603 fps (20.5 FPE), and the knee of the curve shifted one more turn to the left.... so the plateau now starts a couple of turns from coil bind on the Disco spring.... One thing interesting to note is that at lower preload settings, I'm getting more lift on the valve (ie a flatter lift to preload curve).... I think this is a result of the larger throat decreasing the pressure differential across the head of the poppet, and allowing the valve to open further, increasing both the lift and dwell.... I did a small amount of streamlining on the head of the poppet near the valve spring seat, so is also possible that reduced the drag which was reducing the lift.... It shows what happens when you make more than one change, it becomes impossible to determine what causes the results to change....

I once again shot one shot per minute, and at this higher CO2 use that is not enough time between shots.... The velocity is pretty much a linear decline throughout the string, and the gun got progressively colder with each shot.... It would be interesting to shoot another string, waiting like 5 minutes between shots, I'm betting a lot more energy could be extracted from the cartridge.... As it is, I averaged 528 fps (15.9 FPE) over the entire 23 shots which were above 300 fps, with an efficiency of 0.88 FPE/CI, nearly as high as at the 12 FPE level with the stock ports.... The 19 shots which were over 500 fps averaged 549 fps and 17 FPE.... Thats a lot of power from a 22XX platform running on a 12 gr. cartridge.... I'm pretty sure that when I get a 12" barrel I should be able to tune this gun into the high 400s, setting it up as a good non-PAL .25 cal pistol on CO2....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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