I completely sympathise with Terry D and his stance on chrono string tests - it is a deathly dull process!
Two experiments tonight -
First, I tried to bring the power up to about 5 ft/lb, where it was originally.
I did do this, but it doesn't look like I got the full benefit of the SSG. Super-boring shot string follows. Again, all in metres per second, with an 8.44 gr JSB pellet, dry fires denoted by *.
two shots to confirm cartridge pierced, and then
151.2
149.6
152.8
157.2
157.1
156.2
156.7
157.1
158.5
159.1
159.3
157.2
157
157.7
157.6
158.1
159.2
159.9
158.7
159.7
158.8
159.7
160.9
159.2
158.2
158
158.2
159.3
161.4
160.9
160.3
161.8
159.2
161.4
160.7
161.2
158.9
161.5
160.4
160.1
159.2
159.3
160.4
159.3
160.4
159.3
160.5
159.4
159.5
159.9
160.4
160.2
159.7
158
159.8
158.1
158.6
156.4
154.4
153.5
152.5
152.1
151.7
149.5
146.3
141.4
142.2
139.5
137.2
133.2
134.5
127
123.9
120.2
116.5
110.9
107.8
103.8
98.4
93.6
87.8
82.9
77
70.3
64.9
59.6
*****
*****
*****
(empty)
I reckon about 60 shots before a steady decline sets in. Back-of the envelope, I make that about 300 ft/lb generated in total from the usable shots. So, more efficient than the stock spring, but less efficient than my first SSG set up.
What's interesting about this string is the long tail off of about 35/40 shots, when the SSG is meant to get a more sudden decline in power after a relatively flat power curve. I suspect that another design of SSG could get better results at this power level. There really is no standard for these and they are all different.
As this was pushing, at peak, 5.25 ft/lb at room temperature, I wasn't really comfortable leaving it at this power level. So onto experiment number two, splitting the difference.
My patience for recording Combro results was waning rapidly at this stage (and pellets cost money!), so there were a lot more dry fires for this string, which were taken in rapid succession:
**
130.2
129
*****
*****
128.5
*****
*****
*****
*****
129.5
*****
*****
*****
*****
132.7
133.3
*****
*****
132.6
133.7
*****
*****
*****
*****
128.1 (gun was getting quite cold at this point so I had a fag break)
*****
*****
133.9
*****
*****
131
*****
131.8
*****
***** (noticeable sound difference here)
108.5
*****
85.2
*****
*****
*****
*****
(empty)
An average of about 3.5 ft/lb for 120-ish shots before drop off, makes 420 ft/lb generated in total - so I broke the 400 ft/lb barrier, making this the most efficient set up so far! It's good to be right sometimes....but then even a broken clock is right twice a day
I could drive myself mad tweaking this forever and a day, but I am (as I said I would be) chuffed as nuts with 120 shots at 3.5 ft/lb so will leave it here, at least for this gun.
I would be interested to see what our resident engineers can make of this concept, because if a ham-fisted bodger like me can get these kind of results then the sky is the limit for someone with actual skill.
There is a 40-odd page thread on the GTA forum for the interested - it's called "Re: A New Method for Increasing the Efficiency of a PCP - the SSG". Loads of different designs on there - some very intricate - but it can be as simple as my set up of a 4mm hole in the rear plug, a screw, an o-ring (optional!), a spring and a nut.
Please give it a try.
(and yes, this 2240 is officially complete now, promise....)