Nitro Piston specs question...

[ALWIII]

  Does anyone have a definitive answer, or source, for the pressures in the Crosman/Benjamin Nitro Pistons? If I'm going to tinker, (And I most surely will!) I'd like to know my baseline. That's the only way to have the data from testing validated.
 
 Thanks in advance for any help that can be provided!

[mikeyb]

  Does anyone have a definitive answer, or source, for the pressures in the Crosman/Benjamin Nitro Pistons? If I'm going to tinker, (And I most surely will!) I'd like to know my baseline. That's the only way to have the data from testing validated.
 
 Thanks in advance for any help that can be provided!

Based on measurements of dimensions and forces the NP1 pressure is ~80 bar and the NP2 is ~125 bar.
The Hastan Vortex air springs used in the 24 fpe (max) rifles are pressurized to ~125 bar.

If those numbers are not sufficiently accurate enough suggest you contact Crosman directly and ask them what their manufacturing specs are for their different NP gas springs.

Unless you have some specialized equipment and experience working with high pressure gas/air, measuring the pressures directly will be difficult and possibly dangerous.

If you are planning to experiment by varying the pressure in air/gas springs you will need some basic equipment like a HPA hand pump and fittings rated for high pressure.
You can redesign the rifle to allow changing the pressure without disassembly or acquire/build a sturdy spring compressor for disassembly/reassembly between pressure changes.
To really track rifle performance for each pressure change you will also need a chronograph to document/compile pellet velocity data.

If you decide to perform these experiments I (and I'm certain many other GTA members) would love to see your efforts and results documented on this forum :-)

[Roadworthy]

It is my understanding that the pressure in a Nitro Piston cannot be adjusted.  You need to get into more expensive airguns to get an adjustable pressure gas spring.

[Ronno6]

Crosman rams cannot be adjusted in my experience.
Some Hatsan rams can have pressure added, but not relieved... Adjustment is a one-way street on those.
These are found in the Edge and Striker rifles.

Others, such as the mod. 95 can relieve pressure in addition to adding........
Ya can play with those.

My first Edge was low on pressure, so I pumped up with a hand pump.
The gauge musta been a tad off, cuz 1800 PSI gave me a ram that took 2 grown men and 2 teenage boys to cock.....
and was causing the retainer pin to bend........ I replaced with a Crosman ram and waited for the Hatsan to bleed down..

[mikeyb]

Both correct, Crosman gas springs are sealed and are NOT designed to be user adjustable as-is.

I believe those gas springs can be modified to allow discharge and recharge by advanced users. There is some danger during the modification process. Then there is the 100% danger of users OVER-filling to an unsafe pressure to get MORE muzzle energy than their rifle was designed to handle.

Even if i were able to document a safe way to modify these gas springs it would only lead to abuses and possibly injury. I choose the low risk of damage/injury path and bow out now. Best Wishes :-)

[ALWIII]

  I found a source who can supply the necessary sized pistons at whatever pressure I specify, and for a very reasonable price. Granted, I'm at the mercy of the supplier's accuracy claims, but it's the best I can come up with at present. Fitting a NP with a valve is probably possible, and motorcycle shops (especially dirt oriented) often have nitrogen refill stations for shock rebuilds.

 I appreciate the caution expressed by mikeyb, and I'll bear it in mind when publishing results. I don't intend to intentionally go until the point of failure, anyway. The pressure cylinder of a big Crosman shattering/bursting in my face is a risk and expense I am not willing to assume.

  I have a 85 Kg ram installed in my NP XL and the results are so far underwhelming. I got a bigger boost in velocity from a Jim Maccari seal than I got from the new NP. LOL With the new (not broken in) seal and NP combined, however, my 5 shot average, with H&N FTT@ 20.06 gr, jumped from 782 fps to 834 fps. Stock highest velocity was 789.8. Current set up has recorded a high of 845.6 fps.

 The 75 Kg NP in my Crosman Valiant hasn't been tested enough to get any kind of trend recorded, but preliminary chrono checks indicate a 50 fps increase. It has a new Crosman seal installed with about 50 shots on it. The seal replaced a urethane seal from The CO2 Shop on Amazon. As a side note, I found The CO2 Shop to have good quality seals to replace the factory damaged seals on new guns, but the thickness sometimes has to be sanded down. A couple I installed in various rifles sat as much as .040" (or more) above the piston face.

 Overall, these figures aren't of much value until the seals are all broken in and I can get a solid, consistent average over 20 shots, minimum. My chronograph is one of the cheapies from Amazon ($30) that attaches to the muzzle. They don't last long on the bigger NP guns and sometimes require creative mounting on the barrels with the built in "silencers", like the Valiant and Shockwave from Crosman. So, grain of salt, etc.. What I'll mostly be measuring is trends and percentages rather than absolute numbers.

Having said all that;
I do not recommend anyone exceed the manufacturer's specifications on any airgun. There is risk inherent to any mechanical undertaking. That risk increases exponentially when you enter "What if?" territory. I do not endorse or condone any modifications to your airgun intended to exceed specifications. Any efforts to that end are solely the responsibility of the individual. Proceed with caution if that is your goal.

[ALWIII]

  My apologies for the delay in following up. It's been...interesting...around here for a while. I'll try to keep it brief, with more numbers than chatter. Velocities are averages from a minimum 10 shot string, each. It isn't as much as I wanted to have, but my finances took a decided downward turn for the moment.

First, my new Benjamin NP XL725, unmodified.

Predator GTO,               16.54 gr @907.58 fps
H&N Field Target Trophy 20.06 gr @782.12 fps
H&N Crow Magnum        26.24 gr @676.18 fps

NP XL725 w/ARH piston Seal
Predator GTO                16.54 gr @927.68 fps
H&N Field Target Trophy 20.06 gr @807.22 fps
H&N Crow Magnum        26.24 gr @698.77 fps

NP XL725 w/ARH piston seal, new Crosman breech seal, 85 Kg Airmagnum gas piston, and polished and chamfered transfer port sized to 4 mm
Predator GTO                16.54 gr @1016.58 fps
H&N Field Target Trophy 20.06 gr @846.77 fps
H&N Crow Magnum        26.24 gr @733.50 fps         

Crosman Valiant .22, unmodified
H&N Excite                   12.9 gr @875 fps

Valiant w/new Crosman piston and breech seals, 75 Kg Airmagnum gas pston
H&N Excite                   12.9 gr @939.26 fps
H&N Field Target Trophy Green 9.57 gr @1043 fps (unmodified data lost in computer mishap)

I put new seals from the CO2 Shop (Amazon) in my significant other's Crosman Shockwave (.22), with a 70 Kg Airmagnum gas piston. The 9.57 gr H&N FTT Greens at 9.57 gr averaged 900.89 fps, while the 14.3 gr Crosman HP averaged 738.04 fps.

  Overall, I'm happy with my results. I was conservative with the power upgrades, spending more effort in deburring, new seals, and optimizing the transfer ports. Significant power increases with no real felt increases in recoil, with minimal extra effort cocking the rifles. I can't recommend exceeding the manufacturer's power levels by replacing the gas springs, do so at your own risk. I also assume all liability for my rifles and absolutely voided all warranties. If something breaks it's on me, no trying to sneak it past Crosman.  They are inspected regularly for fatigue.