P-rod got stronger as air pressure decreased, go figure.

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bltefft

bltefft

Member
Jan 25, 2017
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West Central Georgia (Columbus, by Ft. Benning), U
Hi all, I was shooting my Crosman .22 cal 1322 pistol over my Caldwell Precision chrony, when I decided to shoot my Benjamin Marauder .22 pistol over it, too.

On my 1322, I was curious about fps vs number of pumps. The max number of pumps I gave it was 12. The pellets I was shooting weighed an average of 15.85 gr each. The highest velocity the 1322 gave me was 478.9 fps.

When I started shooting the p-rod, it had a full tank, so I was thinking that would be the highest velocity that I'd see. Man, was I wrong. I shot it 16 times (2 magazines worth of pellets) and every time, the fps went UP - like I said, I was not expecting that to happen. The fps didn't increase just once or twice, it went up with every single shoot. And by the 16th shot, fps had increased almost 100 fps!

Here's the numbers I got:

Screen Shot 2021-05-26 at 3.26.30 PM.1622057214.png


Are you surprised, too? How can pellet fps go up as the pressure in the tank is going down? I don't understand.

I wanted to keep on shooting to see how high it would go, but had to put things up to visit the doctor. Maybe I'll do that some other time.
 
This just shows that the air pressure in the tank is closing the valve slower as the pressure drops. (Edit for clarification... This is the other end of overfill valve lock. Too high of pressure closes the valve too fast. As the pressure drops, the valve stays open longer, allowing more air to push the pellet. The valve open longer means more push on the pellet, showing higher FPS output as the tank pressure reduces.)

Just like when you overload the tank, you get valve lock. If you put about 3200 psi in the tank, the hammer cannot open the valve to shoot, u til you bleed off enough air.

The hammer has to push the valve against the internal air pressure to release air. You have discovered the root of valves tuning. Weight, vs. stroke (dwell time).
 
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Oh, okay, that makes sense. Would it be good to increase the hammer strength, or just not load it up to 3000 PSI (that's where I usually stop) - or do a combination of both. Now, I wish I had taken the time to record the psi pressure in the tank as I shot, now, that would have been interesting.

I shoots really nice, but I didn't see if accuracy changed as the fps went up, I was just shooting over the chrony in a safe direction.
 
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+1 to what the thomasair said. Your Hammer Spring is set too low. You should increase the hammer spring until you get a bell curve for your shot string. The first few shots will be slower, the fps will increase in the middle and then start to drop off on the last few shots. 

All unregulated airguns will experience this due to the changing pressure in the tank.

Although @JimNM - I have to question your description. The valve lock analogy is correct, and the valve closing slower as the pressure drops I would take as accurate; however, the reason for valve lock is that the hammer is not being propelled with enough force to open the valve enough in the first place. This would be the valve pressing against the air pressure in the tank. I'm know we are ultimately saying the same thing - so no argument - I just believe the slower valve dwell as tank pressure goes down is related more to the HS force being able to open the valve wider, thus a slower return too. 

And as JimNM says welcome to root of tuning an unregulated airgun. Some airgun valve designs, like the balanced valve, are able to overcome this limitation to a certain extent. But you won't find that on a P-Rod.
 
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Oh, okay, that makes sense. Would it be good to increase the hammer strength, or just not load it up to 3000 PSI (that's where I usually stop) - or do a combination of both. Now, I wish I had taken the time to record the psi pressure in the tank as I shot, now, that would have been interesting.

I shoots really nice, but I didn't see if accuracy changed as the fps went up, I was just shooting over the chrony in a safe direction.
Click to expand...

"Would it be good to increase the hammer strength, or just not load it up to 3000 PSI"

6 of 1, half dozen of the other. EITHER increase hammer energy or decrease fill pressure. The latter is probably easier/faster for most to accomplish but would probably result in a loss of uable shots available. Is there enough hammer energy available to overcome a full fill? All relative.
 
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Thanks, ctshooter, I reckon I ought to increase the hammer strength a few turns.
Click to expand...

It's been a while since I've tuned a P-Rod, but it sounds like you may want to increase either the hs a bit or the throw setting. Either will likely help. Start off small - maybe one turn - then chrono a string. It's worth the exercise to see how the bell curve develops - and even deteriorates if you go too far - just for the experience. You'll be teaching others in no time. ;)
 
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Generally speaking you want the useful portion of the bell curve to begin at the maximum safe working pressure (3000psi in the case of the Marauder pistol). That provides the opportunity to achieve the maximum number of shots because you have the maximum amount of energy stored in the reservoir (as opposed to a situation where you have to limit your fill pressure to avoid valve lock).

However some people prefer to tune for a lower fill pressure. For example if you use a hand pump, you may decide you don’t like pumping all the way to 3000psi.

Like most things, it’s a balancing of tradeoffs.


 
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I guess we got a nubie? Lol. That's call a bell curve. Might want to look it up. The p rod isn't a regulated gun, so it has a bell curve. You need to tune it. Not just fill to max 3000 psi and start shooting. It doesnt work like that. You need to find the sweet spot and get like no more than 30 fps spread. Huma also sells regulator for the prod. It helps keep spread low as possible. 
 
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It is not exactly 6 of one and a half a dozen of the other. If you shoot at higher pressures with higher hs pre load, you can get powerful shots that are less noisy and use less air, I think.

what I do is fill past the hump of the curve, and set the hammer preload high, then gradually dial down the preload as the tank pressure drops. So a pre-load of zero at 2600 psi yields the sme energy as highest preload at 4000 psi.

It took shooting through a chronograph a bunch of times and seeing the effect of changing the preload on velocity to get this figured out.


I'm getting the same velocity with the same slugs at both ends of the string, but the noise level is much much higher when the tank is at lower pressure and the very highest energies are only attained with a full tank and lots oF hammer
 
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I jam mine to no more than 2700, and get a very forgiving bell curve. It currently is dialed to bone stock, and my groups are great until it drops into the yellow. 

I have no plans to regulate it, as I have other guns for that. The PROD has a very nice shot string if it's tuned right, and my original factory setting so far has been ideal. I messed with it a little, but just went back to stock. 
 
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