Will increasing spring preload in a spring air rifle produce higher FPE?

[Ontarget58]

It seems easy to add a washer (or more than one) to the back spring guide in my Benjamin Trail NP.  This puts the gas ram under more preload.  Is this a good strategy for increasing fps/FPE?  Is there a downside?

[Roadworthy]

With a mechanical spring, increasing the preload can increase the velocity a bit, and hence the fpe.  I'm not sure how much difference it would make with a gas spring.

Generally you pay for the minimal increase in velocity with a rougher shot cycle.  If you get the pellet velocity nearer the speed of sound you will introduce instability in the pellet trajectory, too.

The limiting factor for power is the displacement of the piston in the compression cylinder.  If you want more power look into a gun with a larger compression cylinder.

[happymecanic]

There's a constant pressure along all the stroke of a gas spring, contrary to a regular spring where the pressure increase with the compression. That would only result in a shorter piston stroke, and possibly an impossibility to cock your gun, since the cylinder of the gas spring would be further toward the front of the gun, and the piston could stop on it before catching the sear. JMHO.

[Scotchmo]

Don't do it.

Even if there is sufficient travel in the spring, it will make little difference since force is almost constant (rises very little) over the full travel.

If there is not enough extra travel in the gas spring, extra spacers will cause the plunger to bottom out before the sear engages.

[rsterne]

Very little to be gained by adding preload to a gas spring, as the force is nearly constant across the compression compared to a wire spring.... Even with wire springs, often the velocity gains are small, compared to the much harsher shot cycle.... The swept volume of the compression chamber is the biggest factor affecting the FPE, most springers deliver about 1 FPE for each 3 cc of swept volume....

Bob

[Ontarget58]

Thanks to everybody for their input.  As a scientist I always endeavor to run an experiment.  I have finally gotten around to testing velocities of four different pellets (177) in the Trail NP, with or without two extra washers added on top of the plain plate in the back spring guide.  Five shot groups were taken with each of the four pellets (Winchester, H&N FTT, H&N Sniper Medium, CP HP).  Each pellet lost velocity after I removed the extra washers; - 4%, -4%, -5%, -3% in the order mentioned.

Conclusion: Small gains in fps can be obtained with a gas ram when you increase preload.  Makes sense as the greater preload builds higher pressure in the cartridge.

[Roadworthy]

Thanks for the update - I feel it's informative.  I do wonder what the overall effect was on shot consistency - whether the added velocity increased group size and by how much.

[Hatsanman13]

I have preloaded a Webely & Scott Vulcan with stock replacement springs new. 
I found it did not help and at the time the springs were going bad preloaded or not.

It went thru 4 factory springs so called 1 was the orginal that out lasted all of the replacement springs.

I last but least replaced the spring with a spring from a Winchester 1000's .

The spring fit inside the spring bore ok but was nealry 6" to long.
I cut the spring down to extend aprox 1.5" out of spring clynder and pushed it in as for preload.
I chrony the rifle at 650 fps with crosman 7.9 gr pellets.  New it was rated at 830 fps.

4 months later I tested the speed with chrony one more time it came up with quite a good gain.  707 fps. with crosman 7.9 gr. Failed to find hobbie pellets but I did find crosman 4.0 gr this did a blazing 950 fps.

I thought it was interesting after the spring had 4 month to set in its new resting place it settled down and did push at a much quicker rate with a 8.8% increase is not bad.....