Portsize vs Efficiency. Why bigger is always better when regulated.

Efficiency can be simplified by valve close time relative to pellet position in the barrel during the shot cycle. Simply put, valve dwell is one of the major variables in efficiency. With that, larger ports simply breathe better, the larger the port (up until a bit over bore size), the more Milligrams of air you can flow per Millisecond. What does this mean? It means a larger port, when compared to one of smaller size, will have significantly less dwell time required to flow the same amount of air. Our goal with airguns is to release as much air as we need for a given power level, in the shortest duration of time before 'elvis leaves the building'.

One reason why going OVER bore size is beneficial, is because during the shot cycle, you have to both lift the valve off the seat, and reseat the valve, two time sensitive operations that both reduce the overall flow rate of your shot cycle, by having a port larger than bore, you are able to get closer to your BORE's max flow rate through-out more of the shot cycle, as this cycle is measured in milliseconds (1-2), and the opening and closure events likely a good fraction of that time that should not be neglected. A valve throat ported to equal .1875" in a .25" bore will have significantly less air flow going through it at low to mid (and peak) lift levels compared to a full/over bored port system feeding the same .25" bore.

Lets take a look into thermal efficiency and how this relates. Thermal efficiency is the energy obtained versus the energy released. I wont bore you with all the math behind it, but simply put you can determine this number by knowing the average air density, and the air volume released (air mass + air volume = contain static values for energy at a given pressure/temp). Say you want to make 60 foot pounds of energy. To do so efficiently, you have to release Y mass of air into the barrel, prior to the pellet being X distance down the barrel. In other words, a larger port will breathe better, and achieve this quicker at a more efficient rate then a smaller port. A valve/transfer port of .1875" feeding a .25" bore at 2000 psi may require upwards of 2.3+ MS valve dwell to release enough air to achieve 60 fpe, where as a valve/transfer port of .25" at 2000 psi would release the same amount of air as the above valve, in about 2/3rds of the time, or 1.5~ MS. This ultimately means the diminishing returns due to the projectiles exponentially increasing velocity and the barrels pressure gradient that chases it down the barrel, will be greatly reduced by having that air mass behind the projectile sooner, rather than later. In other words, the .1875" ported valve with a valve dwell of 2.3~ MS will need to stay open until the projectile is roughly 45-50% down the barrel, and will likely have an efficiency close to .9-1 FPE/CI...where as the .25" ported valve will have closed by the time the projectile is roughly 30-35% down the barrel, with a much greater efficiency of 1.2-1.3 FPE/CI.

Only one IF need apply, and that is IF you're unregulated, this applies in the sense of efficiency still, but not for extending your bell curve or shot count as this would be counter inuitive...with regulated rifles you have a 'restriction' and that is the set point...which allows the above to apply, because you can set your 'working pressure' according to your power level needs, and further adjust it with hammer strike...where as unregulated can only adjust via hammer strike in a full/over bore condition.

I hope this helps explain why bigger ports are ALWAYS better in a regulated air rifle.

**Do note, one should never widen their barrel port beyond 75-85% of their bore diameter, to reduce risk of projectile damage upon loading...rather one should elongate their porting once 75-85% width is achieved if one desires to go larger than 75-85% of their bore diameter.**

-Matt
 
Why bigger is always usually better when regulated (or not regulated).

Most of what you say is true, but you missed a few things.

Main one:
You are making an incorrect assumption about mass flow rate vs port size without taking into account the air velocity through the port. Up until the velocity through the port becomes sonic, the the mass flow rate is limited by the pellet velocity.

port area ratio = 0.1875"^2/0.25"^2 = 0.5625

speed of sound is approx 1245fps, so 1245x0.5625 = 700fps

Whether the gun has a 0.1875" port, or a 0.25" port, the pellet will accelerate down the barrel at approx the same rate, until it passes 700fps. At which point the 0.1875" port is sonic and mass flux is at the maximum. Only after that will the flow advantage of the 0.25" port become obvious.

That is why the bigger port is not always better.

My thoughts on good port efficiency is to size the port based on the intended maximum velocity, not intended FPE. Size the port so that when the pellet hits its intended velocity, the port is still subsonic. And don't go much bigger than that.
 
Why bigger is always usually better when regulated (or not regulated).

Most of what you say is true, but you missed a few things.

Main one:
You are making an incorrect assumption about mass flow rate vs port size without taking into account the air velocity through the port. Up until the velocity through the port becomes sonic, the the mass flow rate is limited by the pellet velocity.

port area ratio = 0.1875"^2/0.25"^2 = 0.5625

speed of sound is approx 1245fps, so 1245x0.5625 = 700fps

Whether the gun has a 0.1875" port, or a 0.25" port, the pellet will accelerate down the barrel at approx the same rate, until it passes 700fps. At which point the 0.1875" port is sonic and mass flux is at the maximum. Only after that will the flow advantage of the 0.25" port become obvious.

That is why the bigger port is not always better.

My thoughts on good port efficiency is to size the port based on the intended maximum velocity, not intended FPE. Size the port so that when the pellet hits its intended velocity, the port is still subsonic. And don't go much bigger than that.

While you made a good point about port efficiency I don’t see how going bigger than the minimum port size required is ever a negative with a regulated airgun.
 
Why bigger is always usually better when regulated (or not regulated).
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While you made a good point about port efficiency I don’t see how going bigger than the minimum port size required is ever a negative with a regulated airgun.

A little bigger than the minimum is OK/good. Just not too much. If there were no negatives, all guns would likely be made with full bore ports.

These are three negatives that you might encounter if you go too large.

1) efficiency - increasing the port diameter also increases the post valve volume. Any extra volume "absorbs" some of the air before it reaches the pellet. If you have a very short run from the throat to the pellet seat, than it won't hurt much. If you have a large enough plenum, you won't notice any drop in FPE, but in either case, it will use a little more air, so efficiency suffers a small amount.

2) energy - the energy will fall a very small amount as pressure in the plenum will fall a little sooner.

3) pellet feeding - pellets tend to feed rougher over the larger transfer port.

There is a right port size for the application, and there is too big.
 
Why bigger is always usually better when regulated (or not regulated).
...

While you made a good point about port efficiency I don’t see how going bigger than the minimum port size required is ever a negative with a regulated airgun.

A little bigger than the minimum is OK/good. Just not too much. If there were no negatives, all guns would likely be made with full bore ports.

These are three negatives that you might encounter if you go too large.

1) efficiency - increasing the port diameter also increases the post valve volume. Any extra volume "absorbs" some of the air before it reaches the pellet. If you have a very short run from the throat to the pellet seat, than it won't hurt much. If you have a large enough plenum, you won't notice any drop in FPE, but in either case, it will use a little more air, so efficiency suffers a small amount.

2) energy - the energy will fall a very small amount as pressure in the plenum will fall a little sooner.

3) pellet feeding - pellets tend to feed rougher over the larger transfer port.

There is a right port size for the application, and there is too big.

1. The extra volume of a larger port has got to be miniscule even with longer ports. I would tend to blame long port runs in general over larger port sizes.

2. This seems like a double dip on your first argument. Ultimately, any air that constricted from getting into the barrel before the pellet exits is a loss in efficiency.

3. Agree there are limits to port sizes which drives the 80% rule of port diameter vs caliber. In some designs there is the opportunity to elongate the port though, effectively increasing flow without messing up feeding. This is true with the FX Impact ports, for example.

On further thought I do see one disadvantage; velocity delta coming off the regulator. A gun, especially one with a larger plenum and lower power, could be tuned to yield a flat curve below the pressure the regulator could produce reliably. For example it could produce near constant velocity to 80 bar while the regulator might need to run at 110 bar to work reliably. In this case the larger port would cause a larger velocity delta, effectively robbing one of shots. 
 
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1. The extra volume of a larger port has got to be miniscule even with longer ports. I would tend to blame long port runs in general over larger port sizes.

2. This seems like a double dip on your first argument. Ultimately, any air that constricted from getting into the barrel before the pellet exits is a loss in efficiency.

3. Agree there are limits to port sizes which drives the 80% rule of port diameter vs caliber. In some designs there is the opportunity to elongate the port though, effectively increasing flow without messing up feeding. This is true with the FX Impact ports, for example.

On further thought I do see one disadvantage; velocity delta coming off the regulator. A gun, especially one with a larger plenum and lower power, could be tuned to yield a flat curve below the pressure the regulator could produce reliably. For example it could produce near constant velocity to 80 bar while the regulator might need to run at 110 bar to work reliably. In this case the larger port would cause a larger velocity delta, effectively robbing one of shots.

1. The extra volume of a larger port has got to be miniscule even with longer ports. I would tend to blame long port runs in general over larger port sizes.

Volume is a function of length and diameter. Diameter more so. If you have a long run, even more reason to NOT go to a bigger port size.

2. This seems like a double dip on your first argument. Ultimately, any air that constricted from getting into the barrel before the pellet exits is a loss in efficiency.

It's not really "constricted" by a larger port volume, it's "absorbed" or maybe "buffered". It's only a double dip if you lose both efficiency and FPE. With sufficient plenum volume, it is possible that the larger port volume will actually improve FPE, but it will still result in a small efficiency hit. In that case it is not "double dipping".

So we are in agreement that the original premise (bigger is always better) is not true?
 
Am agreeing with Scotchmo .... You also need to understand AMPLITUDE of the pressure pulse ( As in how quickly the pressure rises before pellet motion begins ) is also key in have decent efficiency. Excess area downstream from the poppet just absorbs energy while reducing amplitude under a decent tune. * Now if your allowing or purposely over dwelling the valve then bigger is better being it will flush .... Lol ... out more air volume with every shot taken.