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
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
