Velocity adjustment -- Hammer spring or transfer port?

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Adjusting velocity by way of a transfer port restrictor (e.g. a power wheel) is a convenience thing...a shortcut that maintains the balance of regulator pressure and hammer spring tension. It allows you to alter velocity without sacrificing consistency--i.e. maintain a tight extreme spread--but does little to influence air consumption. 

So in contrast, the hammer spring tension has a much greater effect on air consumption (efficiency), but it needs to be in balance with the regulator's setpoint if you want the best consistency. Since adjusting the regulator is generally a more involved process (and sometimes carries with it a risk of damaging the valve seat if adjusted under pressure), it's not well suited for an on-the-fly adjustment.

As a practical matter, what you want to do is set up the gun with wide-open porting (e.g. max setting on the power wheel) and dial in the hammer spring to be in harmony with the regulator's setpoint (95 - 97% of max velocity) to deliver the max velocity you expect to use, and then turn to the power wheel for anything less than that. 
 
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Adjusting velocity by way of a transfer port restrictor (e.g. a power wheel) is a convenience thing...a shortcut that maintains the balance of regulator pressure and hammer spring tension. It allows you to alter velocity without sacrificing consistency--i.e. maintain a tight extreme spread--but does little to influence air consumption. 

So in contrast, the hammer spring tension has a much greater effect on air consumption (efficiency), but it needs to be in balance with the regulator's setpoint if you want the best consistency. Since adjusting the regulator is generally a more involved process (and sometimes carries with it a risk of damaging the valve seat if adjusted under pressure), it's not well suited for an on-the-fly adjustment.

As a practical matter, what you want to do is set up the gun with wide-open porting (e.g. max setting on the power wheel) and dial in the hammer spring to be in harmony with the regulator's setpoint (95 - 97% of max velocity) to deliver the max velocity you expect to use, and then turn to the power wheel for anything less than that.
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Thank you! This is incredibly helpful and easier to understand that what I was finding online.... 
 
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+1 to what nervoustrig said. I'll add some more 'advanced tuning tips' that are of my own.



I run a .225" ported .25 cal with the transfer port limiter removed and jb welded shut (marauder) to keep maximum flow at all times (would be nice to have a non-obstructive transfer port limiter such as a power wheel like some fx rifles, but if the methods of restriction here are to obstruct, they are quite efficient in that not only does the restriction inhibit flow, so does the obstruction).



I then personally tune via set point / pressure (regulated), for the maximum power I intend to ever shoot (57fpe personally, which I obtain 62 fpe at 1950~ psi, so I leave it there as to keep my max power tune at roughly 4% from peak ). For lower power tunes with reduced hammer strike, I use a valve lift limiter with a built in buffer that allows me to obtain consistent, well below the peak shot to shot variance. This allows me to reach a more consistent 'lift' of the poppet off the valve seat regardless of minor variances in hammer energy or pressure holding the valve closed. So there is a work around to achieving better consistency while working well below the nominal % from peak/plateau via hammer spring tension.




As nervoustrig mentioned, restricting the flow path of air in any way, not only doesn't aid in increasing efficiency like reducing hammer strike would, it actually reduces it. Efficiency is a result of how much air you can shove into a specific barrel volume in the allotted time prior to the pellet being so far down the barrel that any more added air doesn't increase its velocity. Thermal efficiency in pcp's have a range thats commonly between 25-30% (very inefficient, but air is known to be very inefficient in terms of delivering available stored energy), while volumetric efficiency generally ranges from 1.0 fpe/ci to 2.0~ fpe/ci, with a few outliers that both go below or above. A longer barrel, with larger ports will always be far more efficient than any barrel that is shorter, with ports of any reduced size, until the barrel length becomes so much that its friction causes more loss than gains from the air behind it, which would take A LOT of barrel, and a little sip of air. HTH.


 
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+1 to what nervoustrig said. I'll add some more 'advanced tuning tips' that are of my own.



I run a .225" ported .25 cal with the transfer port limiter removed and jb welded shut (marauder) to keep maximum flow at all times (would be nice to have a non-obstructive transfer port limiter such as a power wheel like some fx rifles, but if the methods of restriction here are to obstruct, they are quite efficient in that not only does the restriction inhibit flow, so does the obstruction).



I then personally tune via set point / pressure (regulated), for the maximum power I intend to ever shoot (57fpe personally, which I obtain 62 fpe at 1950~ psi, so I leave it there as to keep my max power tune at roughly 4% from peak ). For lower power tunes with reduced hammer strike, I use a valve lift limiter with a built in buffer that allows me to obtain consistent, well below the peak shot to shot variance. This allows me to reach a more consistent 'lift' of the poppet off the valve seat regardless of minor variances in hammer energy or pressure holding the valve closed. So there is a work around to achieving better consistency while working well below the nominal % from peak/plateau via hammer spring tension.




As nervoustrig mentioned, restricting the flow path of air in any way, not only doesn't aid in increasing efficiency like reducing hammer strike would, it actually reduces it. Efficiency is a result of how much air you can shove into a specific barrel volume in the allotted time prior to the pellet being so far down the barrel that any more added air doesn't increase its velocity. Thermal efficiency in pcp's have a range thats commonly between 25-30% (very inefficient, but air is known to be very inefficient in terms of delivering available stored energy), while volumetric efficiency generally ranges from 1.0 fpe/ci to 2.0~ fpe/ci, with a few outliers that both go below or above. A longer barrel, with larger ports will always be far more efficient than any barrel that is shorter, with ports of any reduced size, until the barrel length becomes so much that its friction causes more loss than gains from the air behind it, which would take A LOT of barrel, and a little sip of air. HTH.

Click to expand...

Thanks ackuric. Appreciate that feedback. 
 
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On my Dream Tac Compact I have my best tunes for 16 and 18gr pellets with the TP wheel on 25/30 and my HS wheel on #3. I have my HS adjusted so that when I turn my HS wheel to max the gun is shooting right at the max speed that my gun will produce with my reg setting. When I switch to 14gr pellets, I just turn the TP wheel to 177/22 and it shoots them great. I have yet to move the HS wheel off #3 but I can if I want to shoot a heavier projectile because I know I have gas left in the tank. With the Compact we are crippled with such a short barrel. You can run everything wide open to try to get the gun shooting as fast as its longer barreled brethren but your efficiency will be crap, along with possibly accuracy. When I quit leaning on my gun for max velocity, my accuracy was shocking. FX says its not a long range gun. My groups at 100 and 125yrds will rival most full size guns.
 
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Here are data from my Streamline, in which the only external adjustment is for the transfer port. Pellet was JSB 15.9.

High Port Setting

  • 900 fps
  •  29 ft-lb
  •  50 shots/fill
  •  1450 ft-lb/fill
  • 95.5 dB

Medium Port Setting

  •  720 fps
  • 18 ft-lb
  • 80 shots/fill
  • 1440 ft-lb/fill

Low Port Setting

  •  535 fps
  • 10 ft-lb
  •  120 shots/fill
  •  1200 ft-lb/fill
  •  87.8 dB

I use the High setting for most shooting, including hunting and longer range target practice. Medium power is used for shorter range pesting and target practice. Low doesn't get used much at all, but might if minimum noise was a requirement. I enjoy the feature, it's like having three guns in one with little to no downside.
 
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On my Dream Tac Compact I have my best tunes for 16 and 18gr pellets with the TP wheel on 25/30 and my HS wheel on #3. I have my HS adjusted so that when I turn my HS wheel to max the gun is shooting right at the max speed that my gun will produce with my reg setting. When I switch to 14gr pellets, I just turn the TP wheel to 177/22 and it shoots them great. I have yet to move the HS wheel off #3 but I can if I want to shoot a heavier projectile because I know I have gas left in the tank. With the Compact we are crippled with such a short barrel. You can run everything wide open to try to get the gun shooting as fast as its longer barreled brethren but your efficiency will be crap, along with possibly accuracy. When I quit leaning on my gun for max velocity, my accuracy was shocking. FX says its not a long range gun. My groups at 100 and 125yrds will rival most full size guns.
Click to expand...

Thanks for that. What would you say is the best speed in your experience for those longer distance shots? My Dream Tac is is .177, but at max speed it's shooting 10.3 JSB's at around 860, which I find is pretty accurate. 
 
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Here are data from my Streamline, in which the only external adjustment is for the transfer port. Pellet was JSB 15.9.

High Port Setting

  • 900 fps
  •  29 ft-lb
  •  50 shots/fill
  •  1450 ft-lb/fill
  • 95.5 dB

Medium Port Setting

  •  720 fps
  • 18 ft-lb
  • 80 shots/fill
  • 1440 ft-lb/fill

Low Port Setting

  •  535 fps
  • 10 ft-lb
  •  120 shots/fill
  •  1200 ft-lb/fill
  •  87.8 dB

I use the High setting for most shooting, including hunting and longer range target practice. Medium power is used for shorter range pesting and target practice. Low doesn't get used much at all, but might if minimum noise was a requirement. I enjoy the feature, it's like having three guns in one with little to no downside.
Click to expand...

Thanks, Michigander. Those are great and very usable external adjuster settings. I noticed on my Dream Tac, there isn't much difference in velocity between the medium and high settings (labelled as .177/.22 and .25/.30), but the low does cut it to a very usable speed for backyard shooting. I keep it around 11 ft/lbs for my 10 yard range and it's perfect. I can get around 70 shots before having to refill. 
 
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On my Dream Tac Compact I have my best tunes for 16 and 18gr pellets with the TP wheel on 25/30 and my HS wheel on #3. I have my HS adjusted so that when I turn my HS wheel to max the gun is shooting right at the max speed that my gun will produce with my reg setting. When I switch to 14gr pellets, I just turn the TP wheel to 177/22 and it shoots them great. I have yet to move the HS wheel off #3 but I can if I want to shoot a heavier projectile because I know I have gas left in the tank. With the Compact we are crippled with such a short barrel. You can run everything wide open to try to get the gun shooting as fast as its longer barreled brethren but your efficiency will be crap, along with possibly accuracy. When I quit leaning on my gun for max velocity, my accuracy was shocking. FX says its not a long range gun. My groups at 100 and 125yrds will rival most full size guns.
Click to expand...

Thanks for that. What would you say is the best speed in your experience for those longer distance shots? My Dream Tac is is .177, but at max speed it's shooting 10.3 JSB's at around 860, which I find is pretty accurate.
Click to expand...

Well we are on two different boats for sure. I have my reg set at 150b. What is your reg set at? I would try those 10’s from 850 to 900fps and let the best groups decide. You may have to bump the reg up. If things go south, you can always back it down to what it was. Generally a 177 doesn’t require a high reg set point but with a short barrel, that changes things.
 
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On my Dream Tac Compact I have my best tunes for 16 and 18gr pellets with the TP wheel on 25/30 and my HS wheel on #3. I have my HS adjusted so that when I turn my HS wheel to max the gun is shooting right at the max speed that my gun will produce with my reg setting. When I switch to 14gr pellets, I just turn the TP wheel to 177/22 and it shoots them great. I have yet to move the HS wheel off #3 but I can if I want to shoot a heavier projectile because I know I have gas left in the tank. With the Compact we are crippled with such a short barrel. You can run everything wide open to try to get the gun shooting as fast as its longer barreled brethren but your efficiency will be crap, along with possibly accuracy. When I quit leaning on my gun for max velocity, my accuracy was shocking. FX says its not a long range gun. My groups at 100 and 125yrds will rival most full size guns.
Click to expand...

Thanks for that. What would you say is the best speed in your experience for those longer distance shots? My Dream Tac is is .177, but at max speed it's shooting 10.3 JSB's at around 860, which I find is pretty accurate.
Click to expand...

Well we are on two different boats for sure. I have my reg set at 150b. What is your reg set at? I would try those 10’s from 850 to 900fps and let the best groups decide. You may have to bump the reg up. If things go south, you can always back it down to what it was. Generally a 177 doesn’t require a high reg set point but with a short barrel, that changes things.
Click to expand...

Thanks, yea a bit different for sure. Mine came from set at about 130 bar and I haven't messed with it at all.
 
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Great thread, subscribed!
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Matthias
Click to expand...

Thanks! 
 
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nervoustrig said:
Adjusting velocity by way of a transfer port restrictor (e.g. a power wheel) is a convenience thing...a shortcut that maintains the balance of regulator pressure and hammer spring tension. It allows you to alter velocity without sacrificing consistency--i.e. maintain a tight extreme spread--but does little to influence air consumption.

So in contrast, the hammer spring tension has a much greater effect on air consumption (efficiency), but it needs to be in balance with the regulator's setpoint if you want the best consistency. Since adjusting the regulator is generally a more involved process (and sometimes carries with it a risk of damaging the valve seat if adjusted under pressure), it's not well suited for an on-the-fly adjustment.

As a practical matter, what you want to do is set up the gun with wide-open porting (e.g. max setting on the power wheel) and dial in the hammer spring to be in harmony with the regulator's setpoint (95 - 97% of max velocity) to deliver the max velocity you expect to use, and then turn to the power wheel for anything less than that.
Click to expand...
Great information, thanks for sharing.
 
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3 of my 5 PCPs need careful hammer spring setting for maximum accuracy. So I would not try to use the hammer spring setting as a velocity adjustment on those guns. One of them showed regulator impacts when the hammer spring was turned down too low. The other two do not seem to care from an accuracy standpoint but I cannot change velocity much in these guns with the hammer spring. Less than 100 fps. I use the regulator on my 4 regulated guns.

At least on my unregulated P-rod, a smaller transfer port leads to a more consistent shot string. Not sure if that is true for guns with a variable transfer port. I increased my Prods transfer port to get to the fpe I wanted. So I live with a little more velocity variation.
 
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