Transfer port size for 1377?

[Shadow177]

Hey, New here, but I think I am in the right place. I am trying to determine if I need to alter my transfer port size and if so what size would be appropriate. I have a 1377 and am kind of following the standard blueprint of mods from what I have seen. Currently I have a 14in barrel and flow through bolt from Mellon air along with a steel breach, stock, and scope. I have a flat topped piston, valve, and power adjuster on the way from Mellon air currently. I am currently running 455fps avg with the 10.5g Crosman premier heavy domed pellets.

Found an post from another forum where they had a Canadian model. They plugged the vent hole in the valve and bored the transfer port out to 0.098 and were getting 4.5ft-lb. They drilled more up to 0.110 and jumped up to 5.25ft-lb. However opening it up even more to 0.149 they lost power dropping down to 4.7ft-lb.

I do have a 14 in barrel, and the new flat topped valve has had the port opened opened up to 0.170. I am not sure how the relationship between the valve port size, transfer port size, and bore diameter, and barrel length all interact.

As I understand it the stock US transfer port size is 0.140. I have found transfer port sizes from 0.057 to 0.157 for sale.

How do I figure out what is optimal without buying a bunch of different sizes to try?

Thanks!

[subscriber]

Jonathan,

I do not have a formula for you, but am pretty sure the optimal TP diameter also depends on the air volume and pressure you dump to drive the pellet.  In other words, the number of pumps you prefer to use might alter which TP diameter is optimal.

We like to think larger TP diameters reduce flow resistance, but they also add "lost volume" that instantly drops the pressure of the air driving the pellet.  The longer the TP, the worse the loss in efficiency from increased diameter. 

Ideally, the TP would be barrel bore diameter, but only if the TP was very short.  Otherwise 3 to 4 mm diameters are typical.  For very low power airguns, the TP can be smaller than 3 mm.   Not to act as a power limiter, but as an efficiency enhancer.

If a formula can be determined for the TP, it would almost certainly consider the TP flow area, rather than diameter.  And the volume, and how that affects the starting pressure when the dump valve opens.

[JPSAXNC]

The transfer port hole ideally, would be the same diameter in the valve and the barrel. But .170 would be too big for a .177 pellet, the pellet would hang up on the TP hole. About .160 would be good for .177. As for a formula to make good power, you don't want any pressure restrictions or pressure drops.

[Shadow177]

Jonathan,

I do not have a formula for you, but am pretty sure the optimal TP diameter also depends on the air volume and pressure you dump to drive the pellet.  In other words, the number of pumps you prefer to use might alter which TP diameter is optimal.

We like to think larger TP diameters reduce flow resistance, but they also add "lost volume" that instantly drops the pressure of the air driving the pellet.  The longer the TP, the worse the loss in efficiency from increased diameter. 

Ideally, the TP would be barrel bore diameter, but only if the TP was very short.  Otherwise 3 to 4 mm diameters are typical.  For very low power airguns, the TP can be smaller than 3 mm.   Not to act as a power limiter, but as an efficiency enhancer.

If a formula can be determined for the TP, it would almost certainly consider the TP flow area, rather than diameter.  And the volume, and how that affects the starting pressure when the dump valve opens.

Yes that is a factor, I was looking at it as a constant however especially considering the modification I am doing it definitely is not. Using the stock valve and piston  at 10 pumps would probably be the benchmark but may have no bearing on the new system based on change in flow at the new pressures/volume.

[Psipumper]

The recommended size is 75 or 80 percent of the caliber so not to damage the pellets as they pass the hole. The forward edge of the barrel hole needs to be chamfered and smoothed to not shave lead.
.177 @ 80 percent would be a .142 diameter transfer port. You could drill the barrel and transfer port to that. Since the hole in your valve is.170 now, the  transfer port side facing the valve should be angled or chamfered to.170 to transition to the smaller hole.

[Shadow177]

The recommended size is 75 or 80 percent of the caliber so not to damage the pellets as they pass the hole. The forward edge of the barrel hole needs to be chamfered and smoothed to not shave lead.
.177 @ 80 percent would be a .142 diameter transfer port. You could drill the barrel and transfer port to that. Since the hole in your valve is.170 now, the  transfer port side facing the valve should be angled or chamfered to.170 to transition to the smaller hole.

so the .140 port is it should be fine, and I will just need to use my small files to chamfer the port side a bit, got it.

I assume this is also why .22 can be more powerful? they can use a larger port size to dump more air faster, basically get more energy behind the pellet faster?

[rsterne]

The basic reason that larger calibres can be more powerful is that the base of the projectile is larger, so the air pressure has more area to work on, creating more force.... If the barrel length is the same, and the pressure is the same, then the potential maximum energy (FPE) increases....

Bob