Can you lose performance with a larger Transfer port?

Is it possible the skirts are deeper on the 15.9gr and thus they are not being pushed past the transfer port?

No I can’t say I’ve ever seen that. It can harm efficiency slightly if the porting is mismatched. For example, if the transfer port is bigger than valve exhaust, it does nothing to improve the rate of flow and instead represents a wasted volume that has to be pressurized when the valve is knocked open. And in an unregulated PCP, a large transfer port narrows the useable pressure range over which it will effectively self-regulate. Other than those circumstances, I don’t know of a situation where it causes a negative side effect.



The fact it helped the 18gr but hurt the 16gr is odd indeed. There should be a bigger benefit to the heavier pellet in terms of fpe gained, but they both should gain velocity. 


The only theory I have is the skirt of the 16gr is being more violently obturated, causing bore friction to be greater. However I would not expect it to actually cause the velocity to be diminished. 

What is the size or setting of the transfer port in the breech, or the firing valve? Perhaps they are smaller than the barrel transfer port? If so, the velocity of the high pressure air as it expands into the larger area can slow down. Think of a venturi where air or a fluid goes from a larger area to a smaller, and the speed of the medium increases. The exact opposite could be happening in your gun... Just a thought...

Very strange indeed. Only thing that that I can think of is more volume/space to fill with air before it starts accelerating the pellet. Still can't figure the difference between the two weights?? If you do swap back, seat a pellet before you pull the barrel just to see if the pellets are clearing the port and not getting deformed when hit by the blast of air.

I have the dual TP and pin prope on my compact, and shooting the jsb hades, it gave me about 60-70fps

.22 dreamtac compact

shooting 900fps at 130 reg pressure and 40-45 shots pr fill

Did you match the prope for the TP??

and you need to open it up more, the transfer Hole in the gun is 7mm so to get the most out of it, it has to be 7x7 

No i did not try the Stock prope, but if you don’t match the prope then the rest dossen’t matter.

so open the prope and then you Can always open the TP more later for more power 

Yes but only 1mm pm me if you want some messurments

I will match the back of the probe to the port tomorrow. I worked last night and am in dire need of sleep. I will report back. My goal is to shoot a 18.1gr 850fps with this gun. I have pieces in my lathe and parts in the mail. Just want to iron this out first. The pin probe is tempting because of the 130b reg setting. I will have to dig around for a piece of stainless. Will have to make the pin longer than stock probe because of how it seats the pellet I guess.

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I have done some extensive port work to my FX, similar to porting high performance race engines. Transferring these theories over to airgun ports has given me the best performance increase and fps consistency, and you can get them with a few minutes of work here it apprears. 

Looking at your pictures, to get the best airflow and proper pellet seating, I would remove that metal at the back of the factory probe. Leave it as a curved ramp, blending it into the drilled portion, making it match the brass port. This removes an area of lower pressure and swirling air that is created by the factory drill point pocket. It will flow better air than a round probe too. I use an assortment of Dremel tool bits to do my port work, opening and port-matching the various parts, and smoothing and radius sharp corners to achieve a smooth air path and transitions around corners.

Also, opening up the block's port size to match the new split transfer port is a must. As Centercut said, if ports go from big to small, to big, you are creating a change and or a drop in airflow velocities. By port-matching everything from where the poppet valve opens, on through to the brass transfer port, the Venturi effect and resulting increase in velocity is created at the transition into the pellet probe and directly behind the seated pellet, where it is best utilized.

Have you tried dropping your reg pressure? Opening up your ports should reduce the amount of pressure required to optimize your previous settings, assuming nothing else has changed.