Tuning Loss of FPS with dual transfer port?

Do you have an enlarged probe or pin probe?

Yes I had the same issue with a 30 cal Maverick and have seen others post the same thing in other forums. 

On mine after losing velocity I marked the probe and then pulled the probe and barrel. 

When probe was put into tp to marked line only half the tp opening was used hence the loss of velocity. 

I could not adjust or modify the probe enough to match the dual port TP

It looks to me that the port placement is wrong on the 30 cal Hi flow TP

It will take some time to look thru all the different forums. Will try to post as I find them 

Here is a copy of my post from mid-April detailing the same issue. I only copied my initial post, but there were many responses, most of which were helpful or at least illuminating. Lots of great people and info here on AGN.

Chris



Forums › PCP Airguns › PCP Airgun – Tuning & Repair › Interesting observation on FX Crown Mk2 and new, bigger dual port barrels



April 9, 2021 at 10:45 am EDIT | REPLY | QUOTE | FLAG | LINK

ChRiSiS

Member

United States

Accuracy: 1

I recently picked up a Crown Mk2 with, among other things, a .30 cal 700mm barrel. This barrel is the new style from FX which has the bigger port and a central bar in the barrel, very similar to the Ernest Rowe and Huma Dual Hole Transfer ports.

I also have another Crown MK2 with an older style 700mm barrel with the single hole transfer port. I found that when I installed the dual hole barrel in my original Crown, making no changes what so ever to the settings, that I lost approximately 20 FPS! That seemed so counter intuitive that I installed the older barrel in the newer Crown and found that it then gained about 12 FPS. So, in summary, the dual hole barrel was approximately 20 FPS slower in my original Crown (going from 948 FPS with the single hole to 928 FPS with the dual) and approx. 12 FPS slower on the newer Crown (going from 916 FPS dual to 928 FPS with the single). 

Note that I am using the exact same FX .30 44.8 grain pellets in each gun. The two Crowns obviously have different tunes (I actually haven’t adjusted the newer one in any way yet) hence the different speeds.

My question is, has anyone else seen the same effect, and can anyone suggest a good reason for this rather counter intuitive outcome. I guess I expected that with the improved airflow from the dual hole port on the barrel i would actually see higher speeds, not lower. By the way, the pellet probes are both standard FX Crown .30 probes.

Insights would be appreciated.

Chris

My guess is a increase in volume due to the ports..

I’m just wondering if the hole in the valve body that is feeding the port is not the same size as the factory hole in the transfer port already. So a larger port is going to do you no good. Especially when you stick a larger port in there with a bridge which has to restrict the airflow some. I have a M3 30 and I lost 10-15 FPS also. I am back to running the stock port.

My guess is a increase in volume due to the ports..

Click to expand...

Bingo. This seemed to be the case with one I tested (Maverick). I also used my borescope to ensure the probe I made wasn't blocking the porting. 

As for velocity - I've not personally seen any gains from any of the bridged .30cal transfer ports over the factory part (nearly, if not identical).

So it looks like the 30 cal has the right size from factory, and people are selling these TP but it's not really needed? 

I can at least hit 1024 FPS using a 45gr with the .30 cal OEM single port. I bought the dual Huma port last year, but haven't found a need to install it yet. Now on my .25 call I did see a good gain with a dual Huma port over OEM.

I can at least hit 1024 FPS using a 45gr with the .30 cal OEM single port. I bought the dual Huma port last year, but haven't found a need to install it yet. Now on my .25 call I did see a good gain with a dual Huma port over OEM.

Click to expand...

My stock TP port shooting 135bar @960 w/45gr, just a DIY modified probe and shooting lights out @100. Seems ok with Pdogs out to 150+ so I'm good now, but will Experiment with the Ernest port I have to quantfy my results.

Corect me if I'm wrong, but, there would be a drop in FPS due to the increased volume in the probe/breach area and lack of PSI to compensate making up the needed pressure. Adjust your tune to meet the expectations of your Transfer Port. Just an idea.

If the transfer port in the block is increased in size to bore size for slugs, and the valve port is enlarged, (For slugs), the dual port is the way to go. If none of this is done, all that is accomplished is a larger area for the air to expand into before continuing into the breech. Hence. Loss of fps.

If just shooting pellets, without any other mods, the dual port will of course cost power. Period! It is intended for high power applications. all has to be done in harmony. 

The purpose of the bridge is to allow pellets to still be shot in a barrel by riding the bridge with full bore porting, as other wise the pellets will be damaged and many times jammed in to the port. There is NO advantage in a dedicated pellet shooter what so ever. Unless of course, you are trying to hot rod a heavy pellet shooter such as the .357. (.30), But it will still require the other mods to work.

Knife

Knife, Thank you.

Patrick