Tuning Effect of enlarging the port on a spring rifle.

Jump to latest Follow Reply
OldSpook

OldSpook

Member
Apr 25, 2021
2,104
1,845
Virginia, United States
www.ebay.com
The question seems simple and I do not have a clue what the answer is but I bet you good folks have experience with it so...

What is the effect of enlarging the port on a spring rifle? If you haven't done it, you are speculating. With respect I'd rather hear from people who have actual experience with the exercise. Specifically with enlarging the port and what effect that has on muzzle velocity and accuracy?

Thanks.
 
As someone who has actually done it......

It's not "cut and dry" and very much depends on the rifle and even the fit of the pellets in the bore. Transfer port length and diameter (volume) is also a huge factor here. 

Drilling the port generally doesn't have a drastic effect on power in most platforms, but it's a very big difference in changing where and how much "pulse" the shot cycle has. Use it to tune out bounce basically. Go too big, and you get slam which is worse and harder to remedy. 

Take a gun with a long port, such as a D34, and it has a lot of volume there, so the port effectively acts bigger than it's diameter leads you to believe. There's a lot more "dead airspace" between the powerplant and the pellet than in a short port gun like most underlevers. 

On the HW 77/97 they have a fairly short and small transfer port volume by comparison. These guns when enlarged SLIGHTLY will usually pick up a touch of power. They are about 2.9mm stock, and I usually drill them to 3.4-3.6mm. Much over that and they tend to get slammy IMO unless using very tight pellets. This is the platform I've done the most work with, and the only one I can post good information on. 

Some guns are designed to basically slam, and not work on peak pressure like most spring guns. Two examples or the FWB300 and the HW55. Both have huge ports, low mass pistons, and aren't big power producers. Take an HW55 with a 4mm port and sleeve it down to 3mm and it will make the same power, but piston bounce will be very pronounced and accuracy goes down the drain. Just not enough swept volume and piston mass. Only other example I could think of. 

So again, the classic bull$#it answer I hate to give, but "it depends". 

:)


 
  • Like
Reactions: JayJay
Upvote 0
@thumper - Great post, I really appreciate the practical feedback.

Several questions popped into my head.

  • When sleeving, do you use an interference fit or threads to secure the new sleeve? I would guess it depends on the cylinder wall thickness at that point.
  • Any comments on the HW90 and magnums that push more air through the port?
  • Do you feel that most manufacturers do a good job of optimizing their ports?
  • Is there a benefit to radiusing or polishing the port?

I pulled out my copies of G. Cardews "The-Air-Gun-From-Trigger-To-Muzzle" and "The-Air-Gun-From-Trigger-To-Target". He has a whole chapter on transfer ports. Like you, he says it's a matter of experimentation.

Cardew's books are available on Scribd. The first edition is 'From-Trigger-To-Muzzle'. The later edition 'From-Trigger-To-Target' is about 150 pages longer.

https://www.scribd.com/document/48410185/The-Air-Gun-From-Trigger-To-Muzzle-s

https://www.scribd.com/doc/124078012/CARDEW-The-Airgun-From-Trigger-To-Target. 


 
Upvote 0
I did it enough to learn it usually makes the gun more harsh with little or no gain in power. Only the Whiscombe FB guns (shipped with small ports) can give large increase with a big (no reducer insert) port.

Once it (Finally) became obvious its pretty much a waste of time on a practical level to hop up springers (precharged, pumpup, and even co2 powered guns are way more responsive) I mostly just concentrated on tuning them (springers) for better accuracy and smoothness, and increasing port diameter doesn't give that.

 
Upvote 0
I did it enough to learn it usually makes the gun more harsh with little or no gain in power. Only the Whiscombe FB guns (shipped with small ports) can give large increase with a big (no reducer insert) port.

Once it (Finally) became obvious its pretty much a waste of time on a practical level to hop up springers (precharged, pumpup, and even co2 powered guns are way more responsive) I mostly just concentrated on tuning them (springers) for better accuracy and smoothness, and increasing port diameter doesn't give that.
Click to expand...

Thanks. I believe I will take your advice and leave things like that to folks like yourself. If I want more out of that rifle I'll send it to someone who has the skill and experience to get a bit more out of it without screwing it up.




 
Upvote 0
"and smoothness, and increasing port diameter doesn't give that."

Incorrect. "Might not give that" might be correct but the above isn't. It does sometimes make for a smoother shot cycle and that will usually increase accuracy.
Click to expand...

Agree totally. Been there and done it, and even gave an example. 

If you have a gun that shoots at the velocity you like, but is bouncy, opening the transfer port slightly can remedy this. 

Bounce or "pulse" is the biggest contributor to hold sensitivity. A rifle with the bounce tuned to the minimum and the pulse timed right is sublime to shoot. 
 
Upvote 0
As someone who has actually done it......

It's not "cut and dry" and very much depends on the rifle and even the fit of the pellets in the bore. Transfer port length and diameter (volume) is also a huge factor here. 

Drilling the port generally doesn't have a drastic effect on power in most platforms, but it's a very big difference in changing where and how much "pulse" the shot cycle has. Use it to tune out bounce basically. Go too big, and you get slam which is worse and harder to remedy. 

Take a gun with a long port, such as a D34, and it has a lot of volume there, so the port effectively acts bigger than it's diameter leads you to believe. There's a lot more "dead airspace" between the powerplant and the pellet than in a short port gun like most underlevers. 

On the HW 77/97 they have a fairly short and small transfer port volume by comparison. These guns when enlarged SLIGHTLY will usually pick up a touch of power. They are about 2.9mm stock, and I usually drill them to 3.4-3.6mm. Much over that and they tend to get slammy IMO unless using very tight pellets. This is the platform I've done the most work with, and the only one I can post good information on. 

Some guns are designed to basically slam, and not work on peak pressure like most spring guns. Two examples or the FWB300 and the HW55. Both have huge ports, low mass pistons, and aren't big power producers. Take an HW55 with a 4mm port and sleeve it down to 3mm and it will make the same power, but piston bounce will be very pronounced and accuracy goes down the drain. Just not enough swept volume and piston mass. Only other example I could think of. 

So again, the classic bull$#it answer I hate to give, but "it depends". 

:)

Click to expand...

Well said....I totally agree!!
 
Upvote 0
This quote from G. Cardew's book caught my eye:

"...most of the rifles that we have looked at have a sharp edge at the entry to the port, since it is simply a drilled hole... We took one of the false ports and shaped the entry to a bell mouth; this immediately increased the velocity of the experimental gun by seven feet per second. Other guns that we have modified in this respect have produced better results than this, which goes to prove that the shape and size of the port is individual to each type of gun." Pg 77 'From-Trigger-To-Target'

I don't recall seeing any tuners ever doing this. But it make a lot of sense. I might check it next time I tear down my HW95.
 
Upvote 0
You must log in or register to reply here.
Top Bottom
Back
Home
Forums
Resources
Menu