Author Topic: break barrel exhaust port boring ? (Read 537 times)

Riffmeister · « **on:** February 03, 2012, 04:15:20 AM »

just trying to find out if anyone has done this, and what kind of results came out...

Kailua · « **Reply #1 on:** February 03, 2012, 05:34:24 AM »

Where was this idea originated from? Just curious to learn. Never heard of it.

thekid · « **Reply #2 on:** February 03, 2012, 05:56:50 AM »

Best to leave it alone or you run the risk of wrecking the rifle.
Most exhaust ports are already set for optimum efficiency.
Rob

Riffmeister · « **Reply #3 on:** February 03, 2012, 11:56:19 AM »

i thought of the idea of, and also i figured there was an optimum efficeincy calculation used to determine diameter.

Bentong · « **Reply #4 on:** February 03, 2012, 12:06:16 PM »

Actually there was an article on a break barrel AG that has interchangeable porting. Can't remember now where the article went. No airgun manufacturer ever cloned it, my guess is it's not worth it or the extra process does not justify the cost. You get the port too big and you can throw the pellet farther than shooting it. Too small of a porting and the propulsion suffers. The most efficient one is what's currently in your airgun IMHO.

Paul68 · « **Reply #5 on:** February 03, 2012, 12:13:40 PM »

One of the first things I thought of when I began reading up on increasing AG power was getting a larger and faster flow of air behind the pellet. Since I've done a lot of work with engine cylinder heads and knew that improving flow through ports increased velocity, it was a natural guess. It would seem obvious that working on the port to improve flow rates would result in the desired increases in power, but apparently the physics behind spring gun operation throw curves into what would normally be obvious.

Long story short, after a ton of searching and reading, the conclusion I came to was that there isn't enough air volume that you could create in a practical spring airgun to make any significant changes in port size worthwhile, and any changes past the standard .125 diam are likely to produce negative results.

A good test was done at the following link.

http://www.pyramydair.com/blog/2008/10/air-transfer-port-part-1.html

blind dog · « **Reply #6 on:** February 03, 2012, 12:17:25 PM »

Been there. Done that. It's hard to repair.
Boring it out, more often than not, reduces velocity and can be hard on the gun, too.
As you increase the bore of the transfer port in a spring gun, you increase the volume of air sitting in there that will not be swept by the piston. In effect, it's like reducing piston travel to where it never compresses that last bit of air in the compression chamber. That effect over-rides the effect of any gain by reducing air flow resistance through the port because spring guns don't have all that much volume of air in the compression tube to work with, and need a good quick and full compression of what is there.

If the transfer port has already been optimized from the factory, then inserts could be made to reduce efficiency from that point to give velocity variability, but it might add wear and tear to the mechanicals--but it has been done.

-Rob

Riffmeister · « **Reply #7 on:** February 03, 2012, 12:21:00 PM »

great ! thanks guys , i kinda figured that the whole dynamic of creating high preasure would lost ...... and i threw the notion out in an other conversation. still thought it was worth throwing the topic out there

Riffmeister · « **Reply #8 on:** February 03, 2012, 12:26:57 PM »

now with that being covered..... would it harm the port to lightly chamfer and polish it ?

rsterne · « **Reply #9 on:** February 03, 2012, 01:23:30 PM »

The problem with most port mods is the difficulty in changing back.... they are more or less a permantent change, and if in the wrong direction.... well you get the idea....

Basically, if you make the port too big, it's like shooting a lightweight pellet.... The piston can crash into the end of the chamber, damaging the seal, and possibly the piston and even the spring....

Bob

Motorhead · « **Reply #10 on:** February 03, 2012, 04:00:54 PM »

A good way to think about whats going on is relating to you clapping your hands together. The air trapped between your hands cushions the impact against each other just like the air trapped ahead of the moving piston, it air is allowed to escape to quickly they hit each other with great force. same as hitting your hand against a firm hard surface ... it HURTS !!

layman's simple explanation ....

jz2 · « **Reply #11 on:** February 03, 2012, 04:09:34 PM »

I have two HW35's that have different size transfer ports. Older rifle (1963) slightly larger than the newer (1977). they both shoot within a few fps of each other, so there must be some other parameters at work.

Motorhead · « **Reply #12 on:** February 03, 2012, 04:22:52 PM »

Quote from: jz2 on February 03, 2012, 04:09:34 PM

I have two HW35's that have different size transfer ports. Older rifle (1963) slightly larger than the newer (1977). they both shoot within a few fps of each other, so there must be some other parameters at work.

Spring energy and velocity of the moving piston

?

Tarheel · « **Reply #13 on:** February 03, 2012, 05:25:51 PM »

Quote from: Paul68 on February 03, 2012, 12:13:40 PM

One of the first things I thought of when I began reading up on increasing AG power was getting a larger and faster flow of air behind the pellet. Since I've done a lot of work with engine cylinder heads and knew that improving flow through ports increased velocity, it was a natural guess. It would seem obvious that working on the port to improve flow rates would result in the desired increases in power, but apparently the physics behind spring gun operation throw curves into what would normally be obvious.

A good test was done at the following link.

http://www.pyramydair.com/blog/2008/10/air-transfer-port-part-1.html

Air volume. aka "Swept Volume" is the key to higher velocity, especially with larger calibers and or heavier pellets.

From a "car guy" stand point . . . "There is no substitute for cubic inches"

Dave

Riffmeister · « **Reply #14 on:** February 03, 2012, 11:03:23 PM »

today i was bench shooting with a buddy of mine that has a socom extreme , and i looked at his port and it was noticably larger , and the parameters of his piston diameter and throw must surely be far different than my bone collector (bolth in .22) , to put into perspective (since i didnt have a caliper to measure) my port was roughly the size of .160, and his was .200.......... , a little atenttion getter since we had been on this topic . ps...the socom extreme is very powerful and fairly accurate, and my bone collector is very accurate and slightly less powerful ( not much though after being reworked) even with the supressed much shorter barrel.

Mark 611 · « **Reply #15 on:** February 04, 2012, 12:07:29 AM »

I've done some transfer port work on a few guns and you can get a gain if you go at it correctly but if you go as much as a thousandth of an inch to big you lose power, theirs a procedure to doing it and its time consuming because it has to be done in small increments and tested between each sizing but I have gained as much as 35fps just opening up the transfer port on some guns but some showed no gain and some lost power so its risky if you try it and go to far! Just my 2ct

RedFeather · « **Reply #16 on:** February 04, 2012, 01:54:47 AM »

Transfer ports don't always appear "logical". If you make a transfer port really, really smooth, it won't be as efficient. Some ports have what are, at first glance, weird configurations but these actually set up turbulence patterns that work better. FWIW, messing with ports does not seem to give as much advantage as disadvantage. Like the say, it's easy to take a little off but hard to put it back (those port plates, for example.) I also looked up some links to transfer ports and it seems that most form some sort of cone like flow pattern on the exit side. If you start playing with the port dimensions, you change the shape and length of the cone. The only way to figure this is to model it. For that you need sensors at certain points. OK for big pipelines but totally impractical for air guns. I liken tweaking like this to an old short wave radio I used to have. It had a large outer dial for coarse tuning and a very fine inner dial. Once you got it tuned with the large dial you could turn the fine tune all day long and never get any better reception.

Riffmeister · « **Reply #17 on:** February 04, 2012, 02:38:51 AM »

it is very interesting to compare all of my springers and look at the real differences in port size and angle, but on top of that being said- im going to leave mine be, after the excellent shooting that took place today ....there is no need to ruin my favorite springer just because im curious.

i greatly appriciate everyones intrest and wisdom shared with me on this topic. i thrive on this knowledge. ive sat threw many classes on powder burning firearms of all actions and type.....but the way you gentlemen explain in detail with such patients is just as influencial if not greater than any classroom lecture i thank you all, even if you just call me a dip $hit.

Kailua · « **Reply #18 on:** February 04, 2012, 04:10:30 AM »

It was posted that a pellet seated deeper in the breech would lose fps. My thinking a bigger port may result in the situation. More area to compress the air. Just my guess.

blind dog · « **Reply #19 on:** February 05, 2012, 10:18:18 AM »

Riffmeistrer, that other guy's port may or may not have been the best size from the factory, but assuming it is, or is close, there are other variables like its length and stuff. I think most AG makers figure it out through a lot of trail and error, and kind of stick to what works from gun to gun, model to model.