What about the transfer port?May 16 2010 at 12:05 AM
|david Mainwaring (Login 13dehra)|
from IP address 188.8.131.52
Many moons ago - more than I care to remember, I used to hunt with a Dana 35 and a 27 in .22 caliber. Both great guns, but I wanted more accuracy and consistency and started reading magazines from the UK on air rifle tuning. They all pushed the same stuff about polishing the piston and chamber, lubricating with dri-slide and using silicone oil on the piston's washer. Good stuff, only the washers in both guns were leather and a drop or two of good old engine oil did the trick even though they smoked a bit for the first fifty shots or so. What I did learn from those great magazines (with articles from people like Gerald Cardew) was that the British paid a lot of attention to the transfer port and no tune was complete until the transfer port had been honed and polished. I did this and accuracy improved on those two low powered springers. However, no one seems to care about the transfer port any more, or am I wrong?
all I know
|May 16 2010, 2:00 AM |
is that the transfer port modified (reduced) on a M34 clone (Chinese) in .22 cal with 14.3 gr. pellets got me more velocity, it made the transfer port more efficient
one of the questions I have always had is WHY?? is the transfer port the same in the M34 for .177 and .22 cal, they are different in weight and size but with the same size in the transfer port
not all brands use the same size for their transfer ports and some are angled
I believe a proper size in the transfer port makes a difference and that ONE SIZE FITS ALL is nonsense
and remember "it's 30% the gun and 70% the shooter"
What about the transfer port?
|May 16 2010, 4:12 AM |
I know what you mean about different calibers using the same transfer port. Where does proportion fit into this?
About a decade ago I found a HW35 in a junk shop. It had a .22 barrel that was bent due to barrel-snap I guess, and the previous owner had drilled out the transfer port to around 0.2 of an inch. I only found this out much later when I fitted a .177 barrel and found that when I pointed the gun to the sky the pellets dissappeared into the compression chamber. It ruined my day.
I pulled off the new .177 barrel and after straightening the .22 barrel and refitting it, I shot the gun with the big transfer port. Then I fitted a new brass transfer port (after drilling and tapping the hole to 1/2 inch and screwing in a brass plug which I had previously drilled on my lathe). I can't remember the size of the first hole I made to act as a TP, but I guess it was around one eighth of an inch.
Using the .22 barrel, which shot reasonably well regarding its history, I gradually worked up to the correct spec (measured from a friend's good HW35) and kept opening up the TP bit by bit until I had reached 0.2 inches again. I had no crony, but there was quite some difference in penetration into rolls of toilet paper from the port measuring one eighth of an inch to the correct specification. I mean penetration improved all the way to the correct spec. From there to 0.2 inches nothing much changed. Penetration remained the same. This was with the .22 barrel only. I never bothered to try different size ports with the .177 after I fitted yet another brass transfer port with the correct size opening.
I gave the HW35 away with the good .177 barrel and old .22, because I couldn't stand its weight and lack of power. My old BSF which was two pounds lighter walked all over it in power and accuracy
what was the "correct spec"
|May 16 2010, 6:19 AM |
from the factory for the .22 barrel?
This is exciting!
|May 16 2010, 8:24 AM |
By increasing the eficiency of the air flow we get more MV which helps carry the pellet farther flatter. (?) Therby increasing accuracy.
I could use some of that. Every .01% improvement in available acuracy is benificial to me.
How is this accomplished?
What about the transfer port?
|May 16 2010, 1:10 PM |
I only know what happened when I played with the transfer port diameter and a .22 barrel. This was for one rifle only - an HW35. Results might have been different with another make
Sorry, but I don't remember the regular size of the TP on an HW35. It was pretty long ago and I've never handled one since
The physics remains a mystery to me, but it seems that many manufacturers are happy to change calibers using the same transfer port dimentions
My original blog was aimed at modern ideas of tuning guns - where people have everything done to the chamber, piston, seals etc, but the TP gets no mention these days. WHY?
Re: What about the transfer port?
|May 16 2010, 2:36 PM |
There are so many variables, but the transfer ports that I know are efficient are the Diana 52 and the TX200, because they a short and central transfer port, but guns like the HW80/R1 can be tapped like a funnel slightly without loosing to much lost volume, but there is a lot more, perhaps the tuners might give some input.
Cardew talks about transfer ports in his book
|May 17 2010, 6:58 AM |
I'll post a good link on the subject. Apparently, it was a hot topic a while back, with even JM doing some experimental work. Upshot is, if you want to ruin your gun in short order, fiddle with the transfer port. Be prepared to make some inserts to reline your compression tube, since you'll likely be needing one. As a rule, a bigger transfer port often reduces fps. Cardew's reckoning was that it increases the combustion area ahead of the piston, reducing the gun's efficiency. Also, port shapes can create eddies and even back pressure. But read this link:
And, it seems, a new one:
(Re-inventing the wheel?)
One added thought
|May 17 2010, 7:07 AM |
In the new blog, TG mentions that venturi shapes, which ought to increase flow and, hence, speed, don't seem to work as intended. Thinking about this, a venturi usually dumps into a lower pressure space to achieve a jetting effect. With a pellet still in the barrel, the venturi configured transfer port still must overcome the pellet's resistance, so that squirt of high pressure air/exhaust probably gets washed out.
About all I would try would be to check the back of the transfer port and, if it has a sharp edge (i.e. - simply a straight-sided hole), judiciously round the inside lip a bit to break up any flow resistance.
What about the transfer port?
|May 18 2010, 12:56 AM |
Thanks Red Feather
Your links are great stuff and just go to show how complex the matter is
I am looking forward to your post on the subject, but let me get back to what I originally asked. It seems that no one includes the transfer port when describing a modern tune on their favourite gun, and I wondered why
Is the transfer port really that unimportant?
Back in 1987, I had model 34 diana tuned by a professional in the UK. I wanted it limited to 12 ft lbs - as that was the legal limit there. The pro tuned the gun to just under 12 fpe and polished the port as part of the process
Shot to shot velocity improved
The old girl remains accurate 23 years later with a new full power factory spring
If you look into the spring chamber of a 34, you will see that the opening to the port is located centrally and that it is slightly coned before angling upwards. You might also notice machine marks. The gunsmith polished them out and honed the passage connected to it
I just wondered if anyone bothered about doing this any more
My perspective on the transfer port is...
|May 18 2010, 4:21 AM |
...much different than what I'm reading. And I could be wrong, I'm not a fluids engineer. This is just my opinion. I think what is going on isn't so much about the flow of a fluid (in this case compressed air) but an impact.
To me, a springer shooting a pellet is more like a baseball bat striking a ball that has a rubber sheath over the bat. The rubber sheath being the compressed air.
I think that imaging that the airflow is laminar, and attempts to make the airflow laminar, is a mistake. The airflow is SO turbulent that no matter what you do it will stay turbulent.
Anyone who knows a little about airflow would never consider making the transfer port a venturi. If you want to improve airflow, you don't install a restrictor. Besides, venturis are meant for flows that are mostly laminar, not turbulent. It's just the wrong tool for the job.
Because of the high pressure and high velocity, when the molecules of air try to leave the compression chamber, turn 90°, and enter the relatively small transfer port, what happens is that shock waves are set up in the transfer port. These standing waves are a huge resistance to airflow. If you want to improve performance, then deal with these standing waves first.
As far as tuning is concerned, I think of the transfer port diameter as analogous to the gear ratio on a car's differential. You change the gearing to get the performance you want. If you get it too high or too low, the car is practically useless. Somewhere in the middle is the optimal port size.
Shorter is better
|May 18 2010, 8:02 AM |
Yes, the springer does act more like a baseball bat. You get the slug of compressed air but not simply air. When the piston reaches maximum stroke, the fuel mixture combusts, raising the pressure substantially. (Cardew even charged a gun in a nitrogen environment and found the fps dropped by a couple hundred fps due to lack of combustion.) Unlike a firearm, where you do have some expansion as the powder goes through its controlled burn, the springer uses the fuel up in one instant. The shorter, straighter and smoother the port, the more easily it is transferred to the pellet. Also why deep seating pellets reduces velocity. It's like lengthening the transfer port.
If I remember the links properly, JM experimented with changing the entry profile and abandoned the idea due to diminished returns. And I think I've read where totally smooth surfaces are not always best here, either. My take is, if your gun is shooting well and has reasonable speed, it's best to leave the port alone. More can go wrong than right, and the repair is complicated.
|May 18 2010, 4:07 PM |
some has it RIGHT and someone has it all right
AIRFLOW is a science just look at airplanes, air foes under and over a wing and produces LIFT
1" diameter X 10" long pushed by a spring a piston and a seal up front and all that air has to go through .120 (transfer port) wich is less than 1/2" long
if the stroke of the compression chamber is 10" the first 1" is inconsecuential but the second has to PUSH the first
the 4th has encountered resistance
the 5th is struggleling
then comes the 6, 7, 8, 9 inches
and finally the last inch #10
I have to believe that the 1 to 8 inches has the full power of what happens, # 8-10 is just a stabilized issue adding to the full lenght
remember all of this happens in 3 mil seconds
what happens with the PRE action influences the POST action
I am still trying to figure it out but the BASIC's are there
I might try the machinist approach and waste some compresion tubes on a M34 and find out with some experiments and different transfer ports
and remember "it's 30% the gun and 70% the shooter"
Re: interesting BLOG
|May 18 2010, 5:30 PM |
Lets just say its not always what you think and what you see on paper. Many ways to skin a cat and illusions are just that. Tricks that fool the eye. We trick the air and it fallows suit.
NOT that I know a thing about power!
Transfer port size.....
|May 18 2010, 6:59 PM |
I have done much experimenting with transfer port sizes; and for the Most part, at least on mid-power springers, Gerald Cardew is correct. For low/mid power springers, regardless of caliber [meaning .177/,20/.22] about 1/8th inch is about perfect. I recently reduced the port size of a TF87 from .155 to .135 by: slowly rorating a 45 degree punch as I hit it with a hammer, with the chamber face down on a heavy flat block of steel. Kinda crude; but, it did smooth the port entrance And expanded the metal, reducing the port size. I screwed up [experimenting with a new lube] after maybe 200 shots Still shoots 23-25 ft. lbs. [When I have time to tear that one down again, gonna change lubes, for more consistancy.] Long ago, I experimented with large ports on an R1 and RWS 52=rough! lol
Warren, it's more than the initial air
|May 18 2010, 7:15 PM |
Cardew placed windows in his test springers and could actually see the flash of combustion. This is not dieseling, which is much more violent.