Transfer port size

[daveh]

Hi All, does any body know if the transfer port size has any effect on performance. presumably underlevers are the same diameter all the way through, but breakbarrels seem to have transfer ports different to the barrel size?
Daveh

[clarky]

Hi All, does any body know if the transfer port size has any effect on performance. presumably underlevers are the same diameter all the way through, but breakbarrels seem to have transfer ports different to the barrel size?
Daveh

The transfer port is critical, indeed one of the more critical areas in springer design. As example the Anshutz 335 would have been a full 1.5 ftlb more powerful if only they had not gone for the 5mm transfer port because they thought it might perform better.
As a rule most should be about 3mm or 1/8" but its a balance of compression ratio, stroke length and transfer port diameter which usually arrives at 2.8mm to 3.2mm dependant on the rifle. Its also good if they have quite short and in line transfer ports. The round the bend transfer port of the Webley pistols probably costs 0.5/ftlb as example.
The worst ever is that of the Webley Mk3. We might not have put a nicer, higher quality rifle together in terms of solid steel and sleek walnut stock but all ruined by a transfer port which is actually larger in dia than a .177 pellet. Even been known to lose pellets into the cylinder when firing up into trees. Robs 2ftlbs from the power and has a terrible firing cycle compared to a modern tuned break barrel.
The Airsporter was not subject to the same issue with a moderately good transfer port size, sat between the Mk3 and the ideal at 4.2mm but the long transfer through the loading port length always leaving it behind the BSA Mercury, despite the better barrel/scope alignment potential.
So golden rule would be don't have a port larger than the smallest pellet, or its going down into the cylinder at some point and will be bloody awful to shoot.
Sub 2.5mm starts to restrict flow incidentally..

[daveh]

Hi Clarky,
Thanks for the info. I have Lanes musketeers in both 177&25 and Lanes Bros seem to have produced an air cylinder for a 177 and just stuck a 25 barrel on it. I have managed to make the 25 perform reasonably by respringing it and fitting a new piston washer but did not know anything about transfer port size. Perhaps I will try a slightly larger port & see if it makes any difference. Any improvement will be reported!
Regards
Daveh

[thisisdonald]

just as an aside to this.. I tried to make an excel spreadsheet that you could input your compression tube dia, length, transfer port size etc.. and it would give you swept volume etc.. obviously as you are reading this it was a total failure - I think I just cant get my head round swept volume & compression ratio. If someone could clearly explain this, or make a spreadsheet for us I would be most grateful. I think this is also a critical area of spring guns - understanding them!!

[Airsporter1st]

just as an aside to this.. I tried to make an excel spreadsheet that you could input your compression tube dia, length, transfer port size etc.. and it would give you swept volume etc.. obviously as you are reading this it was a total failure - I think I just cant get my head round swept volume & compression ratio. If someone could clearly explain this, or make a spreadsheet for us I would be most grateful. I think this is also a critical area of spring guns - understanding them!!

Although no expert, my understanding is as follows:

Swept volume = the x-sectional area of the cylinder multiplied by the distance the piston moves.
Compression ratio = the ratio of the volume behind the pellet before firing and the volume behind the pellet after firing (assuming the pellet remained stationary).

No doubt someone will be along shortly to point out my errors, but at least I had a stab at it.

[lam007]

depends on the cal i would think.

[thisisdonald]

Although no expert, my understanding is as follows:

Swept volume = the x-sectional area of the cylinder multiplied by the distance the piston moves.
Compression ratio = the ratio of the volume behind the pellet before firing and the volume behind the pellet after firing (assuming the pellet remained stationary).

No doubt someone will be along shortly to point out my errors, but at least I had a stab at it.

Thanks Paul... I tried marking the piston position next to the cocking slot and cocking the gun with no spring.. trying to contrive a way to see exactly the piston face's length of travel in the compression tube from cocked position to fired + the volume of the transfer port - I ended up with a piece of paper that looked like the back of albert einsteins shopping list... the only instructions I can find online are for car engines.

[Airsporter1st]

Thanks Paul... I tried marking the piston position next to the cocking slot and cocking the gun with no spring.. trying to contrive a way to see exactly the piston face's length of travel in the compression tube from cocked position to fired + the volume of the transfer port - I ended up with a piece of paper that looked like the back of albert einsteins shopping list... the only instructions I can find online are for car engines.

It doesn't matter which bit of the piston you look at - you just need to measure the distance any particular point moves, so if there is any part that is accessible both in cocked and fired position, that would do it.

[clarky]

Although no expert, my understanding is as follows:

Swept volume = the x-sectional area of the cylinder multiplied by the distance the piston moves.
Compression ratio = the ratio of the volume behind the pellet before firing and the volume behind the pellet after firing (assuming the pellet remained stationary).

No doubt someone will be along shortly to point out my errors, but at least I had a stab at it.

No I wont be the bodkin that comes in and corrects this because it is about right but not forgetting the nasty of rebound factors in the whole picture but as an actual ratio of compression your bang on.

Going up to .25 cal should be much more forgiving than going smaller so no worries there. Interesting that the Cobra strike Magnum version of the Mercury was super smooth shooter. Only hampered by the distinct lack of pellet choice at the time.
Incidentally the HW77 was the most perfectly set up bog std springer of all time....the results of entrance onto the FT scene speaks for itself.
Even a modified later version such as the 97 will not give you much more, A better under lever catch system maybe. I believe it had the ideal transfer port at 2.8mm/ I wonder if by accident or design.

[Rob M]

Clarky, what are your opinions on having a radius on the transfer port? I know that some on here reckon it's pointless but I would have though that the air in the cylinder being tightly compressed makes a radius more important, not less.

I remember a while back that T. R. Robb was selling transfer port reamers that would radius the edge of the port and add a slight taper to the port itself, this always seemed like a good idea to me, like you point out, and there are similarities with car engine design, making gases flow around tight corners can sap power.

[clarky]

Clarky, what are your opinions on having a radius on the transfer port? I know that some on here reckon it's pointless but I would have though that the air in the cylinder being tightly compressed makes a radius more important, not less.

I remember a while back that T. R. Robb was selling transfer port reamers that would radius the edge of the port and add a slight taper to the port itself, this always seemed like a good idea to me, like you point out, and there are similarities with car engine design, making gases flow around tight corners can sap power.

Thanks for the question Rob. It did become a real tuning fashion about 1988. There is indeed some evidence that it is beneficial but major offset by the difficulty. The problem is the operation having to be performed all the way down the bottom of the cylinder which can create real vibration problems. This is due to requiring a lengthy cutter system (Bell mouth cutter on some sort of length of rod arrangement) which goes against the good practice of machine work, where the cutter should be presented as close as pos to the work piece to minimise vibration to get the surface finish up to a good standard. Also we have a tiny element of lost volume as the transfer port is potentially made greater in volume/dead space.
I believe one version of the HW35 was tackled professionally and gained slightly but with the much better gains of new piston washer systems, buttoning etc since the noughtys onwards...somewhat lost to history.

[Rob M]

I'm thinking about tuning my old Vulcan (when I get time... ), I've got the delrin to button the piston, will try to get a posh piston head of some sort maybe, and have thought about putting a small radius on the transfer port using some sort of tapered tool and valve grinding paste. I've got a lathe, but as you say, working at length down the cylinder is likely to cause issues with chatter or vibration.

One day maybe...

[clarky]

I'm thinking about tuning my old Vulcan (when I get time... ), I've got the delrin to button the piston, will try to get a posh piston head of some sort maybe, and have thought about putting a small radius on the transfer port using some sort of tapered tool and valve grinding paste. I've got a lathe, but as you say, working at length down the cylinder is likely to cause issues with chatter or vibration.

One day maybe...

Your Vulcan requires the Chambers purple washer and a reasonable top hat. Use a std Webley spring, not a Titan jobby and machine up a better fitting guide rod

[ggggr]

I'm thinking about tuning my old Vulcan (when I get time... ), I've got the delrin to button the piston, will try to get a posh piston head of some sort maybe, and have thought about putting a small radius on the transfer port using some sort of tapered tool and valve grinding paste. I've got a lathe, but as you say, working at length down the cylinder is likely to cause issues with chatter or vibration.

One day maybe...

Mick T20 used a smaller transfer port in a Hawk mk3 and got it up to the legal limit. I think he put pics up on here on a Hawk thread. As power is not really going to be a problem with a Vulcan, I would not bother with the buttoning or a taper on the transfer port. I would go for a decent guide and find a spring/cut down a spring to bring it to legal levels. Without buying the upgraded trigger kit, a bit of work on the trigger and about 3 coils off the trigger spring will help make it better. also do away with the safety and use the sear spring off a Victor. A touch of threadlock on the stock screws might help as well.

found the thread http://www.airgunbbs.com/showthread....highlight=Hawk