Transfer port size !

[diipii]

Don't reduce the port size. I drilled mine out to 4mm. What does Cardew know ? I also lightened the piston by drilling holes in the shaft and raised the point of impact 2 notches on the rear sight and the gun (HW35) shoots very smoothly. p.s. you will get a zillion pet theories about this but that's all they are.

[TonyL]

Don't reduce the port size. I drilled mine out to 4mm. What does Cardew know ? I also lightened the piston by drilling holes in the shaft and raised the point of impact 2 notches on the rear sight and the gun (HW35) shoots very smoothly. p.s. you will get a zillion pet theories about this but that's all they are.

What Bigtoe and T20 will aim to do, on most examples, is not to reduce, but to increase the TP diameter. What they aim to reduce is the LENGTH of the TP, raising the static comp ratio, thus increasing efficiency.
If you look at the most efficient guns on the market, they have short transfer ports, like the HW77/97, HW80, Diana 52 variants and I'd guess the Air Arms TX and Prosport and the HW95.

The HW35, as much as I love them.....I had three until this morning, now four, is an old design and not as efficient. Lovely, lovely rifle with lots of character and charm, accurate and powerful and able to do the business today, but it isn't as efficient as some of these later models.

Until you look at Mick's HW35/LGV 25mm conversion with the short transfer port.

[bigtoe01]

Its not about solely increasing the port diameter or decreasing it, its all related to the static compression which dictates where the piston will bounce on the shot cycle.

Going to 4mm on a 35 being honest will border or slammy UNLESS you lighten the piston,when you lighten the piston you need less static compression to cushion the piston at the end of the shot cycle. Its still way to large imo, its better to reduce the swept volume to 39cc or so and cure the leaky breech and retain the 2.8mm port.

The 35 is not an efficient design though...it can be made to shoot nice however.

[QUIGLEY]

A nice bevel or radius on the cylinder side of the transfer port is a good aid to gas flow.
A smaller port with this works better than a slightly larger one without.
Edd.

[bigtoe01]

A nice bevel or radius on the cylinder side of the transfer port is a good aid to gas flow.
A smaller port with this works better than a slightly larger one without.
Edd.

you gain lost volume...so you are actually dialling in inefficiency, do you feel the gains in port flow outweigh the lost volume?. Air venturing work best when air is being dragged thru the port such as a carburettor on an engine, when air is being forced thru a port i think the gains are minimised somewhat. It actually looks better to have a tiny radius and a port large enough to ensure good flow but small enough to ensure the pressure build up is high enough to cushion the piston. The tiny radius minimises lost volume.

One thing I have considered and I mentioned this to Prof Mike at the Midland is a transfer port utilising the Bernoulli effect coupled with a venturi, there may be a way of increasing the amount of air behind the pellet so increasing the efficiency....not tested it though.

They use this system to blow up evacuation slides on aircraft, they have gas cylinders that if direct coupled would only fill around 1/3 of the slide, using a slick designed venturi they are able to tripple the amount of gas within the slide by dragging air into the system also.

how to fill a large bag with 1 breath... http://www.youtube.com/watch?v=UujAMPv3y-A


The trick would be closing off the ports that drag the air in as the air flow reverses and becomes negative.

[Weasle]

you gain lost volume...so you are actually dialling in inefficiency, do you feel the gains in port flow outweigh the lost volume?. Air venturing work best when air is being dragged thru the port such as a carburettor on an engine, when air is being forced thru a port i think the gains are minimised somewhat. It actually looks better to have a tiny radius and a port large enough to ensure good flow but small enough to ensure the pressure build up is high enough to cushion the piston. The tiny radius minimises lost volume.

One thing I have considered and I mentioned this to Prof Mike at the Midland is a transfer port utilising the Bernoulli effect coupled with a venturi, there may be a way of increasing the amount of air behind the pellet so increasing the efficiency....not tested it though.

They use this system to blow up evacuation slides on aircraft, they have gas cylinders that if direct coupled would only fill around 1/3 of the slide, using a slick designed venturi they are able to tripple the amount of gas within the slide by dragging air into the system also.

how to fill a large bag with 1 breath... http://www.youtube.com/watch?v=UujAMPv3y-A


The trick would be closing off the ports that drag the air in as the air flow reverses and becomes negative.

Tony

Great idea, however I believe that this principal works well when used for high fill volume and reflectively low end pressure requirement - as in an escape slide. The same principal is also used for inducing a lower pressure as in a ring jet venturi manifold used for air / gas evacuation - or even a simple paint spray, however both of these uses provide an induced air flow of high volume but relatively low pressure.

I am however sure that a correctly designed venturi shaped transfer port will make laminar airflow more efficient through it once air flow has 'actually started'

I also believe that sharp edges at the entry to the transfer port will induce extremely high air shear turbulence which will actually slow (choke) air flow to some measurable degree on air flow once it has 'actually started' to flow

It is unfortunately more that 40 years ago since I passed my physics A level and my work with fluid dynamics has been non existent for 30 years so can some one please correct me if I'm wrong.

Carry on with the great work.

All the Best

Steve ( ;-)>

[Lol Moore]

you gain lost volume...so you are actually dialling in inefficiency, do you feel the gains in port flow outweigh the lost volume?. Air venturing work best when air is being dragged thru the port such as a carburettor on an engine, when air is being forced thru a port i think the gains are minimised somewhat. It actually looks better to have a tiny radius and a port large enough to ensure good flow but small enough to ensure the pressure build up is high enough to cushion the piston. The tiny radius minimises lost volume.

One thing I have considered and I mentioned this to Prof Mike at the Midland is a transfer port utilising the Bernoulli effect coupled with a venturi, there may be a way of increasing the amount of air behind the pellet so increasing the efficiency....not tested it though.

They use this system to blow up evacuation slides on aircraft, they have gas cylinders that if direct coupled would only fill around 1/3 of the slide, using a slick designed venturi they are able to tripple the amount of gas within the slide by dragging air into the system also.

how to fill a large bag with 1 breath... http://www.youtube.com/watch?v=UujAMPv3y-A


The trick would be closing off the ports that drag the air in as the air flow reverses and becomes negative.

I have recently been reading all i can find about transfer ports (more interested in PCP though) and have some thoughts:

the idea of a venturi is an interesting one (I get involved with them in my work) but here I dont think you could get them to work as its the increase in velocity through the narrowing of the venturi that produces a pressure drop at that point and thus creates suction which is used to draw more air, here you have an initial increase in pressure with little or no flow so air would pass put of the venturi, which means you would need a pretty efficient none return valve on the suction leg?

One thought that did occur to me is why can you not have a small cylindrical secion on the end of the piston that is sligthly smaller than the port so that it fills the port as the piston reaches the end of its travel which means no where for the air remaining in the cylinder to go so cushioning the piston may be it could seal maybe it could be sized to create some air passage?

I have also been reading some conflicting info on polishing the ports, I assume this is done to "ease" the path of the air? there is some thinking that this is a bad thing as it leads to laminar flow whereas turbulent flow is preferable? what about a rifled transfer port to impart spin to the air?

[tokoloshe]

I have recently been reading all i can find about transfer ports (more interested in PCP though) and have some thoughts:

the idea of a venturi is an interesting one (I get involved with them in my work) but here I dont think you could get them to work as its the increase in velocity through the narrowing of the venturi that produces a pressure drop at that point and thus creates suction which is used to draw more air, here you have an initial increase in pressure with little or no flow so air would pass put of the venturi, which means you would need a pretty efficient none return valve on the suction leg?

One thought that did occur to me is why can you not have a small cylindrical secion on the end of the piston that is sligthly smaller than the port so that it fills the port as the piston reaches the end of its travel which means no where for the air remaining in the cylinder to go so cushioning the piston may be it could seal maybe it could be sized to create some air passage?

I have also been reading some conflicting info on polishing the ports, I assume this is done to "ease" the path of the air? there is some thinking that this is a bad thing as it leads to laminar flow whereas turbulent flow is preferable? what about a rifled transfer port to impart spin to the air?

Been working on this idea for the last month with some interesting results. Still work in progress as I have still to get through various combinations of length/diameter and shape of 'rods' that enter the port. Using a nose extension with 4mm hole in center to accept threaded rods. My TP is 3.9mm dia and initially started using M4 nylon bolts reduced to 2mm for fear of a nasty thwack and one knacked compression tube. Am confidently using M4 brass bolts now and have had them full length of transfer port at 3.4mm in dia.
One reason I wanted to do this was to try and work out where the piston stops and hence cylinder gap. Another reason was to see if I could eliminate bounce on ultra light pistons. Results thus far indicate, on my TX, the piston stops somewhere around 1.8 to 3 mm but need to do further tests. Also want to try with different strokes and piston weights - so many variables! One 'rod' I tried changed my aim points quite substantially at the longer ranges, accuracy bombed after about 30 yds so shape is another headache!
Anyway, it's all fun and keeps me busy when the telly is crap or kids/wife doing my head in!

[QUIGLEY]

you gain lost volume...so you are actually dialling in inefficiency, do you feel the gains in port flow outweigh the lost volume?. Air venturing work best when air is being dragged thru the port such as a carburettor on an engine, when air is being forced thru a port i think the gains are minimised somewhat. It actually looks better to have a tiny radius and a port large enough to ensure good flow but small enough to ensure the pressure build up is high enough to cushion the piston. The tiny radius minimises lost volume.

One thing I have considered and I mentioned this to Prof Mike at the Midland is a transfer port utilising the Bernoulli effect coupled with a venturi, there may be a way of increasing the amount of air behind the pellet so increasing the efficiency....not tested it though.

They use this system to blow up evacuation slides on aircraft, they have gas cylinders that if direct coupled would only fill around 1/3 of the slide, using a slick designed venturi they are able to tripple the amount of gas within the slide by dragging air into the system also.

how to fill a large bag with 1 breath... http://www.youtube.com/watch?v=UujAMPv3y-A


The trick would be closing off the ports that drag the air in as the air flow reverses and becomes negative.

I should perhaps of given a bit more detail.
By radius or bevel I meant only a tiny one..... just enough to remove the sharp square edge.
I agree that this has more benefit for drawn air than pushed air but my opinion is that sharp edges are best kept on knives
Edd.

[evert]

If one wanted to get good power, but still retain a soft and good shot cycle, what would the optimum transfer port diameter be on an old hw35?

And why reduce the swept volume to 39cc if the long port makes the gun inefficient in the first place?

My old hw35 gives me abot 9,2fpe, would like to see around 10,5-11, but still have a good cycle.