I don't believe a TP smaller than 3.0mm ever works in a 12FP springer....

[Black Beard]

It is possible to create a model to calculate flow rates. You need to go to modelling all the components, the forces between them and the air itself in the barrel and cylinder. This isn't easy but I did it about 10 years ago. There is no simple formula you can use.

If the transfer port gets too small, the flow between the cylinder and barrel chokes as the air can't go supersonic (based upon its properties in the cylinder). This is a bad thing as the flow levels off and the pressure in the cylinder goes up a lot. The higher pressure air in the cylinder pushes on the front face a lot harder and makes the forward recoil push horrible.

The pressure difference between the barrel and cylinder causes the choking. Smaller diameter ports have more resistance so make the pressure difference higher, which leads to choking. So do longer or rougher transfer ports.

BB

[TonyL]

I have three formulae for transfer port diameter, and they never agree with each other

There's too many unknowns to make such calculations reliable.

My bachelor's and master's dissertation were based around fluid dynamics, simulation and modelling. One on IC cylinder head flow and one on external aerodynamics. Can be very interesting, but when it comes to transfer ports I would just stick with physical testing. Too many variables and life is too short

Yes, this.

[TonyL]

I was thinking about making a brass insert as a transfer port, my idea is to make it tapered down from compression tube, with a convex entry, so no sharp corners, highly polished [mirror]. My theory is based on - as a youth I used to polish cylinder head ports on my motor bikes & it did improve performance - better intake/exhaust flow. just an idea at the moment?

There has been a number of findings from experiments conducted shared with us over the years and, if I remember correctly, the short answer usually boils down to no fancy shapes / bell mouths etc required. Just a nice, parallel, smooth port with no sharp, jagged edges. I think one of the links that T20 posted onto the thread about HW80 transfer port size to the Pyramydairblog shows a few experiments with differing layouts and I'm sure I've seen plenty of others in the past. That old thread is now on page 1, by the way.. Jim has provided some brilliant information over the years on the subject in Airgun World.

As has been said up above, all very interesting and intriguing stuff.

[look no hands]

Yes, Pete, I also think it's brilliant that we have so many people like Jon who will put the work in, experiment etc. and then share their findings with all and sundry. Like he says, anything tighter than 3mm probably creates too much choking effect on most layouts, but I only base that comment on what I've learned from these many great people who are willing to share their knowledge.

Indeed Tone, I've been looking at long drills now starting at 3mm and working up in increments, may have to purchase some and have a play one day, I suppose finding out what size TP my Mercs and Supersport have already would help.

Pete

[cringe]

There has been a number of findings from experiments conducted shared with us over the years and, if I remember correctly, the short answer usually boils down to no fancy shapes / bell mouths etc required. Just a nice, parallel, smooth port with no sharp, jagged edges. I think one of the links that T20 posted onto the thread about HW80 transfer port size to the Pyramydairblog shows a few experiments with differing layouts and I'm sure I've seen plenty of others in the past. That old thread is now on page 1, by the way.. Jim has provided some brilliant information over the years on the subject in Airgun World.

As has been said up above, all very interesting and intriguing stuff.

OK, thank's for that. so it's back to the drawing board for me!

[Shed tuner]

Out of interest what size TP as the AA Pro/elite got ?.
Les..

3.0 x 15mm, and it's too small, even when the bore is reduced down to 25mm. TOok that one out to 3.2, but the accelerometer data indicates it shoudl be opened up a little more.

I'll be doing some experiments with my FAC one next year - I don't have all the tools and equipment with me on this trip - or even a shrono. Will also bring a machined lighter piston. However I have no doubt that 3.0 will be too small for a 30mm tube... 3.4 woudl be my guess..

[Shed tuner]

Indeed Tone, I've been looking at long drills now starting at 3mm and working up in increments, may have to purchase some and have a play one day, I suppose finding out what size TP my Mercs and Supersport have already would help.

Pete

a taper reamer makes things a bit easier.. just go from both sides

[les allam]

3.0 x 15mm, and it's too small, even when the bore is reduced down to 25mm. TOok that one out to 3.2, but the accelerometer data indicates it shoudl be opened up a little more.

I'll be doing some experiments with my FAC one next year - I don't have all the tools and equipment with me on this trip - or even a shrono. Will also bring a machined lighter piston. However I have no doubt that 3.0 will be too small for a 30mm tube... 3.4 woudl be my guess..

Thanks for the reply.
Like you,I have a reduced cylinder PE with standard TP, I am happy with the way my one performs.
What would I gain from opening up the TP on my gun ?, reduced recoil?.
Les..

[Shed tuner]

Thanks for the reply.
Like you,I have a reduced cylinder PE with standard TP, I am happy with the way my one performs.
What would I gain from opening up the TP on my gun ?, reduced recoil?.
Les..

reduced piston bounce, which then allows for less spring, and therefore less recoil (yes, there's more to it, but basically...).

[les allam]

reduced piston bounce, which then allows for less spring, and therefore less recoil (yes, there's more to it, but basically...).

Ok,cheers for the info.
Les..

[T 20]

The first British built Magnum springer of the 1970s, the Webley Vulcan, had a 2.5mm transfer port.

I've used 2.5mm ports on a number of guns without problems.

[Mr. Gen]

I go 3.5 mainly due to the fact that’s the size drill bit I’ve usually got lying about 🤣🤣

[HW777]

Gasses are indeed regarded as fluids but, being compressible, will behave very differently in a transfer port.

The flow rate link would be spot on if we filled our springer cylinders with water, though.

A springer filled with water, I wonder what the range would be

[look no hands]

a taper reamer makes things a bit easier.. just go from both sides

How do you us one inside the cylinder?

Pete

[Phil Russell]

A springer filled with water, I wonder what the range would be

Crikey, can't resist this. When I was knee high to Ronnie Corbett I had an air rifle ... well maybe I should say 'we had' because my friend and I both used it. No idea where it came from or what it was, maybe a Wobbly Jaguar or a Diana 15/16? I remember firing nails, cocktail sticks and potato bits and ..... don't know how, but using it as a water pistol. It wasn't very good. I can't remember how we loaded it. Maybe trickled water in as we cocked it? We did suss that it needed oiling after a few shots though...
Oh the things you do in the innocence of (very) youth.
Cheers, Phil