Tx200 zero transfer port

[wonky donky]

well I will just machine up some breech seals as they will be the part that does the port reduction size and see what works and hat does not...im thinking 3mm to start and push up from there if the velocity lowers to much...i do know 4mm is to large already

I did all this with my short stroked (68mm) 25mm HW77 & sent you all the results last year some months prior to the springer bash if you think back, I have something like 30 A4 pages listing port sizes/spring dimensions/velocities/different pellet brands etc. I tested various screw in port lenghths sealed, unsealed, protruding into the barrel, away from the piston head, directly on the seal.

I even tried different recess widths & depths on the piston head to create lost volume. The recess was placed directly behind the transfer port & a 10thou recess gave added velocity, cant remember the details off hand.

I did exactly the same thing with my HW80. tedious job I can tell you!

Are you able to make your TPs with a tapered bore, so the air enters at the small end (which should be radiused) and then expands as it leaves the TP?

Mike's point that the expansion brings about the cooling and the increased weight of air behind the pellet is for me the clue.

Also, the whole system needs some resistance to flow to work against, otherwise if it is all down to pellet tightness in the breech, the shot to shot consistency must suffer. A TP that absorbs some work is a constant that buffers those variations in fit.

In the world of PCPs, which may be a sin to mention (!), one of the most consistent rifles ever made was the Pro-Target, and if you look at the air path it is positively tortuous and not conducive to efficiency. It means however that the pellet fit in the breech is not all that relevant. At the other extreme we have the Stealth with its straight through concept, and it's no secret they can be devils to set up and keep stable.

Tapered port; I did this on my Webley .177 Tomahawk many years ago. I'm about to revisit it using the TX as I have four compression plug blanks left over.

Your right about shot to shot consistency without a port, it's not as bad as you may think, I've not had chance to accuracy tests yet but I'm pretty sure the TX won't be hold sensitive however I think it will produce fliers.

As for the Pro Target mine is no better than my EV2 & neither not as good as my HW100 consistently wise, all accurate with the right pellets though. the stealth is another story & I think your being rather kind there!

[Black Beard]

Then again, I did this (5.3 long port sleaved from 3.8 to 3.2mm) with my TX (similar stroke, 76mm) and it didn't work out - I lost about 40fps across the board.. BBs model said it should help as it wasn't choking, and I could go down below 3.0.. apparently not though.

I've tweaked it to add in the drag of the air being squeezed out between the cylinder end ans piston seal when they are close together (low cylinder gap) and doing a 90 degree turn in to the transfer port. This puts a bit more drag in to the system, and means it will be a bit more realistic. Thanks for the pointer.

BB

[Shed tuner]

I've tweaked it to add in the drag of the air being squeezed out between the cylinder end ans piston seal when they are close together (low cylinder gap) and doing a 90 degree turn in to the transfer port. This puts a bit more drag in to the system, and means it will be a bit more realistic. Thanks for the pointer.

BB

Cheers BB - so what does the model now say is optimal TP for that setup ?

Also leads to another question - how large a radius should we be putting on the high pressure side of our transfer ports... i.e. the balance of lost volume vs increased airflow. I'm currently using towards a very small radius (prob around 0.5mm deep, so about 0.7 on the angle) - but a std AA port is more like a 1mm deep radius. And the angle ?

Tks - JB

[Rich]

Instinctively I feel a smooth entry to the port is more important than the lost volume. Roughly speaking, the lost volume in the radius is equivalent to maybe 0.3mm on the port length.

The losses at a sharp edged orifice can be considerable.

[Shed tuner]

Agree sharp edge is bad Rich, but how deep does the radius need to go ? 0.5mm ? 1mm ? 1.5 ?

[Prof51mtw]

Agree sharp edge is bad Rich, but how deep does the radius need to go ? 0.5mm ? 1mm ? 1.5 ?

Rich's observation is spot on. But as to how deep?.... the textbook approach, to avoid corner stagnation at tube entry, is to have a radius with a chord length of 1/5th or more of the tube diameter. For a 45 degree chord angle, this translates to a depth of 1/7th of the diameter. So, for a 3mm TP diameter, where 3/7 = 0.43 mm, 0.5mm depth should be adequate, according to the book. In practice, when I have made port restrictors for Whiscombes, I have used a 1mm radius on the inlet side of a 3 mm diameter parallel port, 5mm in length, and have gained half a ft.lb or so at the muzzle. So radiusing does have a beneficial effect, but bear in mind that the TP geometry in the Whiscombe is very different to that in a standard springer. Hope this helps.
Mike

[Rich]

Just had the chance to do a few sums on this.

I think a bell entry with a radius of a quarter of the port's diameter would be fine.

It gives a good entry, and adds only about 3 percent to the port volume.

From my distant youth I can recall that we used to make diffuser cones to allow air to expand so as to recover some of the otherwise lost velocity pressure, and in the case of a spring gun where the air passing through the port is not actually entering a void space but is meeting further resistance by virtue of a pellet blocking its path, we can assume that the air will soon expand to fill the cone and not break away from the sides, which can happen if the discharge angle is too steep. The optimum would be something like 7 degrees of divergence, so not a lot. A taper reamer would probably manage that.

A sharp-edged orifice acts as though it is smaller in diameter than it actually is, as it causes the flow to neck, occupying the centre of the aperture and not the edges. (vena contracta)


EDIT. I should say, I was typing my post for quite a while and hadn't seen Mike's reply.

[Orlando D.]

I still like the idea of the 1-2 mm port with high compression... but I think you may need that initial very consistent controlled restriction... Most likely the pellets will be a little off from one another and cant be trusted for the job at hand if your trying to stop the pistons braking on point... a fixed variable is probably the best or as close as you can get.. at least something providing just a tad more restriction then the pellet with the least restriction you may be possibly be shooting... I would think its the only way to stop the slam and reach peak efficiency consistently....
O.

[bigtoe01]

Dave

On one of your plug blanks machine in a 6mm deep TP, drill it 3.6mm, then using a small spot drill machine a taper to a depth of 3.5mm on the breech side then using a countersink knock off the front edge to a depth of 0.5mm....then polish it in with fine emery so smooth and round it in. Next flip the plug over and send the spot drill back in to a depth of 1.5mm so we are left with 1mm at 3.6mm.

get a boot lace and some autosol and gently polish the port to just knock off any edges pulling the bootlace thru the port.

If all the theory we are reading is correct this may end up being the ultimate port, if it is drop it in to be drawn up and get the plugs made.

Im thinking something like a 300s seal that allows the barrel to actually touch the port is the way to go or an O ring much like a posted a pic of earlier.

Also remembering that recess that gained you 20fps+ it may be worth machining the plug to 6.5mm depth for TP but the TP is 6mm with the recess 0.5mm like you did before...

I may sketch this up later

[Orlando D.]

Dave

On one of your plug blanks machine in a 6mm deep TP, drill it 3.6mm, then using a small spot drill machine a taper to a depth of 3.5mm on the breech side then using a countersink knock off the front edge to a depth of 0.5mm....then polish it in with fine emery so smooth and round it in. Next flip the plug over and send the spot drill back in to a depth of 1.5mm so we are left with 1mm at 3.6mm.

get a boot lace and some autosol and gently polish the port to just knock off any edges pulling the bootlace thru the port.

If all the theory we are reading is correct this may end up being the ultimate port, if it is drop it in to be drawn up and get the plugs made.

Im thinking something like a 300s seal that allows the barrel to actually touch the port is the way to go or an O ring much like a posted a pic of earlier.

Also remembering that recess that gained you 20fps+ it may be worth machining the plug to 6.5mm depth for TP but the TP is 6mm with the recess 0.5mm like you did before...

I may sketch this up later

Very interesting.. Sketchy please- if you have the chance...
O..

[Shed tuner]

Thanks guys, so 0.5 - 1mm depth for the radius seems like the right range...

And I understand the conventional thinking (except for removing any burs) is not to radius the exit to keep the air column moving straight away from the port, rather than diverging (which the radius would encourage) ?

Tony, I like the sound of that, radiused and tapered towards a choke centrally within the port...

[Leo Bokkum]

A short TP can work very wel. Done that.

Maybe you try to think a bit different about the rocket-science as discribed bij Prof Mike.
What if you see the short TP (with the same dia of the barrel + extreem bellmouth) as the inlet of the rocket, and the barrel as the middlepart & outlet?
Thén you get the same flow and cooling as in a long TP, but without the choke, delay, turbulence and lost power that a small bore TP provides.

My Englisch (or Ingrisch as Bigtoe calls it ) is not good enough to discribe my theoretical thougts what happens inside and what that does for the "rocket-theorie". The odds of misunderstanding because of the language are to big. Sorry for that.
But what I do have for you is some icecold practice to think about:


Rifle: HW97.


Original setup: TP= 3.2 X 5.5mm, stroke 81mm, caliber 4.5mm
Spring: Original FAC OEM.
Piston: Original 265 grams.
Stroke: Original 81 mm.
Piston seal: New type HW.
Spring guide: PTFE, fits the spring in pre-loaded condition.
Power: 19.1 Joules / 14 Fpe with JSB Exact 8.44 g. (No power restriction in the Netherlands)
Everybody who owns one, knows the bad behavior of a (almost) original 97: It kicks like a mule and jumps & turns like a schoolgirl on a Friday afternoon.

After a experimental winter I found the next setup:
TP = 4.5 X 1.8mm (D X L), bellmouth shaped over a length of 1,5 mm. Seal: Leather, 0.75mm thickness when pressed with 100kg.
Spring: The same original as above.
Piston: Aluminium/steel, 202 grams incl. tophead.
Stroke: 75 mm
Piston seal: New type HW
Spring guide: another PTFE, fits the spring in pre-loaded condition.
Power: 20.2 Joules / 14.9 Fpe
Behavior: Zero slam or bounce, stays 100% on target at/after the shot, everybody at the club noticed the very (nice) short locktime, not hold sensitive at all, recoil reduced by at least 70%.

As said, I filled a winter fulltime (and more) with experiments. From ultra light pistons to ultra short strokes, TP's up to 5mm ID and allmost every combination of those 3.
But at the normal lenght of the TP, short/light always reduced the output-power and never gave the smootnes and velocity of this short (bellmouth) TP.


My two cents,


ATB, Leo.

[Shed tuner]

Thanks Leo, good info, and 15 FP from a 75mm stroke/26mm bore is definitely very efficient.

[bigtoe01]

Thanks guys, so 0.5 - 1mm depth for the radius seems like the right range...

And I understand the conventional thinking (except for removing any burs) is not to radius the exit to keep the air column moving straight away from the port, rather than diverging (which the radius would encourage) ?

Tony, I like the sound of that, radiused and tapered towards a choke centrally within the port...

Its not tapered in the centre of the port, just forward of that, I will draw it up over the weekend sometime....

[Rich]

When you think about it, a spring gun needs to get a quantity of air through the transfer port in a very short space of time. Just do the sums and see what the velocity in the transfer port would have to be, if this was air at atmospheric pressure. The numbers are something like 40cc of air going down a 3mm diameter tube in a small handful of milliseconds, maybe just two.

This is why the entry condition is important, all those molecules fighting each other for pole position......