Reply With Quoteengine or air rifle its still a cylinder and piston it would be the same
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As title, can anyone explain the formula to work this out for spring airguns I've googled it & it only relates to engine cylinder capacity. Many thanks.
Regards Graham
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engine or air rifle its still a cylinder and piston it would be the same
may also be employed in conjunction with a drawn reciprocation dingle arm, to reduce sinusoidal repleneration
AFAIK it is the swept volume of the air cylinder divided by the volume of the transfer port. bigtoe01 and Jon Budd are the experts on this, I think the sweet value proposed by Tony is 1:350 for some guns but Jon says up to 1:450 for others, clearly there are other factors involved (spring, piston washer, piston weight blah blah, you can go MAD).Originally Posted by Papa g
Today alot of rifles are made for the over 12 ft/lbs market have static compression ratios which are sub-optimal for non FAC use. And some rifles just have the wrong size port for efficiency.
I'd leave transfer ports to the experts though as arsing around with them is a good way of tuning your gun into a nightmare implement of frustrating horribleness.
Last edited by Hsing-ee; 01-01-2014 at 04:00 PM.
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Hi, its the volume of the transfer port v the volume of the piston stroke. So use the formula pie x r x r x length of transfer port or stroke of piston. R is the radius of the transfer port or the cylinder.
Hope this makes sense!
atb
Max
Plinkerer and Tinkerer
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Yup, except other way round. swept vol / xfer port vol. 400+ is a good number.
*edit you already edited it **
IIRC BT came up with this, my brain just clicked having played with strokes on my TX previously, and ended up emperically with the same ideal numbers. Which was nice
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Hi Graham
This may explain the theory of it a little bit:-
http://www.airgunbbs.com/showthread....69#post5260069
And this may save you getting a calculator out :-
http://www.online-calculators.co.uk/...ndervolume.php
All the best and a Happy New Year - Mick
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Thanks to all who replied to my thread, particular thanks for your reply Mick as I'm no mathematical genius to say the least😊 all the best for the new year to all! Regards Graham
Banned
Micks the master of SCR, not me...
what we all need to remember is you have to measure all recesses and machined grooves etc...99.9% of the time we do not however so what we are really giving is rule of thumb measurement.
300:1 on say a HW35 is more often going to be 120:1 if you measured all the lost volume, however if the simple calculation is giving 150:1 in reality its going to be 1:1 and so slams.
I tend to use 500:1 for 77/97, 300:1 for something like a hw35 or a 95 and 250:1 for something with a machined O ring nose with minimal lost volume.
the shorter the TP the wider it can be (to a given limit) so short ports can have wide ports to promote flow...which explains why they are more efficient at making power.
Once you get the port right, then work on piston weight and spring used, when all in balance the rifle will make power with minimal spring and recoil will be extremely low...it will feel quick but not harsh and follow thru on target will be very easy...IE you can watch the pellet to target, even with .177
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Must type faster.Very interested in the topic and info. provided.
Hope it's OK to ask my own questions here Graham.
I'm very comfortable with the principles and calcs. of compression ratios etc and practicalities in other applications but no experience
when applied to air guns.
So given the relatively tiny transfer port volume it's easy to see how small changes there will give dramatic changes in the ratio and performance
as shown thanks to T20/Mick.
What puzzles me are hands on accuracy problems, highlighted by bigtoe01, like the volume in the hollow skirt of a pellet?
Then with the same action components what happens when barrels are interchangeable. The pellets having different hollow skirt volumes I suppose.
Maybe overridden by pellet size/weight differences.
If I can be bothered to get off my chair I feel a measuring exercise coming on. Has anyone tried filling the pellet skirt and is there any point?
Last edited by deejayuu; 01-01-2014 at 05:58 PM.
may also be employed in conjunction with a drawn reciprocation dingle arm, to reduce sinusoidal repleneration
You make it sound so easy-peasy lemon squeezy!
I'm too afeart to be drilling out ports and that...
The HW35 with a leather washer (even if it isn't a brazing-issue job) has indeed gallons of lost volume, just in the slot of the piston seal screw!
Banned
4mm port, you can get 4mm brass tube with a 2.5mm id, cut a length 25.5mm long and deburr it, then strip and wash the action cleaning the TP out with thinners and ear buddies etc, get it spotless, right now is the same time to cure the leaky breech also, then using a strong retaining compound such as 638 glue the new TP into place, it will stop around 0.5mm short on the inside but that will not make any difference.
Next remove the leather piston head, sell it on herehave a new alu head machined up with 2.58mm wide groove 2.45mm deep for a 2.5CS 20mm ID O ring, at the same time reduce the stroke with the O ring head to 56mm. have the OD of the piston trued up and have the weight reduced to around 240g with the new head and tophat installed.
this will now shoot way better, faster, slicker, than any 35 you buy from a shop, you can get buttons etc fitted also for the final polish.
It is easy, stop messing about buying guns, save up and get a half decent lathe and start self educating, putting myself down if I can do it anyone can...just be thorough and you will do fine.
Holder of nine NSRA gold medals
Not wishing to sound picky but I'm sure it will . . .
The true theoretic static compression ratio is the swept volume of the cylinder PLUS the volume of the transfer port, added together, then divided by the volume of the transfer port. It's the big pot trying to get into the little pot but there is already some air in the little pot.
www.shebbearshooters.co.uk. Ask for Rich and try the coffee
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Now you're just being picky, Rich.Not wishing to sound picky but I'm sure it will . . .
The true theoretic static compression ratio is the swept volume of the cylinder PLUS the volume of the transfer port, added together, then divided by the volume of the transfer port. It's the big pot trying to get into the little pot but there is already some air in the little pot.
You are of course correct but that does complicate the equation a little, as would adding the lost volume in the Piston seal and the tail of the pellet.
The easy equation works for most folks and gives a near enough figure for comparrison, and is simplicity itself compared to the above.
It's interesting to note that Tony finds a SCR of 300:1 works well enough with long transfer ports whereas short ports as on the HW77/97, AA TX200 and Diana48/52/54/56 work well at 500:1.
I found this as well and put it down to a choking effect in the small longer port causing a lower flow rate, so less compression is required to prevent slam.
Jim and Proff Mike did an in depth study on transfer port lengths and in Airgun World --- I must dig it out and re read it to see what they said on the matter.
All the best Mick
Holder of nine NSRA gold medals
Now you're just being pickier........
Seriously though, adding some science to the engineering, it is clear that the rapid compression being adiabatic is what leads to the power being generated. So any increase in transfer port size - in the pursuit of gas flow etc - is taken at the distinct risk of reducing the peak power.
I'm sure there are horror stories from the past where TPs were drilled out massively with poor consequences.
If anyone doubts the value of adiabatic compression over "plain" isothermal, how is it that a stiffer spring gets more work out of the same bore and stroke, with all other parameters remaining unchanged? The swept volume is the same. It's not at all like a PCP where you can just get extra air through.
www.shebbearshooters.co.uk. Ask for Rich and try the coffee
Administrator
Now you're just being pickier........
Seriously though, adding some science to the engineering, it is clear that the rapid compression being adiabatic is what leads to the power being generated. So any increase in transfer port size - in the pursuit of gas flow etc - is taken at the distinct risk of reducing the peak power.
I'm sure there are horror stories from the past where TPs were drilled out massively with poor consequences.
If anyone doubts the value of adiabatic compression over "plain" isothermal, how is it that a stiffer spring gets more work out of the same bore and stroke, with all other parameters remaining unchanged? The swept volume is the same. It's not at all like a PCP where you can just get extra air through.
Hi Rich
In the bad old days everyone was drilling transfer ports out thinking that increasing the flow rate was a good thing.
In most cases all this did was reduce the maximum cylinder pressure and induce slam.
What we're trying to achieve now with transfer port size is just the right amount compression to create air expansion, but a low enough SCR so as not to induce bounce.
The fact that we're now sleeving transfer ports down in an effort to raise a guns power and smooth the firing cycle out shows how far we have come in understanding what makes a boinger tick.
All the best Mick
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