Static compression ratio?

[bigtoe01]

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.

Aye, has a HW80 here with a 6mm transfer port, lad bought it for 40quid, it actually shot but was horrid as you can imagine, I tapped it m6, glued an M6 brass bolt in and milled it off flush when the glue had gone off...then drilled it back to 3.6mm.

I often shoot it, while its typical 80 at sub12 its actually not that bad, its accurate and does what he needs it to.

[Rich]

Quick back-of-the-envelope calc suggests his 6mm TP dropped the peak pressure on the back of the pellet by more than half.

[bigtoe01]

Quick back-of-the-envelope calc suggests his 6mm TP dropped the peak pressure on the back of the pellet by more than half.

he bought it from one of the local trade/swap/buy places here in Manchester, the gun was in reasonable condition and was originally for sale for 150quid, he went to view it and luckily looked it over and had a look at the TP (I advised he did this), he offered 30quid for the gun based on it was poorly modified and he was taking a risk buying it...he ended up paying 40quid which was still a bargain.

Im still waiting for the payment for fixing the gun and the new seal i had to supply (new to him second hand one off me)

Its not a bad rifle now, we fitted a a weaker HW77 spring with a HW77 rear guide, decent preload and 11fpe in .22, it does him and thats all that matters

[Shed tuner]

Now you're just being picky, Rich.

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

All the best Mick

Well as we're being picky....

I can't classify the TX as short transfer port - 10mm is medium, and as such it is has a vastly different sweet spots in terms of stroke and piston weight in this configuration that a 6mm long HW77 port (both run around 3.8mm dia). I guess that's why me and BT were a way out of one another's reasoning a few months ago on heavier vs lighter pistons in terms of my TX tests and his HW tests - I was trying to stop it slamming, and he was effectively trying to stop it bouncing (to some extent both unbeknown to us / each other). Once I realised (from your info IIRC Mick) the TP lengths were so different it clicked. Shortening the TX port down to 8mm makes a huge difference - well, 20% is a lot, obviously (well 20% ignoring Rich's ((tpv + cv) / tpv) pedantry )

At this reduced TP length, I'm pretty much at the stroke and weight that BT was suggesting (76mm and around 225g). It shoots like no TX I've ever shot before - puts my previous TX tune of years ago (which I still believe to be optimal with the stock transfer port and a steel piston) of 84mm/205g to shame.

It's almost as if the science works (but relies on the humans feeding in ALL the correct data)....

Weirdly (or actually not, really) it feels quite like a venom 77 I once shot years ago - might have been Mr Isaac's....

ATB - JB
/pedantry

[loiner1965]

Great thread chaps and at the end of the day accuracy can be totally ruined by an incompetent buffoon pulling the trigger lol

[bigtoe01]

JB, i actually thought the TX TP length was 6mm, just shows I need to measure it myself and not guess

on the 52 the TP length is around 5mm with the seal, its hard to measure it exactly, it is 4mm in dia, i can get 13+fpe from a .177 running 68mm stroke easy ...thats 33.4CC (power is set to 11.2fpe now remember)

lets say the TP is 5mm long, its volume is 0.062857CC

being picky (33.4+ 0.062857) / 0.062857 = 532:1

This is a 25mm conversion, shoots superb although if anything feels to fast so im going for 75mm stroke just to slow it a tad...

This will be (36.8+0.062857) / 0.062857 = 586:1

The bonus on the 52 is that you have all the room you need for the spring, i run a full 52 FAC spring on the .177 here, remember its only compressing the spring a further 68mm over the preload it has, with the UK spec 16J spring i had 8fpe.

You really do need to have a dabble with a sleeved 52, i feel they are better than a TX.

[Shed tuner]

D52 - defo Tony, still on my list.

75mm on your D52 might be too long, my 8mm TP len TX is running 76mm and makes a shade over 12 at full tilt, with it's inefficient long (and 3.8mm dia) port and old fashioned parachute seal Also is there any danger you might end up with too much piston bounce at that stroke ?

Although getting the TX port down to 8mm is pretty easy, going down to ~6mm by moving the barrel back any running just one breach seal should be straightforwards too (going to play with that over the next few days now Xmas is over ) - but getting any less than 6mm would be tricky...

Rgds - JB

[bigtoe01]

D52 - defo Tony, still on my list.

75mm on your D52 might be too long, my 8mm TP len TX is running 76mm and makes a shade over 12 at full tilt, with it's inefficient long (and 3.8mm dia) port and old fashioned parachute seal Also is there any danger you might end up with too much piston bounce at that stroke ?

Although getting the TX port down to 8mm is pretty easy, going down to ~6mm by moving the barrel back any running just one breach seal should be straightforwards too (going to play with that over the next few days now Xmas is over ) - but getting any less than 6mm would be tricky...

Rgds - JB

My 54 airking runs 75mm, .177, measure recoil on the sled was 6mm, so i reduced available travel to 8mm. When you actually run a rifle on a sled like the 54 has you start to see they do move more than you think they do
It has a superb firing cycle, hence I think I will go the same with the 52.

Just thinking about this 54, i shot 10 pellets thru it in stock config then totally reworked it..from brand new...i must be mad lol

[wonky donky]

As some may know i'm modifying my, my new to me, .177 HW95. I have very carefully measured the transfer port & stroke prior to & after modifying them.

the length of the port is 15,55mm 7 the dia is now 3.55. the stroke is 77mm so unless i'm wrong & I often am the static compression ratio in simple terms is 266.47/1

Am I to assume this is too low? I'm reasonably sure Bigtoe01 (Tony) advocates a T/port dia of 3.6mm for the 95 & know Paul (Thevoid) has his port even larger!

Where do you guys feel the break-off point from one dia/lth to another really is.

The way I tend to do it is make an educated calculation, go slightly smaller, feel for piston bounce then adjust the port to fine tune things!

In my case it's possible for two rifles the same & of the same calibre to have different port diameters!

These are a few velocity figures from the 95 prior to modifying & after. Ive used the original spring guides & seal, which was sized to start with so other than the mods mentioned & a lighter piston, 228gms complete with new liner.

RWS Superdome 820 fps…………………………..12.42fpe after mod 794

RWS Hobby 917fps…………………………….13.33fpe ……………………880

JSB Exact 4.52 835fps…………………………….13.26fpe ………………………811

Air Arms 4.51 830fps…………………………....12.91fpe ………………………804

FA Plus 892fps………………………….…13.04fpe ………………………827

Air Arms Express 872fps………………………………13.29fpe ………………………840

H&N FTT 840fps………………………………13.21fpe ………………………810

Yes ok it's still a little hot, the rifles in bits now as I was trialling something. The other thing is there was absolutely no lubrication of any sort in or on any single part inside the rifle. Firing cycle was very very fast, almost as smooth as it was when I first got it, & trust me it was ultra smooth, far far less recoil with a tiny hint of twist & quiet too.

Yes Tony/Paul I have put the antidamper unit in. still not convinced but loath to drop it yet boys. Of course it does have the tapered guide in so will change that next for one disccussed.

Sorry if a little off topic guys but give me the chance to update Tony.

[Shed tuner]

Hi wonky, numbers look good to be honest.

I think the point is the one that both mick and tone have made, which is that longer ports tend to have reduced flow, so you can push the scr down. What we really need is a more sophisticated calc that measures peak pressure for the cylinder and port dimensions, ie incorporates flow rate.

I know if I had 250:1 on my TXs it would be slamming horribly - it's slamming slightly in stock mk1/2 configuration already, and that's an SCR of ~ 360. Moving to 450 is a huge improvement.... ( I think you tuned these to 86/87mm stroke right ? - mk1/2 stock stroke is 82/83mm)

Lightened piston clearly helps too...

** update - we could plot a graph of optimal tested SCR vs port length to infer a pattern. Would probably need two graphs, one for central and one for offset ports **

[Black Beard]

Just to confuse things...

The pellet starts moving when the piston is still going forwards so the pellet will typically be 2" down the barrel when the piston stops. So the real compression ratio is nearer 20:1 (unless the ar gun is over sprung and the piston crashes).

There is too much going on in an air gun to be able to do a simple calculation; too many competing factors that have to be balanced out. That is where lots of experience and a human brain are needed to get a good starting point then the skill of a craftsman to perfect it.

[bigtoe01]

Dave

My theory on the SCR also brings piston weight into the equation, i see you have lightened the piston now in this 95 to around 228g so this will have a bearing also and allow a lower SCR to be used.

At stock weight i would have said stay close to 300:1, at 200g around 250:1 you are at 228g and in the middle (ish) between 250 and 300 for SCR so it will be fine. Some add weight back or take it off to fine tune a SCR feel and bounce, i more often than not adjust stroke by adding or taking away swept volume if i still have enough to make the power needed.

If you are getting twist you need to make sure you have a slip washer in the nose of the piston and polish the ends of the spring.

The last 95 i tuned to the extent you are doing had a 220g piston, 76mm stroke, guide set into trigger block, tophat and slip washer, full bearings, 3.6mm TP, and was set to just on 11fpe with hobby and falcons (.177), everything else was 10.5 to 10.9fpe. Very very very sweet to shoot.

[slugger]

"The pellet starts moving when the piston is still going forwards so the pellet will typically be 2" down the barrel when the piston stops"

wow, I didn't know this...... I always assumed [wrongly it turns out] that the piston stops at the end-of-travel [bottom of compression tube/chamber] just a nano-second before the pellet exits the barrel...
my thinking was recoil/muzzle flip caused by the pistons sudden stop in movement wouldn't affect tradjectory so much as it wouldn't have time [the sheer bulk of a springer slowing felt recoil etc till pretty much after the pellet had gone]..but by the sounds of it theres still quite some time with the pellet traveling down the barrel after everything else has stopped moving

[Black Beard]

Unfortunately, we can't see what goes on within an air gun and it all happens really fast anyway. We can't feel the forces on the piston either (until it hits the end of the cylinder). We mainly feel the force from the back of the spring, the air pressure on the front of the cylinder and a bit of friction in the system.

As long as you have a barrel longer than about 2", hhe piston will always bounce and give you 2 successive recoil-surge events. The TP effects both of these. The pellet will be long gone before the second one

[thevoid]

My 95 TP is 9/64 or 3.6, I'm currently running around 240g piston and the standard spring, stroke wise I've been from 85 to 75 and settled on 78mm and piston as low as 226g.
I have the crude version of top secret antidamper and my thoughts are that it is superb provided you have a stout enough spring (I have the standard spring with 43mm of preload making 11.2 with most pellets)
If I'm honest I had worries that my SCR was way too low, My gun has now had 3000 pellets with the TP that size and there is not a hint of the TP leaving the tell tale impression on the face of the piston seal, my gun is also very quiet with only 60mm of baffles.
Also worth noting that I sleeved it down with some microbore copper to 3.2mm and the feel was identical but lost 40fps.
My trials with this gun have me leaning towards a really light piston, with plenty of spring, the antidamper would allow for a much bigger TP and get really easy power, this however is something I'll refrain from trying as my 95 is sweet enough.
If I ever did find a fixer upper 95 I'd certainly give it a go.

FWIW, I only really chase one thing when tuning the gun is that the sight picture remains completely on target throughout the whole shot cycle.

Edit, another thing to think about. I've had a custom Wonky spring in with the antidamper and could not get it to shoot well unless I Preloaded the spring up enough to put the gun way way over but it sure was sweet to shoot