Different size cylinders and power.

[look no hands]

Hi all,

A question for the shed tuners and smart arses, why is there such a power difference in certain cylinder sizes, I mainly enquire regarding the Theoben spring conversion I've tried to do recently and the complete lack of power from what I would think is a pretty large compression cylinder (27mm) and a pretty beefy spring, yet smaller cylinders like the typical 25mm HW/AA size cylinders make power easily (my Prosport was always going over the limit when it had a V-mach kit fitted), I also read sometimes how owners of Airsporter/Mercury's struggle to get to 10ftp (although mine seem to be able to get over 11ftp ok) I know with the Airsporter it's the tap that sometimes causes power problems but considering they have 28mm cylinders, but then I read about shed tuners (I'm not pointing a finger at you JB but shed tuners in question) conversions were they are doing 22-23mm cylinder conversions with tiny springs and they are making power pretty easily, I know other factors come into it like TP sizes and lengths and more commonly it's the piston head type and size that helps.

Is it black magic and Steve Pope is actually a Voodoo witch doctor or is physics and mother nature just f*****g with us

Can someone please explain this annoying phenomenon to this bewildered and slightly pissed off idiot

Answers on a postcard to......

Pete

[Hsing-ee]

I think the low power from Airsporters and Mercurys is more to do with the 'O' ring than anything else. A tight one will rob 2 fpe or more, thats why some people recommend a Meteor 'O' ring for them. YOu have had your trials with Mercury pistons over the years. Its usually the case that a bigger swept volume will mean more power, as long as the transfer port is set up right.

Bowkett used to do the long-stroke job (lose the buffer washer) and maybe some other trick to the Airsporter and he would have it at 12 fpe with a weak spring and small amount of recoil.

I think that Theoben you have may be cursed, like the Challenger you had. If you want to lift the curse, disassemble it into its components and sell the seperate parts on the Bay. That way you should get a good total yield and the infesting demon will have to leave the rifle.

For 12 fpe the dimensions of the 25mm HW77 cylinder and the 26mm HW99S are supposed to be perfect, I just thought I would mention it as I know you don't like Weihrauchs. The HW35 is a bit 'over square' with a wide bore and a shortish stroke so its not efficient. The fit of the pellet is an issue as well...

[Black Beard]

Because the energy of the pellet is the pressure on the back of it times cross sectional area integrated with distance as it moves down the barrel.

Bigger cylinders make lower pressures with the same spring (as the energy absorbed by the compressing air follows a similar equation for the piston). So to get the same pressure with a bigger piston needs more energy from the spring (in reality, it is more complicated, particularly towards the top end of power for a gun).

BB

[TonyL]

Yep, good answers above.

Although I can't describe the physics involved exactly, something to do with less pressure pushing back on the smaller diameter pistons.

As Jim has shown in his excellent articles, pellet release occurs at about the same percentage of stroke in all layouts. So, with a smaller bore / longer stroke, we have pellet direction and piston travel going forwards for a comparitively longer duration, giving greater efficiency.

I'm often stunned at the seemingly weedy springs required to achieve power on the 25mm guns. That little Slavia was running 14.5 at one point and felt very, very sweet and with a tiny spring and next to no static preload. I'd love to see inside some of the 22/23mm ones!

Other aspects related to efficiency, as Alistair points out, wil be down to various individual design / inefficiency peculiarities - tight seals, over long TPs, inefficient loading taps etc.

[look no hands]

So what you're saying is if the bore had been 25mm on the Theoben then it may have made power easier? I think the other thing is the weight of the piston (heavy) and the piston seals aren't the best and are pretty hard, so probably don't seal very well (so power is lost there as well), not sure what the TP sizes are or what the stroke is, if it was easier to put together and take apart then I'd muck about with it more but trying to get the spring in with my spring compressor and then getting the circlip in place is a pain, it's even worse trying to get it out, what makes me chuckle is that a lot of the shed tuners always seem to revert to O ring heads when doing reduced cylinder conversions, so the Airsporter/Merc heads can't be all that bad, I agree they run better with the Meteor O ring.

Pete

[Hsing-ee]

So what you're saying is if the bore had been 25mm on the Theoben then it may have made power easier? I think the other thing is the weight of the piston (heavy) and the piston seals aren't the best and are pretty hard, so probably don't seal very well (so power is lost there as well), not sure what the TP sizes are or what the stroke is, if it was easier to put together and take apart then I'd muck about with it more but trying to get the spring in with my spring compressor and then getting the circlip in place is a pain, it's even worse trying to get it out, what makes me chuckle is that a lot of the shed tuners always seem to revert to O ring heads when doing reduced cylinder conversions, so the Airsporter/Merc heads can't be all that bad, I agree they run better with the Meteor O ring.

Pete

It should be making a lot more power than a 25mm job if it has a proportionally longer stroke as well as the bore. The 25mm ones are just in the sweet spot for 11-12 fpe. Scirrocos were making 20 fpe on FAC IIRC. Have you tried measuring the transfer port volume by using the shank of drill bit?

Are you saying the piston seals are hard when they are new? It's a shame it is such a b'stard to strip and reassemble, sounds like a nightmare. Would building a better spring compressor help, something with more control like straps or something?

The tuners do seem to like their 'O' rings, but then they have got the gear to true up their cylinders to perfect. I still think parachutes are the best for most guns and most home tunes. The BSA Mercury, Airsporter and Meteor did give excellent consistency when fitted with the 'O' rings, but I think they were robbing power through lost volume and friction.

[look no hands]

It should be making a lot more power than a 25mm job if it has a proportionally longer stroke as well as the bore. The 25mm ones are just in the sweet spot for 11-12 fpe. Scirrocos were making 20 fpe on FAC IIRC. Have you tried measuring the transfer port volume by using the shank of drill bit?

Are you saying the piston seals are hard when they are new? It's a shame it is such a b'stard to strip and reassemble, sounds like a nightmare. Would building a better spring compressor help, something with more control like straps or something?

The tuners do seem to like their 'O' rings, but then they have got the gear to true up their cylinders to perfect. I still think parachutes are the best for most guns and most home tunes. The BSA Mercury, Airsporter and Meteor did give excellent consistency when fitted with the 'O' rings, but I think they were robbing power through lost volume and friction.

To be honest Alistair I've not done any measuring as I knew from reading lots of guff that they are a sod to get up to power so just wanted to see if I could get good power out of it before getting to involved, if it had made decent power then I was thinking of getting a lighter piston made up with a OZ piston seal (apparently they do a 27mm parachute seal) but to be honest now I can't be arsed, the strange thing is it even twanged with a tight fitting delrin guide, I just think it's one of those rifles that has just got to be left alone in its original form, I'll get my summer holiday over with and think about getting another new piston seal (I know how to fit them now so hopefully won't bugger it up like the one I just bought) and a pump and return it to it's former glory.

Pete

[Hsing-ee]

To be honest Alistair I've not done any measuring as I knew from reading lots of guff that they are a sod to get up to power so just wanted to see if I could get good power out of it before getting to involved, if it had made decent power then I was thinking of getting a lighter piston made up with a OZ piston seal (apparently they do a 27mm parachute seal) but to be honest now I can't be arsed, the strange thing is it even twanged with a tight fitting delrin guide, I just think it's one of those rifles that has just got to be left alone in its original form, I'll get my summer holiday over with and think about getting another new piston seal (I know how to fit them now so hopefully won't bugger it up like the one I just bought) and a pump and return it to it's former glory.

Pete

I admire your courage in having a go at that conversion but by Jupiter it sounds like the vexatious conundrum from Hell, and the only sane thing to do it as you say, return it to its gas-ramminess. I'd be wanting to take an angle grinder to it by now.

Now, how about converting your Brocock Independent to a gas-strut?

[Rickenbacker]

I said it'll be "well worth all the trouble", didn't I?

[Shed tuner]

I just think it's one of those rifles that has just got to be left alone in its original form, I'll get my summer holiday over with and think about getting another new piston seal (I know how to fit them now so hopefully won't bugger it up like the one I just bought) and a pump and return it to it's former glory.

Pete

I told you so

(sorry, but the Theoben elves are a tricky lot to battle against)

[look no hands]

I told you so

(sorry, but the Theoben elves are a tricky lot to battle against)

If I'm honest Jon I've not actually put that much effort into it, I obtained a spring that was recommended on another conversion thread I read and got some guides made up, if I had a better spring compressor and could force the whole Titan no14 spring in (I had to chop off 3 coils) then it may have got to more acceptable power but 9.5ftp with a monster spring that I had to fight to get in, just isn't worth a piss in a bucket and I'm not risking an eye and a couple of fingers to try and get those circlips in and out everytime, at least I can say I had a go.

Pete

[look no hands]

I admire your courage in having a go at that conversion but by Jupiter it sounds like the vexatious conundrum from Hell, and the only sane thing to do it as you say, return it to its gas-ramminess. I'd be wanting to take an angle grinder to it by now.

Now, how about converting your Brocock Independent to a gas-strut?

The Indy has a spring in it and that's how it'll stay thank you very much, I've never liked those drop in rams, proper rammers are ok but expensive to keep maintained and pumped up, you could buy loads of springs which would outlive the life of the rifle for what it costs to buy a damn pump.

Pete

[Hsing-ee]

The Indy has a spring in it and that's how it'll stay thank you very much, I've never liked those drop in rams, proper rammers are ok but expensive to keep maintained and pumped up, you could buy loads of springs which would outlive the life of the rifle for what it costs to buy a damn pump.

Pete

I was pulling your leg. The Brocock is a BSA and deserves a spring in it, innit.

I remember seeing the first Scirroco at a shooting show or a range where Theoben had a stand somewhere, it was bulky and a bit crude for the money (I think the barrel was a Webley in those days). Later someone in the club got one, could not get any kind of accuracy from it, and it wasn't the hold sensitiveness, it was shooting dinner-plate size groups at 50 feet. Took it back and got another twice, then got their money back. It was just not as good as a Feinwerkbau Sport or an HW35 Export, and it was fugly to boot. The BIG advantage they claimed was the set of bolted-on scope rings - there was no ramp - which would eliminate scope creep so they said. People were obsessed with scope creep in those days, and with hunter's mainsprings failing because of sitting waiting for beasties to turn up. So these two things made them more attractive then than now. I honestly don't know why they didn't sink without trace like the Jackal Parabellum and the Sabo pellet bullet thing.

Later, one expensive new one (Fenman?) I had a shot with had the detent chisel operating at 30 degrees off, that would not shoot either, unsurprisingly. A BSA Lightning GRT I tried, the same, horrible accuracy. I know there are people that love them and Jon Budd's HW90 is OK (if a bit porky) but I think they are the Devil's work and you will be so much happier when you are shot of it. Even when they are working the shot cycle is like stepping on a rake-head and getting the handle between the eyes. Honestly, if airguns had started out with gas-rams, people would be clamouring to have them fitted with the elegant, robust, simple and pleasant main-spring solution.

Get a nice Diana or summit instead. The Scirocco 2000 looks like a ugly version of the BSA Lightning anyway.

[TonyL]

If I'm honest Jon I've not actually put that much effort into it, I obtained a spring that was recommended on another conversion thread I read and got some guides made up, if I had a better spring compressor and could force the whole Titan no14 spring in (I had to chop off 3 coils) then it may have got to more acceptable power but 9.5ftp with a monster spring that I had to fight to get in, just isn't worth a piss in a bucket and I'm not risking an eye and a couple of fingers to try and get those circlips in and out everytime, at least I can say I had a go.

Pete

Wouldn't it be funny if it turned out that it was oversprung, Pete?

And this may well be the case and certainly wouldn't be the first time it's happened.

Maybe, just maybe, before you decide to convert back to rammer, it may just be worth trying cropping the spring a little? If this worked, it'd give you a nicer firing cycle and make stripping / reassembly easier and safer, too.

[look no hands]

Wouldn't it be funny if it turned out that it was oversprung, Pete?

And this may well be the case and certainly wouldn't be the first time it's happened.

Maybe, just maybe, before you decide to convert back to rammer, it may just be worth trying cropping the spring a little? If this worked, it'd give you a nicer firing cycle and make stripping / reassembly easier and safer, too.

That could be the case but most of the conversion threads I've read (and talking to Tony Bigtoe as well) have said they needed a lot of spring, Tony in particular said even he struggled with the spring and wouldn't ever do another one, the spring is only sticking out the back of the cylinder about 2 inches, so there isn't masses of preload, one of the threads I was reading is HERE, an interesting read.

Pete