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.

[robs5230]

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

Sorry to see that didnt work Pete. A couple of questions...
How old was the piston seal ? They're crap at best.
What spring did you use?
What pellet did you test power with ?
How tight was the spring on the guide you had made ?

I really can't remember much about when I did mine but it made way more power than that.
Rammers work very differently to a springer. Much quicker lock time due to the higher pressure.
One of the issues is maybe the spring itself.
I'd have fitted a Titan or other stiff thick spring for sure in mine and very likely used the hw35 or 80 sized "export type" spring, as they were both springs of choice for me in the early to mid 90's. It may well have been an OX i fitted.

Another valid point may be the "oversprung " thing. When I did mine I did it alone without a spring compressor. I've never owned a spring compressor so it wouldn't have been that hard.

[look no hands]

Sorry to see that didnt work Pete. A couple of questions...
How old was the piston seal ? They're crap at best.
What spring did you use?
What pellet did you test power with ?
How tight was the spring on the guide you had made ?

I really can't remember much about when I did mine but it made way more power than that.
Rammers work very differently to a springer. Much quicker lock time due to the higher pressure.
One of the issues is maybe the spring itself.
I'd have fitted a Titan or other stiff thick spring for sure in mine and very likely used the hw35 or 80 sized "export type" spring, as they were both springs of choice for me in the early to mid 90's. It may well have been an OX i fitted.

Another valid point may be the "oversprung " thing. When I did mine I did it alone without a spring compressor. I've never owned a spring compressor so it wouldn't have been that hard.

The piston seal was in the gun (obviously) so not sure how old it is, I did try fitting a new one but buggered up the dovetail part fixing lug because I didn't use the magic Theoben way of fitting it , I used a Titan no14 spring which I had to shorten by 3 coils as it was impossible trying to get it in the cylinder without doing myself an injury or risk having it torpedo around the shed, I used the usual .177 AA domed pellets, the spring guide is a snug fit, not tight that you have to really push it hard to get it on.

Like I said before I didn't really give it that much time as the circlip that holds the rear guide in looks a bit worn and I didn't want to risk breaking it as with most Theoben parts now they are either hard to get or bloody expensive, I could have tried some preload washers and that may have got the power up into the 10ftp area but like you say, I think the seal may need replacing but I don't really want to spend another £30 at the moment on another seal when my next project is waiting to be worked on (infact I've just bought some stuff for it today).

Pete

[robs5230]

The piston seal was in the gun (obviously) so not sure how old it is, I did try fitting a new one but buggered up the dovetail part fixing lug because I didn't use the magic Theoben way of fitting it , I used a Titan no14 spring which I had to shorten by 3 coils as it was impossible trying to get it in the cylinder without doing myself an injury or risk having it torpedo around the shed, I used the usual .177 AA domed pellets, the spring guide is a snug fit, not tight that you have to really push it hard to get it on.

Like I said before I didn't really give it that much time as the circlip that holds the rear guide in looks a bit worn and I didn't want to risk breaking it as with most Theoben parts now they are either hard to get or bloody expensive, I could have tried some preload washers and that may have got the power up into the 10ftp area but like you say, I think the seal may need replacing but I don't really want to spend another £30 at the moment on another seal when my next project is waiting to be worked on (infact I've just bought some stuff for it today).

Pete

Not familiar with the titan 14 in terms of sizing but it looks like a diana 350 replacement.
Just seen in your signature a scirocco in .177. Was that the rifle you tried springing? Mine was .22

[look no hands]

Not familiar with the titan 14 in terms of sizing but it looks like a diana 350 replacement.
Just seen in your signature a scirocco in .177. Was that the rifle you tried springing? Mine was .22

Yep, my Sirocco's a .177 HE, the Titan no14 is a beast of a spring, you're right it is for a Diana 350 and a Webley Patriot.

Pete

[robs5230]

Yep, my Sirocco's a .177 HE, the Titan no14 is a beast of a spring, you're right it is for a Diana 350 and a Webley Patriot.

Pete

mmm.. Like I said I'd have used an 80 type spring but mine definitely made more power, maybe the calibre helped.
What's the spec on that no14 ? wire thickness, i.d., o.d. ?