Different size cylinders and power.

[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?

[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

[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