Can a heavy piston rob power?

[TonyL]

Oooooh....that's a big, big, sweeping question, Petey Boy!

Are you asking would fitting a heavier piston than optimal in a particular gun would rob power if converting? Maybe. Maybe not. As AB says, it'd certainly give a different feel to the cycle. If the spring wasn't changed, piston acceleration would be slower. But momentum would be increased. Recoil in the first forward stroke would fractionally increase, but piston bounce might decrease. Transfer port size and flow and pellet release pressure would also be big factors here. Depending on how optimal the piston weight was pre-fettle would dictate whether a lighter or heavier piston would be an improvement or otherwise. And it's often subjective and psychological anyway.

A heavier piston combined with more spring force and more preload could well result in greater power output but at a detriment to the firing cycle, usability and accuracy.

Depends on the gun.......I've often heard (but haven't experimented with this) that increasing the piston weight of rodless 25mm pistons (as fitted to Gamos and clones) might be helpful. But I quite like the quick feel of their cycles with their light pistons.

So much to consider here.....

[Neil54]

One of the tuning tips for Chinese airguns was to loose the steel top hat.
I have since learned that is better to leave it in and keep the piston weight to 185g.

25mm cylinder sub 12 ftlbs.

[10/10]

Adding a little weight in the the piston can sometimes increase power and make the gun shoot nicer

[look no hands]

Oooooh....that's a big, big, sweeping question, Petey Boy!

Are you asking would fitting a heavier piston than optimal in a particular gun would rob power if converting? Maybe. Maybe not. As AB says, it'd certainly give a different feel to the cycle. If the spring wasn't changed, piston acceleration would be slower. But momentum would be increased. Recoil in the first forward stroke would fractionally increase, but piston bounce might decrease. Transfer port size and flow and pellet release pressure would also be big factors here. Depending on how optimal the piston weight was pre-fettle would dictate whether a lighter or heavier piston would be an improvement or otherwise. And it's often subjective and psychological anyway.

A heavier piston combined with more spring force and more preload could well result in greater power output but at a detriment to the firing cycle, usability and accuracy.

Depends on the gun.......I've often heard (but haven't experimented with this) that increasing the piston weight of rodless 25mm pistons (as fitted to Gamos and clones) might be helpful. But I quite like the quick feel of their cycles with their light pistons.

So much to consider here.....

I only ask as I'm sure I read JB's tuning of the Airsporter RB2 and he said the piston was too heavy, I was chronoing the Fenman last night and it is running at 10ftp with the drop in Diana gas ram, it was also doing the same with the spring kit I have, but that didn't have any preload slip washers fitted, I had a 5mm thick disk welded to the front of the Fenman piston so I could fit a Diana 35 seal, I first thought it might be too heavy but have just thought that it's been short stroked because of the longer seal and the added disk, so tat might be why it is lower power with spring fitted.

[Segata]

Physics brain says yes and no, if theres not enough room to build up momentum it should but then at momentum its harder to stop thus must transfer more energy and give more power so in short if the Guns designed to utilize it then it will mean more than less.

[Shed tuner]

sure it can.

take an extreme, with the piston weight something stupid like 10 kgs. How slowly would that accelerate ? So you'd not build up that high pressure pulse.
This is why long weak springs and heavy pistons are a bad combination.

too light also robs power, as the piston lacks inertia, so is quickly stopped by the pressure pushing back on it.

So there's a sweet spot in the middle. As Neil indicates, for 25mm bore guns around 12 FP, the sweet spot is 180g - 220g (depending on stroke/port/seal/spring).

[NickG]

Stroke plays a part , the mass needs to get up to speed to deliver the power,if the stroke is too short, for the weight it won't be going fast enough to deliver, extra weight can take you into dieseling territory also .

[look no hands]

I think I read somewhere that Theoben rammer pistons are hefty old lumps because they have thick walls as they have a lot of pressure in them, so most probably heavier than a standard springer piston, and because of the extra weight they need extra high pressure to propell them.

[Shed tuner]

I think I read somewhere that Theoben rammer pistons are hefty old lumps because they have thick walls as they have a lot of pressure in them, so most probably heavier than a standard springer piston, and because of the extra weight they need extra high pressure to propell them.

they are thick and heavy, and seem to need more stroke than expected too.. same with an HW90..

[BTDT]

...too light also robs power, as the piston lacks inertia....

Inertia is a property of a stationary mass, Jon. A moving mass like a piston has momentum, and too light a piston reduces momentum.

[Born Again]

Inertia is a property of a stationary mass, Jon. A moving mass like a piston has momentum, and too light a piston reduces momentum.

Not quite. Splitting hairs, inertia is the tendency of an object to remain in the same state, either stationary or in uniform motion, resisting change.

[BTDT]

Not quite. Splitting hairs, inertia is the tendency of an object to remain in the same state, either stationary or in uniform motion, resisting change.

To really split hairs, inertia is a property of a moving mass only if that movement is uniform, which the piston is not - it's either accelerating or decelerating (apart from an instant before it bounces).

The important point is that it is the reduced momentum of the lighter weight piston that makes it less able to overcome opposition from the compressed air.

[Shed tuner]

Inertia is a property of a stationary mass, Jon. A moving mass like a piston has momentum, and too light a piston reduces momentum.

picky picky.. I've misssed you Jim

Not quite. Splitting hairs, inertia is the tendency of an object to remain in the same state, either stationary or in uniform motion, resisting change.

You are my friend. Not like Jim

[junglie]

I only ask as I'm sure I read JB's tuning of the Airsporter RB2 and he said the piston was too heavy, I was chronoing the Fenman last night and it is running at 10ftp with the drop in Diana gas ram, it was also doing the same with the spring kit I have, but that didn't have any preload slip washers fitted, I had a 5mm thick disk welded to the front of the Fenman piston so I could fit a Diana 35 seal, I first thought it might be too heavy but have just thought that it's been short stroked because of the longer seal and the added disk, so tat might be why it is lower power with spring fitted.

The RB does not need any help making power apart from winding out the restrictor screw in the transfer port in the roller breech, I had to take a coil or so off mine after that mod & new breech seals to keep it below 12.....(RB airsporter carbine in .22) felt slammy & not nice to shoot before, improved it no end & it was the best airsporter I ever shot by far & very accurate too with FTT's.

[look no hands]

The RB does not need any help making power apart from winding out the restrictor screw in the transfer port in the roller breech, I had to take a coil or so off mine after that mod & new breech seals to keep it below 12.....(RB airsporter carbine in .22) felt slammy & not nice to shoot before, improved it no end & it was the best airsporter I ever shot by far & very accurate too with FTT's.

How true yo are, I'm actually confusing the subject by quoting something that isn't connected to this problem, as you say the RB2 doesn't have a problem making power with a heavier than usual piston, JB said that reducing the pistons weight helped with recoil reduction and a smoother firing cycle, I apologise for any confusion caused.