Short stroked HW80 piston ?

[deeps]

Gentlemen, this mornings theory question is...

Why is a sleeved down HW80 generally considered to be better than a short stroked (ie piston head extension) HW 80 ?

With the short stroke option you can be shifting exactly the same volume of air with the same compression ratios as a sleeved variant but with a shorter lock time.

[MattyBoy]

I would guess that a 30mm piston head is far less efficient at delivering air through the TP than a 25mm one.

Matty

[deeps]

Hmm so a 'magic number' for the ratio of TP/Bore CSA ?

[MattyBoy]

Hmm so a 'magic number' for the ratio of TP/Bore CSA ?

Well yes

The recent threads by T20 and Bigtoe01 go a large amount of the way to proving these "magic numbers"

Matty

[Hangtight]

Monday morning thought experiment!
There are a fairly well defined set of conditions you're trying to create behind the pellet ie a certain amount of air compressed to a certain pressure, but you're trying to do that with a lump of metal fired up a hole with a spring.
The bigger the piston the more area for cylinder pressures to act on, so higher force resisting the motion of the piston. So then you need a bigger spring with higher preload which you have to compress in less travel. Which is fine if you can alter the cocking linkage to make it easier. And the loads on the trigger would go up, other components would be more highly stressed and need to be heavier. The accelerations involved would go up would could have a negative effect due to recoil even if the lock time is shorter. Some of this could be rubbish, but I'm just writing down thoughts as they pop up!
Within the constraint of tuning an existing rifle, 25mm bore x 70mm stroke seems to give the best performance/ engineering/ practicality compromise, but if anyone wants to make a 30mm x 55mm I'd love to know how it shoots!

[MattyBoy]

Monday morning thought experiment!
There are a fairly well defined set of conditions you're trying to create behind the pellet ie a certain amount of air compressed to a certain pressure, but you're trying to do that with a lump of metal fired up a hole with a spring.
The bigger the piston the more area for cylinder pressures to act on, so higher force resisting the motion of the piston. So then you need a bigger spring with higher preload which you have to compress in less travel. Which is fine if you can alter the cocking linkage to make it easier. And the loads on the trigger would go up, other components would be more highly stressed and need to be heavier. The accelerations involved would go up would could have a negative effect due to recoil even if the lock time is shorter. Some of this could be rubbish, but I'm just writing down thoughts as they pop up!
Within the constraint of tuning an existing rifle, 25mm bore x 70mm stroke seems to give the best performance/ engineering/ practicality compromise, but if anyone wants to make a 30mm x 55mm I'd love to know how it shoots!

A UKNeil modified 80 piston with 30mm Diameter x 56mm stroke was sold on here yesteday

Matty

[deeps]

A UKNeil modified 80 piston with 30mm Diameter x 56mm stroke was sold on here yesteday

Matty

Yep, that is what got me thinking.

Does this coupled with a larger ID TP = a sleeved piston

[gibboj]

my mates just done a 30mm x 55 mm hw80 in .177 with the std piston head its transformed the rifle from something that was hold sencertive and jumpy and not to accurate into a rifle that has a lot quicker lock time and though shooting at 11.2 flb it feels like 6 flb and tight groups out to 40 yrd now , iv shoot the rifle and looked at the recoil and its v-nice .its got me thinking about wat my hw80 in .22 would be like short stroked [ it should be better than a .177 ? ] iv allready short stroked my smk xs20 and that come out well
atb john

[Spr1nger]

A UKNeil modified 80 piston with 30mm Diameter x 56mm stroke was sold on here yesteday

Matty

It's winging its way to me right now, he typed smugly...

[gibboj]

hi springer dont be to smug yet as you will have some work to do to get it shooting at 11.2 flb it a bit hit and miss
atb john

[Spr1nger]

hi springer dont be to smug yet as you will have some work to do to get it shooting at 11.2 flb it a bit hit and miss
atb john

I can live with a little monkeying if required

I'm interested to see what immediate effect it has and what tinkering may be required to get it nicely set up.
The gun has been deburred and polished, ptfe piston liner, Vortek piston seal, and is lying in bits at the moment because its over the limit as a result

I've a couple of spare Titan mainsprings and one new-old-stock Venom spring, so I should end up with something that shoots ok

ish...

[gibboj]

hi springer if it was striped because it was over ,use that spring first as with all short strokes you need plenty of spring / preload
atb john

[maximus]

my mates just done a 30mm x 55 mm hw80 in .177 with the std piston head its transformed the rifle from something that was hold sencertive and jumpy and not to accurate into a rifle that has a lot quicker lock time and though shooting at 11.2 flb it feels like 6 flb and tight groups out to 40 yrd now , iv shoot the rifle and looked at the recoil and its v-nice .its got me thinking about wat my hw80 in .22 would be like short stroked [ it should be better than a .177 ? ] iv allready short stroked my smk xs20 and that come out well
atb john

I'm in the process of doing my .177 80 at 60mm stroke using a cap and v-Mach seal. My .22 has be done to 70mm but I may well go to 55 with an O ring. Might get an O ring head done for the .177 one too. I think it's cheating making the 80 into a 25mm piston conversion. was shooting my 35e on Sunday, leather seal, if I can get my 80 shooting like that I will be a happy man!

[Spr1nger]

hi springer if it was striped because it was over ,use that spring first as with all short strokes you need plenty of spring / preload
atb john

Thanks for the advice - that's about what I figured.

I've had it stripped out waiting for the right piston...

[gibboj]

to me it looks like the experts on short stroking [ t20 and bigtoe ] are going away from o-ring heads and going back to synthetic heads due to the fact that when thay hit the transfer port there a bit harsh / slamy .i think your write about 60mm on a .177 would be better and easyer to get 11.2 flb
atb john