SpudFiles

In several topics involving pistons, people often suggest drilling out the back to make the piston lighter and therefore increase performance. One problem: this increases pilot volume. I'm wondering if the lowered weight of the piston more than compensates for the increased pilot volume (assume no filler is added to back of piston; ie. foam), or whether drilling the piston is not just only a waste of effort but actually decreases performance.

Obviously this depends on many things including piston weight (or density) before drilling, amount of pilot volume before drilling, and other factors. Has anyone actually observed a significant change in performance by drilling out the back of the piston? If so, was it an increase or decrease, and what were the other variables (pilot volume before and after, piston wieght before and after, piston diameter, o-ringed or not, etc.)?

-goose_man

in my personal opinion, a piston valve open so fast it doesnt really matter if you are off by 1/4'' or if its 1 oz. more than ideal. obviously if it was solid steel then there would be a problem, but most pistons i dont think weigh enough to matter. i think that friction would decrease performance more than weight of the piston (i.e. put some lube on it and it will work really well)

Piston weight or pilot volume typically don't have any significant affect on performance. The biggest advantage of a lighter piston is that it is easier to stop so it puts less stress on the gun.

True clide. You dont want your piston blowing out the back of the cannon like MrCrowly's piston cannon. I would go for lighter but thats just me

Haha yeah thanks for that battlemonkey

My piston isn't exactly light, hence the failing of the PVC fittings even though I did have a bumper.

clide wrote:Piston weight or pilot volume typically don't have any significant affect on performance. The biggest advantage of a lighter piston is that it is easier to stop so it puts less stress on the gun.

Ah, I see. I wonder how fast the piston actually moves though...

-goose_man

why don't you just make a light low friction piston wih a tiny pilot volume?

ALIHISGREAT wrote:why don't you just make a light low friction piston wih a tiny pilot volume?

Isn't that what we are all trying to do?

ooops didn't read t properly. i think the best way would be to drill it out but have a plug or cap over the hole to keep the pilot volume low.

Piston weight or pilot volume typically don't have any significant affect on performance. The biggest advantage of a lighter piston is that it is easier to stop so it puts less stress on the gun.

I disagree there. The heavier a piston, the slower it will accelerate and you can't deny the link between valve opening time and performance. Also, the lower the pilot volume, the quicket the pilot chamber will depressurise and the quicker the piston will start to move.

so in the end what you want is a light piston and a small pilot volume cuz the lighter the piston=faster it will accelerate. the smaller the pilot volume the quicker the pilot volume is exhausted=the faster the piston will start to move

so in the end you want both so they can work together to open your valve faster

open faster = more power

I'd argue that the system is complex enough that it's hard to make hard-and-fast predictions about performance.

Indeed, I've modeled situations with the GGDT in which a larger pilot volume improved muzzle velocity. (said situation was reasonable)

And there probably are scenarios where a heavier piston would increase performance.

Isn't possible that GGDT just glitched a bit? I mean, the program is definitely not perfect...

boilingleadbath wrote:I'd argue that the system is complex enough that it's hard to make hard-and-fast predictions about performance.

Indeed, I've modeled situations with the GGDT in which a larger pilot volume improved muzzle velocity. (said situation was reasonable)

And there probably are scenarios where a heavier piston would increase performance.

I've also noticed that, a pilot volume of 50cm^3 and 250cm^3 has no effect on velocity or anything according to GGDT with some stuff I've modeled, even changing pilot valve flow and other parameters I get no change sometimes.

Well how does ggdt model the piston, because in Mr. Crowleys case his piston doesn't fit perfectly just well. But is dan300 or sgort87 modeled there pistons with o-rings the performance would be different even though ggdt doesn't consider the design of the piston just weight. but not if there are o-rings which as you all know is a pretty big factor.