SpudFiles

Hello all, I'm new to the forums. Some might recognize me from the benheck forums.

Anyways, my attempts at building a piston valve failed, but I'm willing to try again. I don't give up easily. I was wondering, how far does the piston need to move back in order for good airflow? I've heard from Klugeboy that your piston needs to move 1/4 the diameter of the port. So, with a 1 inch port, would it need to move 1/4 of an inch to get good airflow?

Metalcookie wrote:So, with a 1 inch port, would it need to move 1/4 of an inch to get good airflow?

Yes, it's called the D4 rule.

Welcome to Spudfiles.

Metalcookie wrote:how far does the piston need to move back in order for good airflow?

Depends on the kind of performance you hope to achieve. I shoot high pressure (1000psi) in high volume. I make my valves according to the "D1" rule! D2 or D4 will launch a projectile but, D1 gives serious performance.

Which rule would you recommend for a noob like myself?

Well the less room you give your piston to move back the smaller the pilot volume so 1/4 inch sounds good but I wouldnt suggest more than 3/8 inch if you want more space to move back

The 'D4' rule is mainly for coaxials. I like to give a bit more for barrel sealing pistons in a tee configuration. As long as your pilot valve is decent and your pilot volume isn't huge, you should be fine giving your piston a little bit extra room to move back.

Have a read through this post.

well, on mine i used some math. if you have a 3 inch (pretend its ACTUALLY 3.0") barrel, you can figure the surface area of the port is ~7 square inches. the circumfrence of the pipe is about 9.5" therefore, (7/9.5) the piston would only need to move back .75 of an inch, to get (IMO) max air flow. it worked pretty well for me, threw some 4 lb projectiles pretty damn hard, the gun's recoil tilted my stand (which happened to be a forklift)

grock wrote:well, on mine i used some math. if you have a 3 inch (pretend its ACTUALLY 3.0") barrel, you can figure the surface area of the port is ~7 square inches. the circumfrence of the pipe is about 9.5" therefore, (7/9.5) the piston would only need to move back .75 of an inch, to get (IMO) max air flow.

That is pretty much where the D/4 rule came from, 0.75" is 1/4 of 3"

For coaxials:
D/4 + a little
D/4 origins from the math grock posted. However, always give it a little bit more to avoid restriction and to make up for things like piston bounce, sealing face flapping, air friction etc.

For T-valves, porting a bit smaller than the piston OD and T ID.
D/2 + a little
The air can now only come from one side (180*) instead of from all sides (360*), like in a coax.
D/2 should do it, and then throw in a little extra to avoid restrictions.
D/3 or larger recommended, especially on larger portings.

For T-valves, porting close to T ID:
D/1 + a little
When the chamber side approaches the porting (such as porting = 90% of piston dia) you will need to open it completely for full flow, and then a bit more to avoid restrictions.

jackssmirkingrevenge wrote:

grock wrote:well, on mine i used some math. if you have a 3 inch (pretend its ACTUALLY 3.0") barrel, you can figure the surface area of the port is ~7 square inches. the circumfrence of the pipe is about 9.5" therefore, (7/9.5) the piston would only need to move back .75 of an inch, to get (IMO) max air flow.

That is pretty much where the D/4 rule came from, 0.75" is 1/4 of 3"

thought that was just coincidence. didn't know it usually worked. funny though

Yep, I had illustrated the math here.

Thanks for the input guys

I have but one more question: what would be a good material to make the piston out of? I was thinking either a 1 1/4 inch coupler or a piece of molded hot glue.

>>>insert predictable answer here<<<

Guys thanks so much for your help! I built a working piston valve! I used spare parts from mah other cannon for most of the parts, and tested it at around 75 psi. I can't really estimate what kind of performance it has, because It's really dark out, and I didn't have proper spuds. I'm going out of town all this week, but will be back friday. I'll post pics and what not next weekend.