SpudFiles

How much pilot volume is needed for a piston valve to work?

As little as possible.

Thanks! -------- I thought so but wasn't sure.

The volume required would be the minimum amount you can achieve while still allowing the piston to stay closed, and travel far enough to open. So, as flinger said, do your best to make it as small as possible.

Enough to allow the piston to slide back 1/4 of the barrel diameter. Including room for a bumper.

So I do need some. Oh yeah, to let the piston come back.

I edited my post... may be a bit more clear now.

"still allowing the piston to stay closed" how do you figure that? Wouldn't any amount achive that?

"slide back 1/4 of the barrel diameter" So a 1" barrel would require 1/4" of pilot volume.

For most people, yes. There is a point at which the volume could not physically decrease without the diameter shrinking, thus not providing the force needed to keep the piston sealed. This point doesn't exist from a strictly mathematical standpoint, nor would it likely be encountered here, but something in my very rigid mind makes me consider things like this situation, and mention them in posts.

Well thanks again for your input.

For a 1" barrel, 1/4"+ piston lenth+bumper thickness=pilot volume.

jrrdw wrote:For a 1" barrel, 1/4"+ piston lenth+bumper thickness=pilot volume.

And add a lil extra traveling to be sure.
Traveling 1/8" too much isnt bad, but traveling 1/16th" too less is.
The extra travel also makes you have less problems with bouncing problems.
Some pistons may bounce back a short way.

jrrdw wrote:For a 1" barrel, 1/4"+ piston lenth+bumper thickness=pilot volume.

That doesn't look right because the units are wrong. You're adding three lengths and getting a volume. That ain't possible. (Dimensional analysis to the rescue!)

Shouldn't it be, for a solid cylindrical piston;
pilot volume V₁ = (distance piston must move)(area of piston)
The "distance piston must move" is generally taken to be at least 1/4 of the barrel diameter.

For a cylindrical piston open at one end;
pilot volume V₂ = V₁ + internal volume of piston.

If there is a bumper then you need to know what the gas volume is in the bumper region.
pilot volume V₃= V₂ + gas volume in the bumper region

jimmy101 wrote:

jrrdw wrote:For a 1" barrel, 1/4"+ piston lenth+bumper thickness=pilot volume.

That doesn't look right because the units are wrong. You're adding three lengths and getting a volume. That ain't possible. (Dimensional analysis to the rescue!)

Shouldn't it be, for a solid cylindrical piston;
pilot volume V₁ = (distance piston must move)(area of piston)
The "distance piston must move" is generally taken to be at least 1/4 of the barrel diameter.

For a cylindrical piston open at one end;
pilot volume V₂ = V₁ + internal volume of piston.

If there is a bumper then you need to know what the gas volume is in the bumper region.
pilot volume V₃= V₂ + gas volume in the bumper region

Over complicating it a tad bit, is the reason it doesn't look right.

My formula is quick, easy and works.

Doesn't T=1/4 D* apply more to the minimum travel of a piston in a co-axial for maximum flow, not how much pilot volume you should have?

Travel and pilot volume usually go hand in hand, but I've always thought of it as working out the travel of a piston you need to get the best flow, not how much pilot volume you should have.



*Obviously T = Travel, D = Barrel Diameter