SpudFiles

Since I haven't stirred up much lately(partly because of no net access)...

Playing this past weekend, I noticed some rather significant "discrepencies" in performance between my toolie valved ModGun cannon with 6' GB barrel versus DD1(coax) with 10' GB barrel and much larger chamber, both QEV piloted, and both with Identical fills(140psi), the toolie gun is consistently shooting harder and faster even though chamber and barrel are significantly smaller than on DD1.... and DD1 is losing less to recoil due to the tripod than the ModGun(hip fired)...

I don't have a chrony yet, but the plywood'o'truth is consistently telling me the smaller modgun is MOAR than the DD1...

I'm thinking that once I get me a chrony, I'll build another toolie chamber with volume Identical to DD1,and a t-valved chamber with a camlock barrel the same, and a barrel the same length as DD1 to use on the toolie and the t-valved chambers to see which really is the best(performance wise)...

Until then, what's your thoughts on what valve/chamber/barrel layout is more effecient????

I know each has their strength and weakness(compactness/portability/power/flowcapability/etc), so lets hear thoughts on that as well...

well, I'm definatley no expert, but this for me is a recipie for success: a large chamber and a smaller barrel (thus having a bigger c:b ratio. Also, I think the less bends in the pipe the better.

maverik94 wrote:well, I'm definatley no expert, but this for me is a recipie for success: a large chamber and a smaller barrel (thus having a bigger c:b ratio. Also, I think the less bends in the pipe the better.

Less bends is better. Add to the list, the larger the valve orifice the better to increase the affective aperture by keeping the flow lower in a high turbulence area (valve, velocity stack maybe), less dead space between the valve and projectile (oops, maybe the velocity stack isn't good. Testing needed) , and lastly, the faster valve wins.

I know blowing own horn, but these were the reason I progressed from piston to large homebuilt QEV, to Quick Dump Valves. I'll shut up now.

C/B ratio.. Bigger C the better, but diminishing returns. Barrel, Umm, too long you run into max capacity of the pipe for flow when assuming an infinite chamber, so there is more to the formula than just C/B ratio.

When clocking the in barrel launches for a competition t shirt launcher, the theory didn't match the actual results and a shorter barrel worked better than just the gas expansion predicted.

For the new guys; the link to the competition barrel trim testing page;
http://inteltrailblazerschallenge.wikis ... rim+method
In theory, a 2.5 inch barrel over 20 feet long would do best with the 700 cu in tank. Less than 10 feet worked better when tested.

know blowing own horn, but these were the reason I progressed from piston to large homebuilt QEV

A piston valve is a qev. A qev is just a commercially made piston valve.

I'm thinking one of your cannons has a larger valve porting and a lighter more efficient piston with o rings so you lose less air on piloting.

jonnyboy wrote:

know blowing own horn, but these were the reason I progressed from piston to large homebuilt QEV

A piston valve is a qev. A qev is just a commercially made piston valve.

I disagree. Not all piston valves avalanche open and work more like a hydraulic valve. A slow pilot in a large ratio piston means a slow valve (reason for sprinkler valve mods), whereas a true QEV valve will avalanche open due to a large positive feedback when it opens. A slow pilot does not mean a slow opening time in a QEV.

**edit** Chamber sealer valves are not QEV's and are fully dependent on pilot speed. They have little positive feedback. This includes the Supah valve which requires a large fast pilot for it's speed. Don't get me wrong, with a fast pilot, the supah is no slouch in performance.

Link to a thread of supah valve to show internals;
http://www.spudfiles.com/forums/supah-v ... -t299.html
Small pilot and low mass is the key to the speed for this, not high positive feedback.
***end edit***

A 4 inch piston with a 2 inch valve seat has a seat to OD area ratio of 4:1. A QEV typically has an area ratio of under 2:1. (The seat diameter is large in relation to piston OD.)

A Quick Dump Valve has an area ratio of 1:1, thus needing mechanical or external cylinder triggering but highest positive feedback.

I'm thinking one of your cannons has a larger valve porting and a lighter more efficient piston with o rings so you lose less air on piloting.

The high positive feedback provides the speed. No pilot back pressure + the increasing pressure on the front as it opens provides the feedback to avalanche open.


I know, enough with the QDV already.. This time it is on topic. Honest

johnnyboy was talking about outlet (barrel) sealing piston valves. These do have a large jump in force when they unseal from the outlet. Take a look here for a little comparison.

mark.f wrote:johnnyboy was talking about outlet (barrel) sealing piston valves. These do have a large jump in force when they unseal from the outlet. Take a look here for a little comparison.

Sometimes the jump is less than assumed when they have a large area ratio. A 2:1 diameter ratio (4 inch to 2 inch for example) has a 4:1 area ratio. The big jump in force for that is only 25 % and the rapidly compressed pilot area makes the pilot at 75% pressure jump to over 100% in very little travel distance, so some valves don't avalanche open all the way without a large fast pilot. These are NOT QEV's, even though they have some positive feedback.

The large ratio valves are softer opening and don't tend to shatter as much as they don't avalanche open. The supah is one that uses this limited speed to prevent piston breakage.

Again, not all piston valves are QEV's.

As the article states, lighter projectiles and shorter barrels, the valve speed makes a difference. In the 1 inch QEV cannon I built, I was putting gumballs through 1/2 inch plywood. The marshmallow cannon puts marshmallows through empty pop cans and splits open full ones with a short barrel. Valve speed mattered for those applications.

Well, most barrel-sealers I build have a very close surface ratio. For instance, for a 1-1/2" porting valve, I use a 2" diameter (2.06" diameter) piston, and a 1.875" OD sealing face. At 100 PSIG operating pressure, these valves pilot at around 17 PSIG pilot pressure, resulting in a very large jump in force. They open very fast. Here's a video of one opening, although it doesn't prove much.

mark.f wrote:Well, most barrel-sealers I build have a very close surface ratio.

Those are QEV's. You can tell them apart as they open with a bang even with a slow pilot.

The cannons I referenced are both inline(read no bends), they are both piloted by 3/4" qev's, the pistons have zero leakage to the pilot side(piston seals are designed similar to a bike pump, air can go from pilot to chamber but no air from chamber to pilot), the main differnce is indeed the sealing face diameter, 1.69" on the DD1 and 2" on the modgun, the modgun also has about 16cu" dead space between the GB and the chamber which may hurt or help, but I'm guessing it helps on this particular gun, especially since this "dead space" actually steps down from 2" to 1.69" from the chamber to the barrel, and may be acting to allow the inertial mass of the air to further compress/heat the air slamming forward into this deadspace behind the GB...

If I get a chance I guess I need to disassemble and take exploded pictures of everything on both guns...

Technician1002 wrote: I know blowing own horn, but these were the reason I progressed from piston to large homebuilt QEV, to Quick Dump Valves. I'll shut up now.

While sometimes you do drop in the pros of the QDV a little irrelevantly, I have to agree with you here. Apart from an inline combustion, the QDV is about as efficient as you get. The air doesn;t have to change direction much if at all and is relesed quickly too. Apart from the mechanical actuation, there aren't really any disadvantages to the layout that I can think of.

The QDV could do that too. You just replace the piston with a projectile. Instead of pulling back, you push forward to actuate.

Technician1002 wrote:
I know blowing own horn, but these were the reason I progressed from piston to large homebuilt QEV, to Quick Dump Valves. I'll shut up now.

.

Funny thing, My first cannon was a QDV cannon, and I progress to Coax/Toolie/QEV guns... And even using the same barrel on the qdv chamber as on the toolie chamber the toolie way outperforms the qdv...even with almost identical chamber volumes and pressures...

That's odd. I know for the same sizt the toolie has a larger chamber, but I would have thought that the barrel sealing part of the piston would obstruct flow at least a little...