Full bore porting, main valve dimensions..

[Fruit-Dove]

mah I'm so confused right now with these, in 3 hours of searching I couldn't find anything regarding to these, so here we go.

Again I drew some doodles for explanation, omitted tons of parts.. for convenience..

I'll make quick questions, in order not to confuse anyone.


1. 1st image, sure there must be enough room for the airflow, but is there any specific equasion or something to determine the dimensions of the D and d?(sorry, I'm yet not too familiar with those 'valve terms') and how thick the valve stem should be anyway?? is thinner the better??  oh one more, is tapered valve head(second image, source:https://www.airgunnation.com/topic/a-few-big-bore-pcp-projects/) any better than flat one?? I presume it would since it has more smooth curves..

2. in second image, which is the cross sectional view of the first image, IF one is planning on full bore porting it, I understand one should drill 'K' larger than the bore area, to compensate the valve stem's volume. And also, in order to prevent the bullet(by the way I'm considering this is a big bore pcp) getting damaged while feeding, one should drill the 'j'  75% of the bore diameter hole, elongated enough to match the bore area.

so this is the full bore porting isn't it??     but question is, the areas I marked with red circles, definitely those areas would lead to less ideal flow of the air. So is this inevitable or is there any other method to solve this problem??

sorry for the cascade of questions..haha... please cut me some slacks im just a starter and curious of all things;D as always, Thanks!

[rsterne]

When I talk about full "bore area" porting, I mean that every area between the reservoir and the barrel must be of equal (or greater) area to the bore.... You are correct that the width of the barrel port cannot be larger than about 75-80% of the bore, or the pellet may be damaged on loading.... That means that the barrel port must be longer than the bore diameter to get equal area.... Something like 80% wide by 120% long for example.... The transfer port, and the valve exhaust port can be round, and the same diameter as the bore.... The valve throat (k) must be larger than the bore to allow for the area taken up by the valve stem.... The larger the stem, the bigger it has to be so that the annular area (area of the donut shape) at least equals the bore area.... I find that because of the turbulence around the valve stem, it is a good idea to have that area about 10% larger than the bore area.... The stem can be as thin as structurally able to survive the hammer impact and bending loads.... and must be large enough to not "punch through" the poppet head.... Larger calibers, and higher pressures, will need bigger stems to stand the forces on them....

The diameter of the poppet head (d) must be large enough so that the seat "margin" is wide enough that it doesn't compress too much.... How wide, depends on the poppet material and the air pressure.... A material like Delrin needs 0.030-0.040" of seat width up to about 3000 psi.... With a harder material like PEEK, you can get away with narrower, or use higher pressures.... A good rule of thumb is to have "d" larger than "k" by at least 1/16".... The inside diameter of the valve (D) must be such that the gap between it and the poppet OD (d) provides more area than the bore.... I prefer to have this area at least 50% more than the bore area, and double the area is not unreasonable.... Having it larger has no negative impact on performance.... in fact it eases the passage of air into the valve seat area....

There are two things you didn't mention.... The entrance to the valve must also be larger in area than the bore, because you want the air to be able to enter the valve (much) easier than it can get out.... The minimum size would be the same as the valve throat (k), but 50-100% larger in area is fine, again, there is no negative impact on having it larger.... You want the entire reservoir (or plenum) volume to act as part of the valve, to maintain maximum air pressure at the valve seat....

In addition, you must look at the bolt probe that pushes the pellet past the barrel port.... If this is a rod, even a thin one, it makes the area inside the chamber smaller than the bore, just like the valve stem does in the throat.... Since you can't make the chamber larger than the bore, all you can do is retract the bolt (using a flat nose bolt), or the probe, so that the area of the chamber is free from any obstructions....

Angled poppet seats may have better flow, but they can "stick" closed by "wedging" and therefore require more hammer energy to open them.... Most PCP poppets are flat, although Motorhead has been having success with PEEK poppets (very hard) with a small angle (maybe 10 deg?)…. Lastly, those troublesome area you circled in red?.... The flow in the barrel is forward, into the lengthened port, and you can't round that upper corner anyway.... Where the exhaust port meets the throat, they are close in diameter than you have drawn (because the exhaust port is bore size), and you can round that corner, and the front upper corner where it meets the throat, to ease the flow....

Bob

[MJP]

You can make the ports the same size with poppet port and transfer, then funneling it down to barrel port.
Some say its wasted volume but who cares it works.
I use 45deg seat on bigger valves, works just fine.

Marko

[oldpro]

You also have to remember you have to open the valve at some point so having a really large poppet with to much surface area will result in a valve that requires a lot of hammer energy so design the valve with the power in mind you want to reach if you go to large you’ll find yourself hateing to cock and shoot it.

[rsterne]

There is no reason not to use a 45 deg. seat on a balanced valve design, where the opening force is so much less, and therefore little "wedging" effect.... There is a surprising amount of pressure drop that takes place due to the port volume between the valve seat and pellet, particularly in regulated PCPs with limited plenum volume.... Wasting volume there may not make any difference when you put a barrel for a .458 on an action, valve and reservoir made for a 20mm.... but making the ports twice the area (and volume) they need to be on a regulated PCP could cost you quite a bit of power....

Let's say you have a plenum of 50 cc filled to 2000 psi.... In a 50 FPE gun, at an efficiency of 1 FPE/CI, you use 6 cc of that 50 cc of air to create the shot, so the pressure drops to 1760 psi during the shot, for an average of 1880 psi.... Now introduce a transfer port system (total volume from valve seat to pellet base) that is 1/4" diameter and 1.25" long, with a volume of 1 cc.... By the time the air gets to the pellet, the initial pressure has dropped to 1960 psi, the final pressure to 1720 psi, and the average to 1840 psi.... If you double the diameter of the porting system, the volume increases to 4 cc, and the initial pressure at the pellet drops to 1840 psi, the final pressure to 1600, and the average to 1720 psi.... Your 50 FPE gun now produces only 47 FPE....

Yes, this is an extreme example, but there is no question that extra transfer port volume wastes power.... In the example above, you would have to increase the setpoint pressure by 120 psi to get back to 50 FPE.... If your bottle was filled to 3000 psi, the "headroom" of pressure available from that to the setpoint decreases from 1000 psi to 880 psi.... and your shot count would fall by 12%....

In addition, if the poppet is twice as big (4 times the area), it would take 4 times the hammer energy to crack it from the seat.... It is for these reasons that I recommend keeping the porting system as close as possible to bore area.... You need a slight increase above that in the valve throat, to look after the reduced flow past the valve stem.... but otherwise there is no point to use ports a lot bigger than the bore.... and taken too far, you can notice reduced performance, rather than any gain....

Bob

[MJP]

Did you notice the question about the barrel length vs power on my 223 build, we are seeing 53-60% from your lofty goal calculations with diffrent bullets and pressures.
Take a guess on port size 

Marko

[rsterne]

I didn't see it, please provide a link....

Bob

[MJP]

https://www.gatewaytoairguns.org/GTA/index.php?topic=144971.msg1480280#msg1480280

[tronickero]

Very nice post, I'm learning about valve lift and dwell, light and fast hammers.... and after reading a lot of post of Bob i have many doubt.

relation between Ideal valve throat, caliber, and power.

ideal space between valve poppet and main tube or valve body.

this post and the answers are great start point for me.

Im learning the theory about valves because i want build a new valve for my QB79 HPA because in the past i buided one but has bad performance (poor shoot count and low velocity) compare a moded stock QB valve like Jason mini valve or Bob valves.

my dream is a rifle like the "Tale of Two BRods" I want one  !!

Regards from Chile

Maurice

[Fruit-Dove]

wooow..bob, is there anything else you don't know about pcps?? thats...terrific..  lots of things going on here.. It took me some time to really understand all that..


so0 if I understood all that correctly,


1. wherever the air flows, the area must match or larger than the bore area... (dim yellow circles and donuts..)

2. poppet valve should have small diameter as possible(too hard to open if too large), and good rule of thums is to have poppet dia larger than valve throat by at least 1/16.

3. all ports ,including valve throat, have to be kept as close as possible to bore area, and there's no point on having it bigger than that(bore-area), since large transfer port volume is also one of those factors dropping the pressure,thus less efficient.

4. angled seat can be used, but requires more hammer energy due to the 'wedging effect',

5. probeless bolt is necessary for better flow

6. enterance to the valve(marked with red, where area is minimum on sky blue donut) must be larger than the bore area, no negative impact on having it larger.



what I still dont understand is this..

The inside diameter of the valve (D) must be such that the gap between it and the poppet OD (d) provides more area than the bore.... I prefer to have this area at least 50% more than the bore area, and double the area is not unreasonable.... Having it larger has no negative impact on performance.... in fact it eases the passage of air into the valve seat area....

but maybe it has? since it requires larger diameter, thus longer the transfer port... ?? maybe inner diameter of te valve(D) - poppet OD(d) = bore area, might be enough???

those troublesome area you circled in red?.... The flow in the barrel is forward, into the lengthened port, and you can't round that upper corner anyway.... Where the exhaust port meets the throat, they are close in diameter than you have drawn (because the exhaust port is bore size), and you can round that corner, and the front upper corner where it meets the throat, to ease the flow....

I dont think I get this limes completely...'Where the exhaust port meets the throat' what exactly is exhaust port?? is it where the poppet seats?? might rounding those troublesome areas wirh red circle increase volume?? would you please elaborate it..??

You can make the ports the same size with poppet port and transfer, then funneling it down to barrel port.
Some say its wasted volume but who cares it works.
I use 45deg seat on bigger valves, works just fine.

Marko

hmmm I cannot possibly imagine how that would look like.... is it some like cone shaped hole??

and also reading that 45 deg seat would work just fine... as stated..with more hammer energy..

[rsterne]

Wallace, you have summarized things quite well....

If the valve ID is too small, the gap between it and the OD of the poppet will not have enough area for full flow.... There is no penalty for making it larger.... the valve OD is governed by the tube of course.... and the minimum valve wall by structural considerations....

The exhaust port is the vertical passage drilled in the valve, that connects to the throat at the bottom, and the transfer port (and eventually the barrel) at the top.... The transition from the horizontal throat to the vertical exhaust port should be as smooth and unobstructed as possible, with nicely tapered and rounded corners.... I like angling the exhaust port forward 20-30 deg. to smooth the flow from the valve throat into it....

Bob

[MJP]

It looks like velocity stack in miniature. Going from let's say 8mm round holeto 4.5x7mm slot in 5mm distance.

Marko

[Fruit-Dove]

I see now.. to clarify a little...so volume of the 'past the poppet valve' (all transfer ports, valve throat, vertical exhaust port, barrel port, dim yellow, pic included) must be kept

minimum, thus cross sections of those ports are equal to the bore area...

and inside the valve, gap(area) between the poppet and valve ID, including the min area between poppet and valve throat seat(dim purple) should be larger than bore area for better flow..

The transition from the horizontal throat to the vertical exhaust port should be as smooth and unobstructed as possible, with nicely tapered and rounded corners.... I like angling the exhaust port forward 20-30 deg. to smooth the flow from the valve throat into it....

It looks like velocity stack in miniature. Going from let's say 8mm round holeto 4.5x7mm slot in 5mm distance.

maybe some quite like the 2nd pic(source:https://www.airrifle.co.za/threads/33894-PCP-flow-simulation) I guess??

 hmm lack of advanced machining experience makes me hard to imagine how one can even cut that round edges out... (maybe with ball end mill but what about the inner curve??) let alone angling the exhaust port..Im not even sure I understood that.. is there some kind of alien-tech drill bit available??

[Fruit-Dove]

none

[Fruit-Dove]

none

[rsterne]

Here is a drawing of the way I make my barrel oblong ports.... this for a .25 cal. with a barrel port 0.188" x 0.250".... to match up with a 0.222" transfer port....



The rear of the barrel port lines up with the rear of the transfer port.... The front of the barrel port is machined with an end mill angled forward, so that at the boreline the port is longer than the transfer port, but at the transfer port it is the same length.... Then within the thickness of the barrel wall the sides of the port are ground away so that the port transitions from a round transfer port to a narrower but longer barrel port....

The nose of my bolts are flat and retract from the loading position of the pellet to flush with the back of the barrel port where it is locked down for firing....

Bob

[rsterne]

If you double post, or want to change it, open the post using the "Modify" button, delete everything, and then type in the letters "NT" which means "no text".... That is the best you can do, you cannot delete an entire post....

Bob

[Fruit-Dove]

ooh so the exhaust port is actually a round hole with same bore caliber...I thought it was oblong hole from throat all the way through the barrel port.. 

but what about the transition, where the valve throat/exhaust port meets(red circle,pic included)? how can I round the inner curve of it??

The nose of my bolts are flat and retract from the loading position of the pellet to flush with the back of the barrel port where it is locked down for firing....

so thats the J-curve isn't it??  ehmmm just wondering, for optimal flow, shouldn't the bolt face also have to be curved(green)??

[rsterne]

The inside of curves in a pipe are much more important than the outside, that area is relatively stagnant, but is prone to some turbulence.... My "J-slot" bolt retraction is done like this....



The nose of the bolt is flat (no probe) to facilitate easy loading of any pellet or bullet to the same depth (just past the barrel port), be it flat base, boattail, or hollow base like a pellet.... The nose of the bolt is withdrawn flush with the back of the barrel port for firing....

I round the front upper corner where the exhaust port meets the throat with a small spherical burr in a Dremel, working by hand.... The flow is eased if the exhaust port is angled forward towards the valve seat 20-30 deg.... making the throat shorter and less wasted volume as well....

Bob

[Fruit-Dove]

The nose of the bolt is flat (no probe) to facilitate easy loading of any pellet or bullet to the same depth (just past the barrel port), be it flat base, boattail, or hollow base like a pellet.... The nose of the bolt is withdrawn flush with the back of the barrel port for firing....

so flat bolt-face is for versatility

I round the front upper corner where the exhaust port meets the throat with a small spherical burr in a Dremel, working by hand

careful dremel work was the answer... but how thin should it be to reach there??

The flow is eased if the exhaust port is angled forward towards the valve seat 20-30 deg.... making the throat shorter and less wasted volume as well....

?? hmm this is so confusing...please provide me some pics..thanks/