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renderbrandt · New Member

Ok, a little bit of history of my thinking and background.
I've been doing high end turbo charged engine heads, and flow parameters in such. Although not entirely the same, some things does apply, turbulent flow, air velocity, pressure drop issues, etc.

So let the discussion begin, I would love to hear your comments.

Here is a picture of a normal bolt setup. The pluses are simplicity of design and application. Downsides is pressure shockwaves and turbulence which will hurt consistency in air delivery.

My idea, I call it Curved Port Bolt.
I also believe that the bigger bolt provides a better way to position the pellet into the barrel. As well as keeping the air velocity "higher" with the same pressure, it will never be laminar because of the valve system, however smoothing the flow just before it starts to push on the pellet should make the air delivery on the pellet more consistent.

Alstone · Moderator & Site Supporter

Hi renderbrandt, your idea is very similar to the bolt on Qb78/79 rifles plus others. The end probe is hollow and is a good fit in the breach it is fed from a hole just in front of the O ring, this concentrates the air flow behind the pellet. Plus some shooters alter the valve so there is a swept port on the valve exit.

Al

rsterne · Moderator

Alternately, you can simply have the end of the bolt flat, with sufficient travel to push the pellet past the transfer port, and then after rotating to the "lock" position, the handle retracts into a "J" shaped slot, withdrawing the nose of the bolt to the back edge of the transfer port, completely eliminating any obstruction.... the "probeless" bolt as used in my .30 cal Disco Double....

http://airgunhome.com/agforum/viewtopic.php?t=9296

Bob

renderbrandt · New Member

I like this solution over the "pin" obstruction. However my thought goes back to the valve throat areas in engine heads.
You want the airflow to not use a 90 degree wall to change it's direction. Especially a parabolic shaped one, which the roof of the barrel will look like from the port. This will sharply shape the shockwave back towards the port, and not towards the pellet.

Granted my understanding of all the details on air delivery from reservoir - valve - port - barrel. Is theoretical in nature, and not hands on. So I respect your experience and insight.

But at least from a pressurized volume standpoint, turbulent air is unstable and can vary in its ability of delivering the goods. How much it is in real life scenarios, is probably something that needs testing.

But keep this fun fact in mind, air has it's own momentum dictated by it's mass & speed. If you can guide it smoothly towards the pellet, instead of abruptly try to change its direction (hitting the topside of your barrel) then efficiency and power would benefit.

rsterne · Moderator

I think you will find that when you try and implement your design, you will quickly run up against the limiting factor of port area constricting the flow.... My recent PCPs are all running bore area porting straight through, and if you are searching for maximum power, I haven't found anything else that comes close....

If you make the transfer port more than about 75% of the bore diameter, you will have trouble loading the pellets without damage.... The answer is to make the port oblong.... ie for a .25 cal, the barrel port ends up 0.188" wide by 0.312" long at the boreline.... That tapers to a 1/4" round transfer port, the exhaust port is also 1/4", and the valve seat, assuming a 1/8" stem, is larger, about 0.281" or slightly larger, so maintain the same area as the 1/4" bore straight through.... The difference between this and a port of 75% of the bore area (eg. 3/16") is quite spectacular in terms of the maximum FPE that can be obtained at the same pressure....

With the flow-through design you are suggesting, it is difficult to go larger than 75% bore area or the front nose of the bolt gets so thin it is prone to damage and also to damaging the pellet.... Even at 75%, you will virtually lose any "smoothing" that might be possible with a smaller diameter (curved) port through the bolt.... It quickly becomes a matter of which is better, a small, smooth passage, or a large (albeit blocky) one....

As an aside, I have done several experiments with flow-through bolt designs, and in each case I have obtained more power with the conventional extended probe style.... even when working with 75% boresize transfer ports.... Once you go to "bore size" porting, I see no other choice (for a valve under design) than the retracting (probeless) style to obtain full bore area for maximum flow....

Bob

renderbrandt · New Member

Bob,

That was my concern as well, you might end up leaving to much power on the table in exchange for better air delivery.
Really good information though.

Out of curiosity, have anyone tried an angled exhaust port, say 30 degree, using an oblong exhaust port?
I know the port would by it's geometry be a little longer, so you might loose too much pressure filling the longer port.

Thanks for your informative insight.

Regards

Tom

rsterne · Moderator

I angle the exhaust port in my valves about 20-30* aft from the seat to the transfer port.... The transfer port is then vertical.... The barrel port is vertical on the back edge and angled (to get the length) on the front edge.... The combination of those angles helps smooth the flow.... I also smooth the flow between the valve seat and the angled exhaust port, just like doing a "port and polish" job on a cylinder head....

As you suspect, keeping the wasted volume between the valve seat and the pellet as small as possible is important, as any extra volume decreases the pressure....

Bob

Alstone · Moderator & Site Supporter

There is a difference between air that is expanding at high pressure and that which is just flowing at low pressure, when you deliver a shot of air at 1000 psi plus into a chamber it doesn’t flow it expands in the best direction it can escape, the time taken for this to happen is infinitesimal and will have little or no effect on the pellet, by the time the pellet has engaged the rifling it’s on it’s way with 1000 psi all going the same direction.

So I don’t see how different types of probe can make any difference at these small calibres. I have taken readings using a chronograph and a bolt with changeable probes, and to be honest it didn’t seem to make any difference what was on the end of the bolt, what did make a difference was the distance between the inlet hole and the pellet, if the pellet was to close to the hole the velocity did change but ¼” further down the barrel and things stabilised, I think this was because there was initial room for the gas to expanded before having to move the pellet?

renderbrandt · New Member

I get your point, and I don't disagree that expansion vs low speed flow is different. But in this case we are not talking about an expansion as in an explosion.
You got a valve, port chambers and delivery port. which the (expanding) gasses travels trough. There is no static position from where the expanding gas pushes on the pellet.
So flow is involved, but like Bob was explaining, in practical utilization on small caliber guns, the drawbacks on directing the flow (hollow bolt design) outweighs the benefits. Namely total deliverable volume across the system.

AirGunEric · Site Admin

I would tend to go with Al's thinking on this- if we are talking a "high pressure" flow indeed- if Co2, perhaps only 800psi, if PCP, perhaps 3000psi- all others somewhere in-between. Even a turbocharged engine only pushes 8-20psi of boost.

As Bob noted, he saw gains in a .30 caliber setup, and based on previous experiments from Bob and others, changes to the bolt/valve port really only assisted in a noticeable way when the caliber got larger and the setup was better "tuned" i.e. valve flow vs. barrel length, caliber and pellet weight.

My idea here is that the valve and bolt setup are really the last thing to look at as an "improvement" once all other avenues have been exhausted, or the intent it to tune the valve/bolt setup to match the other parameters- which could also go in the opposite direction as well- i.e. restricting flow.

rsterne · Moderator

The biggest gains I have seen to date with increasing the port areas to full bore size are in .224 and .257 cal.... In fact I have not yet achieved that in .30 cal or larger.... My Hayabusa PCP in .224 cal is pushing a 41 gr. bullet at over 1000 fps (it peaked at 106 FPE) and in .257 cal will push an 88 gr. bullet at over 900 (over 160 FPE).... both numbers at 3000 psi.... I wasn't able to achieve even close to those numbers with a 75% port.... To get the big numbers, AIRFLOW VOLUME is everything.... Once you max out the port size equal to the bore, then the route to even more power is more pressure (eg. 4500 psi)....

Bob