A question about exhaust port v. transfer/barrel port size

[Bullfrog]

My Mrod's custom valve has a .18 exhaust port but a stock transfer port and barrel port. My gun gives me 8 shots at or over 70fpe at or near a 1.0 efficiency. However, when I turn the power down to 50fpe my efficiency becomes .8 so I waste air when the power is down.

What would you expect to happen if my transfer and barrel ports were made to be .18 like my exhaust port? Specifically, what would you expect it to do for my air usage and numbers?

Edited to add: I should state that if I stay with heavy pellets then my efficiency will go above 1.0 when I turn the power down, its only when I try to use "normal" pellets like Kings that my efficiency gets out of whack which suggests to me my valve is staying open to long. Unfortunately I can't fix it with any adjustment I try.

[Motorhead]

Generally if the valves throat/poppet/body passage sizes are opened up the flow potential and power can be much greater.
Tho if the transfer tube between valve body and /or barrel port is smaller, it going to act like a throttle and choke the power down.
* No different than a factory valve with passage restriction screw blocking the exit path to some degree.

Can't speak of first hand .25 cal knowledge .... only state that in my own .22 modified gun, it runs an unrestricted .165" passage size throat to barrel.

Think as you reduce spring preload reducing power, the spring rate still remains and the Dwell of valve is eating air.
* Have you tried to Keep the power tune setting an DE-STROKE hammer travel to drop the power ?

[rsterne]

I assume you are talking about a .25 cal.... I would try and get the ports as close to the same size all the way through as you can (ie 0.18"), and insure that the valve throat area (throat minus stem) is at least that big as well.... something like a 1/4" throat to go with the 1/8" stem.... What usually happens when you increase the breathing on a PCP is that the power at a given pressure goes up, allowing you to get the same power at a lower pressure or more power at your previous sweet spot pressure.... If you make no changes to the hammer strike, generally the bell curve becomes a bit "peakier" (ie higher and narrower) at a lower peak pressure.... and the efficiency generally goes up because you get more power without using a lot more air.... If you back the gun down to the previous power level, generally the efficiency is a bit higher.... The only down side, really, is you may get a shorter shot string for the same ES....

Bob

[Bullfrog]

Yes, I'm sorry I didn't specify, I am referring to my .25 Mrod.

Motorhead, I've been running the gun set with a shorter hammer stroke. 7 turns clockwise was giving me a 3300psi fill. However, I discovered 2 days ago that I wasn't running my spring preload at full tension like I thought so I turned it another 3/4 turn CW for max preload. I then turned my hammer 1 turn CW for a total of 8 turns. Weather has been bad here most of the weekend so I haven't run a chrony on my new numbers. All I can tell is that I'm getting a lot more shots. I got 11 shots from 3200 to 2700 yesterday with the new settings. Unless I drastically lowered my fill pressure with the CW turn (which I would think was balanced out by increasing the pre-load), I would predict my power numbers are probably pretty close to what they were originally. Note that earlier this week I re-installed my Gaska air tank extension so that's one reason for the increased shot count but it alone shouldn't give me that drastic of a shot count increase. I figured with my previous air usage the Gaska extension would give me 2-3 more shots, unless my math was off.

rsterne, I think your advice is sound.  It hit me yesterday that the reason for my mysterious decreasing efficiency with decreasing power might be linked to the fact that I simply have miss-matched ports but I wanted advice from the experienced airgun smiths in case I was thinking about it wrong. Before I pull the barrel off to drill it out I'm going to run my numbers. If I can (or already have) increased my efficiency beyond 1.0 at 70fpe I probably need to leave well enough alone for a while until I absolutely can't help myself. I find that is what most of my air gun tinkering is, an addiction to making the gun as "best" as possible, whether best means power, efficiency, quiet, ect. I just can't leave well enough alone! If I can get 1 and a half clips all shots at 70fpe I should probably be thankful and stop. But by golly what if I can make it a clip at 80fpe? Arrrghhh my tinkering addiction will kill me ;-).

[cooter472]

Yes, I'm sorry I didn't specify, I am referring to my .25 Mrod.

Motorhead, I've been running the gun set with a shorter hammer stroke. 7 turns clockwise was giving me a 3300psi fill. However, I discovered 2 days ago that I wasn't running my spring preload at full tension like I thought so I turned it another 3/4 turn CW for max preload. I then turned my hammer 1 turn CW for a total of 8 turns. Weather has been bad here most of the weekend so I haven't run a chrony on my new numbers. All I can tell is that I'm getting a lot more shots. I got 11 shots from 3200 to 2700 yesterday with the new settings. Unless I drastically lowered my fill pressure with the CW turn (which I would think was balanced out by increasing the pre-load), I would predict my power numbers are probably pretty close to what they were originally. Note that earlier this week I re-installed my Gaska air tank extension so that's one reason for the increased shot count but it alone shouldn't give me that drastic of a shot count increase. I figured with my previous air usage the Gaska extension would give me 2-3 more shots, unless my math was off.

rsterne, I think your advice is sound.  It hit me yesterday that the reason for my mysterious decreasing efficiency with decreasing power might be linked to the fact that I simply have miss-matched ports but I wanted advice from the experienced airgun smiths in case I was thinking about it wrong. Before I pull the barrel off to drill it out I'm going to run my numbers. If I can (or already have) increased my efficiency beyond 1.0 at 70fpe I probably need to leave well enough alone for a while until I absolutely can't help myself. I find that is what most of my air gun tinkering is, an addiction to making the gun as "best" as possible, whether best means power, efficiency, quiet, ect. I just can't leave well enough alone! If I can get 1 and a half clips all shots at 70fpe I should probably be thankful and stop. But by golly what if I can make it a clip at 80fpe? Arrrghhh my tinkering addiction will kill me ;-).

sounds like you have the NHRA stock eliminator engine builder curse, the things that haunt us in our dreams

[cooter472]

I will be ordering mine this Friday and I'm already researching to make it a beast, Im a engine builder and do to the economy I closed, but this fits into my current buget and gets my blood flowing. Just hope the predictions I've heard on here that PA will offer a 10% off plus free shipping on valentines day.

[Bullfrog]

sounds like you have the NHRA stock eliminator engine builder curse, the things that haunt us in our dreams

The other side to it is that I've learned that power isn't everything. There comes a threshold where the killing power of the gun is enough and anything more is redundant. My deal is that I'm stuck with a high-power gun unless I figure out how to restore efficiency on low power settings. If I could ever do that then I can have an all-purpose gun... 70fpe for the big jobs and lots of shots at 40fpe for my backpacking trips and squirrel hunting.

As I reason it out to myself it makes sense that I should try to get my gun shooting as efficiently as possible even at low power and with light pellets to make the gun versatile. Yes, that is my justification. Why would I want a gun that only shoots 43.2 Eunjins or bullets efficiently? Surely my gun should be able to do it all. Therefore when I ruin this otherwise awesome 70fpe tune I can tell myself its for a greater purpose in the end. 

[cooter472]

sounds like you have the NHRA stock eliminator engine builder curse, the things that haunt us in our dreams

The other side to it is that I've learned that power isn't everything. There comes a threshold where the eliminating power of the gun is enough and anything more is redundant. My deal is that I'm stuck with a high-power gun unless I figure out how to restore efficiency on low power settings. If I could ever do that then I can have an all-purpose gun... 70fpe for the big jobs and lots of shots at 40fpe for my backpacking trips and squirrel hunting.

As I reason it out to myself it makes sense that I should try to get my gun shooting as efficiently as possible even at low power and with light pellets to make the gun versatile. Yes, that is my justification. Why would I want a gun that only shoots 43.2 Eunjins or bullets efficiently? Surely my gun should be able to do it all. Therefore when I ruin this otherwise awesome 70fpe tune I can tell myself its for a greater purpose in the end. 

I cant wait ot see that tune, I did locate a used A2A complete valve assembly farely reasonble, just wondering that is built into the valve already or if its been hacked and thats why hes getting rid of it

[rsterne]

It seems to me that with all the adjustments on an MRod, it shouldn't be difficult to achieve what you want.... I would start by cranking in the velocity adjuster (port restrictor) and leaving the hammer settings alone (assuming you have the pressure range where you want it).... I have a similar adjuster on my Disco Double, and it certainly does the trick there.... less power and way more shots.... Generally, with lighter pellets, you need less hammer spring preload for the same pressure range, of course....

Bob

[Bullfrog]

I've got my gun depressurized to change out the quick-connect fitting that's been giving me trouble and I'm down for the count until my new Hill pump gets in this week, therefore this would probably be a good time for me to give in to my cravings and drill the barrel port out.

To recap, my valve exhaust port is .18 but my transfer port and my barrel port are stock. I have a .18 transfer port that came with my valve. I just need to get the barrel port drilled out.

How should I do this? In my mind its simply a matter of taking the barrel off the gun, putting it in a vice, finding the right sized drill bit, at drilling out the port with a hand drill. Right or wrong? What material does my drill bit need to be? Also, how do I keep the shavings out of my barrel?

[rsterne]

You can't keep the shaving out of the barrel.... Drill one size at a time, using numbered drills if you have them, and being careful not to hit the top side of the bore.... Then carefully deburr the port on the inside with a needle file.... then clean the barrel before reassembly....

Bob

[Motorhead]

Use a VERY sharp general HS steel drill bit and feed it slowly so it cuts and does not create a burr.
In reality it will burr and your going to need to dress hole from bore side to remove it and make sure no sharp edge is there to shave lead from a chambered pellet.
* This best done with a small stone in the dremel tool, Breech o-ring removed.  Blow out barrel when done, shove a few patches threw and your good to go.

[wwonka]

and being careful not to hit the top side of the bore....
Bob

For those of us just beginning to try some machining, do you mind painting the picture a little more? Once you have the barrel in a vice, and you have the right drill bit on a hand drill - what does it mean "not to hit the top side of the bore" - how do you go about doing that?

[rsterne]

If you push on the drill too hard and it "bites" just as it breaks through into the bore, it can "pull in" and hit the top side of the bore.... I'm just saying to be careful....

Bob

[cambriajohn]

I like to put a wooden dowel rod in the breech end of the barrel as a safety so I can't drill too far. I prefer to use a drill press if it's available.

[wwonka]

OK - got it, understand now - you mean don't hit the opposite diameter of the bore-circle to where you're drilling from. Thanks for the tips...

[Rdsail]

Bullfrog,

One you make the ports the same ID your valve will breath much better and theoretically this should help with opening the valve faster and increasing your efficiency.

I would also open up the intake and the gauge port to deliver the most air to the valve. This will also help with closing the valve faster.

This are pictures of a valve I just finished of a friend.

Gauge block

Valve:




This is very similar to my valve. Which can generate 59.5fpe with 31g with a 12.5# spring at full preload.

[Bill G]

You say that the efficiency goes down with lighter pellets.  What weight had you been shooting before you went to the 25.4gr JSB?  In theory, a lighter pellet will leave the barrle more quickly. This could happen quickly enough to have the valve still barely open compared to the heavier pellet. In a nut shell, It isn't a fair comparison. I think Bob's thought on choking the gun down will result in the lower power output while maintaining or improving efficiency.  Also, I would have thought that the reservoir extension would have delivered more than a couple of extra shots even at 70fpe. I've never had one so I have no clue really.  I don't even know how much additional volume it gives.

Bill

[Rdsail]

In most of my test and observations, the more you choke a gun the more the efficiency goes down. The highest efficiency comes from delivering air as fast as possible for the shortest amount of time. Chocking the air flow does the opposite for both. Although, if you reduce power on the gun then you might have to choke it some to flatten your string. The strings tent to get a higher peak.

[rsterne]

The bottom line is, the efficiency is tied to where the pellet is in the barrel when the valve closes.... The closer it is to the breech, the better.... the closer to the muzzle, the more air you will waste for a given power level.... When you are attempting to get maximum USEABLE power, then you want maximum flow, regardless of the pressure, so that you can close the valve sooner while getting the power you NEED.... Heavier pellets, moving slower, will see the valve close before the pellet has moved as far, for a given valve dwell, hence increasing the efficiency (and usually the FPE).... However, you can also get high efficiency by simply closing the valve sooner.... That will reduce the velocity and energy, but almost always increases the efficiency as well....

The downside to wide open porting is that the shot string gets "peakier", just like putting a hot cam in an engine.... more power but over a narrower range.... This reduces the shot count within a given ES.... If you open the gun right up, and get it producing more power than you need, and then choke up the transfer port to get back to your required power level, you will initially increase the efficiency as the FPE drops.... Eventually, yes, the efficiency will drop again, but this method "smooths out" the shot string, giving you more shots within a given ES.... An extreme example of this is the Hatsan AT44 set up for the Canadian non-PAL rules (under 500 fps).... By restricting the airflow through the valve, Hatsan change a 30 shot, 30 FPE gun to a 170 shot, 6.5 FPE gun, while maintaining the same fill and refill pressures.... and at greater efficiency.... by "choking" up the flow....

Bob