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

[Bullfrog]

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

I shoot 43.2 grain Eunjins for max power and efficiency. If I tune them for 60fpe I can get 1.2 - 1.3 efficiency depending on how tight I have my specific tune. If I max them out at 70 fpe I can get .9-1.0 efficiency. All other pellets lighter than the Eunjins give me poorer efficiency. I regularly shoot Kings and Barracudas. Both of those will equal out at the same FPE, as opposed to the Eunjins which always give me more FPE. Generally the Cudas and Kings give me an efficiency of .8.

My idea of getting two more shots at 70fpe is just a guess on my part. I figured that if I have my gun shooting at 1.0 efficiency at 70fpe the extension is adding just enough cc space for 2 or more shots. Or to look at it even more simply, the extension is adding roughly 25% more volume to my tank, therefore I would get 25% more shots. If I can get a full clip at 70fpe, 25% more of that would be two more shots. Math isn't my strong suit, but nonetheless that's how I reasoned it.

Last week I shortened my hammer stroke to give me a 60fpe tune with Eunjins starting 3100psi. I got 21 shots with a 30fps spread starting at 3100psi. That's not bad, but it is very spikey. One clip at around 2800 will be at the top of the curve from the high 810s to the 830s. The remaining shots before and after the top of the curve hover at 800fps. More than half of my shots are between 800 and 810. So for hunting purposes I have a 20 shot tune, but for "show off" purposes I've only got 8 shots within 2%. That's no different than what I have when I set it for 70fpe it just starts at a lower fill pressure and uses less air.

Thanks for all of the info gentlemen. As I already have a spikey curve that is acceptable for 1 clip at either 60fpe or 70fpe, I don't see a disadvantage in making my ports all the same size. If it makes my curve fall outside of 2% within a clip I can always put a bstaley buffer back on it to try to flatten it out. I'll also try choking down the velocity port to see what that does for me. I'll probably need to remove the valve's velocity port screw. It is custom cut to curve with the valve when wide open instead of being blunt like the factory screw. Therefore I may put the factory screw back it in when I try choking it.

Finally, my intake is already very large and it is made so that I can take the entire head off of it if I want for maximum flow. The builder told me I would get about another 20fps if I did that. I'm not comfortable drilling the gauge block without having a backup on hand. I may order a custom one.

I'm this far into it, I might as well go for maximum air flow and not leave it half baked. I'll update with my progress. I'll probably break my gun down tomorrow. While I'm in there I'll take the valve out and take some pictures so you can have an idea as to how its built.

[Bullfrog]

Something else to add: I definitely notice better numbers and efficiency when my spring tension is jacked up to maximum. I use a 10# spring. I'm considering going to a heavier spring jacked up all the way and shortening my hammer stroke even further.

[CarsonRatSniper]

I am right where you are trying to avoid - I have an 80+ ft lb gun on cast bullets, but it is way too much for pellets!
Kings leave the muzzle at 1140 fps with ZERO preload on the top of a 3000 psi fill. Once I've fired 5-6 pellets there's no point in shooting pellets.

So I built a 30 ft lb pellet shooter specifically for JSB Kings.

I could detune my 25 Challenger Double Tube by using an unmodified RVA, factory Prod hammer and a Prod spring and probably get 40 shots on a 40 ft lb tune...............but the bullets I cast only cost me 1.3 cents each plus the time to make them.
I can't shoot pellets that cheaply!

Why don't you build a different gun? One gun wearing two distinctly different hats is tough without changing quite a bit as far as tuning goes.............unless you have a 'seasonal tune' that makes you happy for a few months each time you re-tune your gun.

[cooter472]

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.

Since were talking in engine theory now can you explain the holes you've added and enalrged and their purpose

[Bullfrog]

I am right where you are trying to avoid - I have an 80+ ft lb gun on cast bullets, but it is way too much for pellets!
Kings leave the muzzle at 1140 fps with ZERO preload on the top of a 3000 psi fill. Once I've fired 5-6 pellets there's no point in shooting pellets.

So I built a 30 ft lb pellet shooter specifically for JSB Kings.

I could detune my 25 Challenger Double Tube by using an unmodified RVA, factory Prod hammer and a Prod spring and probably get 40 shots on a 40 ft lb tune...............but the bullets I cast only cost me 1.3 cents each plus the time to make them.
I can't shoot pellets that cheaply!

Why don't you build a different gun? One gun wearing two distinctly different hats is tough without changing quite a bit as far as tuning goes.............unless you have a 'seasonal tune' that makes you happy for a few months each time you re-tune your gun.

I think you're right, I need 2 Mrods set up differently. Or I need to just detune this Mrod and then buy a stronger gun of another brand for when I need extra power.

Its easy to tune for summer time because the only non-bird animals I can legally hunt during that time are hogs, coons, possums, and coyotes, animals where the max fpe would be welcome (rabbits are legal too year round but I don't count them). When I'm done with this round of mods I'll be leaving the gun as is (hopefully) until fall. Its trickier in the fall because they I can also squirrels where shot count does better for me than power and too much power on the squirrels can actually be detrimental (I found this out shooting some squirrels in the shoulder this year with Polymags).

If I can find a Gen 1 gun for cheap I'll pick it up and make it a dedicated backpacking gun tuned for high shot count. Ultimately I want a backpacking setup that lets be pack out a bullpup .25 with a high shot count for head-shooting game with.

[Bill G]

Just a bit of perspective here.  At 70 fpe, you are delivering what a .22 rimfire pistol delivers and I'm good with that. But how much game do we generally hunt that requires that kind of energy, not much i'm guessing. 60+fpe guns are leaning toward specific application airgun. Just my .02 cents worth.

The spikey curve that you are getting is the norm with enlarger port guns.  You can choke it down and get it to level off a bit while maintaining the higher efficiency.  What do you say BOB?

Bullfrog, Are you using the stock hammer with that 10# spring?

[Bill G]

I was thinking that you may be able to lighten the hammer while choking the port a bit to flatten the curve.

[Bullfrog]

Bob, I'm using the stock hammer. I've never tried a lighter or heavier hammer.

I agree the extra power isn't necessary. My problem is that by the time I had that epiphany, I had already modded my gun to the point of no return and I've been unable to detune it to be efficient on lower power levels. I've shifted my focus from power to efficiency, yet I'm seeing that the way my gun is set up that I'm most efficient on the highest power. If 50fpe and 70fpe both use about the same 65-70psi per shot, I'm not having a big advantage in turning my power down. If I can work out this efficiency issue then I can make a decision as to whether I want to keep my gun at 70+fpe or whether I want to turn it back down to 50fpe for lots more shots.

Here's the progress I've made so far, see the attatched pictures:

1. The valve intake with the end cap on.
2. The valve intake with the end cap removed next to a factory valve. I will be running it without the end cap to see what happens. A2A estimated removing the end cap would add 10-20fps.
3. The valve exhaust at .18 next to a factory valve.
4. The valve exhaust next to my barrel port that I drilled out yesterday to as close to .18 I could.

Today I'll be re-assembling the gun. If my Hill pump arrives before dark and if I'd done with all of my obligations today I'll shoot a string.

I used a wooden dowel as a guard. Thanks for that advice. I also was able to file the jaggies down with a chainsaw sharpener and blow out the dust easy enough. It really wasn't a hard operation.

[rsterne]

Once you get "enough" air into the valve, and that includes through the gauge port, opening up the inlets more is unlikely to add much advantage, if any.... with a 3/16" exhaust port I would think the stock valve inlet is nearly big enough, and any of your other combinations should work just fine.... This would not apply to to a regulated gun, of course, where you have a very limited volume in the plenum between the regulator and valve seat, but there you are actually just creating more volume by removing material (eg. the front of the valve) not increasing the inlet side flow rate.... I pretty much figure if the inlets are all significantly larger than the outlets, you're looking in the wrong place to increase them further.... As an example, I recently converted a Disco valve to .25 cal, and it has 0.219" porting (or equivalent) on the exhaust side straight through, and the inlet side is only 0.266", slightly bigger than the 0.25" throat.... Even so, the gun hit 57 FPE with Kings at only 2000 psi, and will do a nice string in the low 900s, still staying over 900 at only 1200 psi.... If I thought going larger on the inlet would help, I would have done so....

Bob

[Bullfrog]

I've run into an unexpected problem. My stock transfer port orings now have an outside diameter that is too big for the valve's exhaust port when they are placed on the .18 transfer port. I've spent an hour trying to squeeze the stock orings as well my smallest SAE and metric orings on the transfer port into the valve's exhaust port with no success. Tomorrow after work I'll run by the local auto parts store and try to find some smaller outside diameter orings. Has anyone had this problem and find a solution?

[Mr.Bojangles]

I just want to say this thread has been very helpful. I just opened up my disco, and will probably do a post on it this weekend after i get some numbers. After reading this thread, i felt a lot more confident and well informed, so thanks for all the help!

[cambriajohn]

I've run into an unexpected problem. My stock transfer port o-rings now have an outside diameter that is too big for the valve's exhaust port when they are placed on the .18 transfer port. I've spent an hour trying to squeeze the stock o-rings as well my smallest SAE and metric orings on the transfer port into the valve's exhaust port with no success. Tomorrow after work I'll run by the local auto parts store and try to find some smaller outside diameter orings. Has anyone had this problem and find a solution?

I've been using 1/4" OD drip line hose. I found some at home depot that had 0.280 OD and an ID of about 0.153. Using hose you don't need a machined transfer port or o-rings, you just cut it a little long and compress it. I just cut it with a razor blade type knife in one single motion, if It doesn't come out just right I try again.  I'm just going to a bigger port on a different rifle and need an ID of about 0.266. I haven't found a good sized tube yet, but I expect I'll have to go to a .375 OD.

[Bullfrog]

I've run into an unexpected problem. My stock transfer port o-rings now have an outside diameter that is too big for the valve's exhaust port when they are placed on the .18 transfer port. I've spent an hour trying to squeeze the stock o-rings as well my smallest SAE and metric orings on the transfer port into the valve's exhaust port with no success. Tomorrow after work I'll run by the local auto parts store and try to find some smaller outside diameter orings. Has anyone had this problem and find a solution?

I've been using 1/4" OD drip line hose. I found some at home depot that had 0.280 OD and an ID of about 0.153. Using hose you don't need a machined transfer port or o-rings, you just cut it a little long and compress it. I just cut it with a razor blade type knife in one single motion, if It doesn't come out just right I try again.  I'm just going to a bigger port on a different rifle and need an ID of about 0.266. I haven't found a good sized tube yet, but I expect I'll have to go to a .375 OD.

That's an interesting idea. By the time I read it, I had already come to a solution, but had my solution not worked I would have tried your method.

To solve my oring problem I drilled out the exhaust port on the gun's body (the port on the gun itself that the valve's exhaust port sits under) to 9/32. This was an exact fit for my factory port orings when they are flailed out on the .18 transfer port. I didn't have to make any mods to the breech port hole that the barrel port sits on top of.

I got my gun together and pumped up this morning. After working with my hammer stroke I got it adjusted to a 3200 fill and here are my initial results:

43.2 Eunjins
3200psi fill
10# hammer spring
Spring at max preload, hammer stroke at unknown setting (I had to keep turning it ever so slightly to get it set for a 3200 fill and I lost track of the turns).

3200psi
884.2
892.2
884.4
889.4
893.8
902.9
902.0
896.9
2800psi
I stopped here, not knowing if I was on the downward curve or if I was still rising, yet I was happy with that initial clip of 8 shots for a hunting tune.

My tank size is 284.5cc (215cc stock tank + my Gaska extension that is worth 69.48cc). According to Lloyd's calculator (awesome app by the way), my efficiency is 1.28 for an average of 76.5fpe.

No doubt making the ports all the same size was worth it. As ya'll expected, I gained power and saved air. I'm very pleased.

I'm going to shoot some accuracy strings now to make sure none of my mods are causing problems. I noticed when I was manually loading the Eunjins that I was getting flecks of silver on my fingers. Not sure if its just a new tin of pellets or where they might be rubbing a jaggy in the breech.

[wwonka]

After working with my hammer stroke I got it adjusted to a 3200 fill and here are my initial results:

It would be interesting to see how that string ended below 2800 PSI.

I noticed you filled the Marauder to 3200. Is the maximum recommended fill 3000? If so, do you have any reservations about filling 200 PSI above the maximum 3000 recommended fill? Just asking for my own education...

EDIT: By the way, nice work all hand-held without a drill press

[Bullfrog]

I am comfortable filling to 3200psi but I don't recommend it to anyone. Its a risk I'm willing to take. At this time I'm not willing to go above 3200psi for a tune. I have experimented with tunes up to 3400psi and I've tried higher than that for giggles, but I felt nervous the entire time and decided it wasn't worth it after my initial experiments. 3200 is the upper limit as to what I feel is safe based on what I've observed others in the Marauder community tune to. But again, you're taking the risk on yourself if you go above Crossman's recommendation. Part of my reasoning to explain why I am comfortable taking the risk myself is that I figure Crosman makes the gun to actually be safe within a certain range within the recommended fill pressure. Crosman's air gauges on their guns and pumps are so inaccurate that they are begging for law suits if the gun will fail at 3200 psi, where their gauges are often off within 300psi or so. Its realistic for someone to have a gun from the factory that the Crosman gun gauge says is filled to 3000psi, when in reality its 3200 or 3300psi.

So if you're asking me whether I condone YOU filling to 3200psi, I cannot. I must recommend what Crosman does. For myself, I will fill to 3200psi and not think much of it. If it hurts me, then I assumed the risk myself. Myself I'm not filling above 3200psi anymore. Even putting safety aside, there's no need to. I can produce 70fpe at less than 3000psi as it is. I still prefer 3200 because it is a little bit more efficient at the higher pressure.

Its sort of like the you-know-what barrel devices that shall not be named. I'm actually an attorney and I'm comfortable in my own ability to read case law. I have confidence for myself that the you-know-whats are legal for airguns based on the Crooker case. Therefore I'm not very secretive about my airgun you-know-whats. Yet if you were my client and your asking me what's the legally safest thing to do, I would have to say stay away. Not because the law isn't on your side, it is actually. But the law doesn't always mean anything to the government.

So it is with over filling your gun, young grasshopper.  Do as I say, not as I do.

[wwonka]

Yes, you do sound like an attorney.  I don't rely solely on anyone's advice. I'm pretty independent-minded myself, thanks. I just like to ask a lot of questions to see what others are doing (and understand their reasoning) - so this is helpful.

[Bullfrog]

My accuracy tests today aren't good. All of my pellets of all sizes and brands are being thrown all over with the exception of Benji Domes and Barracudas, which are giving me dime sized groups at 30 yards with the occasional flier. One trend I'm noticing is that the fliers go away on the down side of the bell curve near 2400psi. Its too overcast today for the chrony. Some time this week on a sunny day where I have not much going on I'll turn the power down to test both lower power efficiency and accuracy. If my gun is now too hot for pellets, that's ok, I can live with that. I can either try to turn it down or I can go to bullets. I need to figure it out though because I have a fear in the back of my mind that I screwed something up yet unknown when I last modded it. It could also be that I fouled by barrel again, but I gave it a medium cleaning yesterday and it didn't seem that dirty.

Previously at 70fpe (with Eunjins, about 63fpe with other pellets) my gun was dead on accurate at speeds that conventional wisdom would say are at the border of being too much for pellets. It would seem odd to me that an extra 30-40fps made the difference between dime sized groups and lemon-sized groups.

[rsterne]

Looks like you have now found out how fast is too fast....

Bob

[Bullfrog]

Unfortunately I now think there's something else going on besides the pellets flying to fast (at least that being the direct problem). I figured that I could turn the spring tension all the way to do get enough reduction in velocity to tighten the groups up. I shot several groups at an unknown lower velocity, some of them were tight and others were not. I finally shot 2 groups side by side, one at max preload and the second at minimum preload, both with Barracudas, which have been grouping the most consistently good. My results were terrible. Basically the same spread at max velocity and the reduced velocity.

The gun is acting like it did when the barrel was severely fouled. I've put about 750 pellets through it since the heavy cleaning I did a few weeks ago although I've ran patches through it twice since then. Although it didn't seem dirty yesterday it didn't seem dirty before either. I think my next step is to get some fresh brushes and put a good scrub on it.

If this turns out to be the problem, this itself will be instructive. Previously I thought the heavy cleaning was needed simply because the barrel never got a deep cleaning in 2 years of heavy use besides patches and mild solvent. Now I'm thinking that this barrel fouls easily when the pellets run too hot. Others have suggested it. I never discounted it, but now it may be more drastic than I imagined. If I have time tomorrow I'll give it a heavy scrub and shoot some groups.

[bstaley]

Have you checked to make sure you didn't lose the o-ring in the barrel?