.177 B50 - Transfer port size?

[Scotchmo]

I have my new transfer ports and they appear to be the easiest way to tune down the power, but I don't think it is the best way. These thoughts are based on how I know things work. I don't have any previous experience with PCP guns so I may be missing something. The internal passageways appear to be 3.5mm and that was the diameter of the stock transfer port. If I leave everything else stock and use a smaller transfer port to reduce the power, I am introducing a pressure drop across the transfer port. That is not an efficient way to utilize the energy in the compressed air. A weaker hammer spring would have the same drawback. It would open the valve less and that would be introducing a pressure drop at the valve. The most efficient utilization would be to keep the ports as large as possible, open the valve fully, but reduce the the time that it is open. Open it faster and close it faster. I'm not sure how this will affect the "flatness" of the shot curve. Maybe it will be detrimental. But that is the next thing that I'm going to try. I don't really want to modify the hammer. A heavy (34 gram) steel spring guide rides inside the hammer. I could easily knock 25 or 30 grams off of that weight by using aluminum or plastic. I estimate the hammer weight to be about 104 grams (138 grams including the spring guide). The spring guide also controls the spring preload so that would give me something else to play with. I can easily reduce the preload. I can also increase the preload a maximum of about 0.24 inches before coil bind.

After the transfer port, the spring guide is the next easiest thing to change. I am not going to remove or adjust the hammer until I try the lighter spring guide. Tomorrow I will replace the 34 gram steel guide with a 6 gram plastic guide and reassemble with the stock transfer port. According to rsterne, that may not make much of a difference, but I am still going to test it since his data was not from a B50.

I just reread the B50 Technical book and a lot of this has already been tried. I just have to verify it for myself. I read that adjusting the hammer screw out reduces power and adjusting it in increases power. The power is reduced by reducing the kinetic energy of the hammer and thereby reducing the amount that the valve opens. I was thinking that the valve should bottom out when fired. Maybe that never happens and there is a reason that they don't want it to happen. I don't know yet.

Another observation - If the power screw is turned in past flush, then the face of the hammer becomes the limit on how far the valve opens. The valve stem will now project into the hammer and the valve cannot be fully opened. So at some point turning the power screw in further will reduce power.

[John B]

 Man everyone here is a tinker type. I understand the fun but the book + one or two other posts just might be the way to go.
 The "light" spring guide will turn out just as it did when others tried it. But want to tinker, use a section of arrow shaft and plastic washer/spacers, you will not get where you want to be but you will tinker. It's darn easy to pick the right spring, it is the one listed, many many were tried.
 That light Mc spring is THE way to go. The lightened hammer is NOT the way to go, but some of the best in FT (not me folks) and at least one very good engineer tried-didnt get the result desired- fun tinkering I reckon.
 Now I cant keep track of who's heading where here but.
The fill pressure is fine where it is?  Well okay, but, the lower you have to fill for the same or more shots(&or power if that's where you are going) the better to me.
 Now , reducing the valve travel is a nice idea (I never went there). Sure enough you do NOT want it to bottom out as it will #1 not work well, #2 break the stem. Shortening the stem is one option. If you can make your own stems, and make a new valve body this is the idea to spend the most time on.
 So my set up is.
Bam50 .22 McSpring with a washer 1/2 the depth of the space inside the end cap(no tools here buddy, just a yard stick), 2mm off the nose of the hammer, pip screwed flush, there "might" be  2.8mm TP in there (cant recall but just slightly smaller than stock), POLISHED hammer run dry, trigger work-does effect fps spread-, fill 2500psi for 30 shots at 20fpe +/- 20fps using 15.9g JSB.
 Now if you want to tinker just for fun (hey and if you get it all right you will really have a winner) just do all this:
http://www.airgunadvice.net/viewtopic.php?t=7554&postdays=0&postorder=asc&start=10&sid=22708098bd8b9a521a4cf661f06b8477
 It takes 5 minutes to tune a stock one, likely 5 months to perfect the system like Chris did.
I thought the market was saturated or I would have purchased a case & sold them tuned (&crowned). But, it's all fun.
 Which one was the FT shooter? (I shoot my .22 in FT), grab that spring and you are 90% done.
John

[rsterne]

I don't know where to start, so I'll start here....

The most efficient utilization would be to keep the ports as large as possible, open the valve fully, but reduce the the time that it is open. Open it faster and close it faster.

I would agree with that statement in theory.... but I think the only way to achieve that is with an electronic valve like Lloyd used in developing the Rogue.... The reason I say that, is that in a "smack open" system, the amount the valve opens changes in proportion to the pressure (yes, I think it's nearly linear).... I just measured that using an O-ring on the cocking rod on my 9mm Disco and got the following (from that post).... The valve can physically open 0.160"....

At 4.5 turns out and 2400 psi (shot #1) the valve opens about 0.060", increasing to 0.090" at 2000 psi (shot #3) and about 0.120" at 1600 (shot #5).... It's interesting to see the "balancing act" of hammer hit vs. pressure in action....

Incidently, Lloyd got similar results with with dual tube Disco.... the valve opened 0.076" at 2200 psi and 0.150" at 1600 psi.... Let's say I limited the amount the valve opens to 0.060" (which would give the same velocity I'm getting now at 2400 psi).... As the pressure drops, the velocity will decrease as there is less pressure to force air from the valve.... Heck there is even less air (mass) contained inside the valve.... IMO that means that the idea of having a constant valve opening is out the window unless you have a regulated PCP.... In a regulated gun, it would be a completely different matter....

There are only two choices to decrease the amount of air exiting a given valve at a given pressure.... You can decrease the lift, or the dwell (or both).... If you then reduce the pressure (during the shot string), and want to keep the same amount of (mass) airflow, then you have to increase one (or both) of those factors.... Assuming the efficiency stayed the same, it would take twice the volume of air if the pressure was half.... Since the pressure is half, however, you would need MORE than twice the amount of air, because there is less pressure acting through a longer time on the pellet.... This decreases the efficiency, which is proven by the increase in psi drop and noise as the shot string progresses....

I like the way you think, Scotchmo.... and I respect the fact that you won't take anything for granted.... Like me, you are not satisfied with understanding HOW something works, you have to understand WHY it works that way.... and Kudos to you for that.... The simple "smack open" valve system in non-regulated PCPs is a perfect example of the KISS principle in action.... Unfortunately, the numbers of variables is large, and the interaction of those variables is complex.... As an example, changing the weight of the hammer, in theory, should make no difference, as a simple calculation shows that the Kenetic Energy (E= mv^2/2) doesn't change for a given spring setup... The lighter hammer accelerates faster, but the energy at impact with the valve stem is the same.... What IS different is the Momentum (M=mv) at impact.... Practical experiment has proven to me that reducing the hammer weight reduces the velocity of the pellet.... and yet the calculated KE of the hammer is the same in both cases.... Just one more thing to ponder....

Bob

[Scotchmo]

All good information. spysir and rsterne, you both confirmed something that I suspected. That the amount of valve opening may be the "regulator". And it never bottoms out and you don't want it to. The regulating nature of the "smack open" valve depends on the valve opening more as the pressure drops off. I was originally thinking that as the pressure in the gun dropped off, the fully opened valve just closed slower (longer dwell time). Maybe that is partly true. Again, lots of variables.

True, changing the hammer weight in theory should make no difference on how much the valve opens. The hammer spring controls the kinetic energy and therefore the amount that the valve opens. But a hammer with a lower mass should have a shorter dwell time. As the valve tries to close, it will accelerate the lighter hammer rearward at a faster rate than it would with a heavier hammer. Less dwell time = less air = less FPE.

My plan now is to make two lighter spring guides. One 6 gram with stock preload. One 4 gram with minimum preload. And test them individually. I suspect that I will end up using the light spring guide with the reduced preload and then make the final adjustment with the transfer ports. I may end up with a slightly lower fill pressure but I'm OK with that.

I'm leaving for the shop right now. The tinkering is half the fun!

[rsterne]

The lower pressure may indeed close the valve slower, adding dwell time.... However, measurements definitely show the valve opens further as the pressure drops.... and I think that is the major regulating action taking place.... I hadn't thought about the lighter hammer being closed faster by the valve, that could account for the reduced velocity and energy with a lighter hammer.... If might also account for why very large changes in hammer weight don't change the power of the gun a lot....

One thing to watch out for by going for a lighter hammer strike is an increase in the curvature of the shot string along with the reduction in the velocity and pressure at the peak velocity.... compared to keeping the hammer strike the same and restricting the porting.... Think of this as the opposite of what occurs if you leave the hammer alone and modify the porting to allow more airflow.... In that case, the power goes up some, the pressure at peak velocity goes down a bit, and the shot string becomes more curved (and therefore shorter).... Just a thought....

Bob

[Scotchmo]

New results.

I made two spring guides today. The far left guide is the stock guide, (STEEL 38.8 grams). The middle guide is a plastic copy of the stock guide (HDPE 4.9 grams). The far left guide is another plastic guide with 0.3 inch less spring preload (ABS 3.2 gram). I decided to go with the ABS guide since other peoples experiences implied that the lighter weight by itself makes little difference in reducing fpe but reducing the spring energy can help a lot.

Stock total hammer weight = 143 grams
New total hammer weight = 107 grams
25% reduction in total weight

Stock spring average deflection = 1.275 inches
Less preload average deflection = 0.975 inches
24% reduction in spring energy

I ran the test with the stock 3.5mm transfer port. The new spring guide made a big difference. I filled to the previous mid range of the sweet spot at 2500psi.

CPH 10.5gr - 2500psi
458fps
490
What? - I checked the gun over and it looked ok. I shot 10 shots total and rechecked the remaining fill. 2450psi. That is only 5 psi/shot. But it is also less than 6fpe. I bled the gun down to 2200psi and continued:

CPH 10.5gr - 2200psi
749fps
748
....778fps - Still climbing. Kept shooting. After 18 shots, it is shooting around 820fps. And now has 2050psi remaining, 8psi/shot and better fpe. Shot another 27 shots and things started to peak. I rechecked the pressure and it was now 1750psi.

CPH 10.5gr
918
1750psi - I think the shot curve peaked right here.
937
934
924
919
925
920
923
930
927
918
916
909
899
915
907
908
904
898
895
897
890
893
880
882
880
878
1340 psi

So I'm in the ballpark. If I run my gun from 2000psi-1500psi, I should get about 30 shots with about a 920fps mean and an SD of 10. Only four of the shots were a little over 20fpe. I also tried the 8.44gr JSBs and they are shooting a little too fast. If I decide to use the 8.44gr, I will put in a smaller transfer port.

920fps with CPH 10.5gr is right where I want it to be. And the lower fill pressure means that I can fill from my son's Discovery scuba tank setup. I think I will just leave it with this most recent modification. For field target use, I guess that I can just fill it often to keep it near the sweet spot. If I'm just plinking, I can get 100 shots ranging from 750fps up to 930fps and a fill going from 2200psi-1100psi.

I started out wanting to change transfer ports but ended up just changing the spring guide. Result is 25% less hammer weight, and 24% less hammer spring energy. Giving me about 17% less fpe at a 2000psi fill and 1750psi sweet spot. It took a 50% reduction in the transfer port to achieve the same thing and that was with a 2500psi sweet spot. The spring guide modification seems quieter than the transfer port modification. Maybe the smaller transfer port would have given me more shots in the desired fpe range. I don't know since I did not shoot an entire string with the smaller transfer port.

It would be nice to be able to flatten the shot curve some more, but maybe it is just not possible without going to a bigger tank or regulated air. I'm done for now. Now I need to shoot some targets.

Thank you for all the help and feedback

[darryl]

Outstanding!!! You've certainly advanced the knowledge base of the B50. but now you have me wondering about the impact of that lighter spring guide and a smaller TP. I was bout to go off trying to find a softer spring, but your solution seems more elegant. Nice work. Congratulations. I see where you could easily increase power by adding a washer. Impressive.
darryl

[Scotchmo]

I may modify my conclusions some as I learn more but here is what I got:

Transfer port - a smaller transfer port will decrease the fpe. Fill pressure stays about the same. The pressure drop across the smaller port would cause a lower efficiency so shot count may not increase as much as you would expect, but it may have an equalizing affect on fpe. So shot curve may be flatter.

Hammer spring - less energy in the hammer spring will decrease the fpe. Less preload or a weaker spring. Fill pressure is now lower. Shot count is about the same as stock with proportionally lower fpe. The spring guide can be modified to reduce or increase the preload.

Hammer mass - lower mass means lower fpe. It reduces valve dwell time. Shorter lock time is considered a good thing for accuracy. It takes a large mass reduction to make much difference. Fill pressure should stay the same, efficiency should stay the same. So shot count should go up proportionally as expected. The spring guide is about 28% of the hammer mass and is easy to swap out.

Hammer stroke - I did not try this change but did some reading in the B50 Technical Details. Adjusting the hammer screw can shorten hammer stroke and reduce the fpe. I'm not sure what happens to fill pressure. Efficiency probably stays the same. Shorter lock time.

If you want to reduce the fpe and keep the same fill pressure, the easy change is reduce the transfer port size.

If you want to reduce the fpe and want a lower fill pressure, the easy change is reduce the hammer spring energy.

All this might be common knowledge to the PCP experts. If I had not experimented, I would have done the transfer port change and left it at that. I prefer the preload and mass change that I ended up doing.

[Scotchmo]

... I see where you could easily increase power by adding a washer...

You could add washers or change the spring guide to increase the spring preload and hammer mass to be greater than stock. That should increase the fpe over stock. But that assumes that there is room for the valve to open some more.

[john]

You talked to several scenarios there and in each you mentioned "fill pressure." Do you really mean the "optimal" fill pressure? And isn't the optimal fill pressure the one that gives the most shots with the least change in fps? Just trying to get a better understanding.

[Scotchmo]

You talked to several scenarios there and in each you mentioned "fill pressure." Do you really mean the "optimal" fill pressure? And isn't the optimal fill pressure the one that gives the most shots with the least change in fps? Just trying to get a better understanding.

You are correct except you might have to change it to be "the most shots within your acceptable tolerance". That tolerance will vary depending on the shooter and circumstances. The "fill pressure" can be anything above the "sweet spot". The "sweet spot" is the pressure at which the shot string velocity peaks. If you only fill up to the "sweet spot" pressure, the velocity will start to taper off on subsequent shots. So you decide on a "fill pressure" that will allow you to use both sides of the shot curve. The flatter the curve, and the more velocity variation that you can tolerate, the more shots you get. Some of the changes that I tried moved the sweet spot. Stock, the "sweet spot" was at about 2500psi. Now it is about 1700psi. Stock, my "fill pressure" would be about 2800psi. Now it is about 2000psi. The number of shots within an acceptable deviation remained about the same. 30 shots for most of my shooting. If I widen my allowable tolerance, I might use a higher "fill pressure" and get 100 usable shots. I'm bracketing a wider portion of the shot curve. For target shooting, I might use a lower "fill pressure" and only get 16 usable shots. I'm then bracketing a narrower portion of the shot curve.

Fill pressure - how high you choose to fill the gun, determined by you.
Sweet spot - peak velocity in shot curve, determined by gun.

[Scotchmo]

I did some more changes and more tests. One conclusion that I reached earlier from limited data was that a change in the transfer port size did not change the sweet spot of the fill pressure. It might in fact change. I ran another test with the the smaller 2.4mm transfer port, the lighter hammer weight and less spring preload. I also switched to a lighter 7.4 grain pellet so some of the changes may be from the lighter pellet. It was proving too difficult to get the velocity that I wanted with the heavier pellets and still keep consistently under 20fpe.

24fpe - stock - 10.5gr pellet, 1.72 FPE/cuin per shot for middle 30 shots (2500-2000psi)
20fpe - 2.4mm port - 10.5gr pellet - did not test a full string
20fpe - less preload, less weight - 10.5gr pellet, 1.43 FPE/cuin per shot for middle 30 shots (2000-1500psi)
15fpe - 2.4mm port, less preload, less weight - 7.4gr pellet, 1.33 FPE/cuin per shot for middle 30 shots (2100-1700psi)

The attached image is the ChairGun statistics plot of the last string.

[darryl]

Looking very good Scotchmo. I'm having a lighter spring guide made. I hope to emulate your approach, but I want the heavier pellet and a more consistent shot string (Sd) than I've been getting. As you, I've found I cannot reduce power the way i want to without reducing the spring pre-loading.

[Scotchmo]

Looking forward to the results.... I hope you test all of them and draw us a graph.... 

Bob

I remade the transfer ports to more exact sizes and tested three of them. Graph is not too clear but it looks about the same as the trend on your stem diameter graph.

.177 B50 with light hammer spring guide and less spring preload:

3.5mm - 20.0fpe
2.8mm - 19.4fpe
2.6mm - 18.3fpe
2.4mm - 15.0fpe

[rkr]

Did you get different shape of a shotstring curve with smaller port or was it the same with just less velocity?

[rsterne]

The last two points are too close together to give an accurate trendline.... but you are getting close to a linear function that would go through zero FPE at zero transfer port area.... just like the lower section of my stem diameter/area graph.... I think that for further reductions in area, you could make pretty accurate predictions based on anything smaller than 2.4 mm (ie 4.5 mm^2 = 15 FPE).... As an example, a 2mm port should give about 10 FPE....

Bob

[Scotchmo]

This was originally about transfer ports. But, after much experimenting, I'm back to the stock 3.5mm transfer port. I made other changes to reduce the power. There are some valid uses for reduced transfer ports. If you don't want to tinker, do what John B said earlier in this thread. Here is the new thread for my changes and results:

http://www.gatewaytoairguns.org/GTA/index.php/topic,26326.0.html