Author Topic: .177 B50 - Transfer port size? (Read 739 times)

Scotchmo · « **on:** January 25, 2012, 10:21:32 PM »

I finally got a chance to work on my B50. It is currently shooting 10.5g CPH at about 1010 fps. I want to tune it to shoot at about 920-960fps with 7.9, 8.4, or 10.5gr pellets. Whichever gives me the best accuracy. I can make transfer ports on my lathe. The port that I pulled out has a 3.5mm diameter hole. That seems fairly large. I plan on trying a 2.8mm port. I don't have any experience playing with PCP guns yet. Does the 2.8mm sound about right? Is the change in FPE about proportional to the change in port area? If so, the new port should push the 7.9gr pellets at about 930fps.

I'll probably make two ports. A 2.8mm and a 3.0mm.

Gary · « **Reply #1 on:** January 26, 2012, 12:20:48 AM »

Funny you should ask this as I just put two transfer ports in the mail for a bud of mine that has a B-50 in .22 cal that with the stock port is really flinging those 18 gr. JSB pellets. Up around 1000 fps or so and he wants to calm it down a bit and save some air at the same time. One is .093 "or 2.37 mm and the other is .125" or 3.18 mm. He has tried one at .112
' or 2.83 mm. and he felt that that one was a bit small. The reason I sent the .093 one was that he could adjust it up in size if the .125 proved to be to large. He might even chime in here as soon as he gets them and runs some pellets over the chrony.

Gary

Scotchmo · « **Reply #2 on:** January 26, 2012, 12:37:36 AM »

I just checked the passageway in the breech and in the valve. Both are 3.5mm. So the 3.5mm transfer port that came in the gun was essentially maxed out. I was getting 24FPE+ during the middle of the shot string. After doing some reading, I'll forgo the 3.0mm port and try 2.6mm and 2.8mm. I'm hoping to be in the 16fpe-19fpr range with those.

I would still appreciate any feedback on transfer port sizes in the .177 B50. Everything else in the rifle is stock. I did read about changing the hammer spring, but that seems to reduce the fill pressure. The fill pressure is fine where it is at right now. I may change that later.

rsterne · « **Reply #3 on:** January 26, 2012, 01:28:17 AM »

I can't help with the actual size.... but restricting the transfer port size will have an added benefit.... it will flatten the shot string, and therefore lengthen it.... Initial changes to the transfer port diameter will have little effect, but eventually the effect on the FPE is dramatic once you really strangle it down.... I changed the TP diameter from 0.140" down to 0.109" in a gun that shot 22.3 FPE and it only dropped to 19.9 FPE.... but the shot count went from 9 shots to 11.... The same gun, tuned for 17.1 FPE with the 0.140" port shot 15.7 FPE with the 0.109" port.... but the shot count went from 15 shots to 20 shots.... 0.140" is 3.56mm and 0.109" is 2.77 mm so these sizes are very close to what you are talking about.... The gun I was working on, however, only filled to 2000 psi (less in the case of the 17 FPE tune)....

Bob

Scotchmo · « **Reply #4 on:** January 26, 2012, 02:01:58 AM »

rsterne,
OK. If I make them too big, I'll have to make new ones. So now I'm thinking maybe a 2.2mm and a 2.4mm. I can just drill them out if they are too small. If the process goes fast on my lathe, I may just make a bunch of different sizes.

From your information, the port area is not be proportional to the FPE. Do you know how big the passageways were on your breech and valve? Could they have been smaller than .140?

rkr · « **Reply #5 on:** January 26, 2012, 05:14:01 AM »

12 fpe Scorpions have used 1.8-2.4mm - new ones have 1.8mm with stiff spring and lighter hammer.

rsterne · « **Reply #6 on:** January 26, 2012, 02:34:16 PM »

All the other ports were larger than 0.140".... the Transfer port was the smallest point.... especially once I restricted it.... Here is a chart I did on a Hatsan AT44 where they change the valve stem diameter to vary the FPE....

You can calculate the area of the space around the stem if you care to.... I would guess by looking at the graph that the hole in the valve seat is about 0.160".... at least that is where the FPE would approach zero on the graph.... The dots are actual measured FPE levels.... at constant (generous) valve lift.... At a quick glance, it looks like once the FPE starts to decline rapidly it is proportional to the TP area....

Bob

Scotchmo · « **Reply #7 on:** January 26, 2012, 04:07:24 PM »

rsterne,
That makes sense. When the transfer port is large, it may not be the main thing that is restricting flow. As we reduce the transfer port size, it starts to become the primary restriction. If the other restrictions become negligible, the FPE should be proportional to the transfer port area. I don't know if I will test that theory in detail, but it is nice to know what is happening when I make any changes.

Ribbonstone · « **Reply #8 on:** January 26, 2012, 05:42:59 PM »

For transfer ports, think area of a circle rather than just the diameter of the hole, and the change is a bit more predictable.
.15 = .0177
.10 = .0079
.075 = .0044

rsterne · « **Reply #9 on:** January 26, 2012, 06:01:51 PM »

We are using the area in our calculations.... and I think that may be valid once the port gets small enough.... However, the relationship is not linear (relative to the area) once the port gets big enough.... even though it is still the smallest point in the system.... I presume this is because there is still a difference to the total resistance to flow between having just one choke point (the transfer port) or several points which, although larger than the transfer port area, still restrict the flow to some degree....

It's like when you bore out and smooth out the ports in a valve.... Even though they are slightly larger than the transfer port, there are still some gains to be made.... just not as much as if the rest of the passages were already larger.... It always pays to work on the smallest area (with the greatest restriction) first....

Bob

Scotchmo · « **Reply #10 on:** January 26, 2012, 08:13:45 PM »

I made some transfer ports to try. I attached some photos taken with my iPhone so excuse the quality. I plan on trying the 2.4mm (2.39) port first. My selection of drill bits in those sizes is limited. I'm always breaking those small bits. I used the bits that I had left in my index. That is why the sizes may look a little odd. Time to get a new set of drill bits.

The first photo is my trusty 1928 Southbend lathe.

The second photo is a transfer port being cut.

The third photo shows the original and four new transfer ports.

I will post results after I get a chance to assemble and test the B50 with the new transfer port.

rsterne · « **Reply #11 on:** January 26, 2012, 11:21:26 PM »

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

Bob

darryl · « **Reply #12 on:** January 27, 2012, 01:03:57 PM »

good luck Scotchmo. Looking good with the TPs.
darryl

Scotchmo · « **Reply #13 on:** January 27, 2012, 06:04:18 PM »

First results of new transfer port:

Stock:
3.48mm port
9.51 square mm
CPH 10.5gr
1026fps
24.6fpe

New transfer port:
2.39mm port
4.48 square mm
CPH 10.5gr
934fps
20.3fpe

I shrunk the transfer port area by 50% and only got a reduction in FPE of 17%. However, the new 20.3fpe peak is close to my goal. I can try the 2.24mm port next and it should get me under 20fpe. The readings were taken at the peak of the sweet spot (2400psi) where I averaged the four shots that had the highest readings.

I'm starting to think that shortening the hammer stroke may be the better route. The gun is still too loud. It sounds like a lot of extra air is blasting out the muzzle after the pellet exits. I think that the shorter hammer stroke will allow a shorter burst of higher pressure air. Maybe that will be a more efficient use of the energy in the compressed air, rather than choking it off with a smaller port.

darryl · « **Reply #14 on:** January 28, 2012, 11:40:37 AM »

How many shots are you looking to attain per fill? I'm testing various TPs with the goal of 30 shots per fill where I can get as many to fall within 4% of the fastest recorded shot, along with the smallest Sd across those shots. I'm looking at the shot curve to see how flat I can make it (because I hope that will be reflected in the smallest Sd, and so -maybe- best accuracy). Your idea to reduce the hammer stroke will help you. I have a friend who thinks I should look at a lighter hammer. I think he's 100% correct for my goal.
keep it coming! Fun ain't it?

rkr · « **Reply #15 on:** January 28, 2012, 11:56:34 AM »

Stiff spring with little preload, light hammer with short stroke and adjust the power with transfer port size? That's what I've been working with lately. Making a new transfer port without lathe is PITA I can tell you.

rsterne · « **Reply #16 on:** January 28, 2012, 12:39:05 PM »

Lightening up either the hammer, hammer stroke, or spring preload will all have the same effect.... You will move the peak of the velocity curve to a lower pressure.... Yes, it will make the gun quieter, and if combined with a smaller transfer port, you MAY be able to still run the same fill pressure.... When you are at the peak of the velocity curve, I think for the most part you will find that the noise is pretty much related to the FPE level.... PCPs are quieter (and more efficient) at higher pressures (with short valve timing) and get progressively louder as the shot string progresses.... A Disco is nearly twice as efficient for the first few shots as it is for the last few in a 30 shots string.... If you go too light on the hammer hit (by any method) you will not be able to use the first part of the shot string (the velocity will be too far below the peak) and you will have to fill to (and refill from) a lower pressure....

Bob

darryl · « **Reply #17 on:** January 28, 2012, 06:58:57 PM »

well there's the rub, Bob. How much lighter to go on the weight of the hammer? I admit I have no clue. So I guess I'd better order a couple of spares. With my latest testing I've decided that using a 0.125 TP will give me the FPE I want, but I'll need to lighten the hammer's weight to give me the flat shot string I want. I tried the McMaster-Carr spring mod, and unless I want a 12 FPE B50/.22, it's not getting the job done, even with the adjustment set for max power. It's too weak. Oh wait. I should have tried lower start pressure. I just thought of that. BUT! life goes on.

Anyway, I've turned in the hammer (stroke) adjustment as far as I dare (9 turns CW) and the stock spring is still overcoming the smaller TP, even at 195 bar of start pressure. So again, a hammer of lighter weight seems the way to go. (Gary?)

rsterne · « **Reply #18 on:** January 28, 2012, 07:30:04 PM »

I have found that it takes 25-33% reduction in hammer weight to make much of a difference.... I have even reduced it by 50% in my 1750 on CO2.... It does improve the shot-to-shot consistency, but it doesn't take a lot of hammer spring preload to get back the loss.... so as a power reduction feature you may be disappointed.... In one 22XX PCP, reducing the hammer from 60 grams to 40 grams (33%) reduced the FPE from 38.8 FPE to 36.0 FPE.... In another one, reducing the hammer from 60 grams to 45 grams (25%) reduced the FPE from 32.8 FPE to 32.2 FPE.... Is it worth the bother?.... Spring preload is sooooooooo much easier, and reversible.... I think you will find that the more you are asking from the gun for power, the more important hammer weight is.... eg. lightening the hammer had more effect on my 39 FPE gun than on my 33 FPE gun....

Strangling the transfer port is the BEST way to reduce the power while flattening (and lengthening) the shot string....

Bob

darryl · « **Reply #19 on:** January 28, 2012, 11:05:23 PM »

Coax would agree with you, Bob about replacing the spring over the hammer. It's just not been that clear-cut about how to select the right spring. Interesting,though about your results in reducing the hammer weight. I guess it's all a matter of trial and error, eh?