13XX/2289 Piston Testing

[rsterne]

A while back I had a discussion about which was better, an adjustable metal Flat-Topped Piston and Valve or an adjustable metal piston carrying the original Crosman pump cup, meeting with a shortened (but still cone shaped) valve front end.... It is fairly well known that the tapered front on the Crosman 13XX/2289 valve is slightly too long, and hits the bottom inside center of the pump cup before the edge of the cup can hit the shoulder on the front of the valve, resulting in greater headspace than necessary.... Therefore, the first step was to machine 0.030" off the front of the cone on the valve so that the rubber pump cup touches the end of the cone at the same time the end of it touches the valve flange near the O-ring.... Once done, the pump cup conforms very closely to the front of the valve.... The idea of the adjustable metal piston is to put some pressure on the pump cup, squeezing it tight against the valve front to create minimum headspace.... This minimum headspace is already achieved, of course, with a flat-topped piston and valve because you have two flat surfaces touching....

I made an adjustable piston with two heads, one flat-top, and one carrying a Crosman pump cup.... I also had a stock plastic Crosman piston for comparison purposes.... In order to measure the pressure inside the valve, I drilled out the back half of a valve and threaded it 1/8"-27NPT for a 3000 psi gauge with 100 psi increments.... I machined up a Delrin spacer for inside the valve to provide a spring seat (in the original location) with a tiny (3/64") hole through it for the pressure to the gauge, and adjusted the dimensions until the internal volume of the valve was the same as a stock 13XX/2289 valve.... I shortened a 1377 pump tube just behind the stake that locates the valve (thanks to Skanzy for donating one to the cause and sending it to me) so that the gauge cleared the back, and added an 8-32 setscrew on the side to prevent the back half of the valve from turning when I mounted and removed the gauge.... The front trigger screw (actually a 1/4" long high tensile 8-32) was used to secure the valve in addition to the rear stake.... Here is a photo of the parts....



and here is the assembly.... I used a stock 2289 pump arm and linkage, and a 3/16" solid steel pivot pin (easily removable)....



I started with the stock piston and recorded the pressure at each pump to 20 pumps, then at 22, 25, and every 5 pumps up to 50 pumps.... I then removed the stock piston, adjusted the length of the one with the Crosman cup to put it under slight compression (you can feel the resistance when the handle is 1" from closing) and repeated the test.... I then removed the valve, replaced the valve front end with the flat one, installed the flat-topped piston (adjusted so that resistance on the handle is felt at 3/4" from closing) and tested one more time.... I actually ran each test 3 times and used the average, although the pressures were quite close on each run, basically within my ability to read the gauge (about 20 psi).... I then graphed the results, as shown below.... EDIT: Testing done at 2500' altitude, so subtract about 9% from the number of pumps at sea level for any data presented in the graph....



Obviously, either of the adjustable metal pistons is a HUGE improvement over the stock one.... Using stock springs, a 13XX/2289 starts to retain air at about 1500 psi, which you can't reach with a stock piston.... The flat-topped design performed the best, although at 10 pumps and below there is very little difference.... in fact there is only about 1 pump difference at 12-13 pumps.... Above that, however, the more you pump the greater the margin for the flat-topped piston, which is what I expected to see.... As the pressure rises, even with the adjustable metal piston, the Crosman rubber pump cup starts to compress from the air pressure, and the headspace gradually gets greater and greater, lowering the compression ratio of the pump, and requiring more pumps to reach the same pressure.... It tops out at about 2400 psi at 50 pumps, which is 50% more pumps than the F-T-P takes to reach that pressure (~32 pumps).... I stopped at 40 pumps with the F-T-P (2520 psi), although the pressure was still building slowly....

I didn't test a "stuffed" piston, where metal rods are glued into the slots in the side of a Crosman plastic piston to stiffen it.... This mod is often accompanied by fitting an O-ring between the cup and piston, and a washer under the cup, to reduce the headspace, in conjunction with shaving the cone on the end of the valve.... Obviously this can't be any better than the adjustable aluminum piston using the Crosman cup which I tested.... It is just a cheap way to gain some benefit, how much is gained being in proportion to the care used.... It would be better than stock, for sure, and it wouldn't take much improvement to reach 1500 psi where you have to worry about retaining air....

I am very pleased with the results of these tests.... Not only do they clearly show the improvements that can be made, but they also give a plot of valve pressures inside a stock valve, which is a piece of information I haven't seen in this much detail before.... I hope you all find this information helpful....

Bob

[mafatone]

Whoa! That's what I call research. I have a stuffed piston 1322. Thanks for this.

[hutnicks]

Excellent info there. I am a little surprised the flat top and the metal piston with cup are so close in spec.

Have you done or are you planning on doing anything with oversized piston and compression tube configs?

[rsterne]

I'm not surprised they are close at below 1000 psi (<5%), and I'm also not surprised that they are quite different above 2000 psi (16-50%).... Rubber compresses, just that simple....

Nope, I did that when I built my Disco Carbine Pumper and Millennium Pumper, using the 7/8" OD tube and Benji 39X pump linkage with a home made F-T-P.... All the data on that exists....

Bob

[hutnicks]

I'll dig around for that thanks.

How was the heat buildup?  Surprised the cup can take 50 pumps.

[rsterne]

I was only pumping in sets of 5 with reading the pressure in between, so you could feel some heat in the tube but not a lot.... Also, my hand was on the top of the tube, acting like a big heat sink.... *LOL*.... The cup looked perfect....

http://www.gatewaytoairguns.org/GTA/index.php?topic=37401.

http://www.gatewaytoairguns.org/GTA/index.php?topic=39328.

Bob

[plinker99]

Wow Bob!!!!
   Thanks for doing this test & making the graph.  This really helps me to understand what the mods are actually doing.  This is one of your posts that I can really relate to.

   One question.....how would the pressure build on a reduced valve volume like on the 2100?  I believe it is around .072 cu.in as compared to the .1122 cu.in.of the stock 1300 valve.

Thanks

[hutnicks]

Thanks. Had the Millenium tagged but not the .25 Disco thread.

What I am really liking about that graph is it looks like a pretty consistent 100 psi per stroke up to number 10 for the ft and alu with cup. Not to bad a deviation from there up to 20 as well.  That is incredibly useful data and really shows up the advantage over the stock setup.

Now if someone will just make a "Stroke Dot" reticle:)

[rsterne]

I didn't test other valve volumes, but if you leave the pump swept volume the same, the number of pumps to reach a given pressure should be in the ratio of the volumes.... In other words, if you use a valve that is half the size, it should take half as many pumps.... double the size, double the pumps....

I think your volumes on both valve are slightly too large.... did you subtract the volumes occupied by the spring and the head of the poppet and the check valve?.... Your ratios of volumes are pretty close, though, I have a note in my old files that a 2100 valve is 60% of the volume of a 2289 valve.... your numbers give 64%.... Basically it should take 6 pumps instead of 10 (or 12-13 instead of 20) to hit the same pressures, assuming you have the same headspace....

Likewise, if you use a 2100 pump on a 2289 valve it should take you about 70% the number of strokes that it takes with a 2289 pump.... ie 7 instead of 10, 14 instead of 20....

There is a bit of a curve in the first 10 pumps.... It's about 600 psi at 5 pumps, and about 1100 at 10 pumps, for the F-T-P.... 1900 at 20 pumps....

Bob

[nervoustrigger]

Great information you have put together here, Bob.  I know a good bit of time went into this so thank you for sharing!

Your data clearly shows the benefits of the FTP and FTV combo and in reducing headspace to a minimum.  I know some will go to great lengths to stretch the limits of energy the 13xx platform will produce and I respect that.

I just want to share my perspective with others as to why I prefer shimming the pump cup and stuffing the piston.  Doing that netted 11fpe on 15 pumps on a long-barreled .177 cal pumper for almost zero cost using miscellaneous parts in the shop...a washer, O-ring, and some 1/8" steel rod.  Also, while the pump cup being rubber and having some give to it limits the ultimate pressure, it has a benefit in terms of reliability.  The flat top setup puts a lot of stress on the pump linkage and pivot pin, many people reporting and posting pictures of their worn-out pistols and carbines after relatively low numbers of shots.

And as a practical matter owing to the enjoyment of shooting a pump gun more than five times a day, I lose interest fast if it takes more than 15 pump strokes.  Well, even that feels burdensome compared to the reward:effort ratio of a springer or even a hand-pumped PCP.  Of course I realize I don't [/i]have[/i] to pump 30 times just because I have a FTP/FTV but still, that's a lot of pumping to do even occasionally and proportionately each successive pump yields less FPE per stroke, reaching a point of diminishing returns.

So each approach has its advantages and disadvantages but what constitutes "best" has a lot to do with the individual's criteria for best.  That's how I like to look at it.

[rsterne]

You are absolutely correct that much of the benefit of an F-T-P can be had for little to no cost, particularly at low pump numbers.... I disagree, however that a PROPERLY ADJUSTED F-T-P is responsible for wearing out the linkage and pivots.... If you set the piston into too hard a contact with the valve  that can happen, but not it is is adjusted to just touch, just enough to hold the linkage closed after the gun is fired.... After that, by the time you have 1000 psi of air in the gun virtually all of the load is from the air pressure, NOT the piston contact, and the load is the same at the same pressure, regardless of whether you use an F-T-P or a rubber cup.... It's just easier (not as critical) not to screw up with the cup.... It is the increased pressure, not the F-T-P, that wears things out (including your arm!)....

In terms of minimizing pumping even at low pump numbers, there is NO reason (other than cost) not to use an F-T-P.... even at 3 pumps, they are better.... virtually an unmmeasureable difference, but better.... No question there is diminishing returns on continuing pumping, but for those who want high power, the F-T-P offers the way to reach high pressures with the fewest pumps.... On the other hand, a stuffed piston with and O-ring and washer, pinning the pump cup, and file off the end of the valve, will give you 90% of the benefit, up to 10 pumps, at virtually no cost....

Bob

[nervoustrigger]

I agree about the _properly_ adjusted FTP.  At least logically that makes sense.  Apparently it's not easy to get right though, just judging by the anecdotal evidence.  Don't you need some amount of deliberate interference in order to create a cam-over effect to help keep the pump arm closed?

It may indeed be mostly because of the increased pressure but I would be surprised to learn that most users of FTP/FTV-equipped guns tend to use over 15 pumps for most of their shooting.  A 500ct tin of pellets at 20 pumps each is 10,000 pump strokes.  They'd be easy to spot though.  It's the airgunners with forearms like Popeye.

[rsterne]

It's actually easy to get the piston length correct.... You want the pump arm to feel resistance when it is 1/2"-3/4" from closing, with the gun fired.... Less than 1/2" the pump arm will fall open after firing.... More than 3/4" and you are unduly stressing the linkage and pivot pin.... The biggest problem is people using high pressures without replacing the roll-pin with a smooth steel pivot pin.... The roughness of the roll-pin really takes a toll on the pump arm hole at higher pressures....

I was one of those (Popeye), during the development and testing of my Uber Pumpers and Uber Carbine.... *LOL*.... Now most of the shooting I do with my .177 Uber-Pumper is 3 pumps of the 2200 tube, giving >500 fps.... Occasionally I'll use 10 pumps when I want 800.... I sold the .22 cal version, as although I got over 900 fps the 1400 pump wore me out past 10 pumps, where I could get 750 fps (18 FPE).... The only pumper I use much is my .22 cal Uber-Carbine, using a 2200 pump tube, which is tuned for two shots at 600 from 20 pumps....

Bob

[plinker99]

Thanks for the reply Bob.
   That confirms that I'm on the right track.  I mostly shoot my 2100 with 3 pumps for around  540fps & my 760 with a reduced volume valve about the same fps with 5 pumps.
  As for my valve numbers, that is what I got by filling with water he backhside of the valve with the poppet then   the front side with the check valve then adding the 2.  I think I left the spring out.
Thanks!!

[Tater]

I saw this was a "Bob thread" and knew it would be worth reading. Excellent tests and very valuable info, especially the pressure gauge. Thanks for taking the time to run the tests, this should be a sticky.
I have never dug in that deep with mine, but I have a flat top I received 2 years ago waiting to go in. Maybe this winter.
Great post Jason as well.

[rsterne]

plinker, the two valves overlap where they thread together, it would be hard to get that exactly right....

Bob

[K.O.]

Hi Bob at times I feel I should say Sensi...

What was the intake passage volume? On the modern 13xx I have seen them vary from .06 to .085 and I use brass 1/16 brass tubing to sleeve it down to .03

most of the passages have been right at .55 long pre matching to the cup and .5-.51 after...

at

.08" x .5" you have headspace that is equal to about losing ~1 pump in 40 pumps which is about 45-60 psi in 20 pumps using .1 for valve volume.

.06"x .5" = losing  ~ 1 pump in 70 or about 20-35 psi in 20 pumps

.03 x .5  = losing  ~ 1 in 283 or  5 psi

Have more I wonder about but the Seahawks are playing....

[K.O.]

And is it possible to get you to give the actual figures I can read the graph to get a general idea but I am very interested in a very close way of figuring just how the pump efficiency curve goes at psi/pump...

also freshly oiled versus not...   oops commercial over...

[rsterne]

The intake passages in both valves were 0.060", I couldn't get a 1/16" drill into them.... I realize there is a tiny gain to be made there on BOTH valves, slightly more on the full length (after shortening) than on the flat top.... but we are talking 140 psi difference at 20 pumps (1760 vs. 1900) between the cup and the flat-top (both adjustable metal), which is over 2 pumps.... In fact the F-T-P did 10 psi more at 18 pumps than the cupped did at 20 pumps.... The valve holes are 0.200" difference in length between the two front ends x 0.060" D = 0.000565 cu.in = 0.0093 cc.... If you cut the hole in half to 0.030", the new volume would be 1/4 that, so the gain would be 0.75 x 0.093 cc = 0.0070 cc.... Using your valve volume of 0.1 CI = 1.64 cc (which I agree with) that represents an improvement in the compression ratio of 0.43% for the cupped piston relative to the F-T-P if both were sleeved.... We don't really care about how much we are losing now, only how much of that loss could be reduced in one setup compared to the other.... Using your 20-35 psi estimate for the total loss of a 0.060" hole 0.5" long, the relative gain we are talking about if both are sleeved down to 0.030" would be 6-10 psi....

I think that I have shown that the differences between the cupped piston and the F-T-P is insignificant below 10 pumps, which you would expect, as they both eliminate most of the headspace in the stock design.... However, as the pressures increase, the cup compresses from the load on it, and the headspace increases, reducing the pumping efficiency relative to the F-T-P.... At 2400 psi, I found that difference to exceed 50% (50 pumps for the cupped piston vs. 32 for the F-T-P....

Bob

[avator]

WOW! The first Rsterne post that I understood after only reading it once. And the first I completely understood at all. Thanks for lowering to my understanding level Bob. Those are some money saving results. Time to pull the 1377 carbines apart.