C9/392/397 low pump count mods

[DanD]

Inspired by posts from psipumper, tackdriver10, Robert 5mm, rsterne, and others,  I've been experimenting with a 1999 C9 Blue Streak.
My goal was reaching 600 fps on 6 pumps with 14.3 gr .20 cal pellets.
I am not a machinist....

I recorded velocity with 14.3 Benji cylindrical pellets at 6 pumps unless otherwise noted. My motto is pump less, shoot more.

Stock gun (discovered it had no rear o-ring when I removed the valve)
550 fps

New o-rings and 2 spring mod
580 fps

113 middle o-ring.
590 fps

Added 3 washers to decrease valve volume (yeah, broke 600 but just barely!)
601,     8@660

Added lots more spacing (too much of a good thing)
586

Lightly port inside front transfer area w small dremel bit, keep 3 washers.
603

Port it more, keep 3 washers, thin valve stem.
610

Add 4 more random washers for spacing and trimmed exhaust spring to dump all air. I'm guessing erratic velocity was due to washers shifting.
606-620,  8@ 665 to 680.

Sanity check, put stock rear valve half back in
550

Settled on 3 brass #10 washers and get consisted velocity. I need to buy more washers.
3@ 470, 5@570, 6@610, 8@670

My new goal is 600fps on 5 pumps. I have various Crosman PCP poppets and a spare valve on the way. Seems like I'll need a combination of volume reduction and porting.

Should I drill out the valve TP? If so, to what size?

Should I do anything to the TP in the barrel?

Is there a better way to reduce volume than stacking washers?

Any other advice on reaching 600fps on 5 pumps with only a small drill press, a hand drill, and a dremel?

Thanks for reading and any advice!
Obligatory gun pic attached...

[Tack Driver 10]

600+ FPS with 5 strokes should be very easy seeing you get 570 FPS currently.

Here's what I would do:

Replace the stock piston with an adjustable cup or better yet, flat top piston/valve with 90 duro seals.

You'll need to reduce the valve volume to maximize pressure at low pump counts with spacers.

Delrin valve stem with an 1/8" shaft sitting on a concave valve seat.

Enlarge and port the transfer port opening in the valve.

A balanced valve would be nice with all the available internal valve real estate.

Should look something like this:

[Rob M]

shortening the valve stem will also help.. more travel distance for the hammer hence more cracking force .. one can assume that more you reduce internal volume the higher the pressure in the valve and  requiring more cracking force to open rapidly and dump the full valve.

[Robert 5mm]

I disagree with the idea of reducing the valve volume.
I know it increases internal valve pressure but it may also increase needed pumping force for each pump.
I prefer to add volume to the valve, decrease pumping effort, and pump 1 or 2 more times to increase energy.

[DanD]

I disagree with the idea of reducing the valve volume.
I know it increases internal valve pressure but it may also increase needed pumping force for each pump.
I prefer to add volume to the valve, decrease pumping effort, and pump 1 or 2 more times to increase energy.

I've been thinking about this, too- whether I want a gun that pumps really stiff at 5 pumps, but I already have the C9 ACP and a few older solder valve guns that fill the sweet and easy niche. I can always remove spacing or drop in a shorter check valve to increase volume, and hopefully the porting will help either way.
This is all fun new stuff to me!

[DanD]

600+ FPS with 5 strokes should be very easy seeing you get 570 FPS currently.

Here's what I would do:

Replace the stock piston with an adjustable cup or better yet, flat top piston/valve with 90 duro seals.

You'll need to reduce the valve volume to maximize pressure at low pump counts with spacers.

Delrin valve stem with an 1/8" shaft sitting on a concave valve seat.

Enlarge and port the transfer port opening in the valve.

A balanced valve would be nice with all the available internal valve real estate.

Should look something like this:

I appreciate the guidance and illustrations. I'll try shaping the transfer port some more as a start.

[Psipumper]

It is fun to make forward progress. Headspace is very important. How much space do you have between the valve halves? The pump handle should be like the photo when it touches the valve.
The square oring does wonders but careful deburring of the tube is a must to use it. Needle files , riffle files, 3/4 “wood rod with adhesive backed sandpaper ,does a good job of prepping the tube for the square oring.
Sleeving  the air inlet hole of the valve to a 1 mm hole can help also. The sleeve can be made with a hand drill and a file .
The transfer port size for .20 caliber  may be ok . Experiment with the valve cc but in .20 caliber slightly smaller .
Post a photo of your undercut poppet.

[DanD]

It is fun to make forward progress. Headspace is very important. How much space do you have between the valve halves? The pump handle should be like the photo when it touches the valve.
The square oring does wonders but careful deburring of the tube is a must to use it. Needle files , riffle files, 3/4 “wood rod with adhesive backed sandpaper ,does a good job of prepping the tube for the square oring.
Sleeving  the air inlet hole of the valve to a 1 mm hole can help also. The sleeve can be made with a hand drill and a file .
The transfer port size for .20 caliber  may be ok . Experiment with the valve cc but in .20 caliber slightly smaller .
Post a photo of your undercut poppet.

Thanks for the advice, Charles.
I'm currently running an X oring in the front, a round in the middle, and a square in the back.  I did notice the squares got chewed up real easily in the front.  Now I know I'll need to prep the tube for them.
I don't think my pump arm was open quite as much as yours, but I did at one point unscrew the valve halves so I felt definite contact without making other changes, but it didn't gain me anything, so at that point at least headspace didn't seem to be the limiting factor.

[Rob M]

the reason you noticed nothing by unscrewing the valve halves was that you increased the valve volume at the same time..Charles is spot on , you need serious camover at the end of the stroke to assure the least headspace possible

[Psipumper]

I would try to narrow the stem closer to seal. Shortening the brass threads on the valve helps flow around the head of the poppet. The diameter of the poppet is .425 and enters back into the .437 hole in the valve when opening . I shorten the threaded section to 7/16 length. Even though the valve volume will be increased, you can add a spring pin to reduce  it back.
You can make power with the original parts. 600@5 pumps should not be a problem.
I have speculated a 392 could be set up to get 600 on 2 or 3 pumps.

[19Sheridan57]

   You have done a lot of work to gain a little extra FPS. I am assuming that you are using Benjamin-Sheridan 14.3 gr. pellets. If you switched to JSB Exacts 13.43 pellets, your FSP would increase around 20 + FPS & you would have your desired speed with a better quality, more accurate pellet. In my testing of many, many Sheridans, this is the case every time.

[DanD]

Last night I reduced volume by replacing the intake spring with a piece of inlet valve. I also did very mild TP opening.
5 pumps 580, 6 pumps 615.

Spun the exhaust valve in a drill and filed some brass off it.
585, 620.

Filed valve head more and measured it - stock valve was 10.7mm, now 10.2. Not much more room to file without hitting the seal.
585, 625, 8 pumps 680.

I did not see Charles ' stem recommendation at the time, so did not try that yet.

To answer Steve, I am getting 605 fps with H&N FTT 11.4 gr on 4 pumps, and they shoot pretty well.  They do 660 fps on 5 pumps. I'm all out of JSB and have more on the way.
This work is interesting and rewarding to me. I have other guns I'm happy to enjoy unmodified.

[Tom Tucker]

I disagree with the idea of reducing the valve volume.
I know it increases internal valve pressure but it may also increase needed pumping force for each pump.
I prefer to add volume to the valve, decrease pumping effort, and pump 1 or 2 more times to increase energy.

This is the way I prefer to go as well.  Is 5 pumps better than 6 if the 5th pump is becomes just as hard as the 6th pump used to be?  Sometimes you just have to try it both ways and decide for yourself which way you enjoy the gun more.

[DanD]

I nicked the exhaust seat when trying to grind out the threads. Although it still seals,  I started a new rear half.
On the new half,  I used an epoxy putty to fill in the recess around the seat and cover the threads. I don't expect it to last, but I'm having fun. I used extra intake poppet material for spacing and used gasket sealer to build up around the tp.

5 pumps 596 (so close!), 6 pumps 630, 8 pumps 670.

So I'm at the point where the low volume is increasing lower pump count velocity,  but higher pump counts are staying about the same or lower than previous configurations.

[Tom Tucker]

I wonder how fast it shoots at 2-3-4 pumps now that you've reduced the volume even more.  I have a feeling it would really send the lighter pellets flying...

[Tack Driver 10]

Perhaps it's time to address the elephant in the room.

These guns were never designed to produce high power.
The long pump stroke and brass tube will flex when pushed and limit performance.

To test this simply pump the gun and release the pump arm.
If it springs open more than 30 degrees then there is too much dead air space.

[DanD]

So, headspace.
What's the easiest, cheapest way for a two bit hack to reduce it?
I have the middle oring, teflon tape, and the halves closed lightly so there is plenty of cam-over, but there is still lots of rebound on the pump arm.

[Tack Driver 10]

So, headspace.
What's the easiest, cheapest way for a two bit hack to reduce it?
I have the middle oring, teflon tape, and the halves closed lightly so there is plenty of cam-over, but there is still lots of rebound on the pump arm.

There's only so much that can be done since at some point the tube/link will flex and self limit power.

Regardless of skill and tooling, this is the best approach:

A flat top piston and valve will maximize pump stroke and minimize dead air space.

A new valve front half will be needed.
The current design has several limitations.
It has a square seal gland instead of a round gland.
A square seal can be used but......
The square gland is set back .100" from the front of the valve introducing dead air .
The valve diameter is .767" but the .100" setback is .750" again, more dead air space.

The solution is a FT valve front with and o-ring seal set back .025" with the full diameter of .767".

The best approach to build high power is a short stroke which obviously will require more pumps.
My modified 1322 with an 18" 22 cal barrel makes 600 FPS (actually 596) with 10 pumps using 14.3 g CPHPs.
For those that like to pump, 27 pumps  yielded 821 FPS 21.4 FPE, that would be equal to 9 streak pumps.
It's only about 1/3 the swept volume of the streak but the short stroke limits power robbing flex.

https://www.gatewaytoairguns.org/GTA/index.php?topic=153687.msg155691545#msg155691545

[DanD]

I pulled one of the intake spacer disks out and regooped the TP area with gasket sealant and let it dry. Instead of trying to hand trim the sealant, I just shoved the valve in and let the tube edge trim it, then shot some air to blow any debris out.
I met my goal. 605-616 on 5 pumps with vintage 14.3. I also got my JSBs delivered.
Gun leaves barely a whisper of air at 7 pumps and probably not enough to shoot another pellet at 8.

Here are some numbers from 1 to 8 pumps with different pellets:

Benji 14.3 cyl-
1@289, 2@413, 3@500, 4@559, 5@614, 6@648, 7@678, 8@706

JSB 13.7
1@310, 2@423, 3@516, 4@586, 5@631, 6@672, 7@701, 8@724

H&N 11.4
1@344, 2@467, 3@562, 4@627, 5@685, 6@719, 7@753, 8@787.

I was thinking of trying some gasket goop to fill the area in front of the front oring, but for now I'll just enjoy my hack job valve until it breaks. JSB breaking 600 @ 4 pumps might be my goal next time. 

Thanks for reading and all the advice!

[Psipumper]

I pulled one of the intake spacer disks out and regooped the TP area with gasket sealant and let it dry. Instead of trying to hand trim the sealant, I just shoved the valve in and let the tube edge trim it, then shot some air to blow any debris out.
I met my goal. 605-616 on 5 pumps with vintage 14.3. I also got my JSBs delivered.
Gun leaves barely a whisper of air at 7 pumps and probably not enough to shoot another pellet at 8.

Here are some numbers from 1 to 8 pumps with different pellets:

Benji 14.3 cyl-
1@289, 2@413, 3@500, 4@559, 5@614, 6@648, 7@678, 8@706

JSB 13.7
1@310, 2@423, 3@516, 4@586, 5@631, 6@672, 7@701, 8@724

H&N 11.4
1@344, 2@467, 3@562, 4@627, 5@685, 6@719, 7@753, 8@787.

I was thinking of trying some gasket goop to fill the area in front of the front oring, but for now I'll just enjoy my hack job valve until it breaks. JSB breaking 600 @ 4 pumps might be my goal next time. 

Thanks for reading and all the advice!

Dan that is a big improvement for what you did. Good numbers in .20 caliber @8.
Charles