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Slavia · Moderator

O.K., so I admit I'm a springer guy, so bear with me. These questions relate to pumpers only, Crosman to be specific:

1. What would be the advantage of increasing the valve body's empty volume? (Same barrel.)

2. How effective are techniques like angling the transfer port, and decreasing the diameter of the exit valve?

3. What are the effects of increasing the I.D. of the transfer pathway?

4. What are the relative advantages/disadvantages of increasing the hammer spring tension (and reducing the valve spring tension), or vice versa?

AirGunEric · Site Admin

The same questions, and the same answers apply to Co2 guns, for the most part.

The answer- "it depends". Depends on the number of pumps you plan to use the majority of the time and/or what the target optimum shot power/velocity is intended, caliber and weight of pellet also play a role in this.

To get an idea, check Bob's "uber grouse gun" post.

What is it you want to do?

rsterne · Silver Status Member

Completely different answers for .177 and .22.... which one are you insterested in....

Bob

Slavia · Moderator

The plan is to learn on my daughter's model 66, and then move on to a 1377 carbine conversion. Both will remain in .177.

The goal for the 66 is to give it some more zip and accuracy without making it beast to pump. My daughter has had it since she was little, so it's now a nostalgia piece. That means it's not shot a lot, so the longevity of plastic parts is not an issue. It also means the outward appearance should remain the same. It will be used for plinking only. Also, as I said, it is a learning tool.

It does have a rifled barrel. Velocity increases to a peak of 645 FPS (with 8.2 grain Superdomes) at 11 pumps, and then trails off down to 625 FPS at 15 pumps. I stopped there because it didn't feel right to stress the plastic frame any more than that. My daughter has decent upper body strength, but I don't think she's going to want to pump more than 6-8 times. A second dry-fire shot after a 15 pump shot didn't move a piece of tissue paper, so I think it dumps the entire charge.

I have a metal piston for it. I have some ideas for the barrel, trigger, and stock. It's the valve where I have no experience.

rsterne · Silver Status Member

In .177, here is my experience (based on a 13XX valve):

1. Increasing the valve volume willl require MORE pumping and will only benefit at over ~950 fps.... Shocked

2. Increasing the volume between the exhaust valve seat and the pellet can (and in my experience, usually does) reduce the velocity.... It apprears that in .177 a short, high pressure pulse works better than a longer push of more air (which helps in .22).... The only effective mods I have heard of in that area are filling and redrilling the exhaust port on an angle to reduce turbulence without increasing volume.... with about a 50% success rate.... The exception is at over ~850 fps (or in .22 cal) where streamlining can pay benefits....

3. I found that opening up the barrel port to 0.141" or a couple of thou bigger is a benefit (ie making it the same size as the transfer port ID or ONE number drilll larger).... Until you are past ~850 fps, enlarging the transfer port loses a fraction rather than gains.... Once you replace the springs to allow the gun to be pumped past 20 pumps or so, then increasing the transfer port size may be of benefit.... and the further you go, the more benefit although in .177 the gains are MUCH less than in .22.... The barrel port will be the ultimate limiting factor as if you go much larger than 0.145" the pellet will begin to hang up on loading.... Rolling Eyes

4. Increasing the hammer spring tension is again, only useful when you get to the point of the gun retaining air (~650 fps @ ~15 pumps with a stock valve spring - right where you are now).... and between ~650-850 fps, installing a lighter valve spring is a better choice.... Using a Disco hammer spring in my Uber-Pumper (which already had a weaker valve spring) produced the same results at ~600 fps.... gained about 20 fps @ ~800.... and moved the air retention point from about ~850 fps up to ~990 where the gun peaked.... This was at 30 pumps with a 2200 pump (which is the equivalent of ~45-50 pumps with a stock pump with F T P)....

What does that leave you with to get more performance below 15 pumps?.... not much, unfortunately.... A weaker valve spring will show some benefit at anything over about 5 pumps, and will move the point at which the gun retains air up to over 20 pumps with a properly adjusted F T P.... That produces ~600 fps in .22 cal (~650 in a 2289 with 14" barrel) and ~750-800 fps (~850 fps with a 24" barrel) in .177.... It's about the only mod other than an F T P that I would bother with in your situation.... If you are really interested in getting the most velocity with 6-8 pumps, you could actually try REDUCING the volume inside the valve with a sleeve in the front end around the spring.... The gun will build pressure faster but you will limit the top end, of course....

In conclusion.... Unless you are striving for over 850 fps (longer barrel probably required) at over 20 pumps.... forget about the hammer spring, and don't modify the valve or transfer port other than to replace the valve spring.... You can drill the barrel port to 0.141"-0.143" (and deburr, of course).... and fit the lightest valve spring that will allow you to pump the gun without pre-cocking it.... If you go too light, the hammer will hold the valve open and you will have to cock the gun before pumping (no benefit at these levels).... Use a Flat-Topped-Piston and flat-faced valve, of course, properly adjusted so that the pump arm feels resistance when about 3/4" from the closed position.... Mr. Green

Bob

Slavia · Moderator

Thanks for the advice - it all makes sense. Aside from the FTP and opening the transfer port slightly, it looks like my efforts would be better invested in areas I already know.

Thanks again for the detailed explanation!

rsterne · Silver Status Member

T191032 · Member

I get lost in all the Crosman Modding' Tech details on the Green Forum. It's all those purty pictures everyone posts of their creations. Of course, I'm basically swapping parts around on the 1377 "Carbines" I'm preparing to make. The Crosman Green Forum is a bad, bad thing - makes you want all kinda things, Crosman 1377 and 2240 Target Pistols, 1377 and 2240 Carbines. . . It just goes on and on Mr. Green

LOL

Slavia · Moderator

Barrel port. Got it.