More QB-tests:

More QB-tests:

Joined: January 30th, 2005, 11:00 pm

June 25th, 2010, 10:55 pm #1

Today I got a chance to test another set up on my Co2 Powered QB-78. My rifle was one of the first QB's and was Origanaly tuned by Craig Pitts, and Gene Sunday. I have used this Rifle for several years for shooting Varmits. And it was set up for power.
It is a 22 cal. and although powerful, it was a gas hog, I modified it by lighting the striker, and this helped. I did not have a crony at this time and after my work on the striker, I did buy one. After my striker work, this rifle still had two washers behind the spring and was shooting slightly over 700fps on a warm day. After changing the origanal ( modified )valve stem out for a Roy from Mountian Air Delrin stem, I removed the two washers and am using only a stock spring. The fps went to 725, on a hot day. I had a request from Mike to see what it would do using a stock valve stem, today I took the rifle apart and installed a new stock stem, the crony shows same modded rifle with stock stem, 670fps. All crony fps were 5 shot averaged, waiting two minutes between shots, day was hot. Now I am waiting for my order from Archer for more O-rings to change it back to the Delrin stem.
So looks like the Delrin stem is good for 55fps in my rifle.
Bill...
Last edited by Billcurtis on June 25th, 2010, 10:59 pm, edited 1 time in total.
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Joined: November 17th, 2006, 3:51 am

June 26th, 2010, 2:19 pm #2

...shape related than material. Do get a little bit more from the harder material unsealing/sealing more quickly, but it isn't much.

Usually Delrin stems are a little smaller in diameter, letting gas more easily flow past the sides and to the valve outlet. OFten they are a little differntly shaped at the sealing end, which also seems to help a little bit. IF the Delrin part if a different length, then the valve return spring tension is also different. ANd lastly, the length of the metal rod part of the valve stem that sticks out to get smacked by the striker may be a different length, which can change the dwell time.

Guess I could going to have to hop on the lathe and make one that is the same size, shape, radius.etc. as a factory one and see how it does (but have to get more white Delrin).
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Joined: January 30th, 2005, 11:00 pm

June 26th, 2010, 2:46 pm #3

Robert the Delrin stem I used is from Roys Mountian Air and is the same size and shape of the stock stem. Only difference is that it is lighter and much harder.
Bill...
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Joined: November 17th, 2006, 3:51 am

June 26th, 2010, 3:03 pm #4

Small changes to the valve do have larger results than you'd expect... thanks for the information. Next one I have to make, will be made easier/simpler.
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Joined: March 28th, 2002, 6:54 pm

June 26th, 2010, 5:36 pm #5

Robert the Delrin stem I used is from Roys Mountian Air and is the same size and shape of the stock stem. Only difference is that it is lighter and much harder.
Bill...
your results are pretty much in line with my experience, but better controlled.
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Joined: January 30th, 2005, 11:00 pm

June 26th, 2010, 6:31 pm #6

Scott I am thinking maybe I should shape the Delrin stem, to be like the modified one I got with the rifle. I bet that would up the speed a bit and it would also make the stem lighter and quicker to open and close.

Bill...
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Joined: March 28th, 2002, 6:54 pm

June 27th, 2010, 12:21 am #7

has a valve head shaped like a truncated cone, but with the narrow end at the seat, just barely wider than the seat mark. The other end is full size, with just a nipple to center the return spring. My theory is that the tight wall clearance gets the valve shut sooner, with more response to reservoir pressure variation, and the narrow seat end gets good flow there.

The other factor is that this stem is in a cut away style body, with a tiny, pedestal mounted spring. Don't know how big a difference that makes, but it's some...
Last edited by classicalgas on June 30th, 2010, 3:28 pm, edited 1 time in total.
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Joined: May 12th, 2001, 1:29 pm

June 27th, 2010, 2:32 pm #8

Today I got a chance to test another set up on my Co2 Powered QB-78. My rifle was one of the first QB's and was Origanaly tuned by Craig Pitts, and Gene Sunday. I have used this Rifle for several years for shooting Varmits. And it was set up for power.
It is a 22 cal. and although powerful, it was a gas hog, I modified it by lighting the striker, and this helped. I did not have a crony at this time and after my work on the striker, I did buy one. After my striker work, this rifle still had two washers behind the spring and was shooting slightly over 700fps on a warm day. After changing the origanal ( modified )valve stem out for a Roy from Mountian Air Delrin stem, I removed the two washers and am using only a stock spring. The fps went to 725, on a hot day. I had a request from Mike to see what it would do using a stock valve stem, today I took the rifle apart and installed a new stock stem, the crony shows same modded rifle with stock stem, 670fps. All crony fps were 5 shot averaged, waiting two minutes between shots, day was hot. Now I am waiting for my order from Archer for more O-rings to change it back to the Delrin stem.
So looks like the Delrin stem is good for 55fps in my rifle.
Bill...
...numbers - e.g., shots per 24gm fill.

The reason I say that is it's well known that a hard seat material like Delrin will open further and release more gas with a given hammer strike energy than a softer one like the stock QB seat.

So it may be that those extra 55fps are being bought simply at the cost of more gas expenditure, and you could get the same velocity with the softer stock seat and more spring preload to make up for the difference in required energy to open it.

Do you think running a full chrony string might be possible sometime?

Steve
Last edited by pneuguy on June 27th, 2010, 2:34 pm, edited 1 time in total.
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Joined: November 17th, 2006, 3:51 am

June 27th, 2010, 3:39 pm #9

The other way to look at it is that you may be able to take off some more pre-load (or slightly strangle off the transfer port) to get back to the stock valve stem's velocity.

Ether way, finding he gas consumption is a good idea. If you're ging to get all OCD about it, get the weight of the 2 full 12gr. than the weight of those 2 12gr. when empty (12gr. is an aproximation...some hold a noticable amount less). Will find there is still gas (by weight) in the gun when the vel. starts to decline; even all gas (no liquid) co2 has weight.

The tougher nature of Delrin more durable, and that alone(even if no vel. difference) makes it worth the replacement. Seldom have to fix one of the Delrin stems, but crap can get embedded. Stock seal is pretty well shot when that happens, but can usually pluck out the offending bit of "gunk", clean the delrin stem, re-lap it, and keep using it.
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Joined: January 30th, 2005, 11:00 pm

June 27th, 2010, 9:09 pm #10

...numbers - e.g., shots per 24gm fill.

The reason I say that is it's well known that a hard seat material like Delrin will open further and release more gas with a given hammer strike energy than a softer one like the stock QB seat.

So it may be that those extra 55fps are being bought simply at the cost of more gas expenditure, and you could get the same velocity with the softer stock seat and more spring preload to make up for the difference in required energy to open it.

Do you think running a full chrony string might be possible sometime?

Steve
Steve;
I was getting only about 20-25 shots with the modded original valve with two washers spacing the stock spring. Now I am getting more shots with no spacers, and it is making almost the same power. I also noted while testing, a couple of my 12 gram cartrages were empty. I was using only one at a time testing the valve for leakage. I will when I get the rifle going again, try and run a full string using two new carts, just to see when it drops off. I am waiting for a new supply of O-rings and other parts from Archer.
Bill...
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