Seem to be stuck...

Seem to be stuck...

Joined: August 20th, 2006, 5:36 am

May 26th, 2012, 11:12 pm #1

I've got my QB up to 30 fpe with Kodiaks, but no matter what I try, I can't get past that. I've gone from a hollow probe to a needle, no gain at all. I've tried a variety of valve head shapes and sizes, I've come up with some that cost me power, but a couple of others have brought me back to the 30 fpe level. Diameter seems more important than shape of seat, going from a 5/16" diameter to 3/8" knocked the power down to the low 20's. Tried a few other things, but I either stay at 30 or go backwards. The only constant has been my regulator, it's at 1200 psi. Do I need to go to higher pressure to get more speed? Or have I reached the limit of the QB valve in .22 cal.?

And before someone asks "Why do you need more speed/power?", the answer is simple, I don't. This is about testing the limits of the platform, and testing my ability to wring the last fps out it that I can get . So for you guys that have been down this road ahead of me, if you have some suggestions or opinions, I'd love to here them. Thanks.

Dave
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Joined: March 28th, 2002, 6:54 pm

May 27th, 2012, 3:29 am #2

(capacity between the reg and the valve) That can make a difference. How open is your port work? An oval port may help. How open is the front of the valve? Less metal is better, down to the minimum that will still support a pedestal. My exhaust valve spring is less that 1/4" OD..that may help too.
Last edited by classicalgas on May 27th, 2012, 3:29 am, edited 1 time in total.
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Joined: August 20th, 2006, 5:36 am

May 27th, 2012, 5:47 am #3

I've got the full volume of a 78 tube between the reg and valve, call it something very close to 50 cc's. Port work, I assume you're referring to the transfer port, the valve exhaust port is stock, the barrel is at .166" IIRC, or about the same as the poly tubing. Don't see how opening it up more will gain me anything. I could take more metal off the front of the valve, but again, I don't see how I'd gain anything, it already has far more free flow than the valve can use. I'm using a tapered spring, .346" OD at the big end down to .251" at the small end, which fits on a spigot turned on the valve head. Pretty soft, wire is only .027" vs. .035" for stock, and 13 vs. 6 coils. My pedestal is really more of a spring guide, just a short Delrin piece that sits in the recess over the hole in the front of the valve extension and keeps the big end of the spring in place.

I will say, I was surprised at the very large difference in velocity I got with differing valve head diameters. The actual seat profile doesn't seem to matter, I've tried flat, conical, and "toilet plunger" style, the only difference was in quality of seal, the plunger style seems to give me the best seal, I've had valve leaks of varying severity with the other styles.

I've had to do some back tracking, I have a terrible habit of making more than one change at a time . I'd opened up the valve port on one valve to .228" (#1 drill), and used a 3/8" plunger style valve head. Power dropped from 30 to just over 20 fpe. Two changes at once, didn't know where the problem was. So I tore it down again, went with a stock valve port and the big valve head, no change. 5/16" valve head, back up to 30 fpe with either size valve port. This series of tests, more than anything else, is what has me thinking I may be at some kind of limit, and logic tells me that the limit is regulator pressure. But I'm at 1200 psi now, I don't think I want to push a stock QB tube much higher than that. And besides, 30 fpe from that is pretty respectable, don't you think ?

BTW, don't take anything I've written here as meaning I'm NOT going to try your suggestions, I almost certainly will. I also still have one untouched valve on hand, which I may do some more back tracking with, kind of a reverse base line thing. Later, and thanks.

Dave
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Joined: March 28th, 2002, 6:54 pm

May 27th, 2012, 5:18 pm #4

... respectable!

The reason I think you want the front of the valve wide open is twofold...

The Philippine co2 guns with no front end to the valve at all, make great power...more than anything else I've seen on co2.

It seems to me that when working with lowish air pressure, ANY restriction to flow upstream of the valve head reduces the average pressure of the charge delivered during valve dwell, and the key to getting good power with decent gas consumption is keeping that average as high as possible. Even your conical spring may be messing with peak flow..it's in the air stream.

Head diameter is a huge factor, tiny changes make noticeable differences..seat type not so much, in my experience. The undercut head seems to be the most reliable seal.

On my most potent QB, I used a section of 1/4" nylon air line; it's a few thousands bigger ID than the poly tube..then an oval twin port in the barrel above that..that may have helped

You might consider a longer wire probe. I haven't done a lot of experimenting with length, but my feeling is that with short dwell and lowish pressures, putting a larger percentage of the charge past the port helps.

I'm like you, in making more than one change at a time....it does make it tough to pin down what happened.
Last edited by classicalgas on May 28th, 2012, 5:55 am, edited 1 time in total.
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Joined: May 29th, 2004, 2:30 am

May 27th, 2012, 9:18 pm #5

I've got the full volume of a 78 tube between the reg and valve, call it something very close to 50 cc's. Port work, I assume you're referring to the transfer port, the valve exhaust port is stock, the barrel is at .166" IIRC, or about the same as the poly tubing. Don't see how opening it up more will gain me anything. I could take more metal off the front of the valve, but again, I don't see how I'd gain anything, it already has far more free flow than the valve can use. I'm using a tapered spring, .346" OD at the big end down to .251" at the small end, which fits on a spigot turned on the valve head. Pretty soft, wire is only .027" vs. .035" for stock, and 13 vs. 6 coils. My pedestal is really more of a spring guide, just a short Delrin piece that sits in the recess over the hole in the front of the valve extension and keeps the big end of the spring in place.

I will say, I was surprised at the very large difference in velocity I got with differing valve head diameters. The actual seat profile doesn't seem to matter, I've tried flat, conical, and "toilet plunger" style, the only difference was in quality of seal, the plunger style seems to give me the best seal, I've had valve leaks of varying severity with the other styles.

I've had to do some back tracking, I have a terrible habit of making more than one change at a time . I'd opened up the valve port on one valve to .228" (#1 drill), and used a 3/8" plunger style valve head. Power dropped from 30 to just over 20 fpe. Two changes at once, didn't know where the problem was. So I tore it down again, went with a stock valve port and the big valve head, no change. 5/16" valve head, back up to 30 fpe with either size valve port. This series of tests, more than anything else, is what has me thinking I may be at some kind of limit, and logic tells me that the limit is regulator pressure. But I'm at 1200 psi now, I don't think I want to push a stock QB tube much higher than that. And besides, 30 fpe from that is pretty respectable, don't you think ?

BTW, don't take anything I've written here as meaning I'm NOT going to try your suggestions, I almost certainly will. I also still have one untouched valve on hand, which I may do some more back tracking with, kind of a reverse base line thing. Later, and thanks.

Dave
Dave,

Have you shortened the threaded section of the of the valve body? You can shorten it to about 0.375" above the shoulder where the o-ring is sandwiched.

The valve body bore can be bored out to 0.475" to allow more air flow around the pop-it valve head.

You might want to try a search and see if you can find anything on the subject by Rich from Michigan. I know he has done a few big bores (.308) based on the QB with a modified stock valve that have produced about 100 fpe.

Good Luck!



Dan

''If everything seems under control, you're not going fast enough!'' Mario Andretti
Last edited by VWDan on May 28th, 2012, 11:20 am, edited 1 time in total.
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