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Author Topic: 13XX/2289 Piston Testing  (Read 16504 times))

Offline rsterne

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Re: 13XX/2289 Piston Testing
« Reply #60 on: December 25, 2021, 11:13:36 PM »
Look at the photo in the first post.... The stock valve has a cone on the front (top right).... The aluminum piston is on the bottom left and fits onto the front half of the adjustable piston at center left (instead of the flat top piston with O-ring).... I don't know of anybody who makes them commercially....

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Offline VinnyG

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Re: 13XX/2289 Piston Testing
« Reply #61 on: December 26, 2021, 07:20:05 AM »
and is the blue line you graphed for stock valve + mod piston or were both modded??

it really is not clear to me, prolly I'm missing something
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Offline nervoustrigger

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Re: 13XX/2289 Piston Testing
« Reply #62 on: December 26, 2021, 07:59:56 AM »
Yes, the blue line labeled "aluminum piston with cup" is with the stock valve...meaning the end of the valve is the original conical shape that matches the rubber cup.

Only the red line labeled "aluminum flat top piston" has both parts modded...meaning they each have flat faces so they meet with minimal headspace.
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Offline VinnyG

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Re: 13XX/2289 Piston Testing
« Reply #63 on: December 27, 2021, 04:30:40 AM »
Yes, the blue line labeled "aluminum piston with cup" is with the stock valve...meaning the end of the valve is the original conical shape that matches the rubber cup.

Only the red line labeled "aluminum flat top piston" has both parts modded...meaning they each have flat faces so they meet with minimal headspace.
"dang" so the valve mod unless it has more volume is pointless(at low pumps)
and i guess that piston figure would be even better with the poly tube mod you described in your pdf?

so much mods info here, is there a more basic-technical thread list for beginners on modding crosman pistols?

Thanks nervous trigger
« Last Edit: December 27, 2021, 04:36:44 AM by VinnyG »
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Offline nervoustrigger

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Re: 13XX/2289 Piston Testing
« Reply #64 on: December 27, 2021, 12:36:52 PM »
The volume of the valve is unchanged in Bobís comparison so we can see how much pressure is achieved with X number of pumps depending on the type of piston and head geometry.  If any of the configurations had a different valve volume, we could not directly compare them in this way.
 
Incidentally, in most cases you would not want to increase the valveís volume.  Perhaps you might for a 13xx that has been retrofitted with a long .22 barrel but it would be counterproductive for a short barrel in either caliber.  It would just take more pumping strokes to build the same pressure yet it would yield no benefitÖjust louder because the valve would be belching air well after the pellet has left the barrel.  Some folks have actually gone the other direction and inserted something in the valve to reduce its volume so it takes fewer strokes to build pressure.
 
From my perspective, the big takeaway from Bobís excellent study is that there is a massive benefit to using a stiffer piston and placing a shim of some kind behind the pump cup to take up any headspace (make it so the cup squeezes against the valve when you close the pump arm).  At anything up to 10-12 pumps, thereís no need for a flat-top piston or valve.  For example, we can see an almost 50% increase in pressure over the OEM arrangement at 10 pumps.  The reason that is appealing to me is because thatís about as much pumping effort as Iím willing to invest.  I mean, sure it might be fun to pump 40 times and see the max power I can get.  Iíd do that onceÖokay, twice so I can see if itís repeatable.  But thatís about it.  From then on, itís never going to see more than a dozen pumps.
 
Regarding your question about the poly tube transfer port, I think it had been established as a slight benefit for .22 cal and a slight detriment to .177.
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Offline VinnyG

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Re: 13XX/2289 Piston Testing
« Reply #65 on: December 27, 2021, 03:38:40 PM »
The volume of the valve is unchanged in Bobís comparison so we can see how much pressure is achieved with X number of pumps depending on the type of piston and head geometry.  If any of the configurations had a different valve volume, we could not directly compare them in this way.
 
Incidentally, in most cases you would not want to increase the valveís volume.  Perhaps you might for a 13xx that has been retrofitted with a long .22 barrel but it would be counterproductive for a short barrel in either caliber.  It would just take more pumping strokes to build the same pressure yet it would yield no benefitÖjust louder because the valve would be belching air well after the pellet has left the barrel.  Some folks have actually gone the other direction and inserted something in the valve to reduce its volume so it takes fewer strokes to build pressure.
 
From my perspective, the big takeaway from Bobís excellent study is that there is a massive benefit to using a stiffer piston and placing a shim of some kind behind the pump cup to take up any headspace (make it so the cup squeezes against the valve when you close the pump arm).  At anything up to 10-12 pumps, thereís no need for a flat-top piston or valve.  For example, we can see an almost 50% increase in pressure over the OEM arrangement at 10 pumps.  The reason that is appealing to me is because thatís about as much pumping effort as Iím willing to invest.  I mean, sure it might be fun to pump 40 times and see the max power I can get.  Iíd do that onceÖokay, twice so I can see if itís repeatable.  But thatís about it.  From then on, itís never going to see more than a dozen pumps.
 
Regarding your question about the poly tube transfer port, I think it had been established as a slight benefit for .22 cal and a slight detriment to .177.

agreed, less pumps the better
 if there was no loss I'd go for bigger valve/TP

in a case like this tho: https://www.gatewaytoairguns.org/GTA/index.php?topic=45838.msg156257639#msg156257639


would shimming be enough or do you need to file the valve's tip?

just curious, the gun shoots perfectly out of the box(unlike a 2100 i had to return)
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