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Comparison of Transfer port Diameter


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#1 Guest_bmp01_*

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Posted 24 May 2006 - 08:52 PM

Hi there,

Looking for some help / guidance on the above. To expand the subject:

The transfer port on a HW97 / 77 should be 3.2 mm, maybe 3.3 as a maximum. Radiused insdie edge, polished etc also part of the spec. My HW97 is done to 3.2 and shoots nicely, good balance between short firing cycle but not too harsh. I should add it is a 0.22 calibre.

I have more recently acquired a TX200 mk 2 in 0.177 which I believe to be in original condition. I was pleased to see the end of the compression tube could be removed and not being able to help myself pulled it apart for a look. So first off the tranfer port is a sleeve insert, pressed in. It has an attempt at a radius entry into the port but the port diameter is 3.8 mm which surprised me and might help to explain the harsher firing cycle compared to the HW. Area-wise 3.8 mm is 40% up on 3.2 mm.....

Anyone done any experiments on a TX mk 2 transfer port to make it smaller and does it help make things a bit less harsh?
Also, does a 0.177 calibre by itself warrant a larger transfer port compared to the 0.22 ?

BTW I already removed the piston weight which helped a chunk.

Hope someone can help.

Cheers,

BMP01




#2 rich

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Posted 24 May 2006 - 09:52 PM

It would seem, if you've removed the piston weight then the piston is more likely to rebound off the compressed air as it approaches the end of its travel. The larger transfer port would therefore be necessary to let the air through to the pellet more readily.

It's a while since I had a 77/97 apart but I think I'm right in saying the Weihrauchs have rather more preload on the spring than the TX series. The TX always seems easier to reassemble anyway.

#3 Guest_bmp01_*

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Posted 25 May 2006 - 10:47 AM

Hi Rich,

Thanks for your observations.
I should add - prior to me getting my hands on the TX, it's standard spec was with the large transfer port AND piston weight. In that state the rifle was really quite harsh to shoot. It was also slightly on the naughty side power wise. It's pretty common for the piston weight to be discarded to smooth things out - so that's what I did and obviously with less spring preload the power came down, to an acceptable level (low - mid 11's).

IMHO the the large transfer port is wrong. While the pellet is still in the barrel it has limited effect but after the pellet has left the barrel and the piston is still boinging around I think there is need for the port restriction to bring the piston in for a softer landing. But rather than reinventing the wheel i was hoping someone else may have been through the same optimisation process.... And I fully understand anything done after the pellet leaves the barrel can't affect the pellet flight but it might make the shooter (me) more settled and less likely to anticipate the recoil .... it'd be good for my scope and mounts too.

You are quite right about the spring preload HW vs TX. The highly regarded Venom kits tend to head towards very low preload and are noted for lower recoil - you'd think that would be a plus for lower recoil on the TX though.

Regards,

BMP01

#4 stoogey

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Posted 25 May 2006 - 12:21 PM



not a 100% sure here but a .177 porting on a .22 produces more power?...if thats any help...(a fraction two big and its its pooped but you can insert 'special shaped ports')













#5 rich

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Posted 25 May 2006 - 09:58 PM

When Cardew analysed the various parts of the operation of a springer to assess where the greatest power losses were, he found that the transfer port was one of - if not the largest - component that robs efficiency.

#6 Guest_bmp01_*

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Posted 27 May 2006 - 11:14 PM

Stoogey - Thanks. I heard that 0.177 spring in a 0.22 added performance but wasn't aware the same was true with transfer ports.

Rich - Yep. I have read the same.

Anyone else any comments on reducing the transfer port diameter on a TX200 ? Put another way what dia should it be?

Thanks,

BMP01

#7 Jon

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Posted 28 May 2006 - 12:29 AM

Just measured an untouched standard one at 3.74mm.
I always thought out of the box the TX MK1&2 were better, but better things could be accomplished with an HW. Try 3.3 to 3.5mm.
Best thing you can do on them is make a spring guide swage it into the rear block and make the spring a very tight fit over it. Power goes up with no need for any sound deadening fiddling, barely any lubricant and certainly no grease- no twang or resonance.


#8 Guest_bmp01_*

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Posted 28 May 2006 - 12:51 PM

Jon,

Thanks for the measurement - seems like 3.7 - 3.8 might be normal spec after all.
I'm thinking 3.2/3.3 is ok on the HW and that 3.8 on the TX is too big. Area wise 3.8 gives a third more area than 3.3... that's alot.

BMP01

#9 Guest_wchilton_*

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Posted 20 June 2006 - 02:54 AM

You need to consider transfer port length as well as area. While area affects the flow rate from the comp tube into the barrel, transfer port volume is also important to building up pressure and creating that air cushion in front of the piston. If you ever had a FWB 300 apart you'd see that the transfer port area is very near the bore diameter (4.5mm) but it's length is near zero. The effect of this is that "lost volume" is only the volume in the pellet skirt. That's about as efficient as you're gonna get with a spring gun.




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