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DON"T DRILL OUT YOUR TRANSPORT VALVE ON 1377's,etc.
https://www.airgunforum.ca/forums/topic50590.html
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Author:  MyCrosman [ Fri Nov 02, 2012 2:37 pm ]
Post subject:  DON"T DRILL OUT YOUR TRANSPORT VALVE ON 1377's,etc.

Here's something interesting: Taken from an article on air flow through a reduced opening:

"Again, at the exit end of the gun, a nozzle reduces the diameter of the exit, increasing the speed of the air and abrasive."

So I guess if someone drills out the restricted transfer port on 1377's,etc. the air speed to the pellet is REDUCED.

Author:  rsterne [ Fri Nov 02, 2012 4:01 pm ]
Post subject:  Re: DON"T DRILL OUT YOUR TRANSPORT VALVE ON 1377's,etc.

Except that past the transfer port the passageway expands again, slowing the air.... In most cases, having all the ports matching (ie constant in area) works the best.... With a bottom port like on a Crosman, that limits you to about 3/4 the bore size or the pellet may be damaged on loading.... For a .177 that is 0.0134", which is the size of a stock Crosman barrel port.... 22XX and 13XX transfer ports are usually 0.140" unless restricted to detune the gun.... Therefore in most cases, there is no benefit to going larger than that in .177.... In a .22 cal, however, you can often go up to about 0.162" and get worthwhile gains....

Bob

Author:  MyCrosman [ Fri Nov 02, 2012 4:46 pm ]
Post subject:  Re: DON"T DRILL OUT YOUR TRANSPORT VALVE ON 1377's,etc.

If there is no point in drilling out the Canadian transfer port ,then why does the U.S. 1377's have a BIGGER transfer port?Why reduce the Canadian one if it has no effect in pellet fps? There's a vid on you tube and this guy is machining 1377 valve ports and boring them out to 3.95mm from what he says start out at 1.9mm.It supposed to give you 50fps from an otherwise stock gun.I think we need a physicist. I'm confused.

Author:  rsterne [ Fri Nov 02, 2012 5:46 pm ]
Post subject:  Re: DON"T DRILL OUT YOUR TRANSPORT VALVE ON 1377's,etc.

Quote:
22XX and 13XX transfer ports are usually 0.140" unless restricted to detune the gun....

Read that again.... Canadian 1377s use a transfer port that is about 0.078" IIRC.... You are the one that starting this thread by stating you would lose velocity by drilling out the port.... That is incorrect, providing you don't go too far.... No Physicist required, just practical experience....

Bob

Author:  MyCrosman [ Fri Nov 02, 2012 6:06 pm ]
Post subject:  Re: DON"T DRILL OUT YOUR TRANSPORT VALVE ON 1377's,etc.

Well this science journal says air flow increases as its pushed through a smaller orifice.So HOW would anyone else consider it a restriction of air velocity? Obviously I don't see the whole picture.Anyway,I am new here. Lots of conflicting ideas on this site.Guess I'll have to do trial and error and find out for myself.I want to start modding.

Author:  notec [ Fri Nov 02, 2012 6:11 pm ]
Post subject:  Re: DON"T DRILL OUT YOUR TRANSPORT VALVE ON 1377's,etc.

I agree with Bob.....no need for any edumacated :lol: xperts.......we have many people(myself included) that have many hours(years) of tooling time into our Crosman platform guns. There are people on here that know Crosmans guns better than Crosman themselves, so I wouldnt call out the cavalry just yet :mrgreen:

Author:  rsterne [ Fri Nov 02, 2012 6:38 pm ]
Post subject:  Re: DON"T DRILL OUT YOUR TRANSPORT VALVE ON 1377's,etc.

Quote:
Well this science journal says air flow increases as its pushed through a smaller orifice

I'll bet it doesn't.... It says the VELOCITY of the air flow AT the restriction increases (and incidently the pressure reduces as well).... The FLOW will very likely actually decrease from the added resistance.... In addition, as soon as the airflow passes the restriction and enters the barrel, the VELOCITY of the airflow will slow down again.... Since airguns are driven by the MASS of air moving through them, we want lots of FLOW.... That means, generally, no restrictions in the ports....

Bob

Author:  AirGunEric [ Fri Nov 02, 2012 6:42 pm ]
Post subject:  Re: DON"T DRILL OUT YOUR TRANSPORT VALVE ON 1377's,etc.

Bob is correct- Velocity increases, not volume. Whomever used the term "airflow" in whatever article obviously equates "flow" with "velocity" which is misleading.

Think the of the transfer port like a venturi (i.e. like on a carburetor)- same thing.

Author:  MyCrosman [ Fri Nov 02, 2012 7:14 pm ]
Post subject:  Re: DON"T DRILL OUT YOUR TRANSPORT VALVE ON 1377's,etc.

Here it is here.maybe I just misunderstand the physics of it I guess.
http://www.ehow.com/about_6535612_sandb ... ation.html

Author:  rsterne [ Fri Nov 02, 2012 9:52 pm ]
Post subject:  Re: DON"T DRILL OUT YOUR TRANSPORT VALVE ON 1377's,etc.

No, you understand it perfectly, and that article is correct.... The nozzle in the case of a sandblaster is the LAST part in the system before the abrasive is expelled.... The air accelerates (velocity, not flow), and the abrasive with it.... The air then expands and slows down, but the abrasive, being much denser, carries on at the now higher velocity....

An airgun is different.... The transfer port is NOT the last piece in the system, the barrel is.... The transfer port is smaller than the barrel (in the case of a restricted one, a LOT smaller).... The air accelerates through the (restricted) transfer port, then slows down again before getting to the pellet.... which so far is AT REST.... The air reaching the pellet then has to accelerate it down the barrel.... It is not the velocity of the air at any given point in the system before it gets to the pellet that matters, but the MASS of the air available to accelerate the pellet down the bore.... More MASS (and a longer barrel to accelerate in) results in a higher velocity....

Bob

Author:  MyCrosman [ Sat Nov 03, 2012 5:10 am ]
Post subject:  Re: DON"T DRILL OUT YOUR TRANSPORT VALVE ON 1377's,etc.

Ok.then I will assume it is safe to drill out the transfer port a little and I should expect close to 500fps without any other power mods.Thats what I'm looking for. thanks.

Author:  rsterne [ Sat Nov 03, 2012 10:54 am ]
Post subject:  Re: DON"T DRILL OUT YOUR TRANSPORT VALVE ON 1377's,etc.

If you don't have a Chrony, I would strongly suggest you don't mod a Canadian non-PAL pistol....

Bob

Author:  MyCrosman [ Sat Nov 03, 2012 1:55 pm ]
Post subject:  Re: DON"T DRILL OUT YOUR TRANSPORT VALVE ON 1377's,etc.

Oh..I didn't realize one could go over the limit so easily.I DO have a chrony on the way. thanks for the heads up.

Author:  doodlecat [ Tue Nov 13, 2012 7:13 pm ]
Post subject:  Re: DON"T DRILL OUT YOUR TRANSPORT VALVE ON 1377's,etc.

MyCrosman wrote:
Oh..I didn't realize one could go over the limit so easily.I DO have a chrony on the way. thanks for the heads up.


Get a # drill index. #1 to #60 any man who does not have a set of # drills should turn in his man card (jokes). Did you ever price a letter drill index? :shock:

Author:  mes4u2c [ Sun Nov 18, 2012 4:13 pm ]
Post subject:  Re: DON"T DRILL OUT YOUR TRANSPORT VALVE ON 1377's,etc.

I have modified my co2 gun, 2240 and increasing the air transfer port definitely increased the fps. However I have a BSA meteor springer, which is a .177. THe transfer port was .125 and it shot at 650fps. So I thought by increasing the air transfer port to .170, close the the barrel size of .177 it should yield an increase in fps. It actually decreased it to 510 fps. I was shocked :shock: and very unhappy. Tested it on a chrony 10 more shots and it was around 510fps. a decrease of 140fps. Luckily I was able to tap the transfer port, put in a threaded brass insert, with a hole set to .125, inserted it in with loctite and the fps increased back up to 650 fps all with the same 9.6g pellet.
Still trying to figure it out.

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