Tuesday, June 17, 2008

The 1377 Project: Valve and Port Tinkering

So I started looking for ways to make the air flow...

I could see too much of the bolt tip through the barrel port.

So I turned it down even more.

Now there is less restriction.

I replaced the stock valve spring(L) with a much lighter spring(R). The only down side is that the gun must be cocked, thus taking the hammer off of the valve stem, in order to start pumping air in.

I turned down the diameter of the valve stem just a tiny bit as well, where the air flows around it into the valve.

I also drilled out the valve port a smidgen bigger.

I put it back together. Velocity with 10 pumps was 733 on average, a slight improvement from 726 fps. The gun also dumped all the air with each shot, which makes it more consistent. I felt I could achieve more though.

So I drilled out the transfer port steel tube to .157" (#22), from the stock .144" This increases the area of the port by .041 sq/in. Every little bit helps. This is, in theory, the smallest orifice that air must flow through in the system now.

I also drilled out the barrel port. I jammed a couple of pellets in just at the port so as to reduce the burr left by the drill. Pushing them out removed the burr.

So, now it averaged 739 fps at 10 pumps (CPL's) with some shots finally breaking into the 740's and the velocity spread was 736-742, which is very consistent. At 5 pumps I got 572 fps. on average, which is no improvement. This makes sense as the flow at lower pressures probably didn't change as much as at high pressures, over the time it took for the pellet to leave the barrel. While I don't have much interest in making an ultra-maxi-steroid-pcp pumper, I did try 12 pumps and 15 pumps just to see...at 12 pumps I got 780 fps. and at 15 I got 838 fps., with air left over in the valve.


Anonymous said...

This is neat. I like reading your 1377 posts. Thanks for doing this.


Anonymous said...

I liked your idea about using a couple of lead pellets to prevent the burr forming.
Thanks for the information.

Nick Carter said...

I got the idea somewhere on one of the forums, seems to work pretty good! It would probably be even better if I'd used a couple of round balls rather than pellets.

E in V said...

Super information on the 1377. Do you know what fps you were getting before installing the longer barrel? I was wondering what differences would be from that change (I don't have fancy machinery to work with like you do). I like that "sci-fi" muzzle brake you added to it. Very informative.

Nick Carter said...

I didn't measure that particular 1377 before the longer barrel, but I did get 481 fps. with Crosman lights (7.9 gr) on another old one. The fps. they list for a new gun is 600 fps. but that's likely with a lightweight pellet.

Anonymous said...

How increase fps ,change the much lighter spring into valve ?

Nick Carter said...

There are a bunch of tweaks for improving velocity, best roundup I've found is over on the green crosman forum.