Drilling out the air ports and/or more psi In a QB for more velocity ?

[Austringer]

Mackrel5

You think if I drop the regulator pressure I would see an increase in velocity? That's still a hard concept for me to grasp, but I think you might be right. I'm guessing you think I'm tuned to the knee and lowering the pressure will let the valve open a little more?? If it wasn't so time consuming to make all the changes, I'd probably try it.

In any case, I have updated velocity readings taken this morning under adequate light. With the RVA turned in quite a ways (to the point it's kinda hard to cock the bolt), I got a 4 shot average of 880fps with 14.3's but I was wasting air. I backed it off a few turns to 860 and it sounds a little better.

William. I think we both would benefit from increased plenum volume (as is the obvious case). My target is 29 fpe with 18.1's, so if drilling out the tank block within safe limits and careful RVA tuning doesn't work, I'll be cutting down a QB78 tube to get me an extra 10 cc's or so. Anyone know where I can find a reasonably priced 78 tube?

troy

[Mr. Panther]

Thank you for the information Troy.

[nervoustrigger]

Troy, 880fps with 14.3gr CPHP sounds about right for your regulator setpoint and airflow modifications.  The 79 I set up for Chris was running a little over 920fps with a setpoint 75psi higher and a bored out tank block.  If you do the tank block, that will probably put you right at 29fpe with JSB Heavies.

From there, you can check efficiency to see if it's acceptable for your needs. 

[wll2506]

William. I thought you already built the springless valve? Is this valve different? I finished my valve last week. Need to work on my bolt probe tonight. If we do everything right, I see no reason we should not be able to achieve performance close to what Jason (nervoustrigger) attained with his 1510psi QB79.

Troy

Troy I posted a pic on another thread, but this is what I did today and it works out great, info on other post


wll

[Austringer]

Well, I drilled out my tank block with a 15/32 drill to a depth of about 1/2” leaving plenty of beef behind the block screws. I also beveled the  circumference of the hole a little to pick up a tiny more plenum volume. By eyeball, I’m guessing I gained about one cc. My max velocity went from 880 with the stock tank block to about 892 with the drilled block. No safe way I can see to hog out more volume in the block. Not sure, but I believe the gun is not wasting quite as much air while maxed out with the modded block as opposed to the stock block with minimal volume. But thats just guessing by ear.

Now I have to decide if I will be satisified with the gun in this condition or if I would be happier with a 78 tube shortened to 1.5 inches longer then the QB79. If I picked up 12 fps with 1 cc, would I pick up 108 fps with 9cc’s? I’m sure it doesn’t quite work out that way, but I’m still asking the question as I know there are talented folks out there dying to mathematically explain the answer.

Troy

[rsterne]

As you increase the plenum volume, with no other change, the gun will get quieter and more efficient, because the AVERAGE pressure during the shot is slightly higher, so the valve closes a fraction quicker.... However, that higher average pressure does increase the velocity, so you gain twice.... More FPE and less dwell = higher efficiency....

I compared a QB79 to a QB78 and then to a 78 with the tube shortened to half way between.... That is plenums of about 10 cc, 50cc, and 30cc's.... Regulated at 1500 psi, shooting 18.1 gr. JSBs, with a valve that had been opened up for better flow, but still had about half of the front section (the sides were completely milled away).... The QB79 got 39 shots at 26.4 FPE, the QB78 got 40 shots at 31.5 FPE, and the one with the half-tube got 40 shots at 30.5 FPE.... This was one of the first PCPs where I decided that about 1 cc of plenum per FPE was the right place to be....

bob

[BigTinBoat]

Now I have to decide if I will be satisified with the gun in this condition or if I would be happier with a 78 tube shortened to 1.5 inches longer then the QB79.

you need to get a hold of Troy M on the yellow. He's got 8 of the 78 tubes for $15 each

http://www.network54.com/Forum/79574/thread/1456886660/last-1456886660/--%3E+Expand+Thread

[Austringer]

Thanks bigtinboat. I just sent him a PM.

Unfortunately now I have hogged out my tank block, I have no space for additional pinning to the tube (that I can easily see). However I have thought I might inlet the base of the tube insert on the block to allow deeper insertion of the tube before I drill my new holes for the block screws. This could give me more space between the screw holes and the end of the tube making it safer after upgrading the screws.

Oh and Wll, nice looking valves. I love those spring clips. Wish I would have found them before I got e-clips.

Thank you!
Troy

[wll2506]

Mackrel5

William. I think we both would benefit from increased plenum volume (as is the obvious case). My target is 29 fpe with 18.1's, so if drilling out the tank block within safe limits and careful RVA tuning doesn't work, I'll be cutting down a QB78 tube to get me an extra 10 cc's or so. Anyone know where I can find a reasonably priced 78 tube?

troy

Troy, right now my '79 is getting 810fps with 18.1's and I'm happy with that "for this gun"  I'm guessing it gets above 850fps with CPHP's, more than enough for starlings in the trees at a max of 60 yds, and farm houses 500yds away... A 18.1 gr pellet at higher velocity may put that pellet around their living zone, and I don't want to go there.

The cool thing is we can taylor these guns to a certain need, and do another for another purpose and not be out huge $$$$$$ ----- just time, and frayed nerves !!!!

My '79 in .22 is the general purpose gun.. Starlings, plinking, etc and it has some serious power too :-) This gun is no slouch at 26 fpe and 17 fpe at 60 yds using Exact Heavies if I decide to use them !

My '78 in 177 is my pest bird getter, shooting 10.5 gr Kodiaks at 885fps, it is very flat shooting (important for threading the needle with branches), and penetrates very well.

My '78 in.22 is the most powerful of the three and ment to be that way..this is the gun I would use for serious pest control..... at close to 17 fpe at 75 yds this gun has plenty of poop for ground squirrels, jacks, possum and the like !

All of these have good size tanks (my 17ci arrives Tue) and get a fair # of shots at high fps... They leave my Disco's in the dust
(just say'n) :-)

wll