Air Gun Home :: View topic - QB 78 on HPA

rsterne · Moderator

I've been working on the QB78 I recently purchased that was holding CO2 but not working.... The hammer strike was too weak to open the valve at all at over about 85*F.... I tore it apart, couldn't find anything wrong, polished everything up and reassembled it and it worked like a charm.... I assumed it was some dirt hanging the hammer up....

Anyway, I made an HPA fill fitting for it and proceeded to do some testing as a non-regulated PCP, just using the stock air tube (which is about 44cc).... Before the initial testing I pulled the filter out of the valve (there was no screen), drilled the bolt flow-through out to 5/32", and I got 34 shots within 10% of peak (23 within 4%), shooting from 1600 psi down to 600, and the velocity peaked at 603 fps at 1200 psi.... I had heard that QBs had a very strong hammer spring, but there was no question it wasn't capable of doing the job at over 1600 psi at all.... the velocity fell to only about 300ish there.... The efficiency at this stage was a staggering 1.96 FPE/CI....

Next I deciced to get rid of the very restricted transfer port, so I bored a shoulder in the valve and breech and fitted a poly port.... This gained 100 fps, but dropped the shot count down to 18 within 10% and 13 within 4%, and dropped the top end pressure for the 10% range down to only 1500 psi (with the bottom still at 600).... The peak velocity was now 701 fps at only 1100 psi.... and the efficiency was still very good at 1.58 FPE/CI....

Today I pulled the valve out and did a major chop job on it.... I machined two 5/16" wide slots about half way down the sides of the valve to effectively eliminate the valve having any restriction to the flow at all.... I got rid of the peircing pin, inner sleeve and washer, I machined spring seats in the end of the valve and on the poppet, fitted a longer spring, tapered the poppet and the surrounding housing, drilled the exhaust valve throat out to 7/32" and rounded the corner on the exhaust port.... Basically, the biggest restriction is right at the valve seat, everything ahead of that is wayyyyyyyyyy bigger.... The idea was to increase the average pressure during the shot by using the entire 44cc tube as the valve....

I'm pretty pleased with the results, especially at the low pressures involved.... The shot count dropped to only 9 shots within 10%, and 5 within 4% of the peak velocity, which was 851 fps at just 1100 psi (fill to 1400, refill at 600).... Shocked

.... The efficiency is still 1.28 FPE/CI, a fraction better than what a Disco returns at the same power level.... Here is a graph of the results so far.... JSB 14.3 gr. Exact Express pellets....

OK, so I've got a gun that breathes VERY well, producing 23 FPE at only 1100 psi.... The shot count is short, but a regulated tank would fix that in a big hurry.... Here is the problem.... There has to either be something wrong with this hammer spring (not hardened?).... or all the rumours I have heard about the hammer spring in a QB being good to about 2000 psi (some say more) aren't true.... Comments?....

Bob

AirGunEric · Site Admin

Who are the "all" that claim the QB hammer spring is good to 2000psi?

I've always advised people that 1600-1700 is the maximum that can be done with it, and I believe this was arrived at through the extensive experimentation of DerekVinyard with the chrome-moly tubes for HPA use.

No, I'm not trying to be funny- but who/where are all these experts claiming 2000psi is workable with the stock spring?

One thing I have noticed over the last few years, many people either guess at things, or exaggerate things to make their efforts seem that much more spectacular. And while exaggerating the capabilities of a stock hammer spring in a QB hardly seems like a real ego building sort of thing- people can be odd.

All that being said, I do believe I had one QB that worked to around 2000psi using a stock hammer spring- but it was the exception, not the "rule".

rsterne · Moderator

Interesting, I got a response on the CAF that a stock spring works to 3000 psi.... Shocked

When you say 1600-1700 psi, do you mean that is the TOP of the sweet spot (ie where the maximum velocity occurs).... or the highest pressure that can be used before the valve is locked so badly the velocity is dropping like a stone?.... I was literally getting in the 300s at 1700 psi....

Do you have stock QB hammers springs available?....

Bob

AirGunEric · Site Admin

Yes, I was meaning complete drop off by 1700psi- just as you are experiencing.

Do not have QB hammer springs- but would very much like to find some that are stronger than the stock units for use with the HPA tube kits.

I did find where all the "experts" were getting 2000+ psi with a "stock" hammer spring- but I also noted that it seems to be the same guy who "tuned" all the rifles- quite likely meaning he replaced the spring. In my experience, this tuner will do all sorts of things and just not mention it so as to not create a name for himself making 'firearms' out of 'non-firearms'.

SMP · Silver Status Member

Excellent stuff Bob.

I'm going to read over this in great detail because as you know I'm just about to start the same journey

1.96 fpe eh....See I told ya my 1.5 was a solid guess Mr. Green

rsterne · Moderator

Said "tuner" claims the hammer springs are dead stock.... velocity peaking at 2600-2700 psi.... no shims or preload adjuster....

I'm beginning to wonder WTF is going on here.... Rolling Eyes

Bob

AirGunEric · Site Admin

Talk to DerekVinyard- he did alot of testing and experimentation a year or two ago and found the same thing we're finding- not 2000 or 3000psi as far as I recall.

Perhaps the factory changed the springs at some point- my testing was done about 2 years ago, DerekV's over a year ago, perhaps if your rifle is about the same age, and the others are newer...

Can you pull the hammer spring, take a picture and post dimensions? I can perhaps rip a new QB apart on the weekend to compare.

Gippeto · Member

Or perhaps someone might be attempting to protect what they view as an income?

Magnum and RVD used to do a heck of a lot of qb tunes...maybe they're still at it...don't know.

So...that leaves...what DO I know.(Never know...might be less than I think 'Tard

) I'm re posting this from elsewhere, so the velocity and pellet weight aren't the same. Still makes a valid (and interesting) point, I think. Smile

Wanna do some math Bob??? Smile

Calculating the pressure required to achieve a given velocity with some known variables:

MV = 900fps
Pellet = .218" dia with a mass of 15.9gr
Barrel length =23"

Some derivations/conversions from the given information:

Bore area =.0373 in2
Pellet mass = .00227 lbs
Barrel length = 1.917 ft

Average velocity will be 1/2 of muzzle velocity...450fps

Time to travel the length of the barrel will be 1.917ft/450fps... .00426 seconds.

Acceleration = change in velocity divided by time ..... 900fps/.00426 seconds ..... 21126.761 fps2

Force = mass * acceleration .... .00227 lbs * 21126.761 fps2 ....47.96 lbs of force.

Pressure = lbs force divided by area upon which it acts ..... 47.96 lbs / .0373 in2 ....1285.791 psi ....lets just call it 1286 psi eh.

So IF we could maintain 1286 psi in the bore behind the pellet, we could theoretically achieve 900fps with a 15.9gr pellet in .22cal. We're ignoring bore friction and drag loses throughout the system, but this is a "rough idea" only..so I'm not gonna sweat that.

Factoring those loses in would be a pain...let's just say that to hit 900fps in a 24" barrel with a 15.9gr pellet, we need MORE than 1286psi.

If the math is correct..and without re-crunching, I'd say your tune is a bit more than "good", it's damn near perfect. Cool

Al

rsterne · Moderator

HI Al....

According to my spreadsheet, to achieve 850 fps with a 14.3 gr. pellet from a 24" barrel driven by a 44cc chamber at 1100 psi.... only requires 59% effiency.... The valve would only be open for the first 7" of pellet travel, and when it closes the pressure would be 1000 psi and the velocity would be 600 fps.... At the muzzle, the residual pressure would be down to only 300 psi....

So I don't think my tune is quite as good as you do.... but on the other hand, 59% ain't half bad from my experience with other guns....

PM to follow....

Bob

rsterne · Moderator

It's been an interesting afternoon.... I discovered that my hammer spring is 0.046" wire instead of 0.051", hence the weak hammer hit.... I found out that there is room for five 1/16" thick #10 washers on the spring guide.... I tried various setups and decided that three washers (3/16") under the back of the spring gave me what I wanted.... Here are the results (red line)....

As you can see, the velocity now peaks at 1800 psi, at 1016 fps with 14.3 gr. pellets (32.8 FPE).... Since I plan to run a tank regulated to about 1600 psi and fitted with a 1.8K burst disc the hammer spring is now up to the job.... Here are the peak velocities with heavier pellets....

18.1 gr. 940 fps 35.5 FPE - The efficiency is 1.16 FPE/CI with this pellet....
25.2 gr. 841 fps 39.6 FPE
32.1 gr. 762 fps 41.4 FPE

During experimenting I actually tried 2 washers, then went to 4 before deciding on 3.... With 4 washers, I got the following results....

14.3 gr. 1050 fps 35.0 FPE
18.1 gr. 971 fps 37.9 FPE
25.2 gr 875 fps 42.9 FPE
32.1 gr. 800 fps 45.6 FPE

Nearly 46 FPE is a whack of power from a QB.... I would imagine that with a higher pressure tube and a stronger hammer spring the gun could at least equal my Disco, where I got 52 FPE in .22 cal.... I did some quick calculations, and using a 13 CI tank regulated to 1600 psi the gun should be good for about 40 shots at 35 FPE.... Mr. Green

Bob

AirGunEric · Site Admin

What was the maximum pressure you could get the valve to release with the shimmed hammer spring? Or, this being the stock tube, you didn't go higher than 2000psi?

rsterne · Moderator

I didn't go higher than 2000 psi....

Bob