Thoughts about HDDs

[rsterne]

I was just thinking about HDDs.... Is it possible that they work better on CO2 than on HPA?.... It is a given that all hammers bounce.... The problem is that sometimes there is enough energy in that bounce to allow the hammer to open the valve a second (or subsequent) time.... When you see shot strings published that show the effectiveness of an HDD on a PCP, they usually seem to be most effective as the pressure drops, and pretty much unnecessary on the first half of a shot string in a PCP.... That is because when the pressure is higher, it is harder for the a subsequent hammer hit to open the valve....

I'm wondering if with CO2, there is a fraction of a second where the pressure in the valve is lower, not only because a shot has occurred, but because the liquid CO2 in the reservoir hasn't boiled off to bring the pressure back to "normal".... I don't know the time scale involved, but if the hammer strikes the valve again before the pressure has returned to what is usual for the temperature, it would be easier for the hammer to open the valve again, wasting CO2 in the typical b-uu-rrr-pppp we hear in a 2240, etc...  This time lag during boil-off wouldn't occur with HPA, and the better the flow into the valve on the inlet side, the less likely it would be to occur as well....

Just a theory....

Bob

[Ribbonstone]

YOu've worked a lot with regulated HPA bottles, so i suspect from a pressure stand point, it is a lot like tuning a co2 gun.

Dio think tuning a constant pressure gun (HPA or co2) for efficency is easier than tuning one that has to work over a wide change in pressure.  Also seems that tuning a gun with a large volume of "on deck" gas (be it HPA with a larger volume between valve and regulator or co2 with a large volume of already converted to gas) is easier than working with small volumes.

Really do not know what the time frame from the liquid to gas co2 conversion...but would it kind of equate to the regulator's operating time frame?

[PakProtector]

hey-Hey!!!,
Pressure restoration time would be based on an energy input; it will take some energy to boil the liquid, so how fast can you put it in? Or through what temperature drop will it take place and then rise slowly as the heat soaks into it again. Don't make me bet on timeframe comparison...

Now that I am shooting at about 300 psi less than previous tune with the new valve, I wonder what if any bounce is costing me. I can certainly tell the diff in report tone as it gets to the last shot. Instead of considering some anti-bounce device, I would prefer to examine hammer spring conditions that will offer least risk. For that matter, this could wrap back into the valve dwell discussion before...

Store the energy in the hammer spring, then release it to accelerate the hammer. Hammer hits valve. What happens to the force acting to close it? In one ideal condition, we have zero pressure drop at the tank side of the valve...and along with any valve closing spring we get a maximum rate closing. Looks good for efficiency as long as there is enough loss to keep the hammer from opening the valve again...
cheers,
Douglas

[CitySniper]

Bob-

I have to agree in my case I find an HDD not needed for my .22 Mrod nor do I see the need for a Bstaley mod,10lbs spring,valve work, or regulator.. I am getting a pretty good shot string by just playing with fill pressure, factory adjustments, and pellet selection.

Here are 34 shots from my .22 Marauder shooting 21.30 Baracuda's from 2,900psi down to 1,950psi.

813.00   847.30   849.70   821.90
817.70   846.60   850.00   818.80
818.60   848.20   844.60   815.10
819.80   848.90   839.80   810.50
838.50   853.00   837.70   
823.10   849.80   837.30   
828.80   861.80   839.40   
828.40   844.40   826.60   
834.70   854.90   830.70   
838.70   852.00   824.00   

[rsterne]

The regulator response time may well be similar to the CO2 boil-off time.... so if you are running a small valve/plenum volume they may act in a similar fashion.... If the plenum is large (and perhaps if the gun is bulk-fill CO2 with an open valve inlet) then the pressure change during the shot will be small, and the hammer should act much like it would in an unregulated PCP....

I think that if the pressure is constant, it should be easier to tune for no bounce (ie no re-opening of the valve), particularly if you are operating at high pressure and light (relative) hammer strike.... If you are right on the peak of the velocity curve (unregulated) or just off the velocity plateau (regulated) then you may get a small amount of bounce but from my experience the efficiency doesn't seem to suffer much.... The time you get bounce which wastes air is when the hammer strike is MORE than it needs to be.... That occurs on the low pressure part of the curve (unregulated) or if you are operating up on the plateau (regulated).... and occurs in any CO2 gun that is venting liquid or otherwise wasting CO2 (like a stock 2240)....

As I typed this, I realized how important that statement was.... Perhaps this is the key to understanding if an HDD will help.... To repeat:

You can get hammer bounce which wastes air is when the hammer strike is MORE than it needs to be

Bob

[Ribbonstone]

Might be able to do better depending on how much tank pressure was used.  That looks like 3% of max coming out to about 18 shots (4% coming out to about 23 shots).

Problem with much of the HDD tests is this:
1. Its often comapared to a none efficient non-HDD tune.
2. Installing the HDD usually gives a bit LESS energy.  But the tester doesn't take out the HDD and retune the rifle to run at the SAME energy for comparison.


With a wel tuned rifle, adding an HDD usually works out to 5-10% more shots once the rifle is tuned back up to the original energy (or once the rifle is tuned down without the HDD to the same energy as with it).

Now 10% isn't garbage; it is certainly something.  In co2 terms, it's like paying for a package of 10 12gr. but getting 11.

[PakProtector]

Might be able to do better depending on how much tank pressure was used.  That looks like 3% of max coming out to about 18 shots (4% coming out to about 23 shots).

Problem with much of the HDD tests is this:
1. Its often comapared to a none efficient non-HDD tune.
2. Installing the HDD usually gives a bit LESS energy.  But the tester doesn't take out the HDD and retune the rifle to run at the SAME energy for comparison.


With a wel tuned rifle, adding an HDD usually works out to 5-10% more shots once the rifle is tuned back up to the original energy (or once the rifle is tuned down without the HDD to the same energy as with it).

Now 10% isn't garbage; it is certainly something.  In co2 terms, it's like paying for a package of 10 12gr. but getting 11.

I didn't even go back and re-tune  to HDD levels once I took it out. Installed the HDD( that confounded AoA one ) and got the same number of shots but at lower energy. With the AoA thing there are also other issues that make things difficult like its change to the bolt travel and the mess this makes for magazine removal. 'just slide the trigger group one way or the other'...well, in addition to not suffering inefficiencies that could be remedied with the HDD mine has effectively no trigger group travel with the bolts loose...

I would say that it is the poorly tuned ones that seem to benefit that 5-10%, and even a tyro's tuning efforts can do better than that de-bouncing device.
cheers,
Douglas

[TimmyMac1]

The effectiveness of any device controlling the hammer bounce will have a lot to do with how far off the hammer momentum is from the ideal.
I've seen instances where the geniuses of HDD induced hammer bounce by swapping hammers for heavier units so the numbers would be more dramatic and commercially viable to suggest a HDD was needed.
A bit like a tire guy throwing nailes into the street. I suppose if you can induce demand & if the product is well received you have created a demand for your own widget. Like salty chips makes you thirst, heavy hammers need taming. If the hammer weight and springing are spot on hammer debounce has little effect.
I always come back to accuracy which is a function of consistency. Drag on a hammer almost always induces a wider spread between readings. We always have avoided greases and viscosity variable products as well as thermoelastic products affecting hammer movement.
When drag is a greater percentage of the total forces the accuracy suffers despite the increase in shot count and/or consistency of velocity. For that reason I want to see someone set a world record or win siomething with a gun fitted with an HDD and then I may change my tune. SHOW ME by BEATing ME!
Since bounce extends valve duration it may be that systems balance themselves better when they are bouncing a bit. I know my gun in Genration 1 trim was bouncing. Bouncing to an FT Nationals and more.

HDD's are the answer to a Question nobody was asking. Look at the $ savings in air assuming a 10% improvement in air usage and the HDD would never pay for itself. Can you say "RUBISH?"

[PakProtector]

There are a few issues I have with the currently available Marauder HDD. First, there is no evidence published that they are more effective than adjusting travel/preload settings. Also, a lot of the 'proof' I see is missing critical bits, like 'before' shot strings. That it is possible for Crosman to deliver them in a state that they'll suffer count reducing hammer bounce is likely. That that behaviour needs a +$40 device is not clear at all...

I would also complain about the term, 'drop in' for something that needs serious fiddling if( more often, when ) the bolt position changes and magazine removal gets difficult or impossible.

What I'd like to see is decent experimental method delivered in the fashion of Consumer Reports...but I don't think most of the customer base cares. Getting that to happen would be most useful...
cheers,
Douglas

[Bill G]

I strongly agree with Tim.  The short number of years that I've been into PCP's has afforded me an ongoing and enjoyable education.  I have been a big fan of the B-staley tune process and have recently decided to put it aside.  The HDD never setteled with me.  I would see post after post of how they got 2,3,5 extra shots and always, the velocities suffered by 4% or much more.  Douglas is right when he questions the drop-in aspect of the HDD.  On another forum, dedicated to the Marauder, I may have irritated some on this subject because I wasn't sold.  The fact is, no-one has conducted a serious study of the product and I think the study would be for nothing.  The use of the B-staley method has educated me on the workings of hammer and valve interaction.  Tim is very accurate in his belief of a good tune over a marketed device.  The HDD imparts drag on the hammer and Tim points out the poor out come of such.  The points that Tim and Douglas make are exactly what I had tried to convey elsewhere.  That is why I chose the B-staley method so readily.  It didn't induce hammer drag nor did it cost $40 but what it did do, was allow me to grasp the mechanics mentally.  I now see no reason for the B-staley process unless you are very new to the process of tuning your PCP.  I believe that it gives a more realistic look into the dynamics of the valve mechanics, by allowing you to somewhat seperate the spring tension from the hammer travel aspect of generating power output.  This method also affords the opritunity to have imperical proof of valve stroke.  By having a spacer that is equall to the total amount of the valve stem travel, you can see an exact fps drop when the calculated prediction of stroke equalls what you have adjusted into the tune with stroke adjustment.  Example; The spacer allows .250" of stroke but if no stroke is added to the adjustment, the valve will not open(dependant upon the material used as spacer.  Soft materials compress).  3 CW turns will add .150" of stroke.  In my case, with an unballanced tune, this will show up in the recorded velocity percisely when the calculated stroke and shot indicate stroke is at .150".    Bob, Lloyd, Douglas, myself and others have had excellent discusions on the subjects involving hammer v.s. spring and the like.  Infact, I was going to start a thread this evening concerning hammer mass, distance hammer traveled before impact with the valve stem and spring tension.  I have a couple of thoughts that I was wanting to clear up. 
As for CO2, I can't offer any opinion due to lack of experience.  With the temperature sensitivity that CO2 offers, I have always found it less than desirable. From my limited knowledge of PCP's, HPA is the way to go.  Focus on the correct combination of Porting, hammer mass, hammer spring and hammer travel.  This will render best results.  Addind some new device when all the other dynamics aren't understood, only complicates things and slows the gain of good knowledge. 

Bill   

[PakProtector]

hey-Hey!!!,
While I'd like a magic bullet, I am petty sure no such thing exists. The post-peak velocity air use in an unreg'd PCP is of intrest. I am at a bit of a loss as to trading pre-peak velocity hammer behaviour for good behaviour past the peak. Bob mentions having more energy than is needed at the low pressure operation, and I am wrestling with how to get it reduced w/o shorting the high pressure shooting.
cheers,
Douglas

[rsterne]

That exists.... It's called the bstaley O-ring mod.... There is an excellent explanation of how it works on the Green.... I'll see if I can get Mr. Staley to post it here....

Bob

[Ribbonstone]

have become a believer in the BStaley o-ring mod.  Not for every application as it does not give the highest possible energy, but it does do a nice job of evening out the shot count graph into more of a plateau than a rounded hill.

[Motorhead]

Did not wish to clutter thread with my minds wanderings .... started another thread on HDD ideas.
See: http://www.gatewaytoairguns.org/GTA/index.php/topic,37692.msg350674.html#msg350674

[PakProtector]

have become a believer in the BStaley o-ring mod.  Not for every application as it does not give the highest possible energy, but it does do a nice job of evening out the shot count graph into more of a plateau than a rounded hill.

Thanks for the encouragement for the bstaley mod. Will get some 211 o-rings shortly. Now I am not going for maximum power...though that is a matter of opinion; I am going for 880-910-880 with 21.3 gr Barracuda...

Read a bit, and will put moly powder on the o-rings. I have a 10# spring in case it is needed. I will be Dancin' in the Street if this gets me 30 shots... 20 within 3% is more likely. More fun to be had. I saw one set of instructions that looked remarkably simple; dial up spring preload for a peak in the 2600 psi neighborhood, and then travel to deliver the required velocity. I can did that...
cheers,
Douglas