Lift / Dwell Quiz

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Stubbers said:
Do you propose that if I try to shoot a 3 gr pellet at 20 fpe it will take the same lift/dwell relationship as a 30 gr slug shot at 20 fpe in 22 cal?

-Matt
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3gr at 20fpe is way into supersonic. That would result in choked flow and greatly reduced pressure behind the pellet. You’ll likely need to increase dwell to make up for that.

The tendencies that we have been discussing only hold true for subsonic.

Long dwells with small ports can result in choked flow even with subsonic muzzle velocity.
 
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Scotchmo said:
3gr at 20fpe is way into supersonic. That would result in choked flow and greatly reduced pressure behind the pellet. You’ll likely need to increase dwell to make up for that.

The tendencies that we have been discussing only hold true for subsonic.

Long dwells with small ports can result in choked flow even with subsonic muzzle velocity.
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3 gr @ 20 fpe is obtainable but you're right, it would take a lot of dwell for such a light pellet, but it greatly demonstrates the point....pellet weight can work both ways, too heavy, too light, just right.

a 3gr @ 1750 fps
a 10gr @ 950 fps
a 30gr@ 550 fps

All 3 will have very different lift/dwell profiles...

-Matt
 
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Stubbers said:
This is a trick question for some, and a simple one for others.

1).What is the biggest contributing factor that:

a) Changes the relationship between lift/dwell in a pcp at a GIVEN energy level (changing a variable while keeping energy output the same, so for example a 20 fpe .22 cal, what can you change while keeping power at 20fpe that alters the lift/dwell relationship the most)

2) Whats the second biggest contributing factor that does the above?

I'd make this multiple choice, but that is rather not fun.

-Matt
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Without reading to many comments..
I’d saying changing pressure behind valve.

For #2….I’m going to go with transfer port size,
 
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corny said:
Without reading to many comments..
I’d saying changing pressure behind valve.

For #2….I’m going to go with transfer port size,
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You got the top 2 first try, kudos.

RScott said:
Transfer port restrictor settings. Restricting the TP cause a back-up of pressure that keeps the valve open longer.
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Ding ding! That is #2. Transfer port restriction / choking flow!
Either this or increasing/reducing the valve throat/seat diameter have near identical effects.

Plenum volume would be #3 and pellet weight #4.

If you setup a .22 with a 3 cc plenum, it will technically be about a dump shot making 20 fpe, opposed to a .22 that has a 40 cc plenum, it will only take a blip of air, much shorter pulse of dwell, with increased average shot pressure, resulting in a more efficient shot that required less air mass to produce equal power.

One could argue the order but within the nominal scope of use in airguns, I think pressure comes first, choked flow second, plenum volume third and pellet weight last, although I could have one or another mixed around in order, they all greatly contribute to effecting the relationship of lift/dwell, opposed to altering hammer weight and spring alone to maintain the same power, which primarily only effects the lift curve (barely).


Also note the top two reasons are how bell curves in an unregulated gun works, you generally choke the flow and shoot between two pressure set points that while choked with hammer spring leaned on, results in desired fpe at your highest fill pressure, and as pressure drops and valve opens further, that flow becomes less choked for the pressure and hammer begins opening the valve further, resulting in your bell curve.

-Matt
 
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Stubbers said:
70 gram hammers, reminds me of my stock marauder. Sadly to greatly reduce hammer weight without making any other changes you have to greatly increase the spring rating to restore power. It's one reason people either run balanced valves, thinner valve stems that reduce the closing force imparted on the head of the poppet during the shot cycle, pilot valves, lower reg pressure, or simply lesser rated/no valve spring and thinner poppet sealing margins which all require reduced hammer weight/spring ratings to keep power the same upon changing.

For the .20 / .22 caliber..

You'd be best reducing the regulator pressure so that your target velocity is 97-98% just like the 25 cal, so 930 fps plateau for a 900-910 fps tune, and running lighter hammers or lighter hammer springs (hammer weight is better to chop off first). Either that or choke the flow with a smaller transfer port on those two smaller calibers, but this is the less efficient approach than regulator adjustment, just less work.

-Matt
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I just shaved 8 grams off the hammer or about 11.6% - let's see how much of a difference that makes all other things - K.
 
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Stubbers said:
..

… I think pressure comes first, choked flow second, plenum volume third and pellet weight last, although I could have one or another mixed around in order, they all greatly contribute …

-Matt
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All things considered…agree.

But pressure from where? Choked flow and insufficient plenum volume each result in reduced pressure in the barrel. So basically, #1 could be just pressure. And #2 is pellet weight (projectile mass).
 
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SpiralGroove said:
I'm going to reduce the reg. to 95 bar or a 10 bar drop and see where she's at.

I also want to shoot the 15.89 grains at 900/910, so trying to find a happy balance - even though the 13.73's so far are more accurate.
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A really good trick to shoot different pellet weights without changing reg pressure is any method of choking the transfer path, usually adjustable dials located in the transfer path, these are most repeatable, not sure if RAW comes with one... I need to develop one for my marauder as I don't use the stock metering screw (pain to be super repeatable with).

I do run a valve lift limiter at times which also works great but relies on the hammer, it basically chokes the lift if I so please, fx runs similar. Its more efficient but less reliable than the above as it relies on consistent hammer strike, still pretty neat if you dial in these. This is most efficient but least repeatable, I'll rate it #2 method of choking air.

(Also Scotchmo's electric valve could do the same, limiting the lift of his valve and/or dwell, choking the valve and obtaining less fps, although better for higher/sharper lift for efficiency)

The last place you can choke but last place you want to choke is at the bolt probe, you can have one that is adjustable by design or swappable, where one restricts the path flow for your desired fps and another for another fps...although this is least efficient of the three hence why its last.

-Matt
 
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.20 caliber at 95 bar -
13.73 max 972 fps; - 15.89 max 922
Hence .97 = 943 fps & 894 respectively.

ES on either pellet should be about 1.5%, I bet the 13.73's will be OK at around 925.

I'm calling it good enough until my shooting says otherwise.

On the .22 caliber I will reduce set-point from 125 bar to 115 bar tomorrow.
 
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Went out to the range today and shot the .20 caliber at 50/100 yards; the wind was blowing a little.
At 50 yards the 13.73 (3 groups) were <1 MOA (4 shots - I can't count) 2 very near MOA .58, while the 15.89 were six in the absolute, same hole and then a flyer 3/4" off:cautious:. I don't shoot many groups or for very long.

Went to 100 yards with the 15.89; they had little command. One 5 shot group was 1-1/16", but the other two were like a shotgun.
Hence, nothing to conclude at 100 yards, except for a .20 cal. & 15.89 gr pellets - you better be on your game:whistle:.

I'm finding my (same forward/back) head placement at the scope is turning out to be critical.

Not a wasted day, but I hoped for better:unsure:.
 
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Shooting airguns outdoors on non-calm days is not forgiving, especially at distances you're shooting!

Really hard to determine fliers in such conditions unless you know its steady calm and you have surrounding elements helping!

I'd say landing a 15 gr pellet within 1-2" at 100 yards is always going to be a pretty sweet victory, the bc of the lighter pellets is not favorable for 100yds.

-Matt
 
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Stubbers said:
I am looking for dwell duration, not necessarily fpe, but the two do go hand in hand, you cannot achieve a large shift in your lift/dwell relationship without effecting the plateau power output of your pcp, no?

Very glad you chimed in with pellet weight, it's remarkable how much more this weight effects the relationship than hammer weight and spring when dialed in for the same power.

-Matt
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So let's say I'm going to a faster twist rate barrel in order to shoot heavier slugs, I would need to increase the dwell. Heavier hammer spring ?
 
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MEC17670 said:
So let's say I'm going to a faster twist rate barrel in order to shoot heavier slugs, I would need to increase the dwell. Heavier hammer spring ?
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Added preload with a spacer on for the spring, or heavier hammer, but this is provided you're not already at your plateau for the guns ability.

-Matt
 
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