Misconceptions About Valve Lift

Almost every week I get a PM asking about increasing (or decreasing) valve lift to change the performance of a PCP.... People seem to have the idea that how far the striker/hammer can physically open the valve before hitting the back of the valve is the lift, and 90% of the time that is NOT the case.... Only with the Korean guns where the power wheel adjusts a buffer on the back of the hammer, or a gun running a bstaley O-ring buffer, does the hammer ever hit anything except just the end of the valve stem, unless the air pressure is very low (compared to the hammer strike).... Some CO2 guns, such at the QB7X or XS-60c, have only about 1/8" of valve stem sticking out behind the back of the valve (or rear block), and when running CO2, and the low pressures involved on a cold day or when nearly empty, then the hammer does hit that stop, to prevent wasting even more CO2.... but in a PROPERLY tuned PCP, the valve simply doesn't need to open that far.... Here is a device I made to measure the valve lift in a PCP....

http://i378.photobucket.com/albums/o...psf3b2824b.jpg

It is a piece of 1/8" aluminum rod, with a nut on the front end captured between the valve spring and the hammer, that travels with the hammer.... On the back end is an O-ring that is snug on the rod, and travels with it.... When installed in a Disco running a Challenger style RVA, it projects through the center hole in the adjusting screw like this....

http://i378.photobucket.com/albums/o...ps82051661.jpg

To determine the lift, you simply push the O-ring up against the back of the adjusting screw, cock the gun, and fire it.... The distance between the O-ring and the end of the screw is how far the valve opened.... In the photo, you can see it is less than 1/16".... This is typical of a PCP that is operating efficiently.... Incidently, if you measure the distance between the O-ring and the screw when the gun is cocked, you will also know the hammer stroke/travel....

Over the last couple of years, I have made a lot of lift measurements on many different PCPs, and the lift that is required is related to the diameter of the valve throat.... This makes sense, because a poppet valve has maximum flow when the valve is clear of the seat by 1/4 of the throat diameter.... Beyond that, the flow RATE cannot increase, because the area of the throat is smaller than the "curtain area" between the valve poppet and the seat, measured around the perimeter of the valve.... Here is a typical graph, showing the actual valve lift in my recent .30 cal Grizzly regulated PCP....

http://i378.photobucket.com/albums/o...ps5fpyblzo.jpg

You can plot a velocity curve like this for any pneumatic airgun (PCP, CO2, or pumper), at any pressure, and provided you can hit the valve hard enough with the hammer, you will get similar results for the velocity.... You will note that adding hammer spring preload past a certain point does NOT increase the velocity further, all it does is waste air.... This is because eventually, the valve is open after the pellet has left the barrel, and that is occurring on the flat part of the plateau on the left side of the graph.... Note that where the plateau starts is slightly different with different pellets.... as the weight increases, you need a bit more preload.... When the valve closes when the pellet is about half way down the barrel, you have in fact only lost about 3% of the velocity in most PCPs, so that point is where the velocity just starts to decrease.... I call this the "knee" of the velocity curve, and in the example above, it is at about 2 turns in (from zero preload) with the 45-50 gr. pellets, and 3 turns in with the 70 gr. bullets....

Now, look at the measured lift at those points.... It is between 0.080"-0.095".... The valve in the Grizzly has a throat diameter of 0.266", so that means the lift is about 1/3 of the throat diameter.... Opening it further than that only wastes air.... This is very consistent over all the PCPs I have checked, regardless of pressure, and the maximum lift required is always between 1/4-1/2 of the throat diameter for any usable tune.... In a detuned gun, it can be even less.... In an unregulated PCP, tuned for decent efficiency, the lift is typically about 1/4 the throat diameter at the beginning of the shot string (high pressure) and about 1/2 the diameter at the end (low pressure).... Therefore, you really will never use lift greater than about 1/2 the throat diameter, regardless of the pressure you tune for, if you have a reasonably efficient tune.... The only time the valve will open more than 1/2 the throat diameter is when you have the gun set up to waste air.... Therefore, allowing for more travel than that is basically useless.... I like to make sure that the hammer doesn't impact the valve, just so that I can be 100% sure that the lift is behaving in the self-regulating fashion we want from our PCPs, so in an unregulated PCP I allow about 75% of the throat diameter to insure such contact never occurs.... However, in every case, if I measure the lift, it turns out I allowed for way more lift than was actually needed....

Increasing the distance the valve can open can actually waste air.... When I got my FD-PCP, I noticed that the valve could only open about 0.10", which I thought was marginal (that is about 1/2 the throat diameter).... so I shaved 0.050" off the backing block, just to make sure the hammer didn't hit it.... The result was that I lost 1 shot and ended up with a shot string that dove more quickly after the peak velocity.... This could only be because the hammer was hitting the block once the pressure dropped below about 1100 psi, and preventing extra lift below that point.... When I machined off the back of the block, and the pressure was below 1100 psi, the valve opened more than 0.100", and the gun used more air, even though the velocity of those shots didn't increase.... The result was that I lost a shot....

I hope that this post will give you a better understanding of how much maximum lift you need to allow for in a PCP.... and how much lift is actually occurring when the gun is tuned efficiently....

Bob

How about valve "duration"- as in how long it stays open at maximum lift? Seems to me an airgun would be much like any valve-operated mechanism, it's rarely about "how far" (i.e. maximum valve height/extension)and more about "how much" (as in dwell/duration of it being open).

Lift and dwell are completely interrelated.... Once the valve is open 1/4 of the throat diameter, the flow RATE cannot increase.... However, as the valve coasts open further, it takes additional time to do that, and the dwell increases, so that maximum flow rate occurs during that extended time.... If the valve lifts twice as far, it is open for about twice as long.... The dwell, which is typically 1-2 mSec, is important in terms of the total amount of air that is released through the valve.... The longer the dwell, the more air is released.... However, once the dwell exceeds the length of time the pellet spends in the barrel (typically about 3 mSec), no further acceleration of the pellet it possible.... In fact, having the valve close when the pellet is only about halfway down the barrel only loses about 3% of the maximum velocity possible, while conserving a large amount of air....

Is it possible to drive a valve open more than half it's diameter?.... Of course it is.... With valves of normal proportions for the caliber, however, when you do that the dwell becomes so long that you are wasting a lot of air.... That is why I stated that for guns tuned to operate with reasonable efficiency, the lift ends up being 1/4-1/2 the throat diameter.... In fact, by the time the lift is 1/2 the diameter, the pellet is likely very close to, or has already exited from, the muzzle....

Bob

So I would have something to add.

Yes ago the airgun God's tested that a piston powered airgun did not need a barrel longer then 24" [ now that was for piston powered airgun.

now we have PCP air gun we can adjust the valve timing lift and duration so lets say we install a 32" Barrel on a PCP air gun could we adjust for best velocity and air conservation.

Let's say this set up is for hunting and power for ethical take down is important could we gain FPS with a 32" compared to a 24" barrel and is it worth it?

I'm Asking this my airgun God's LOL

FRANK

If you are talking a powerful PCP, then absolutely they can gain from a longer barrel.... In fact, the gains in FPE can be nearly proportional to barrel length, so going from a maxed out 24" to a maxed out 32" PCP you could gain as much as 33% in FPE.... The same thing applies to pressure, a PCP running 4500 psi can develop 50% more power than one running 3000 psi, assuming both are tuned to the maximum....

Springers, on the other hand get most of their velocity in the first 8-12" of barrel, and can actually be losing velocity past about 18"....

Bob

ok I need a 32" QB79 .30Cal or .303 or .308 barrel :))

and a valve and hammer combination that can take advantage of it....

Bob

you have already set up my valve in my QB79 to be very good so most of the work is done well ok ok some of the work is done, but this is just a pipe dream.

A QB79 will never be able to make use of a .30 cal barrel, let alone at that length.... The volume between the regulator and the valve is just wayyyyyyyyyyyyyy too small.... in fact it's only a fraction of the barrel volume....

Bob

Yes but My Disco on the other hand would, but I'm running that is a custom FT setup.


Sorry to change subject.

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