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Discussion in 'Beginners Forum' started by Pammo9, Sep 18, 2008.
Can someone please explain to me what a bore up is?
I have no clue. Never even heard of it. where did you hear of it?
Bore-Up cylinders have a greater volume than standard cylinders.
This is due to a slightly larger inner diameter.
Bore up piston heads have a larger outer diameter to match.
WARNING! SCIENTIFIC KNOWLEGE AHEAD!
In another thread, I posted the formula for figuring the volume of a cylinder.
Specifically, it's: Radius X Radius X Pi (3.1415) X Length.
As you can see, since you are essentially squaring the radius in the formula, volume increases exponentially as the radius increases. In other words, a small change in the radius equals a large change in volume.
A six inch pipe, one inch long, has a volume of 28 Cubic Inches.
A twelve inch pipe, one inch long, has a volume of 113 Cubic Inches.
In this example, when you double the diameter, you quadruple the volume.
The purpose of a bore-up set is to provide a greater volume of air to push a BB down a longer barrel.
I do not recall specifically, but I think the bottom end to use a bore-up kit is 590mm. I could be wrong on that, though.
If you do go with a bore-up kit, make sure you buy the kit. I have heard some people have issues trying things like a Guarder Bore-Up piston head in an ICS Bore-Up Cylinder.
Sometimes bore up kits create barrel suck no?
No, barrel suck is a myth. It doesn't exist.
I wrote this up for a local forum:
thaks for adding words to explain the math... i suck at math....
so i can now officially call bullpucky on a fellow much bigger than me talking about bb suck, thanks
+1 sparky d
now i have no fear of buying a bore up kit
well that helped me alot thanks.
Great Science but your math is off... Pi = 3.1415
Im sure it was just a typo
It's Canadian Pi after the exchange rate.
Yep, it's a typo.
I'd fix it, but it's too old to edit
I did get it right in the top post, though.
OK i like your theory on this however your MATH is wrong. remember that the BB starts from rest and accelerates to its muzzle velocity. the bb does not immediately start at the muzzle velocity as you assume. to find the actual time the bb takes to travel down the barrel you must use kinematics.
the math would look like this.
Assumptions; first the barrel is frictionless and therefor the bb will accelerate in a parabolic motion and effected buy the back spin or the bouncing around in the barrel. this is not the case and friction would cause the bb to have a longer in barrel time.
with these asumptions we know from high school physics that
X(final)=X(initial)+(Velocity(initial))x(time) + (1/2)x(Acceleration)x(time^2) (1)
we also know from calculus that velocity is the first derivative of displacement so that
Thus the time for the 400 FPS bb to travel down a 2 foot long barrel from rest is.
V(in)=0 V(fin)=400 ft/s X(in)=0 x(fin)=2 ft
we plug that into the equations and solve for t.
since we do not care about the acceleration we solve for that in the first to get
and we plug that into (2)
to get 2=(1/2)(400/t)(t^2)=200t
so the time for the BB to travel down a 2 foot barrel is .01 seconds not .005 seconds.
So if we take the worst case of 250 FPS, and a insane rig of 25 BPS that is the time down the barrel is .016 seconds with a per round cycle of .04 seconds we see that this is still not a problem.
So while your math was not spot on your conclusion was.
and oh ya we can also solve for acceleration, that for a 400 FPS BB and a 2 foot barrel the acceleration is 40000 ft/s^2 or 1242 Times that of gravity 1242 G-forces.
with a .2g bb that makes the relative weight of the bb when it is fired as if it were 248 grams or .55 pounds.
Well, I never was a wiz at math, but thank you for reinforcing the theory with real math skilz, lol.
I'm gonna have to bookmark this because there's no way I'm ever going to be able to remember all of that stuff.
Thanks, though. That could come in real handy.
And thats why I love physics.
(Still hate studying it)
So what would happen is you use a bore up kit for let's say a 200mm inner barrel? Would that in essence increase my velocity without the need to swap my spring? Any adverse effects?
Not your fps would stay the same. However it will put more wear on your gearbox.
You know how dry-firing harms the gun? It's because the piston slams forward into the cylinder head without having the resistance of the BB in the barrel there. The BB gives the air something to work against, causing back pressure in the barrel and cylinder.
The port in the cylinder is there to ensure the useable volume of the cylinder matched the volume of the barrel. If you have a short barrel, and use a full cylinder, the BB will be pushed out of the barrel during the first half of the piston's travel in the cylinder. This means that the piston is now free to travel the rest of the way forward until it smacks into the cylinder head.
Actually, barrel suck is the reason you would want a bore-up kit in the first place. I agree with you guys that the piston sucking air from the barrel is a myth, but I've heard the term barrel suck used to describe a different phenomenon.
Just to be clear on where I stand here:
Your math is right, and even if it wasn't the nozzle does indeed come out of the hop-up before pulling in air. The piston doesn't ever pull air from the barrel. But, the term barrel suck has been used to correctly describe a different phenomenon that does indeed exist.
When using a barrel that has a larger volume of air than your cylinder/piston pushes, the area in the barrel behind the bb becomes a vacuum at some point before the bb leaves the barrel. This is why bore-up kits are used. This vacuum effect can slow the bb anywhere from 5-50fps depending on the barrel/cylinder ratio. Though this is not the piston pulling air, it IS often referred to as barrel suck.
Not disagreeing with Sparky or Jlawrence here. Just wanted to clear up some semantics.
Uhm, ok, but you're adding to the confusion. There is no vaccuum effect. It's a simple matter of lack of air volume.
No vaccuum, no suck, no barrel suck, so don't call it barrel suck, because it isn't.
I'm sorry to make things more complicated than they have to be, but I'm not saying it should be called barrel suck, just that it has been called that a lot by other people. Also, in my defense, when the bb reaches and passes the point where the air pressure behind it is equal to the atmospheric pressure outside of it, the area behind it becomes by definition a vacuum. A vacuum is any area with lower pressure than the standard atmospheric pressure. The reason your fps drops when you add a longer barrel without a bore up instead of staying the same is because forces pull back at it just before it leaves the barrel.
I want to make it completely clear that I agree with you about the impossibility of the piston pulling air out of the barrel. However a phenomenon that is rightfully being referred to as barrel suck by people other than myself most certainly does exist. I think it would be a lot better for everyone if we make sure this phenomenon is understood. The last thing we need is for someone to read this and then pick a fight with someone over it when it could just as easily be a misunderstanding. Barrel suck does exist, and is legitimately a vacuum effect that pulls back at the bb before it leaves the barrel. However, it is caused by a bad cylinder to barrel ratio, and not by the cylinder refilling. It is also caused by failure to install a bore up when needed, and never by installing one. Installing a bore up when not needed can cause fps loss and damage to your gearbox for other reasons, but not due to barrel suck.
I guess what I'm trying to say here is that if you are going to upgrade your barrel or any part of the piston set, the only things you really need to be concerned with are the cylinder to barrel ratio and if/where the cylinder should be ported. All of this info is available on the net somewhere if you look for it.
Back to the thread's original topic, a bore up kit and other cylinder related upgrades are complex methods of fine tuning your gun's efficiency to get the best fps you can. If you find it hard to understand, you might want to have a professional handle your internal upgrades. Or you could just avoid changing your barrel length. (this paragraph was directed at general form readers and not at sparky)
You are confusing vaccuum with drag