[cantonjason]

We've just built a new Keezer (new to kegging) and I'm wondering how full we should fill the kegs.

The Keezer will hold three 5 gallon and one 2.5 gallon corny kegs and I'm thinking that they shouldn't be filled all of the way to the top (i.e. 2.5g in the 2.5g keg) to leave some headroom for the CO2.  However, I'm not sure

[eaholljr]

I just fill mine till my secondary is empty, minus the gunk. I still have headroom for CO2.

[troybinso]

There is a seam near the top of the keg that I think is the 5 gallon mark. I typically try to fill it up to there, and keep it below the level of the gas dip tube.

[Pinski]

I try to fill them between the top seam and the bottom of the IN tube.

[jeffy]

I get some satisfaction when I have exactly enough beer to fill the keg to the top.
I'm not sure how much it matters that you have any headspace in your keg.

[denny]

I get some satisfaction when I have exactly enough beer to fill the keg to the top.
I'm not sure how much it matters that you have any headspace in your keg.

For me, having some headspace is a necessity.  I don't keep the kegs constantly hooked up to CO2, so it's the CO2 in the headspace that gets absorbed into the beer.

[roguejim]

Still using those cobra taps, eh Denny?   

[jeffy]

I get some satisfaction when I have exactly enough beer to fill the keg to the top.
I'm not sure how much it matters that you have any headspace in your keg.

For me, having some headspace is a necessity.  I don't keep the kegs constantly hooked up to CO2, so it's the CO2 in the headspace that gets absorbed into the beer.

I use the high pressure and shake method.  I don't think the head space makes much difference this way.

[majorvices]

definitly leave some some headspace if you are force carbonating. the more headspace the faster it will carb.

[jeffy]

definitly leave some some headspace if you are force carbonating. the more headspace the faster it will carb.

Why?
Let's say I fill my keg to the top, then put 35 psi of CO2 on it and shake the keg for a minute or so until it stops accepting gas.  What difference would the headspace make?

[tubercle]

definitly leave some some headspace if you are force carbonating. the more headspace the faster it will carb.

I don't think it would matter with a regulator set as long as the surface area is the same.

[Mark G]

I fill to just below the gas-in dip tube. Getting beer sucked into your gas lines is a pain in the rear.

[bluesman]

The beer will continue to absorb CO2 until it has met equilibrium. If you apply 10psi of CO2 to the surface of the beer, it will absorb CO2 until the volume of CO2 in the beer is equal to the applied pressure of CO2 on the surface of the beer.

The rate of dissolution of CO2 in beer depends on:

Temperature and Pressure
Degree of under or over-saturation
Presence of mixing
Interfacial surface area between the CO2 and the beer (the more surface area...the more transfer of gas)

When the partial pressure of the undissolved gas meets the partial pressure of the dissolved gas, equilibrium has been met and the beer is then carbonated.  This is all assuming that the correct amount of CO2 pressure at a given temperature has been met.

[oly]

I fill to just below the gas-in dip tube. Getting beer sucked into your gas lines is a pain in the rear.

+1

[dak0415]

1) If you are using the set-and-forget method, the amount of headspace is irrelevant the surface area of the keg (when vertical) is the same whether you have 3 gallons or 5.15 gallons in the keg. 
2) If you are shaking then more headspace will carbonate faster since there will be more surface area as you move the keg from vertical.  If you connect and disconnect (with 35 or 40 psi) then more headspace will carbonate faster as there is more volume at 40 psi and the pressure will drop more slowly as the beer absorbs the CO2 trapped in the top of the keg.

My preferred method:
Beer should be chilled to 31-32 deg.
Fill keg to just below IN tube.
If beer comes out of the IN tube while filling then pressurize and draw a pint off.
Set regulator to 15lbs, connect the gas in tube, shake on side with gas tube toward the floor.
Shake for 3 minutes.
Put back in 31deg cooler for 1/2 hour to allow foam to settle.
Vent keg to 2-3lbs(ish) Put keg in keezer (still at 32 deg) and hook up to gas.

The beer will be drinkable immediately and will reach optimum carbonation in 2-3 days.

My $.02